Materials & Process

Assignment

Submitted By Anglia Ruskin University Student ID

1227201

Index

Title Leaf

Chapter 1 - Industry Visit 2

Truck-lite Global 3

Company Preface

Products and Services

Activities performed at truck-lite 4

Truck-lite Harlow testing facility 5

List of Raw Materials 6

Recycling 7

Bonding Mechanisms

Other techniques 8

Encocam 9

Company Preface

Products & Services

Raw Materials 10

Aluminium Honey comb production

Test Procedures to evaluate products 12

Range of clients & Application

Chapter 2 – Welded and Brazed Specimen 14 - 19

Fusion Welding 20

Non fusion welding

Effects of poor welding of joints

Conclusion

Chapter 3 – Defects & Failures 20 – 24

Chapter 4 – Corrosion 26

General Corrosion

Pitting Corrosion

Cervice Corrosion

Intergranular Corrosion

Erosion Corrosion 27

Environmentally Induced facture

Prevention methods of Corrosion

Working of Cathodic protection

Corrosion Inspection & Detection method 28

Stress Corrosion

Boiler Tube Stress Corrosion Cracking 29

Two new effective ways of corrosion control

References 31

Appendices 32 - 37

Abstract:

In the present world, every other product we use starts with the selection of raw materials and goes through a process of refining,

design to last for year. In a hypothetical world this should last for ever. But ideally, we deal with a number of problems. From the

raw materials to the designing and then manufacturing takes a lots of process involved in it which includes the materials going

under the process of stress and strain analysis, tensile and forces acting on it. Neither does this, even in manufacturing them

involves the process of joining as welding the parts, forging to get them in shape and finalise them. Before finalization these

products go under different test and standards. Each and every product needs to pass it. This does not only limit, aftermaths

include the corrosion, yielding of products and finally in an ideal world it collapses and goes into recycling or waste. There have

been trends and analogies to mends this and make the product last long more. Every product can be analysed before it is made,

tested and fabricated. This research assessment will focus on achieving the longevity, analysis of the problems and make materials

and process more productive.

1

Chapter 1 Industry Visit

Industries include:

&

Truck-lite Global

Company Preface:

Truck-lite is an industry manufacturer for vivid applications of vision systems of the automotive industries. Spread across

worldwide with 11 manufacturing locations, truck-lite has the very old 50 years of reputation in the fields of OEM supply,

incandescent lighting technology, mirror manufacturing and trailer assemblies, servicing the truck, trailer, off-road and military

sectors as well. Much of its clients include the Daimler, Volvo, Man and you name it. Established by A A Sandbrook, rubberlite is

another trade mark of the truck-lite. Truck-lite records the innovation of the first sealed wire harness system, LED life span of

100,000 hours.

Products & Services

As the Truck-lite is one of the biggest manufacturers for the vision systems and mirroring in the automotive industries, much of

their products include the following:

Trailer Components:

Plugs / Sockets / Coils.

(Figure 1 right: Shows a web shot of truck lite connection product)

Looms

Junction Boxes

Harnesses

Lighting Equipment:

which included the following :

White Light

Warning Lamps

Signal Lighting

Reflectors

Rear Lamps

Numberplate Lamps/ Holders

Headlamps

(Figure 2 on the left : Shows the web

shot of the products from Truck-lite)

Bulbs

Acoustics

These include : Reverse Alarms & the horns

(Figure 3 right: Shows the examples of the reverse alarms and horns)

Wide range of the Accessories for the automotive industries & Military Applications too.

Advice/ Technical Support

Finding a Stock

This specialization of the products in the Truck-lite makes it a global leader in the vision and lighting field. Much of its accuracy

is achieved by the through industrial standard tests before even the product is launched in the market by the company. The

researcher has made the next topic on the activities that is usually practised in Truck-lite

Activities Performed in the Truck-lite:

Truck-lite employs engineers from wide background from design to development of products, test engineers and validation of the

products apart project management for managing the project into the right timeframe and meeting the global needs of its client.

The following flow chart represents the working of Truck-lite.

(The above flow chart shows only the engineering activities that takes place in the Truck-lite)

Truck-lite Harlow based in Essex in the United Kingdom is the Headquarters of our European arm. With 15,960m² of space and

320 employees it is the home of tried and trusted Rubberlite brand. The factory is used for moulding, assembly lighting, mirrors

and distribution centre and specialises in manufacturing our range of rear lamps, trailers harness kits, reflectors and many of our

OEM production.

