Inorganic Zinc vs Galvanising
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A B C D E www.international-pc.com Page 1 Release Date:28-Oct-02 Ref: JT/els Inorganic Zinc vs Galvanising D EF Most people in our industry know that galvanising is the dipping of prepared steel into a bath of molten zinc metal. The zinc then adheres to the surface of the steel forming an amalgam with pure zinc on the surface. The zinc is usually treated to prevent white rusting of the zinc in wet oxygen deficient atmospheres. This treatment used to be done with chromates but these days it is likely to employ thin coatings of acrylic or polyurethane. Inorganic zincs are a wet paint coating formed from the mixing of very high levels of zinc powder with a silicate resin in either a solution of water or solvent. The silicate resin is a type of glass. It is quite reactive with the bare steel surface and with the surface of the zinc particles to give a coating chemically bonded to the steel surface. This process is quite distinct from the adhesive forces generated by normal paint coatings. Those are unable to react with a metal surface and only adhere by dint of their electronic forces. These fall off quite rapidly if the surface has a coat of dust or grease on it. How the two systems work: Surprisingly, zinc galvanising and inorganic zinc coatings work by quite different mechanisms. Zinc protects steel by galvanically sacrif icing itself in place of the more noble iron on t he galvanic table. Hence zinc has a limited life. It does, however, have the advantage that, in non-aggressive condit ions, the life of a zinc coating can be calculated on the basis of depletion rate. Inorganic zincs work in a much more economical fashion. In the initial service life, the zinc particles do act galvanically, protecti ng the steel sacrificial ly. In this process, the zinc forms slightly soluble zinc hydroxi de, which spreads over the coating and fills up the pores being formed by the reacting zinc. The zinc hydroxide reacts with carbon dioxide in the atmosphere to form zinc carbonates. These are quite insoluble and form an imperv ious barrier to further moi sture or corrosive iron penetrat ion. Thus, after the initi al dissipation, the coating no longer protects sacrificially like galvanising but by forming an impermeable barrier. In effect inorganic zinc coatings give you two protective coats for the price of one.
Transcript of Inorganic Zinc vs Galvanising
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Ref: JT/els
Inorganic Zinc vs Galvanising D EF
Most people in our industry know thatgalvanising is thedipping of preparedsteel into a bath of molten zinc metal. Thezinc then adheres to thesurface of the steelforming an amalgamwith pure zinc on thesurface. The zinc isusually treated toprevent white rusting of the zinc in wet oxygendeficient atmospheres.This treatment used to
be done with chromatesbut these days it is likelyto employ thin coatingsof acrylic or polyurethane.
Inorganic zincs are a wet paint coating formed from the mixing of very high levels of zinc powder with a silicateresin in either a solution of water or solvent. The silicate resin is a type of glass. It is quite reactive with the baresteel surface and with the surface of the zinc particles to give a coating chemically bonded to the steel surface.
This process is quite distinct from the adhesive forces generated by normal paint coatings. Those are unable toreact with a metal surface and only adhere by dint of their electronic forces. These fall off quite rapidly if thesurface has a coat of dust or grease on it.
How the two systems work:
Surprisingly, zinc galvanising and inorganic zinc coatings work by quite different mechanisms. Zinc protectssteel by galvanically sacrificing itself in place of the more noble iron on the galvanic table. Hence zinc has alimited life. It does, however, have the advantage that, in non-aggressive conditions, the life of a zinc coatingcan be calculated on the basis of depletion rate.
Inorganic zincs work in a much more economical fashion. In the initial service life, the zinc particles do actgalvanically, protecting the steel sacrificially. In this process, the zinc forms slightly soluble zinc hydroxide,which spreads over the coating and fills up the pores being formed by the reacting zinc.
The zinc hydroxide reacts with carbon dioxide in the atmosphere to form zinc carbonates. These are quiteinsoluble and form an impervious barrier to further moisture or corrosive iron penetration. Thus, after the initial
dissipation, the coating no longer protects sacrificially like galvanising but by forming an impermeable barrier. Ineffect inorganic zinc coatings give you two protective coats for the price of one.
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Inorganic Zinc vs Galvanising D EF
Which, where?
In general, inorganic zinc coatings are the most economical form of corrosion protection for steel members,which have a surface area of less than 25 square metres per tonne. As a rule of thumb, if you can pick it upyourself, then galvanise it. If you cannot lift it unaided, then coat it with inorganic zinc.
Aside from the cost aspect, inorganic coatings and galvanising share the virtue of being substantially immune totransportation and erection damage.The application of inorganic zinc coatings can now be controlled to the same degree of quality as galvanising.The product is controlled to high composition and performance standards under AS2105. The surfacepreparation, application and the cure of coatings are well controlled by AS1627.4 and also the new AS3894standards for field-testing of coatings. With a correctly worded specification, your quality is assured.International Paints would be happy to assist you in writing a suitable specification.
Appearance
Would you drive an unpainted car? Of course not! Why erect a dull grey metallic finish when it is possible thesedays to ensure your project is aesthetically pleasing as well as protected from corrosion. When it comes to topcoating, inorganic zincs are far more economical than galvanising.
Galvanising requires brush blasting for correct preparation of the surface for painting. Inorganic zincs require nofurther preparation. Correctly chosen topcoats can go straight on to the inorganic coating. Painting over galvanising can be problematical in that moisture can always penetrate the coating and react with the pure zincsurface, leading to paint flaking and some-times premature rusting. Whereas topcoats actually bond into theinorganic silicate surface of inorganic zincs and the adhesion is much more stable and intact. It is almostunknown for paint to flake from an inorganic zinc coating.
Unique feature
These days, when it is impossible or prohibitively expensive to blast-clean an existing structure, the applicationof inorganic zinc has an additional unique feature. Because it binds chemically to the steel surface, it protectsthat surface from corrosion like no other treatment.
Inorganic zinc coatings do not undercut with corrosive products. Any corrosion due to damage to the paintcoating is contained until repair can be effected at the owner’s timing.
Where do you use them?
Inorganic zinc coatings are used alone or top coated on offshore oil rigs, coal washeries, road bridges, oilrefineries, wharf facilities, mineral processing plants and on steel exposed to temperatures up co 450ºC.
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Disclaimer The information given in this sheet is not intended to be exhaustive and any person using the product for any purpose other than that specifically recommended in this sheet without first obtaining written confirmation from us as to the suitability of the product for the intended purpose does so at his own risk. Any warranty, if given, or specific Terms & Conditions of Sale arecontained in International’s Terms & Conditions of Sale, a copy of which can be obtained on request. Whilst we endeavor to ensure that all advice we give about the product (whether inthis sheet or otherwise) is correct we have no control over either the quality or condition of the substrate or the many factors affecting the use and application of the product. Therefore,unless we specifically agree in writing to do so, we do not accept any liability whatsoever or howsoever arising for the performance of the product or for any loss or damage (other thandeath or personal injury resulting from our negligence) arising out of the use of the product. The information contained in this sheet is liable to modification from time to time in the light of experience and our policy of continuous product development .
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Inorganic Zinc vs Galvanising D EF
There is a downside
Inorganic zinc coatings should not be used on small section steelwork such as purlins, or where there is a highchance of acid spills onto the coated surface.
Conclusion
It is now being recognised that inorganic zinc coatings are the most economical method of protecting largesection steel against corrosion, especially when putting a decorative finish on it as there is no need for further surface preparation.
