Galvanizing Versus Galvanizing

Galvanized coatings are NOT all the same - Galvanizing versus Galvanizing

Introduction There are many types of coatings that are specified as hot dip galvanized. The process involves immersing steel in molten zinc. The zinc reacts with the steel to form the galvanized coatings. The time the steel is immersed in the zinc along with post-galvanizing treatment controls the coating thickness, appearance and other characteristics

Hot dip galvanized coatings are applied to steel to improve the anti-corrosion performance of the steel to ensure that it lasts as long as possible with a minimum of maintenance.

Standards currently being developed for the housing industry have set a benchmark of at least 50 years as the acceptable life of structural building products. Only hot dip galvanized steel products with the heaviest galvanized coatings are capable of meeting this requirement.

The Australian Standard AS 4680 - 2006 , Hot Dipped Galvanized Coatings on Ferrous Articles, includes galvanized coating standards on sheet, wire, tube and general articles. A great deal of confusion exists through the inclusion of galvanized coatings with significantly different coating characteristics within the same Australian Standard.

Coating Thickness Counts ... All sheet,wire and many tube products are CONTINUOUSLY galvanized. This means that the coating is applied at high speed and the coating thickness is controlled by the process. Immersion time in the zinc is measured in seconds. Alternatively, in the BATCH hot dip galvanizing process steel items are immersed for periods ranging from 3-10 minutes, depending on the mass of the items being galvanized.

These completely different methods of applying galvanized coatings produce different types of coatings.

There are 4 main differences that impact on anti-corrosion performance of BATCH galvanized steel compared to CONTINUOUSLY galvanized steel. These are:

1. Coating thickness - BATCH galvanized items of the same section thickness are typically at least 3 TIMES thicker than similar CONTINUOUSLY galvanized coatings on sheet and tube.

2. Coating hardness - BATCH galvanized items have much thicker zinc/iron alloy layers in the coatings which gives BATCH galvanized items 5 TIMES the abrasion resistance of CONTINUOUSLY galvanized coatings.

3. Coating integrity - BATCH galvanized coatings apply a uniform heavy coating to all internal and external surfaces, edges and cavities. CONTINUOUSLY galvanized coating will always have exposed bare steel at cut edges. CONTINUOUSLY galvanized hollow sections are fully galvanized on the external surfaces only.

4. Coating mass - The cathodic protection of exposed steel by zinc depends of the mass of the zinc in relation to the area of exposed steel. Because of the drainage characteristics of BATCH galvanized coatings, the coating mass on BATCH galvanized products is

significantly higher (typically 3-5 times) in proportion to thickness than CONTINUOUSLY galvanized coatings. Hot rolled medium structural sections commonly achieve coating mass levels exceeding 1000 g/m2. More Coating Thickness = Longer Coating Life

150 years of field testing has determined that all things being equal, galvanized coating life is equivalent to galvanized coating thickness. When comparing BATCH galvanized coatings to CONTINUOUSLY galvanized coating, all things are not equal.

The Cut Edge Factor All CONTINUOUSLY galvanized sections have exposed steel at cut edges and rely on the adjacent zinc in the coating to provide cathodic protection to the bare steel. This requirement accelerates the rate of corrosion of the galvanized coating at cut edges. The thicker the CONTINUOUSLY galvanized section, the faster the rate of coating corrosion at cut edges because of the greater area of bare steel exposed. Even if it was possible to apply a CONTINUOUSLY galvanized coating to a steel item to the same thickness as a BATCH galvanized item, the cut edge factor gives the BATCH galvanized coating a life typically 1.5 TIMES greater.

Comparison of Galvanized Coatings CONTINUOUSLY galvanized coatings comply very closely to their specified coating mass. BATCH galvanized coatings on hot rolled steel sections almost always exceed their minimum specified coating mass.

Galvanizing and Coal - 40 Years Experience

The use of hot dip galvanized coatings in contact with coal has been questioned by some involved in the specification of protective coatings for the coal industry. Industrial Galvanizers has been galvanizing steelwork of all kinds for the coal industry for nearly 30 years. The Hunter Valley, NSW is the centre of power generation for NSW and the use of galvanized coatings for conveyor steelwork in the power industry from the 1960's laid the foundation for the acceptance of hot dip galvanized coatings for the export coal industry.Industrial Galvanizers involvement with the coal industry has expanded to the central Queensland coal fields and the Victoria brown coal operations as the company has expanded structural galvanizing services into these areas.

The performance of the hot dip galvanized coatings in a wide range of coal industry applications has been monitored since the 1970's. This has provided a valuable insight into galvanized steel's performance and allows reliable predictions to be made on galvanized coating life. It has also highlighted high risk areas, design criteria to minimise stress on coatings and the coal operations that impact on coating performance and durability.

There are three factors that impact on galvanized coating life in coal mining and processing operations.

These are:

The nature of the coal, particularly sulfur levels and the form that the sulfur takes in the run of mine product and the processed (washed) coal.

The nature of the process water - salinity, pH, hardness. The local environments in which the galvanizing operates - housekeeping standards, time

of wetness.

The corrosion rate of hot dip galvanized coatings will be influenced by these factors individually and in combination.

Case History Performance Hot dip galvanized steel used on conveyor systems and associated above ground coal handling has given very good performance. Conveyor systems such as the No. 4 Liddell Power Station overland conveyor, installed in 1968, still retains approximately 20% of the original galvanized coating in the most severely affected areas, which are buried under coal spillage.

The stockpile conveyors at Kooragang Coal Terminal, currently one of the world's largest, shipping in excess of 50 million tonnes of coal annually, have been in service since 1983. All of the external conveyor steelwork remains in good condition, with all test sites still having over 75 microns of galvanized coating. Some sections under the conveyor belts, which are frequently wet by the de-dusting sprays, reached the end of their service life and were replaced after ten years. The Kooragang site is classified as an industrial marine environment

The Macquarie Coal Preparation Plant was commissioned in 1982, and has been monitored regularly since start-up. All the structural members, including the column bases subject to spillage and wash down runoff are still in good condition. The hardness of the process water has contributed to this good performance because of its scaling effect on the galvanized coating.

The worst areas for galvanizing have been in the areas immediately adjacent to the washery screens. These areas are constantly wet and are subject to spillage of low pH wash water. Rapid corrosion of the pre-galvanized purlins and Bondek continuously galvanized formwork supporting the concrete floors has occurred where they have been subject to wet spillage. Heavily galvanized purlins have been supplied as a replacement for the less durable pre-galvanized products with the expectation of 200% increase in service life.

Projects undertaken in Central Queensland, such as the North Goonyella Mine development, which commenced operation in the early 1990’s have has galvanized coating performance audits conducted after 12 years of operation. These audits found no significant deterioration of the coating with the likelihood of a maintenance free service life of the structural steelwork in the coal treatment plant exceeding 50 years.

Metals in contact – Avoiding Electrochemical Incompatability.

The derivation of the term ‘galvanizing’ has absolutely nothing to do with protecting steel from corrosion. The name comes from the Italian physiologist, Luigi Galvani, who identified the effects of electric current on the nervous system of dead frogs. In the formative years of electrical science, zinc was the most widely used metal for producing galvanic electricity. In

1837, French scientist Sorel took out a patent in France for a process of dipping steel in molten zinc and provided the process with the name `galvanizing’ in honour of Galvani, who died in 1798.

The Electrochemical Series of Metals When metals are in electrical contact, one metal will give up electrons and oxidise (the anode) while the current generated will prevent oxidation of the other metal (the cathode).

All metals have an electrochemical pecking order that determines whether they will act as an anode or a cathode to other metals in the Series. The following table illustrates the relative position of the common metals in the Electrochemical Series.

The electrochemical protection provided to steel by zinc coatings is a vital element in the effectiveness of galvanized coatings in protecting steel from corrosion. All pre-galvanized products rely on the cathodic protection provided by zinc to prevent corrosion of exposed steel at cut edges.

Industrial Galvanizers has frequent requests for information about the effect of galvanized coatings in contact with other metals. The most commonly asked questions involve the use of stainless steel fasteners in contact with galvanized coatings and contact between galvanized and ungalvanized reinforcing bar in concrete.

Electrochemical Corrosion and Galvanized Coatings Tables of electrode potentials are of value in drawing the attention to the dangers of electrochemical corrosion between dissimilar metals but such tables can be misleading. While the potential difference between metals is the prime driving force providing the corrosion current, it is not a reliable guide to the rate and type of corrosion occurring at a particular point of contact.

Metal / Alloy Potential (Volts)*Magnesium - 1.55Zinc - 1.10Aluminium - 0.86Cadmium - 0.77Cast iron - 0.68Carbon Steel - 0.68Stainless Steel - 0.61Lead - 0.57Solder - 0.52Tin - 0.49Copper - 0.43Aluminium bronze - 0.41

* All voltage values with respect to copper sulfate half cell.(Zinc Handbook, Porter 1991)

The severity of bi-metallic corrosion also depends on the ratio of the areas of metals in contact, the duration of wetness (bi-metallic corrosion can only occur in the presence of a conductive solution) and the conductivity of the electrolyte. The presence of oxide films on the surface of one or both of the metals can greatly inhibit bi-metallic corrosion.

In general, galvanized surfaces may safely be in contact with most aluminium alloys, stainless steel 304 and 316F, chrome steel (>12% chrome) and tin, provided the area ratio of zinc to metal is 2:1 or higher and oxide layers are present on the aluminium alloys and the stainless steels. Bi-metallic corrosion rates are greatly reduced if electrical resistance is high due to the presence of insulating films or other non-conductive membranes.

Where the points of contact between galvanized coatings and other metals are not subject to wetness, no bi-metallic corrosion will occur. This is important with galvanized reinforcing bar in contact with uncoated rebar. The points of connection are inevitably deep within the concrete mass and after curing of the concrete, are maintained in an inert environment.

The use of stainless steel fasteners on hot dip galvanized items in well drained atmospheric exposure conditions will also cause minimum stress to the galvanized coating, because of the very high zinc/stainless surface area ration and the short periods of wetness to which the assemblies are exposed in Australian weather conditions.

Electrochemical Protection and Coating Mass In any situation where zinc is corroded sacrificially to protect exposed steel, the mass of available zinc will determine the anti-corrosion performance. Corrosion rates of zinc coatings required to cathodically protect uncoated steel in aggressive environments (saltwater/marine) may be 25 times as high as the normal zinc corrosion rate.