The site also houses the lab with its stringent testing centre, measuring everything from flammability, water resistance and dust

and particle resistance, to testing against shock, impact, vibrations and extreme weather conditions. The Harlow site is the only

plant in Europe to measure photometric from 30 metres with our state-of-the-art optics and reflex bench. (Truck-lite, website)

Conceptual Phase for Clients

(Designing and Generation of the

concept design) Generation or Manufacturing

prototype of the concept Test of the product

in test labs

Manufacturing the products in

factory using manual labour or

automated process Use of the statistical Process Quality

control of the products to check and

test mark every product ok Validation of the tested by

Truck-lite, packaging and

shipping / dispatching to

client

Truck-lite Harlow Testing Facility

Truck-lite’ laboratory and test facility has been validating products for over 40 years working directly the certification bodies,

VCA and BSI. At Harlow the equipment necessary to certify products for European legal compliance and customer specific

validation plans are equipped. They design and develop new robust products. Truck-lite’s testing capabilities include shock and

vibration; electrical development; environmental; photometry; European certification and international certification.

Water - IPX4, IPX5, IPX6, IPX7, Jet wash, Leak

(Figure 4 & 5: Shows the Water test on the lighting of the trucks IPX 7 & IP6 tests respectively)

Dust

Corrosion

(Figure 6 right: Shows the corrosion test equipment)

The product is kept under the corrosive atmosphere for a time

frame and the analysis is done for the product.

(Figure 7 left: Shows the white corrosion of aluminium under

test lab)

Chemical

(Figure 8: Shows the test of the covers of the light using the

leaded petrol and anti-freeze mixture test)

Measuring

Vibration – Random, Sinusoidal

(Figure 9: Use of vibration to test the automotive truck mirrors)

Thermal – RH(Relative Humidity), Shock , Temperature(-40°C to +50°C), LED Endurance

Endurance – Mechanical

Optics – Reflex, Colour, Signal functions, Number plate

Impact – Drop ball impact, Mirror knock

Apart from the research and the laboratory activities Truck-lite performs a range of manufacturing activities which include the

following:

Modern multicomponent plastic injection moulding

(Figure 10 & 11: Shows the automated injection moulding)

Gas assisted injection moulding

Two colour moulding

Plastic welding

Metallization

Gluing techniques

List of the raw materials used at Truck-lite:

Optical Materials used are:

Acrylics - PMMA

Polycarbonates – PC

Housings / Covers:

ABS

Talc filled Polypropylenes

ASA

Structural/Heat Resistance Materials:

Nylons

Glass Filled Polypropylenes

Synthetic Rubber, ABA, Thermoplastic elastomers, PMMA, Propylene with long glass fibres, nylon 66 glass filled are some of the

prominent and most used raw materials used by the Truck-lite.

(Figure 12: finely chopped ASA for the plastic injection moulding Figure 13: Raw materials at Truck-lite)

Recycling:

The waste or the faulted products are a make of polycarbonates and can be shredded before recycling. Most of the waste

or the faulty products are not reused to make new products instead they are sold as scraps to the recycling companies at

lower rates.

Shredding: The confirmed waste and the excess or faulty polycarbonate products are shredded into chips or smaller units before

recycling. The recycled polycarbonate is less resilient, having decreased impact resistance when compared with newly manufactured

polycarbonate. The addition of fillers and pigments can also decrease the plastic's resilience. This problem can be addressed by the

use of chemicals to modify impact resistance in recycled polycarbonate. (Polycarbonate recycling, online)

PIC Bins: Use of the PIC labelled bins to throw the waste and excess product

Examples of the properly labelled plastics.

Other methods that include are:

Thermal Depolymerisation: Includes the conversion of the poly carbonate into the petroleum products.

Heat compression

Land Filling: Properly sealing land filling can be used for non-recyclable wastes.

Heating and melting to form other products : This may include can convert the molten polycarbonate products into other

form of products

Bonding mechanisms used in manufacturing products at Truck-lite:

Plastic joining/ Bonding Mechanisms:

Ultrasonic Wielding: Use of high-frequency ultrasonic acoustic vibrations are applied to

work pieces being held together under pressure to create a solid-state weld.

Advantages are: Clean, Quick, Low Cost Tooling

Limitations: Size, Material’s compatibility

(Figure 14: Showing the mechanism of ultrasonic plastic wielding)

High Frequency Welding: High frequency welding uses this property to soften the plastics for joining. The heating can be

localized, and the process can be continuous also known as Dielectric Sealing, R.F. (Radio Frequency) Heat Sealing.