How to Prepare Hot Dip Galvanized Coatings For Painting

There are many instances where hot dip galvanized coatings need to be painted. Industrial Galvanizers has been directly involved in the painting and powder coating of a large number of hot dip galvanized structures and items. There are well established quality assurance procedures for the painting of hot dip galvanized components in a controlled environment, but it a common requirement to apply paint coatings to hot dip galvanizing under separate contract arrangements or on site.

When a steel item is first hot dip galvanized, its surface is free from oxidation and contamination and is in the best condition for coating. It is also highly susceptible to oxidation, particularly reaction with atmospheric moisture. Most galvanizers quench the work in a weak sodium dichromate solution to passivate the surface. This chromate passivation film weathers away with time and is replaced by a stable complex carbonate oxide film. This dynamic set of surface conditions needs to be considered when painting galvanized steel.

In addition, surface contamination can occur that will interfere with paint adhesion. Diesel fumes are a common source of surface contamination that are very difficult to detect, as the galvanized coating may still appear clean and bright.

Where close control of surface condition is not possible, the best alternative to ensure a high quality paint application is to brush or sweep blast the galvanized surface immediately prior to painting.

This is a poorly understood technique with many paint contractors. Incorrect technique will cause serious damage to the hot dip galvanized coating.

The following specification is recommended for abrasive blasting of hot dip galvanized surfaces prior to painting.

Compliance with this specification will ensure that not more than 10 microns of zinc will be removed from the galvanized coating during the blasting process, and that the coating will not be damaged by fracturing of the alloy layers through excessive impact energy of the blast media on the galvanized coating.

Brush or Sweep Blasting Procedures for Preparing Hot Dip Galvanizing for Painting

1. Blast nozzle pressure 40 psi (280 kpa) maximum 2. Abrasive grade 0-2 - 0.5 mm 3. Abrasive type - clean ilmenite or garnet 4. Distance of nozzle from surface 400 - 500mm 5. Nozzle type - 10mm minimum diameter venturi type 6. Blasting angle to surface - 45 degrees

The aim of this blasting procedure is to remove any oxide films and surface contaminants from the surface. It is NOT to produce a profile similar to that required on bare steel. The brush blasting of the relatively soft zinc will automatically produce a fine profile, giving the clean surface a satin appearance.With inexperienced operators, a test section should be evaluated by measuring coating thickness before and after blasting with an approved magnetic thickness gauge.

A 5-10 micron reduction in galvanized coating thickness indicates an acceptable technique. Over 10 microns of coating removed indicates an unacceptable technique.

On reactive steel, the coating may already have a matte grey or satin appearance. This indicates the presence of the zinc-iron alloy layers at the surface, which also indicates a thicker than standard galvanized coating.

The micro-roughness of the alloy layers already provides a good mechanical key for appropriate paint, and only very light brush blasting is required on galvanized coatings of this type.

Grey galvanized coatings are more susceptible to mechanical damage than shiny coatings and should be treated accordingly.

White Rust on Galvanized Coatings

Introduction One of the commonly encountered problems with galvanized coatings of all kinds is ‘white rust’ or ‘white storage stain’. It is manifested as a bulky, white, powdery deposit that forms rapidly on the surface of the galvanized coating under certain specific conditions.

White rust can cause considerable damage to the coating and is always detrimental to the galvanized coating’s appearance.

The surface of galvanized coatings is almost 100% zinc. It is the durability of the zinc that provides the outstanding anti-corrosion performance for steel, yet zinc is a relatively ‘reactive’ metal. It is the stable oxides that form on the zinc’s surface that determine its durability, and these oxides are formed progressively as the zinc is exposed to the atmosphere. Carbon dioxide in particular is a contributor to the formation of these stable oxides.

With newly galvanized steelwork, the zinc’s surface has been subjected to little oxidation and is at its most vulnerable. For this reason, galvanizers use a chromate passivation in conjunction with their galvanizing operations to provide protection to the galvanized coating during the ‘youth’ period of the coating. This passivation coating provides short term protection to the zinc to give the stable oxides time to form on the surface.

White rust formation Pure water (H2O) contains no dissolved salts or minerals and zinc will react quickly with pure water to form zinc hydroxide, a bulky white and relatively unstable oxide of zinc. Where freshly galvanized steel is exposed to pure water (rain, dew or condensation), in an oxygen deficient environment, the water will continue to react with the zinc and progressively consume the coating. The most common condition in which white rust occurs is with galvanized products that are nested together, tightly packed, or when water can penetrate between the items and remain for extended periods.

Avoiding white rust formation There are a number of simple steps that can greatly reduce or eliminate the formation of white rust. These are:

1. Keep the packed work dry 2. Pack the items to permit air circulation between the surfaces3. Stack the packed items on an angle to allow water to drain out4. Treat the surface with proprietary water repellent or barrier coatings to prevent moisture

contact with galvanized surface.

Treating galvanized surfaces affected by white rust Once the galvanized surface has been attacked and the zinc hydroxide compounds have formed, it is desirable to remove the oxide products from the surface because:

a.their presence inhibits the formation of stable carbonate based oxides and b.they are unsightly

The effect on the galvanized coating can range from very minor to extremely severe and various levels of remedial treatment are available to deal with white rust problems at the various levels at which they are likely to occur.

The following treatments are recommended to deal with white rust on galvanized products.

1. Light white rusting. This is characterised by the formation of a light film of white powdery residue and frequently occurs on freshly galvanized products during periods of heavy rain. It is particularly evident on areas that have been buffed or filed during quality assurance operations. These treatments remove the passivated surface from the galvanizing and expose unoxidised zinc to attack from rainwater. Provided the items are well ventilated and well drained, white rust rarely progresses past this superficial stage. It can be brushed off if required but will generally wash off in service with normal weather. No remedial treatment is generally required at this level.

2. Moderate white rusting. This is characterised by a noticeable darkening and apparent etching of the galvanized coating under the affected area, with the white rust formation appearing bulky. The galvanized coating thickness should be checked to determine the extent of attack on the coating. In the majority of cases, less than 5% of the galvanized coating will have been removed and thus no remedial work should be required, as long as the appearance of the affected area is not detrimental to the use of the product and the zinc hydroxide residues are removed by wire brushing. If appearance is unacceptable, the white rust affected area can be treated as follows: a. Wire brush the affected area to remove all white corrosion products b. Using a cloth pad wet with aluminium paint, rub the surface with the pad to apply a thin film of aluminium paint to the affected area to blend it with the adjacent unaffected galvanized surfaces.

3. Severe white rusting. This is characterised by very heavy oxide deposits. Items may be stuck together. Areas under the oxidised area may be almost black of show signs of red rust. A coating thickness check will determine the extent to which the galvanized coating has been damaged. Remedial treatment to reinstate the coating should be undertaken as follows: a.Wire brush or buff the affected area to remove all oxidation products and rust if any. b.Apply one or two coats of approved epoxy zinc-rich paint (see section “Repairs to Hot Dip Galvanized Coatings) to achieve required dry film thickness of 100 microns minimum.

Chemical removal of white rust Zinc hydroxide dissolves readily in chromic acid. Bulky white rust deposits can be quickly removed by washing with a 5% solution of chromic acid. Precautions must be taken in the handling and containment of this solution and its residues. This treatment has the advantage of re-passivating the zinc surface and is well suited to treating batches of small parts. Other

chemical treatments based on phosphoric acid can be used, however, these will darken the coating in the process.

Re-passivating the galvanized surface Where white rusting has occurred and the item may be subject to continuing exposure that may propagate similar corrosion, re-passivating of the surface can be done by treating the surface with a solution of 5% sodium dichromate, 0.1% sulfuric acid, brushing with a stiff wire brush for 30 seconds before thorough rinsing of the surface.

Conclusion White rust is a post-galvanizing phenomenon. Responsibility for its prevention lies in the manner it is packed, handled and stored prior to the galvanized product’s installation and use. The presence of white rust is not a reflection on the galvanized coating’s performance, but rather the responsibility of all those involved in the supply chain to ensure that the causes of white rust are recognised and the risks of its occurrence minimised on newly galvanized steel.

Galvanizing Special and Alloy Steels – what to look out for.

Introduction From time to time, galvanizers get inquiries about galvanizing special steels or unusual steel sections that are outside the normal range of steels processed on a day to day basis through jobbing galvanizing operations. While these steels can present problems for galvanizers, it is sometimes possible to accommodate these problems in the design of the components or through modifications to the hot dip galvanizing process.

Galvanizing alloy steels While high strength alloy steels are rarely used for structural applications, they are sometimes used as performance-critical components in assemblies or as individual manufactured items. There are three factors that effect the ability of steel to be galvanized. These are:

1. The chemical composition of the steel 2. The strength rating of the steel (the yield strength in MPa) 3. The steel’s section thickness*

* This is a factor in that it determines the immersion time of the steel in the molten zinc.

When a request is received to galvanize an unusual type of steel, the chemical composition is first checked. Most special steels contain carbon (C), phosphorous (P), manganese (Mn), silicon (Si), sulfur (S), chrome (Cr), nickel (Ni) and may also contain copper (Cu), Vanadium (Va) and other elements that are used to give the steel particular performance characteristics.

What is in alloy steels?While there are hundreds of different types of special steels, there are generic chemistries that fit particular applications and these are useful in determining what is likely to happen when these steels are hot dip galvanized. The following list is a guide to the most prominent alloying elements likely to be in these special steels that will have an effect on their ability to be galvanized:

1. Spring steels . These steels contain high levels of silicon which can be up to 2.0%. 2. Tough steels . These steels contain high levels of manganese which may be over 1.0% 3. Hard steels . These steels contain high levels of carbon which may be over 0.8% 4. Free machining steels . These steels contain high levels of sulfur 5. Electrical steels . These steels contain high levels of phosphorous 6. Stainless steels . These steels may contain high levels of nickel, chrome and manganese

The galvanizing characteristics of these steels are as follows:

1. High silicon steels will produce thick galvanized coatings which may be brittle because the steel reacts very rapidly with the zinc. The effects of high silicon content can be minimised by keeping immersion time in the zinc as short as possible. This becomes increasingly difficult as section thickness increases.

2. High manganese steels will produce brownish coloured coatings that may be brittle and easily damaged in handling compared to galvanized coatings on conventional steels.

3. High carbon steels can be successfully galvanized as long as their yield strength is within an acceptable range (see note below).

4. High sulfur steels are used for high speed machined components (threaded fasteners, sockets etc) and should not be galvanized. The high sulfur steel can be severely eroded in the galvanizing process, rendering threaded items unserviceable.

5. High phosphorous steel are rarely encountered in galvanizing operations but are unsuitable for galvanizing. They react rapidly with the zinc to form thick, dark coatings that are easily damaged and may delaminate from the surface.

6. Stainless steels can be galvanized but are susceptible to liquid metal embrittlement and can fracture under load after immersion in molten zinc. Stainless steels are only galvanized incidentally if they are attached to mild steel assemblies.

Steel strength and galvanizing High strength steels (around 800 MPa yield strength and over) are susceptible to hydrogen embrittlement arising from the pickling process in galvanizing. Pickling should be avoided for steels in this strength range.

Steel size and galvanizing While all conventional steel structural sections can be galvanized, from time to time, unusual sections arise that may present problems. Very thick sections over 100 mm in thickness may be difficult to galvanize acceptably in a conventional galvanizing bath. The mass of these items per unit of volume is very high, and as the zinc in the galvanizing bath is only about 35 degrees above its freezing point, the zinc freezes around the item when it is immersed, and may form a layer of frozen zinc 50 mm or more in thickness.

This zinc has to be re-melted and then the item itself heated up by the zinc bath to bath temperature for the galvanized coating to form. This sequence of events may interfere with the performance of the flux on the surface of the item and cause uncoated areas on the surface.These defects can be minimised by pre-heating the item or operating at higher galvanizing bath temperatures, which requires special galvanizing bath design.

Conclusion It is possible to successfully galvanize difficult steels, particularly spring steels which are the type most likely to turn up for galvanizing. As long as the section thickness is not sufficient to be the determining factor in immersion time, and the springs can be abrasive blasted rather than pickled, an acceptable galvanized coating can be produced on certain types of coil and leaf springs that will not affect their performance.

What Tolerances are required on Moving Parts that are hot Dip galvanized?

Introduction: There are many applications where galvanized components have to fit together after galvanizing and galvanizers receive regular inquiries about dealing with the clearances required. There are many factors that interact and the following information is to provide some basic rules for determining tolerances on moving parts.

Fasteners and threaded components There are a large number of threaded components presented for hot dip galvanizing in the form of U bolts, rag bolts, foundation cages, studs and threaded attachments. Where possible, threaded components are centrifuged to ‘spin’ off excess zinc. Where this is not possible because of the size of the item, mechanical cleaning of the threads is required after galvanizing. This is done by heating the threaded area* until the excess zinc starts to melt and then wire brushing the threads to remove excess zinc.

*Note: The hot dip galvanized coating, because of its alloy layers, has a higher melting point that the free zinc that makes up the excess coating. Provided the item is not heated above about 550-600oC, there will be no damage to the majority of the galvanized coating.

It is not recommended that nuts or internally threaded components be galvanized. Nuts must be tapped oversize for use on galvanized bolts. The dimensions for over tapping of female threads are as follows:

• 12 mm and smaller 0.40 mm• Over 12 mm to 25 mm 0.53 mm• Over 25 mm 0.79 mm

On larger diameter threaded items, where standard taps may not be available for over tapping, cutting threads on bolts 0.79 mm undersize will allow accommodation of the galvanized coating.

Theoretical clearances On most galvanized items that need to accommodate moving parts, the thickness of the galvanized coating is typically 100 microns or 1/10 mm. In an axle/socket arrangement, there are 4 such galvanized surfaces, which add a theoretical 400 microns (0.4 mm) to the surfaces in contact. On the full range of sections, from under 3 mm to over 10 mm, galvanized coatings may range from 50 microns on the lighter material to 150 microns on the heavier sections.

Practical clearances Galvanized coatings are specified in Australian and international standards in coating mass (grams per square metre) and not in thickness. They are commonly converted to thickness because it is easier to relate to for most specifiers as all paint coatings are specified in terms of thickness. In practice, galvanized coatings are not of uniform thickness. Hot dip galvanized coatings in particular can vary widely in coating thickness due to localised variations in steel composition, surface condition and orientation during the galvanizing process. These variations need to be accommodated with moving or closely fitting parts.

Radial clearances Radial clearances in socket and shafts should be not less than 1.6mm and preferably 2.0mm. Double sided surfaces (hinges) should provide not less than 0.8 mm clearances prior to galvanizing to allow correct closing or mating of surfaces after galvanizing.

Moving part assemblies: Galvanizing moving part assemblies is not recommended. All components should be separated prior to galvanizing and assembled afterwards. Even if the assemblies are designed with adequate clearances, the surface tension effects of the molten zinc will trap excess zinc in joints. This will ‘solder’ moving parts together and is very difficult to remove without risking damage to the coating.Closer fitting parts: Where tighter tolerances are required, machining of the bearing area of pins and shafts may be required after galvanizing. Because of the nature of the galvanized coating, which consists of a series of hard (harder than 250 grade steel) alloy layers coated with a soft free zinc layer, it is often possible to ream holes and sockets with correctly sized reamers to remove excess zinc without removing the galvanized coating entirely.

As long as there is an adequate zinc coating on one surface, and the uncoated steel surface is in intimate contact with it, then the zinc will cathodically protect the uncoated steel and prevent corrosion inside the assembly. This phenomenon is used universally with fasteners, where the close contact between the threads on a galvanized bolt and the uncoated female threads on the nut provide acceptable protection from corrosion.

Conclusion When designing for clearances after galvanizing, a factor of a minimum of 4X over the expected coating thickness should be applied, after adding up all the surfaces involved in the assembly.

Example: Item - Heavy shaft and bush. Number of surfaces - Shaft 2, bush 2 = 4Expected coating thickness - 125 micronsClearance required - 4 x 125 x 4 = 2000 microns (2.0 mm)

How to Minimise Distortion when Hot Dip galvanizing

When steel sections or fabrications are immersed in molten zinc, their temperature is raised to that of the molten zinc which is typically 455oC. The rate at which the steel will reach this temperature across its entire surface will depend on:

the thickness of the sections used in fabricating the item the rate at which the item can be immersed in the molten zinc the total mass of the item the dimension of the item - large items exceeding bath dimension require double-dipping.

At galvanizing temperatures, there is no change to steel’s metallurgical micro-structure and the galvanizing process is not hot enough to have any affect on the mechanical properties of the steel after galvanizing.

However, at galvanizing temperatures, the yield strength of steel is lowered by approximately 50%. If the adjacent steel is not at the same temperature and any stresses exist, the weaker area will be subject to movement by the stronger area. There is a responsibility on the designer, the fabricator and the galvanizer to co-operate in ensuring that distortion risks are minimised or eliminated.

Use design and fabrication techniques to avoid distortion

Design and fabricate sections of uniform steel thickness. Use symmetrical designs where possible, and avoid asymmetrical designs where cleats or

plates are welded to one side only of a beam or RHS section. Avoid designs which require fabrications with a large surface area of thin plate to be

double-dip galvanized. During fabrication use balanced or staggered welding techniques to avoid uneven locked-

in stresses. If cutting a plate to size, ensure all sides are cut using the same technique. Guillotine is

the preferred cutting technique. Ensure that the structural design of the item is sufficient to support its own weight at

50% of the steel specified yield strength. Consider temporary bracing if potential to yield exists.

Ensure that venting and draining holes are adequate. This will allow the item to be immersed and withdrawn from the molten zinc as quickly as possible.

During fabrication, accurately perform parts to avoid force or restraint during welding. Consider (or consult your galvanizer) the hanging requirements for the hot dip

galvanizing process. This will ensure the fabrication is adequately supported throughout the process.

Items that are prone to distortion Most steel sections and fabrications that are hot dip galvanized never give rise to distortion problems. However, certain types of products have a high risk of losing dimensional stability during the galvanizing process. Some examples are:

Thin (6 mm and under) flat sheet and plate will almost always ripple or buckle unless it is ribbed or corrugated. Flat steel sheet used for box trailer floors will always buckle when the trailer is galvanized. The ribbed sections of the same thickness (1 mm) used for the side sections will rarely buckle.

Long lengths of light wall pipe (spiral or seam welded) or other long, thin sections can be prone to bending during the galvanizing process. As the yield strength of the steel is halved at galvanizing temperature, long lengths of light section can bend or distort under their own weight. This can be avoided by designing dipping equipment that supports the product or by adequate hanging or support points along the length of the section.

Floor plate welded to heavier structural framework render the fabrication prone to distortion because of differential expansion and contraction of the plate and structural sections. The framework and the plate should be galvanized separately and then mechanically fixed.

Welded beams with a flange to web thickness ratio of greater than 2:1, particularly long beams that need to be double-dipped, may present a risk of distortion. Your galvanizer should be consulted at the design stage to ensure satisfactory processing of these types of products.

Fabrications containing large areas of asymmetrical welds (e.g. crane beams). The welding stresses built into these fabrications will significantly increase the risk of distortion as the temperature of the galvanizing process will stress relieve the fabrication.

In most cases, distortion risks can be minimised or designed out of fabrications in consultation with the galvanizer. Efficient design that allows the fabrication to be quickly immersed in the molten zinc to minimise differential temperature stresses that are the major cause of distortion.

Improving Galvanized Purlin Performance

Introduction Industrial Galvanizers Corporation introduced its PermagalTM Purlin service in 1997 provided a heavy-duty solution for galvanized steel purlin applications that operate in more aggressive environments such as indoor swimming pools, chemical plants of other industrial facilities that generate high levels of humidity inside their structures. PermagalTM Purlins were developed to meet the need for a galvanized purlin with significantly improved durability over the pre-galvanized cold formed purlins. PermagalTM Purlins are hot dip galvanized after slitting, punching and cutting that produces a galvanized purlin free from exposed, untreated edges. In addition, PermagalTM Purlins have a zinc coating mass significantly heavier than that applied to a Z350 pre-galvanized purlins.

Facts About Permagaltm Purlins

PermagalTM Purlins have a zinc coating mass typically three times greater than Z350 pre-galvanized purlins.

The maintenance-free life of a PermagalTM Purlin is guaranteed to exceed that of pre-galvanized purlins in proportion to the zinc coating mass. This is typically three times the life.

PermagalTM Purlins are hot dip galvanized to comply with the Australian Standard for Hot Dip Galvanized Coatings on Ferrous Products AS 4680, and the International Standard ISO 1461.

All edges and holes are completely protected on PermagalTM Purlins as they are hot dip galvanized after manufacture. This significantly reduces the corrosion stress on the

coating. Heavy (3mm) pre-galvanized purlins will show evidence of cut edge corrosion almost immediately after installation in aggressive environments.

The availability of PermagalTM Purlins allows the specification of a completely hot dip galvanized package of structural steel, purlins, girts and fasteners.

The hot dip galvanizing process does not affect the structural strength of the sections. Industry standard load tables are available for use with PermagalTM Purlins.

Typical Applications For Permagaltm Purlins PermagalTM Purlins have already been used in a wide range of heavy duty applications. Details of these case histories are available from Industrial Galvanizers if required. These applications include:

Coastal or marine structures where standard purlins would require a multi-coat paint system for acceptable durability

Aquatic centres, indoor heated pools subject to high levels of humidity. Mining and mineral processing plants - coal treatment plants, concentrate storage

buildings, refineries subject to chemical fumes, high humidity and frequent wash-down. Conveyor stringers to bring the durability of the supporting structure in line with the hot

dip galvanized conveyor systems. Water reservoir roofing structures. The under-roof areas of water storages are subject to

high levels of humidity.

How Does the Durability of Other Zinc Coating Compare with hot dip galvanizing?

Introduction Consumers at the domestic and industrial level are frequently confronted with the need to select steel products that are already galvanized. The fact that they are ‘galvanized’ is used as a major selling point. In many cases, the standard of the ‘galvanized’ coating may not be not clearly represented, and in some cases, misrepresented.

Claims may be made by the manufacturer that can not be substantiated in the field. With other products, particularly those that are zinc plated, descriptions such as ‘galvanized’ are used on the packaging that deliberately mislead buyers into expectations of durability that will never be realised.

More and more products are being introduced that are galvanized by high-speed, in line galvanizing technology. This allows a thin zinc coating to be applied to the steel at low cost. These thin zinc coatings are frequently coated with clear polymer topcoats to enhance their storage characteristics and in some cases, claims have been made that the addition of these polymer topcoats significantly improves the durability of the coating compared to a conventional galvanized coating. The addition of organic coatings to zinc plated parts is also a common technique that the manufacturers claim improves the corrosion resistance of their products. What are the facts?

Coating characteristics Zinc plating involves the electrolytic application of zinc by immersing clean steel parts in a zinc salt solution and applying an electric current. This process applies a layer of pure zinc that ranges from a few microns on cheap hardware components to 15 microns or more on good quality fasteners. Technical and cost issues prevent the economical plating of components with much heavier coatings.

In-line galvanized coatings are applied during the manufacturing process of hollow or open sections, with the cleaned steel section exiting the mill and passing into the galvanizing bath. This process applies a coating of zinc to the surface that can be controlled in thickness. This coating is usually measured as coating mass in grams per square metre and ranges from a minimum of about 100 g/m2 upwards, with an average around 175 g/m2 .

Accelerated weathering testing of coatings has traditionally been done in salt spray cabinets. This testing technique has been largely discredited with respect to metallic coatings as it does not reflect the way metallic coatings weather in atmospheric exposure conditions where the development of stable oxide films gives these coatings there excellent anti-corrosion performance. The addition of polymer topcoats to metallic coatings will significantly improve their apparent performance in salt spray tests but field performance will not necessarily reflect this.

Finding the facts The South African Bureau of Standards has undertaken accelerated weathering trials of polymer coated in-line galvanized coatings and compared them with conventional in-line galvanized and hot dip galvanized coatings to evaluate the effect on durability of the addition of these this polymer topcoats. A summary of this report follows. (A full copy of the report is available from the Galvanizers Association of Australia, 124 Exhibition St, Melbourne, 3000. Ph: 03 9654 1266, Fax: 03 9654 1136).

S.A.B.S. Report The samples were subjected to Salt Fog Testing, Damp SO2 Atmosphere testing and QUV Weatherometer testing as well as Hardness Testing. The conclusion of the SABS report states the following:

"The results of the accelerated corrosion tests indicate that the expected life of the continuously galvanized and lacquer coated samples will not be essentially different from the commercially continuously galvanized sheet material. Test results demonstrate that the expected life exhibited by the standard hot-dip galvanized panels (zinc coating thickness approx. 100 microns) can be considered to be significantly superior to the continuous galvanized/lacquer samples. The lacquer coating appears not to be fully effective in inhibiting the onset of corrosion under damp conditions due to porosity.It is well known that the zinc/iron alloy layers of standard hot-dip galvanized coatings are hard in nature (in excess of 200HV - often harder than the base steel itself). Conventional hot dip galvanized coatings, consisting of alloy layers with a soft zinc outer layer, therefore provide in essence a buffer stop coating which withstands knocks and abrasion. The soft nature of continuous galvanized lacquer coating (75 HV) coupled with the low coating thickness indicates

that these coatings will not have the same ability to withstand rough handling compared to conventional hot-dip galvanized items." Poor performance from plated coatings

Zinc plated coatings are not suitable for exterior exposure applications. Zinc plated bolts and hardware fittings such as gate hinges will not provide adequate protection from corrosion, and will rarely last more than 12 months in exterior exposures in most urban coastal environments.

Zinc plated products have an attractive appearance when new as the zinc coating is bright and smooth, where a hot dip galvanized coating has a duller and less smooth surface. There is typically about 10 times as much zinc applied to small parts in the hot dip galvanizing process than is the case with zinc plating. A bright, shiny smooth zinc finish on builders hardware (bolts, nuts, hinges, gate latches, post shoes) indicates a plated coating that will not provide adequate corrosion resistance and will rarely provide more than 12 months protection in most of the coastal population centres.

What Size Vent and Drain Holes are Needed for Hot Dip Galvanizing Hollow Sections

Introduction One of the most common issues in designing fabrications for hot dip galvanizing is ensuring that fabrications are vented and drained correctly. All steel to be galvanized needs to be immersed in molten zinc and the zinc needs to be able to flow freely into and out of all hollow sections and corners.

The flow of molten zinc into, off, and out of the fabrication is one of the most important factors in determining the final quality of the coating. Inadequate venting and draining can cause the following galvanized coating defects:

misses in the coating caused by air locks preventing molten zinc contacting the steel surface.

puddling of zinc in corners, wasting zinc and interfering with subsequent assembly ash trapped on zinc surface causing surface defects irregularities in surface appearance caused by erratic immersion and withdrawal because

of item floating or trapping zinc internally thick zinc runs on surface caused by zinc freezing during draining steel is only about 15% heavier than zinc. A relatively small amount of air trapped inside

a hollow section will prevent the section from sinking in the molten zinc any water trapped inside a hollow section will expand 1750 times its original volume as

steam and generate pressures as high as 50 MPa (7250 psi).

Basic Venting Rules

no vent hole should be smaller than 8 mm the preferred minimum size is 12 mm

about 200 grams of zinc ash will be produced for each square metre of steel surface galvanized.This ash is a solid powder and will not pass through small openings. Venting large internal areas required larger vent holes to allow ash to escape

hollow vessels require 1250 mm2 of vent hole for each cubic metre of enclosed volume. This means that a 40 mm2 diameter hole is required for each cubic metre of volume

hollow sections such as tube, RHS and SHS require minimum vent hole area equivalent to 25% of the section’ diagonal cross section

vent holes should be at the edges of hollow sections

Basic Draining Rules

no drain hole should be less than 10 mm preferred minimum drain hole size is 25 mm large hollow sections ( tanks, pressure vessels) require a 100 mm diameter drain hole for

each cubic metre of enclosed volume drain holes should be at the edges of hollow sections. hollow sections such as tube, RHS and SHS require minimum drain hole area equivalent

to 25% of the section’ diagonal cross section. The preferred design option is to leave the ends of tubes, RHS and SHS open.

Circular Hollow Section

Nominal Bore

rectangular hollow section

size mm

Square hollow section size mm

vent hole diameter

single hole Double hole

8 8

10 10

15 10

20 13 x 13 10

25 16 x 16 10

32 19 x 19 10

40 38 x 19 25 x 25 10

50 38 x 25 32 x 32 12 2 x 10

65 64 x 30 – 76 x 38 51 x 51 16 2 x 12

80 76 x 51 - 89 x 38 64 x 64 20 2 x 14

100 102 x 51 – 102 x 76 76 x 76 25 2 x 18

127 x 51 – 127 x 64 89 x 89 25 2 x 18

125 127 x 76 – 152 x 76 102 x 102 32 2 x 22

150 152 x 102 127 x 127 38 2 x 27

200 203 x 102 – 203 x 152 152 x 152 50 2 x 35

250 254 x 152 202 x 203 63 2 x 45

300 305 x 203 254 x 254 75 2 x 54

350 305 x 254 305 x 305 88 2 x 63

400 100 2 x 70

Table of vent and drain holes for tanks and pressure vessels

Capacity - litres Single drain hole diam. mm

Double drain hole diam. mm

Vent hole diam. mm

500 80 25

1000 115 2x 80 40

1500 140 2x100 45

2000 160 2x115 55

2500 175 2x125 60

3000 200 2x140 70

3500 225 2x150 75

4000 225 2x160 80

4500 240 2x170 85

5000 250 2x175 90

5500 265 2x185 95

6000 280 2x200 100

7000 300 2x220 110

8000 325 2x225 115

9000 350 2x240 120

10000 350 2x250 125

Does the hot dip galvanizing process affect steel strength?

Introduction Over the past 10 years steel makers worldwide have developed new structural grade steels with higher yield and tensile strengths. These developments have enabled manufacturers to design their steel products using lighter-section steels which in turn reduce the production, transport and erection costs of the finished product. Prior to these developments, the steel fabrications which were most commonly galvanized were manufactured from Grade 250 MPa hot rolled structural steels. Since the early 1970’s, the results from research and testing centres around the world have shown that the hot dip galvanizing process does not affect the tensile and proof (yield) strengths of the Grade 250 MPa structural steels. But does the hot dip galvanizing process affect the yield and tensile strengths of the newer high-tensile grades of structural steels?

Galvanizers have been asked these questions on a number of occasions following claims made by others that galvanizing of these steels affected their tensile strength and performance. To ensure that factual information was available, the industry has undertaken a number of testing programs to verify the effects of hot dip galvanizing on steels with yield strengths up to 500 MPa

Objective The aim of these test programs was to establish what effect the hot dip galvanizing process of dipping steel in molten zinc has on the strength properties of a typical high tensile steels using standard hot dip galvanizing practices. These practices include duplicating the immersion time of the steel in the molten zinc (this does not exceed 15 minutes under normal conditions) at a

temperature of 455 degrees Celcius.Test 1 Product: ha70t-p hot rolled, with black finish manufactured by BlueScope steel.

HA70T-P hot rolled steel has a guaranteed minimum yield strength of 450 MPa and a minimum hardness of 70 HRB. The typical yield strength is between 520 to 610 MPa. The typical tensile strength is between 530 to 620 MPa. This steel is normally used in shelving, automotive parts and more recently for purlins.

ProcedureThe test procedure involved cutting eleven pieces from a of 3.0mm thick black HA70T-P steel coil. Six of the pieces were hot dip galvanized in accordance with AS 4680-1999.

Test 2 Product: galvaspan G450 zinc coated, structural grade manufactured by BlueScope Steel.GALVSPAN G450 has a guaranteed minimum yield strength of 450 MPa and is an in-line hot dip zinc coated structural grade steel. The typical yield strength is between 470 to 550 MPa. The typical tensile strength is between 510 to 600 MPa. This steel is normally roll formed into products such as purlins, girts and light structural profiles.

ProcedureThe test procedure involved cutting six pieces from a single length of a roll-formed Z25024 purlin, which had been roll formed by BHP Building Products. The steel thickness was 2.4 mm. Three of the pieces were acid pickled (to completely remove the Z350 mill applied zinc coating) and hot dip galvanized in accordance with AS 4680-1999.The remaining pieces were left in the mill applied Z350 Zinc coating (as rolled) finish.

All sections were then delivered to the BHP Port Kembla Technical Services for testing.

Test 3 Product: OneSteel Grade 500 Plus (Microalloyed and Tempcore) reinforcing barOneSteel Grade 500 PLUS reinforcing bar is manufactured in straight lengths using the TEMPCORE process (quenching and tempering) and in coil for using micro-alloying of the steel. This product has a guaranteed minimum yield strength of 500 MPa. All steel reinforcing products are designed to be bent in accordance with relevant design codes for concrete construction.

ProcedureSamples from a number of different steel heats, in sizes of 12, 24 and 36 mm diameter with both Tempcore and mico-alloyed chemistry were tested, with samples galvanized in both straight lengths and after bending. Control samples from each batch were tested in conjunction with the galvanized samples.

Results Averaging of the results of the yield strengths of the uncoated sections and the results for the galvanized sections of the HA70T-P indicated a difference of 0.4%. As this variation is less than 1% it is considered to be within the accuracy tolerance of the testing and steelmaking procedures.

Averaging of the results of the yield strengths of the uncoated sections and the results for the galvanized sections of the Galvaspan G450 indicated a difference is 0.6%. As this variation is less than 1% it is considered to be within the accuracy tolerance of the testing and steelmaking procedures.

Averaging the results of the OneSteel 500Plus reinforcing bar yield strengths indicated a small increase in yield strength from 592 MPa to 602 MPa or 1.45%, after galvanizing the reinforcing bar. This variation is within the acceptance limits for this product.

SummaryThese tests verify that hot dip galvanizing has no effect on the mechanical properties of standard grades of steel. This is consistent with principles associated with steel metallurgy as the temperatures involved in the galvanizing process are well below the transition range fort structural steels.

How long do galvanized coatings last?

Introduction Galvanized coatings have an unusual characteristic compared to other protective coatings in that they fail by weathering and oxidation from the surface. Paint coatings, once breached, deteriorate through under-film corrosion and can suffer rapid failure as a result.

Because of the electrochemical protection provided to steel by zinc (galvanized) coatings, no corrosion of the steel will occur while there is any zinc present, regardless of the thickness or condition of the galvanized coating.

Galvanized coatings, in atmospheric exposure conditions, corrode at an approximately linear rate. Once this rate has been established for a particular environment, the expected life of the coating can be defined by relating the rate of corrosion to the thickness of the coating.Factors affecting galvanized coating life

The durability of galvanized coatings depends on a number of environmental factors. These include:

Time of wetness Ambient temperature pH of moisture Chloride levels in atmosphere Sulfate levels in atmosphere Contact with other chemicals Contact with dissimilar metals Orientation of exposure (vertical, horizontal) Nature of exposure(sheltered, open) Ventilation conditions

Corrosion engineers take these factors into account when assessing the life-cycle performance of galvanized coatings. Organisations such as the CSIRO have developed environmental assessment techniques based on atmospheric computer models that facilitate the accurate assessment of metallic corrosion rates.

A number of international (ISO) standards have also been developed that use combinations of the parameters listed above to tabulate corrosion rate data for zinc (galvanizing) and other metals.

Classification of environments Most standards and documents associated with coating performance use exposure classifications to define corrosivity of the atmosphere. For metallic coatings such as galvanizing, factors such as UV exposure do not impact on coating life, where with paint coatings, UV levels are an important factor in their durability.

For galvanized coatings, common Australia exposure classifications are arid/rural, mild/urban, industrial, marine and tropical. Much exposure testing has been done to obtain corrosion rate data in these environments, and this work is ongoing.

Testing done by Industrial Galvanizers in a number of long-term case studies has indicated that hot dip galvanized coatings in service may have lower corrosion rates than those of zinc coupon samples exposed in test facilities.

Reasons for this apparent lower rate of in-service corrosion have not been quantified, but are thought to be related to the quite different characteristics of a hot dip galvanized coating compared to pure zinc, typical of the samples used in exposure testing.

The hot dip galvanized coating contains alloys of iron, aluminium and sometimes nickel, each of which may modify the way the coating reacts with the environment.

The following table shows typical corrosion rates of hot dip galvanized coatings in the various environmental classifications.

Table 1 Corrosion rate – microns per year Arid/rural < 1 Mild/urban* 1 - 3 Industrial 3 - 5 Marine** 5 - 15 Tropical 1- 3 * Metropolitan and urban areas within 25 km of the Australian coastline outside the ocean surf spray zone. ** Within the ocean surf spray zone from 0 – 1000 metres from ocean surf, depending on topography.

Coating thickness versus coating life All continuously galvanized and after-fabrication galvanized steel products have coating thickness specified in various Australian, New Zealand and international standards. By relating this coating thickness to the corrosion rates in the table, an accurate estimate of galvanized coating life can be obtained.

Hot dip galvanized coatings that comply with AS/NZS 4680 – 2006 are those that will give the longest life, as they are typically 3 –5X the thickness of zinc coatings applied to continuously galvanized products.

On structural steel sections, 50 years life before first rust in other than marine or heavy industrial environments is a reasonable expectation. Case history studies of existing installations in tropical and industrial environments indicate that 100-year life is achievable with galvanized coatings applied after fabrication.

Cold galvanizing is not galvanizing – Facts about zinc rich paint

Introduction For many years, there has been debate over the relative merits of zinc rich paint and hot dip galvanizing. There has also been debate within the paint industry about the relative merits of one type of zinc rich paint compared to another.

This has generated a degree of confusion with end users of these corrosion prevention products as much of the information requires interpretation or may, in fact, be misleading.

Making valid performance comparisons Hot dip galvanized coatings have been widely used for nearly 150 years. The technology involved in their application has not fundamentally changed in that time. The main coating component (zinc) has also been a consistent component of the coating since its invention.

Thus, a hot dip galvanized coating applied to a piece of steel in 1900 is technically identical to a hot dip galvanized coating applied to a piece of steel in 2000. There is no difference in adhesion, metallurgy or durability.

For this reason, hot dip galvanized coatings have established an international reputation for consistent performance based on case history observation of the coating in service for over 100 years.

Zinc rich paints were invented in Australia in the 1930’s. Since that time, the technology has gone through a number of manifestations in terms of binders, fillers and curing technology. The original inorganic zinc rich paints were heat-cured products. This technology was followed by acid-cured, lithium water based, potassium silicate water based, colloidal silicate water based, lithium/potassium (high ratio) water based and solvent based ethyl silicates.

Each of these inorganic zinc rich paint technologies has its own characteristics for hardness, durability, film-build and ease of application and comparison between them is not valid. The

zinc rich paint industry commonly uses examples such as the Morgan-Whyalla pipeline as a long-term case history. The technology used on this project has not been used for forty years!

Australian Standard AS/NZS 2312:1994 Guide to the Protection of Iron and Steel Against Exterior Atmospheric Corrosion , lists only two types of inorganic zinc rich paint of the six mentioned in AS 3750 - Inorganic Zinc Rich Paint.

It is thus important to verify that the type of zinc rich paint being specified is the same as the type of zinc rich paint being used as a case history example.

How much zinc do you get? The most important anti-corrosive component in both galvanized and zinc rich paint coatings is zinc. The mass of zinc present is the standard method of rating the durability of metallic coatings and all international standards use mass per square metre (or mass per square foot in the USA) to define coating durability for a wide range of galvanized products.

The amount of zinc in a zinc rich paint coating is not clearly defined and the method of specification is misleading. Zinc rich paint specifications nominate the percentage of zinc by weight, in the dry film of the paint coating.

Thus, inorganic ZRP may nominate 78% zinc in the dry film and a high quality organic (epoxy) ZRP may nominate more than 90% zinc in the dry film. Because zinc is approximately 7X as dense as the organic binder material, the volume of zinc in a ZRP coating is much less.

The mass of zinc per square metre will thus be significantly lower than that of a galvanized coating of the same thickness. Tests done by South Australian Roads Authority in testing the zinc content of various types of zinc rich paint has found the following:

Inorganic Zinc Rich Paint - 75 micron coating Solvent Based - 185g/m2

Water Based - 280g/m2

Organic Zinc Rich Paint - 75 micron coating Solvent Based - 185g/m2

Hot dip galvanized coatings range from 450 g/m2 on thinner steel sections, to well over 600 g/m2

on heavier structural sections.

Reliability factors There is no question that properly specified and applied ZRP coatings give excellent performance in many applications. However, as with most paint coatings, the quality of the application is a major factor in determining the long-term performance of the coating.

Using statistical methods, reliability factors of coatings can be estimated, where factors affecting coating quality are considered. With paint coatings, these factors include:

Initial steel surface condition (new, rusty, contaminated)

Surface preparation ( blasting equipment, operator skill, access, design) Weather conditions (wet, dry, dew point) Paint application (equipment, operator skill, paint mixing, pot life) Paint curing (humidity, temperature, time) Handling (paint hardness, full curing time, handling methods)

With galvanized coating, the process involves chemical pre-treatments and metallurgical reaction between steel and zinc which is process, rather than operator dependent. The reliability of hot dip galvanized coatings in protecting steel in a given environment is an order of magnitude higher than that of paint because galvanized coatings never fail in service through application related factors

What causes Gray Hot Dip Galvanized Coatings on Steel?

Introduction A common phenomenon with hot dip galvanized structural steel is the gray appearance of part or all of the coating after galvanizing, where the expectation of the customer is for the galvanized steel to be shiny.

Gray coatings are often a cause of contention between galvanizers and their customers as a result. This information has been produced to explain the phenomenon of gray coatings, their cause and effect on the performance of hot dip galvanized steel.

Why are some galvanized coatings gray Hot dip galvanized coatings are the result of a metallurgical reaction between the zinc and the steel. This reaction forms a series of zinc-iron alloys in the form of needle like crystals that grow from the steel’s surface.

With conventional galvanized coatings, the alloy layer makes up about 80% of the coating and the upper 20% of the coating is zinc. This surface layer gives produced the shiny appearance.Where this surface coating of free zinc is not present, the zinc-iron crystals are visible and it is the appearance of these that gives the coating matte silver or gray appearance.

When the steel emerges from the galvanizing bath, the coating is always shiny. The appearance of the coating changes to gray as the residual heat from the galvanizing process allows the reaction between the steel and the zinc to continue until all the fee zinc on the surface is consumed, leaving the coating with 100% alloy layers.

What causes some steels to produce gray coatings? The reaction between zinc and steel in the galvanizing process is a function of a number of factors. The most significant of these with respect to gray coatings are:

The chemical composition of the steel The steel section thickness The galvanizing bath temperature The cooling rate of the steel after galvanizing

Of these, the chemical composition of the steel is the most important. Two alloying elements in particular, silicon and phosphorus, will increase the reaction rate of the zinc with the steel. If the silicon content exceeds 0.20% or the combination of the percentage of silicon plus 2.5 x the phosphorus level exceeds 0.25%, then the likelihood of gray coatings forming is increased. Most Australian-made steels are ‘galvanizer friendly’ in this respect with silicon and phosphorus levels controlled within acceptable limits. As about 35% of steel used in Australia is now imported, the variation in steel chemistry makes control of gray coatings a more difficult issue.

The steel section thickness is a factor with relatively thick sections (over 20 mm) because the greater mass of steel retains heat longer. The zinc-iron reaction will continue even when the zinc has solidified (at 420 degrees C) as a solid-state reaction until the temperature falls below about 390 degrees C. For this reason, heavy plate fabrications will produce thicker, gray coatings regardless of the steel chemistry.

The galvanizing bath temperature will only have an effect where it is possible to operate the galvanizing bath at above the normal 455 degrees C level. This can only be done in special ceramic lined galvanizing baths, as high operating temperatures will damage conventional steel galvanizing baths.

The cooling rate of the steel after galvanizing can affect the coating appearance. Galvanized items that are air-cooled are more likely to develop gray or partly gray coating than items that are quenched immediately after withdrawal from the galvanizing bath. This occurs because the quenching halts the solid-state zinc iron reaction before all the free-zinc on the coating’s surface is consumed.

What effect do gray coatings have on coating performance Without exception, gray coatings are thicker than shiny galvanized coatings on equivalent steel sections. Australian and international galvanizing standards require that on structural sections over 6 mm in thickness, the minimum galvanized coating thickness is specified at 85 microns.

Gray galvanized coatings are more typically almost double this thickness, and on heavier sections will frequently exceed 200 microns in thickness. As galvanized coating life is almost directly proportional to coating thickness, a significant increase in service life can be expected from these heavier coatings.

The main problems associated with gray coatings are their aesthetic acceptability and the fact that the zinc-iron alloy layers are hard and inflexible, and may be prone to mechanical damage if subjected to impacts during transport and erection, where conventional shiny coatings have excellent resistance to quite severe impacts.

One fringe benefit of gray coatings on galvanized steel is that they provide a good substrate for painting, because of the matte surface. BHP produces a galvanized sheet product called Zincanneal where the mill produced shiny galvanized coating is converted to a 100% alloy layer coating by post heat treatment to improve the paintability of the product for whitegoods manufacture.

Gray coating micrograph – zinc-iron alloy crystals form all of the galvanized coating, producing a non-shiny gray surface appearance.

Standard galvanized coating micrograph – the zinc-iron alloy layedris coated with smooth layer of zinc producing a shiny surface.

How ‘Green’ Is Hot Dip Galvanized Steel? - Zinc as an environmentally sustainable coating material

The issue of environmental sustainability is becoming increasingly significant at all levels of our society. It is not only on the political agenda as ‘green’ candidates represent an increasing proportion of the political landscape at local, state and federal level, but is also a high priority for the design professions and their clients in the 21st Century.

A simple method of rating materials is to compare them on the basis of their Gross Energy Requirements (GER.). This accounts for all the energy used in mining, smelting, refining and forming the material. For metals in particular, another factor called Gibbs Free Energy (GFE) is a measure of the energy required to convert the ores to the metal. Nature always seeks equilibrium at the lowest energy levels and the GFE makes all metals intrinsically unstable. Their stored energy constantly seeks an opportunity to get out. The GER and the GFE are not necessarily related. Some metals like copper have high GER requirements because of the nature of their ores, and low GFE requirements because of the nature of the material.

The following table illustrates this relationship:

Table 1. Gross Energy Gibbs Free Material Mineral Requirement Energy (MJ/kg) (MJ/kg)

Aluminium AI203 270 29.00 Copper Cu2S 115 0.70 Zinc ZnS 70 3.00

Steel Fe203 35 6.60 Lead PbS 30 0.45

It can be seen from this table that in the context of protective coatings for steel, zinc has double the GER of steel but has less than half the GFE.

Zinc, when used as a component in a protective coating for steel is by its nature, sacrificial. All zinc used as a protective coating for steel will be returned to the environment as it oxidises or corrodes sacrificially to prevent corrosion of the steel. Protective coatings of all kinds work on the principle that a small amount of coating can protect a large amount of steel. On hot dip galvanized products, for example, the galvanized coating mass is typically about 5% of the mass of the steel that it is protecting. If unprotected, the steel would corrode at rates typically 20 times faster than zinc. Using adequate protective coatings systems on steel to delay the escape of its Gibbs Free Energy as long as possible is thus a major factor in determining environmental sustainability.

Zinc as a sustainable materialCompared to other base metals zinc occupies a favorable position as an environmentally sustainable material. Energy consumption for primary zinc production is 25-50% higher than that of steel and only about 20% of aluminium.

About 20% of zinc used is recovered as scrap and this is likely to increase to over 60% as recovery process technology improves.

The galvanizing of steel as sheet, wire, tube and fabrications offers very good corrosion resistance on steel and greatly increases its life. On average, about 70 kg of zinc (which consumes 250 kWh of energy to produce) is consumed to prolong the service life of 1 tonne of steel as sheet, which consumes about 2900 kWh of energy to produce, by a factor of between 3x and 5x. At the end of its service life, the galvanized material can still be recycled, except for the zinc lost through corrosion and run-off.

As weathering occurs with these zinc-based coatings, the zinc is consumed in two ways. These are:

1. Oxidation of the zinc and physical removal of the zinc oxide products by washing or erosion.

2. Electrochemical dissolution of the zinc adjacent to exposed steel when an electrolyte (water) is present.

These zinc corrosion products are transported into the surrounding environment. It is their impact in this context that determines their viability as coatings into the foreseeable future. The rate at which zinc moves into its surrounding environment from the weathering of coatings is obviously determined by coating life.

Galvanizing Threaded Sections

Introduction The galvanizing of threaded fasteners is well established, and is done in specialised galvanizing facilities that centrifuge the fasteners to remove excess zinc from the threads. Australian Standards (AS1214) nominates the required clearances on nuts for use with galvanized bolts to accommodate the additional coating applied to the threads.

The problems associated with the galvanizing of internal threads on nuts are solved by galvanizing the nut blanks and tapping them afterwards. The intimate contact between the galvanized thread on the bolt and the uncoated steel on the thread on the nut provides an acceptable level of protection from corrosion.

The galvanizing of other threaded components such as bolt cages and threaded assemblies, tapped holes and socket attachments, is an issue of concern to both galvanizers and their clients. These items are frequently included in fabricated assemblies and may be rendered unserviceable unless provision to deal with the cleaning of threaded item is dealt with.

Methods of dealing with threaded components for galvanizing

External threads When items are withdrawn from the galvanizing bath, the excess molten zinc drains off the work. On threaded items, much of this zinc is trapped in the threads and forms a thick buildup on the bottom side of the thread.

There are a number of options available to the galvanizer for dealing with the cleaning of these threads.

These are:

1. Fettling the threaded sections while the zinc is still molten to bump or brush the threaded section to shake the free zinc of the threads.

2. Heating the threaded section with a gas torch to re-melt the free zinc and wire brush the thread clean. This does not affect coating durability as most of the coating is a zinc-iron alloy with a higher melting point (650oC versus 420oC) and provided the area is not overheated, only the free zinc will be removed.

3. Re-tapping the threads. This will remove the coating and is time consuming and access issues may make it impractical.

4. Protecting the threads prior to galvanizing to prevent them being galvanized. This can be done with proprietary stop-off materials. High temperature tapes from specialist suppliers like 3M can be used to mask threads. These procedures will leave the threaded elements ungalvanized.

Internal threads Internal threads on sockets, nipples and tapped holes will always fill up with zinc on the down-side of the hole as it exits the galvanizing bath. The options for cleaning internal threads are limited to tapping out the threads after galvanizing.

The best methods of preventing zinc build-up in internal threads, is by preventing the zinc coming in contact with the threads. This can be done in a number of ways. These include:

1. Inserting a stud or bolt in the hole prior to galvanizing. This can be removed after galvanizing and may require heating with a gas torch to free the fastener and allow it to be screwed out.

2. Using a suitable high temperature sealant in small threaded holes that can be used to block off the hole. This can be mechanically removed after galvanizing.

3. Using a proprietary stop-off product.

Special threaded items Galvanizing of larger manufactured threaded items such as roof bolts, threaded rod, foundation bolts and post-tensioning rods can be done successfully if the volume justifies the set-up cost for specialised thread cleaning operations. Some of Industrial Galvanizers’ plants in Australia, Asia and the USA have the capability to efficiently galvanize long, threaded items to a high standard.

This is done by either installing special progressive galvanizing equipment that removes the excess zinc from the threads as part of the galvanizing process, efficient post cleaning systems that remove the excess zinc by heating the threaded sections and brushing, or vibrating the items.

Hot dip galvanizing remains the best way to provide a heavy duty anti-corrosion coating to threaded items, with the additional advantage that the zinc has self-lubrication properties along with a hardness that equals or exceeds that of the base steel.

Cautionary noteSome common pipe fittings such as threaded nipples and sockets are sometimes manufactured from free-machining steel and not from the parent steel from which the pipes or tubes to which they are attached are made. Free machining steel contains high levels of sulfur to deliberately weaken the steel so that it machines easily and forms small chips during the machining process. This type of steel is not suitable for hot dip galvanizing as it may be attacked by both the acids in the pickling pre-treatment operations, or by the molten zinc. In some cases, all the threads may be eroded off the fitting in the galvanizing process.

What is the Standard for hot dip galvanized coatings?

AS/NZS 4680:2006

IntroductionAustralian Standard AS/NZS 4680:2006 – Hot dip galvanized (zinc) coatings on fabricated ferrous articles is the defining standard for hot dip galvanized coating specifications in Australia. It is closely aligned with the International Standard ISO 1461:1999 – Hot dip galvanized coatings on fabricated ferrous products , in keeping with the Australian Governments policy of aligning all Australian standards with appropriate international standards.

AS/NZS 4680 defines the minimum requirements for coating mass (thickness) for various steel sections, methods of test and repairs to galvanized coatings, as well as containing informative

information on design for galvanizing, surface preparation for painting and metallurgical information.

Coating thickness requirements of AS/NZS 4680

The coating thickness on significant surfaces shall be not less than the values given in Table 1 or Table 2. If an article includes a number of different thicknesses of steel, each thickness range shall be regarded as a separate article and the relevant values in Table 1 or 2, as appropriate, shall apply.Where large articles, e.g. structural steel fabrications, are tested by the magnetic method, the local coating thickness shall be the average of 10 determinations performed randomly over an area of 20 cm2 (the reference area). The average thickness shall be the mean of the values taken from three separate test areas (i.e. the mean of 30 determinations).

Unless otherwise specified. thickness measurements shall be carried out at positions not less than 10 mm away from edges, flame cut surfaces and corners.

TABLE 1 - requirements for coating thickness and mass for articles that are not centrifuged

Articlethickness

mm

Local coatingthicknessminimumµm

Average coatingthicknessminimumµm

Average coatingmassminimumg/m2

51.5 35 45 320>1.5 <3 45 55 390>3 <6 55 70 500

>6 70 85 600

NOTE: 1 g/m2 coating mass = 0.14 pm coating thickness.

TABLE 2REQUIREMENTS FOR COATING THICKNESS AND MASS FOR ARTICLES THAT ARE CENTRIFUGED

Thickness of articles(all componentsincluding castings)mm

Local coatingthicknessminimumµm

Average coatingthicknessminimumpm

Averagecoating massminimumg/m2

<8 25 35 250

>8 40 55 390

NOTES: 1. For requirements for threaded fasteners refer to AS 12142. 1 g/m2 coating mass = 0.14 µm coating thickness.

Zinc and health – the facts

How much zinc migrates into the environment?

Emissions of zinc from point sources (factories, sewage treatment) have decreased dramatically since the 1970’s to a level expected to be reduced by more than 80% by the early 21st Century.

Significant decreases from diffuse sources (coatings, run-off from zinc coated products) have also occurred in developed countries, particularly in Northern Europe, due to dramatic reduction in sulfur dioxide emissions, again since the 1970’s. Acid rainfall arising from high sulfur dioxide levels has not been such a problem in Australia as has been the case in the Northern Hemisphere, where environmental controls have reduced sulfur dioxide emissions by up to 90% in some industrialised regions.

It is estimated that in Sweden, where a major study has been done in the 1990’s, zinc gets into the environment from the following sources and in the following volumes:

Source Tonnes/year

Corrosion and run-off 400tTyre wear 150tAsphalt wear 50tBrake lining wear 50tSewage 50t Landfills and mining activities 400tSundry sources 50t. Zinc is the 24th most common element in the earth’s crust and is always present naturally at various levels depending on local soils and other environmental factors. In Sweden the average zinc concentration in soil is around 60mg/kg. In Australia, with its ancient soils, very low zinc levels occur in many areas that contributes to the infertility of these soils for cropping, in particular.

The Swedish investigators have estimated that about one quarter of the zinc that ends up in the waterways and lakes comes from natural sources while the balance derives from human activity.

An interesting observation made by the researchers is that the atmospheric deposition of zinc in the forested areas of the country from industrial activity has contributed to maintaining the zinc levels in the humus layer in forest soils. As pollution controls continue to reduce the atmospheric zinc levels, there is now a risk of zinc deficiency becoming an issue in the future in Sweden’s

forest soils. In Australia, where 95% of the continent is not subjected to industrial activity of any kind, most soils are severely zinc depleted. As a result, most of the fertilizers used in Australia in cropping and agriculture have zinc as one of their major constituents.

Zinc Requirements for Plants, Animals & HumansIn human and plant biology, there are both essential, non-essential and toxic metals. These individual characteristics may not apply to all organisms and at all concentration levels, and even the most beneficial compounds may be toxic if their biological uptake is excessive.

Zinc is one of the essential metals and plays a central role in the function of a number of proteins in living organisms. Zinc participates in many vital biochemical reactions such as detoxification, maintenance of DNA and RNA genetic codes, protein synthesis and particularly in reproductive functions. It also plays a major part in the health of the human immune system.

Many plants are prone to zinc deficiency that dramatically reduces their fertility and productivity. The higher up the food chain, the greater the organism’s ability to regulate its zinc intake and even in high natural zinc environments, mammals (including humans) and birds do not accumulate zinc in their tissues.

It appears from research done to date, that the life forms most susceptible to toxic effects from zinc are lower forms of plant life (micro-organisms, algae). This phenomenon is used deliberately through the addition of zinc chemicals to coatings and cosmetics as an anti-fungal treatment.

It is for this reason also, that zinc is used in many ointments and medications, particularly for the treatment of skin disorders.

While the research is incomplete, current findings indicate that levels of five times the background zinc level may have detrimental impact of these lower plant life forms.

While zinc is often classified with the so called ‘heavy metals’ such as lead and cadmium when environmental standards are discussed, zinc is in fact one of the most beneficial metals. There would be ‘no life without zinc’ to quote Prof. Heinrich Vahrenkamp from the Institute of Inorganic and Analytical Chemistry, University of Freiburg, Germany from a paper of the same title presented at International Zinc Day, 1994.

The human body contains about 2.5g of zinc and more than 200 enzymes are known that require zinc to function correctly. This is a far higher number than any of the other metals essential to healthy body functions. (e.g. iron, magnesium, calcium, sodium and trace metals such as copper). Zinc has been identified as essential in wound healing, digestion, reproduction, kidney function, breathing, diabetes control, inheritance functions, tasting and skin health.

High levels of zinc are not required in humans or plants and they do not accumulate zinc, with one or two notable exceptions. Oysters have 10 times as much zinc as the next highest source (red meat), and a small flower, silene vulgaris can accumulate up to 3% of its dry weight of zinc. Some plant species can tolerate very high levels of zinc, and vulgar knotgrass (polygonum) has

been found to extract over 300 kg of zinc per hectare per year from soils containing high levels of zinc..

In humans, zinc is found in the highest concentrations in the reproductive system and lowest in the nerves and brain. Mother’s milk, sperm and ova have very high concentrations of zinc and humans require around 20 mg/day of zinc, which is available in a normal balanced diet with a supply of fruit, vegetables, cereals, red meat and seafood.

The most dramatic effect of zinc deficiency is on the reproductive system in humans, particularly in the Middle East. Extreme growth retardation in adolescents and the skin disease, acroder-matitis enteropathica, which is a very painful and potentially lethal condition, have both been immediately cured by simply adding zinc supplements to severely zinc deficient diets.

Zinc in soil is vital for cereal crops and the well being of a wide range of vegetation. For this reason, zinc compounds are widely used as additives in fertilizers.

How much does hot dip galvanizing cost?The cost of hot dip galvanizing is determined by a number of factors. These are:

1. The cost of zinc. 2. The type of item being galvanized - light, medium, heavy, 1, 2, or 3-dimensional, hollow

or solid section. 3. The size of the item. 4. The current cost of labour, chemicals, power and gas. 5. Shareholder requirements – profit

The cost of zinc has the greatest impact and can make up 40% of the total processing cost. The zinc price is volatile and from 2004 to 2006 it increased 300%.

The fixed costs of a structural galvanizing plant are high. The furnace must operate on a 24-7 basis and for this reason, large tonnages of structural steel can be processed at a much lower cost than smaller, 3-D items such as trailers.

For this reason, the cost of hot dip galvanizing steel can range from $700/tonne for large tonnages of structural steel, to well over $2000/tonne for small cash-sale items.

It is long-standing industry practice to charge hot dip galvanized coatings on the basis of ‘white weight’.

This is based on the weight of the steel after it has been galvanized. This weight is recorded by the galvanizer using certified weighbridge scales and the customer will be charged at a rate per kg or rate per tonne, based on that weight.

In some cases, a unit rate may be negotiated where repetitious manufactured products such as fence panels, star pickets or building products are processed on a regular basis.

The tonnage of steel that can be processed through the galvanizing bath in a given time will have a big impact on how it is costed. All the fixed overhead costs of the galvanizing business are allocated to the galvanizing bath. Steel fabrications that can be processed at 6 tonnes per hour will incur a lower bath cost component than those than can only be processed at 2 tonnes per hour.

For this reason, attention to detail design can significantly reduce galvanizing costs. A fabrication that can be assembled from 2-dimensional sections will be less costly to process than if it was a 3-dimensional fabrication, as more fabricated sections can be loaded onto the galvanizing jigs in 2-d form. Fabrications that have dimensions within those of the galvanizing bath will avoid the additional cost associated with double-end dipping that is required with items that are longer or deeper than the bath dimensions.

While the costs of galvanizing lighter steel sections appears to be significantly higher than heavier sections, there is a significant cost saving when compared on a surface area basis. Hot dip galvanized coatings are the most cost effective heavy-duty coatings when compared on this basis.

For example, medium structural steel (12mm section thickness) could be galvanized for say, $800/tonne. Steel of this thickness has a surface area of about 20 m2/tonne, so the cost of galvanizing per square metre is $40/tonne. Light steelwork, commonly associated with box trailers or light hollow section that averages 3 mm in thickness may cost $1500/tonne to galvanize. Its surface area is 85 m2/tonne so the cost of hot dip galvanizing is then less than $18/m2.

Can my ‘old’ items be galvanized?

Steel that has previously been galvanized can be easily re-galvanized. If the base steel is still in good condition, the re-galvanizing will restore the item to as-new condition. Boat trailers are regularly re-galvanized but must be stripped of their running gear and electrical equipment, with moving parts separated, to be satisfactorily galvanized.

Items such as wrought iron that has previously been painted should be cleaned of old paint and excessive rust prior to galvanizing. This is best done by abrasive blasting, but grinding or wire brushing is suitable for smaller items.

Care must be taken with some vintage wrought iron of heritage value, that was manufactured from iron castings in the 19th Century. This older wrought iron may be porous and have casting defects that allow moisture to penetrate inside the metal. This can cause damage to the item, or injury to the galvanizing plant staff because of the risk of explosion when the material is immersed in the molten zinc.

Old motor vehicle parts – fuel tanks in particular – may have been assembled originally by soldering. The solder will melt in the galvanizing process and render the items unusable. Soldered items cannot be galvanized.

Any items that have been riveted with aluminium pop rivets are also unsuitable for galvanizing, as the aluminium will be dissolved during the pre-treatment process.

Some fabrications may have stainless steel fittings welded to them. While the stainless steel fittings may galvanize satisfactorily, there is a risk that they will be adversely affected by immersion in the molted zinc, which may cause embrittlement. They may fracture under loads if this occurs. Such fittings should be attached after the fabrication is galvanized.

Excessively rusted items can be re-galvanized, but the heavy rust nodules should be removed by abrasive blasting prior to galvanizing, as the time taken to remove this excessive rust in the chemical pre-treatments used would otherwise be excessive. The ‘old’ steel will be pitted and the rough surface will be reflected in the new galvanized coating, although the durability of the steel will be restored.

If older fabrications made out of hollow sections (bike frames, handrails, etc) that have not previously been galvanized are required to be hot dip galvanized, they will need to have vent and drain holes put in appropriate locations to ensure safe and effective galvanizing. It is advisable to consult with the galvanizer for advice on venting and draining of these types of fabrications. This will ensure that vent and drain holes are in the right places and are kept to a practical minimum.

One feature or re-galvanizing ‘old’ steelwork is that the resultant coating is likely to be thicker than what would have been applied to new steelwork because the rougher surface will react more vigorously with the zinc. This means that, second time around, the durability of the re-galvanized item is likely to be better than the original.

How much heat can a galvanized coating tolerate?

Galvanized items are sometimes used for applications where the steel is subjected to either intermittent or permanent higher temperatures. Zinc has a relatively low melting point – only 420oC. When it reacts with steel to form the galvanized coating, the zinc-iron alloy that is formed has a higher melting point of around 650oC.

Zinc is also unusual among metals in that it vaporizes (turns to gas) at the relatively low temperature of around 950oC. This characteristic is used in the manufacture of zinc dust and zinc oxide.

Galvanized coatings can handle higher temperatures (up to 250oC) in dry heat conditions in intermittent exposures, but are not well suited to continuous exposure to temperatures of this level. It is for this reason that items such as automobile mufflers are generally manufactured from aluminized coated steel sheet.

When galvanized coatings are heated to over 350oC, a solid state reaction will be initiated between the steel and any free zinc (the shiny surface zone) in the galvanized coating. This will convert the coating into 100% zinc iron alloy, gives it a frosted gray appearance. This is done deliberately in the sheet galvanizing industry to produce a 100% zinc-iron alloy surface which is

better for pointing because the microscopically rougher surface provides very good coating adhesion.

This type of coating is widely used in the white goods industry and is marketed by BlueScope Steel as its Zincanneal coating.

Galvanized coatings do not perform well for hot water storage, where water temperatures are in the order of 70 oC. Corrosion rates of the zinc are increased and there is evidence that a polarity reversal takes place with the steel at that temperature and the zinc becomes cathodic to the steel and can increase the corrosion of the base steel.

In short-term exposures to much higher temperatures, galvanized coatings perform very well. Significant research has been none by the Bushfire Co-operative Research Centre for BlueScope Steel to determine hot dip galvanized steel poles performed when exposed to bushfires.

Bushfires can produce very hot fires but they are generally for short local duration as the fire front moves forward. Typical exposure times to maximum bushfire flame temperatures is less than 2 minutes. The combination of the reflectivity of the galvanized surface, and the heat sink provided by the mass of the steel to which the hot dip galvanizing is applied has shown galvanized steel to give excellent performance, with virtually no effect on the coating.

This superior performance of hot dip galvanized power poles has seen power distribution authorities moving to the use of galvanized steel power poles in bushfire prone areas in preference to timber. Although the steel poles have a significantly higher cost, their ability to maintain power supplies in bushfires far outweighs other factors in the selection process.

Galvanizing and chemicals – What is OK and what to avoid.

The majority of galvanized steel products are used in atmospheric environments and the galvanized coating’s performance has been well established for such applications.

There are many instances where galvanized steel is in contact with other common materials and chemicals. Some of these materials may be very aggressive on zinc coatings.

Galvanized coatings perform well in contact with most petroleum products – petrol, diesel and oils, and organic solvents such as methanol, turpentine, mentholated sprit, alcohols,etc. The only durability problems with these types of products arise because of water in the fuel being precipitated to the bottom of the container and giving rise to localised corrosion, often caused by microbiological corrosion.

Dry bulk products such as grains can be stored in galvanized facilities. Some food products, particularly fruit, can cause problems with galvanized coatings because of the acidity of their juice, should it come in contact with the galvanized steel.

Most fertilizers will aggressively attach zinc coatings and can quickly remove a galvanized coating if damp fertilizers are left in contact with galvanized surfaces. Exceptions are products high in agricultural lime.

These are relatively benign to both zinc and steel.Building materials can react rapidly with zinc when wet. Gypsum plaster is an example, where initial corrosion rates can be quite high, but are negligible one the plaster has dried. Cement behaves in a similar manner, with a reaction with the zinc being possible during initial contact, with virtually no corrosion occurring after concrete cure is complete.

Some timber products can be aggressive to zinc also. Some wood species have sap that had pH less than 4 and exposure to unseasoned wood products can cause rapid attach on the zinc coating. However, with most timber, including CCA treated timber, rapid decreases in corrosivity occur once the moisture content of the wood drops below 20%.

Most detergents and cleaning chemicals that are phosphate-based are not good for galvanized containers, nor are other trisilicate-based detergents.

Very soft water can be very aggressive to zinc-based coatings. This was a problem in the days of galvanized rainwater tanks (all steel water tanks are now made of polymer coated steel over a galvanized or Zincalume base). If new tanks were rapidly filled with storm rainwater, it was found that they suffered premature failure, typically within 2 years or so. Tanks that filled slowly had a normal life expectancy of 20 years. The dissolved salts in the slow-filling tanks provided the carbonates and other compounds to allow the zinc patina to be established, while the rapidly filled tanks, containing only pure water, did not.

Another material that is aggressive to zinc coatings and is often overlook it graphite. Graphite is one of the many forms of carbon, and in this for, it is the most cathodic material on the electrochemical series of metals, and is one of the few non-metals that is a good conductor of electricity. Soot from oil or wood fires, and lubricants or rubbers containing graphite that are in contact with zinc or aluminium coatings or materials can cause rapid attack on these metals on the presence of moisture. Graphite is at the far end of the electrochemical series of metals and generates very high corrosion currents when in contact with metals, like zinc, at the other end of the scale

Can hot dip galvanized steel be friction grip bolted?

When designing friction grip bolted connections, engineers need to ensure that the Coefficient of Friction between the connecting surfaces exceeds 0.35.

It is possible for hot dip galvanized structural steel to meet this coefficient of friction requirement, provided the connecting surfaces are treated appropriately. Industrial Galvanizers has developed its QA procedures to ensure that friction grip bolting requirements are satisfied, as a result of a comprehensive research project that was conducted by the University of Newcastle’s Engineering Research facility (TUNRA) in the 1908’s.

This research was done on to support a major construction project involving several thousand tonnes of railway bridge steelwork for the NSW State rail Authority. These QA procedures have subsequently been used on a large number of structural projects since that time with 100% satisfactory outcomes.For the 0.35 Coefficient of Friction requirement to be met, the following information should be noted.Australian research findings……

1. A conventional shiny hot dip galvanized coating will have a Coefficient of Friction in the order of 0.20 – 0.25.

2. A reactive steel (gray or matt colour) hot dip galvanized coating will have Coefficient of Friction in the order of 0.50.

3. Abrading the hot dip galvanized surface with an appropriate grinding disc will result in a Coefficient of Friction in the order of 0.40.

4. Abrasive blasting (brush blasting) the hot dip galvanized surface will result in a Coefficient of Friction exceeding 0.50.

5. Where free zinc is present on the connecting hot dip galvanized surfaces, the slip factor can increase over time under cyclic loading through ‘cold welding’ of the free zinc under high compression loadings.

To achieve a coefficient of friction exceeding 0.35 for the friction grip bolted connections on the galvanized structure, the following procedures are recommended.

1. Procedure for mechanical buffing. Buffing the connecting surfaces to remove any surface irregularities and to roughen the surface. This should be done using an air grinder of a type equivalent to a Model SP-1222SD 125 mm high-speed grinder (max rpm 15,000). The grinder should be used with a flexible backing pad and 36 grit resin grinding discs. This process should not remove more than 10% of the galvanized coating. This proportion of the coating comprises the free zinc layer. The balance of the coating is made up of much harder zinc-iron alloys, which are not subject to plastic deformation. This ensures that bolt relaxation will be minimal after torqueing of the friction grip bolts.

2. Procedure for brush abrasive blasting. The following procedure should be observed when sweep blast cleaning is carried out to ensure that a good surface is produced for painting, without severely damaging the existing galvanized coating.

1. Use fine abrasives of a size which will pass through a test sieve of nominal aperture size 150 pm to 180 pm (80 to 100 mesh), e.g. ilmenite or garnet.

2. Use a venturi nozzle which has an orifice diameter of 10 mm to 13 mm. 3. Set the blast pressure at 275 kPa (40 psi) maximum. 4. Keep the venturi nozzle at a distance of 350 mm to 400 mm from the surface of

the work piece and at an angle no greater than 45° to the surface.

Galvanizing Versus Galvanizing

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