Heat Staking: Heat staking is the process of connecting two

components by creating an interference fit between the two

pieces

(Figure 15: Left shows riveting a plastic using heat staking)

Silicones: Advantages are Good Material Compatibility, Easy to apply, Flexible Joint While disadvantages are Cure time, Mixed

Material.

Injection Wielding: Injection welding is similar/identical to extrusion welding, except, using

certain tips on the handheld welder, one can insert the tip into plastic defect holes of various sizes

and patch them from the inside out. (Refer Appendix 1)

(Figure 16: Right Showing the injection wielding )

Plastic injection moulding:

Material for the part is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the

configuration of the cavity. After a product is designed, usually by an industrial designer or an engineer, moulds are made by a

mould maker (or toolmaker) from metal, usually either steel or aluminum, and precision-machined to form the features of the

desired part.

Other techniques are: Hot Melts; Adhesives; spin wielding and laser wielding.

End of Truck-lite company visit

Company Visit & Observations

Company Preface:

Huntingdon, Cambridgeshire based company, Encocam Ltd is the parent company of 10 divisions with over 25 years’ experience,

spanning across a large range of industries from energy absorption, safety testing solutions, composite panels, interior design and

architecture, motorbike distribution and a range of manufacturing and engineering services . Encocam prides itself on being

leaders in innovation and creativity as well as being forward thinkers in technology and manufacturing, believing strongly in

continuous improvement.

Products & Services:

Automotive: specializes in testing equipment, R & D prototyping and manufacture of components.

(Figures 1 & 2 showing crash tests at

Encocam)

Composite Panels: Crash test barriers, Honey comb

structures

Interior Design & Architectural

Motorcycle Distribution: Includes low cc motorbikes

sales in UK

Precision Engineering: This involves the manufacturing of the crash test dummies.

Rail : Doors, Floors, Energy absorbers & Furniture’s

Road Safety Solutions: The highway hard fasteners and crash barriers for the trucks.

Dummy testing: It provides dummy heads and legs for test analysis (Figure 3 right)

Other applications include:

• Clean room panels

• Exterior architectural curtain wall panels

• Air, water, fluid, and light directionalisation

• Heating, ventilation, air conditioning equipment and devices

• Skis and snowboards

• Energy absorption protective structures

• Electric shielding enclosures

• Acoustic attenuation

• Wind turbine blades

(Figures 4 & 5: Encocam’s crash test barrier & airplane floor panel)

Raw Materials used in the company:

Its major raw materials include:

Aluminium, Quenched steel, mild steel, Polymer, silica.

Aluminium Honeycomb Production:

The aluminium honeycomb production is done in a very sophisticated process where its starts as sheets of Aluminium and after

the various mechanical process and use of adhesives the aluminium is later converted into the honeycomb structure. The steps are

explained more elaborately:

Aluminium Foil

Printing of the adhesive lines

This is where the glues are pasted on the aluminium sheets

Piling up of the aluminium sheets using a stacking machines

As the figure below shows the piling of the glued aluminium sheets are stacked over one another

Pressing using heat to form a block

at least more than 1500 sheets are stacked together to make a block which is pressed

under the hot presser upto 2400 ’C

Slicing the block into the client’s thickness requirements

The blocks are later sliced according to the thickness requirement of the client by a cutter.

Expansion of the sliced blocks in to honeycomb structure.

Finally the Honeycomb of Aluminium is ready from the production unit which goes to the different stages of finishing according

to the client’s prospective fields of interest.

A finished Honeycomb out of Aluminium is shown above: Figure 6

Test Procedure to evaluate products (Aluminium Honeycomb & Crash test dummies)

Weather ability

Cladding panels are exposed particularly intensively to the elements for protracted periods. To examine whether the panels

produced by the new method are equal to the ever-increasing climatic stress, alternating climate tests in a humid climate followed

by very low temperatures were performed in very quick succession.

Comparison of mechanical values

Creep strength

Impact strength

Range of Clients and application of Encocam’s products:

Encocam’s clients range from the test facilities to on highway application. Most of the automotive clients include Chrysler,

Mercedes for crash test barriers. Trucking companies include for the rare barrier.

Amongst the many applications for energy absorbing honeycomb are passive automotive safety, subsystem component testing,

dummy neck calibration, energy absorbers as impact countermeasures in vehicle interiors, and deformable crash test barriers.

Cell bond is the only manufacturer to produce the full range of deformable crash test barriers for both frontal and side impact

testing and has been supplying to the majority of car manufacturers and test institutes for many years.

End of Encocam industry visit.

Chapter 2

Wielded & Brazed specimens

Objective: To determine by macroscopic and/or microscopic inspection the soundness or otherwise of the joint

& to determine the reasons for faults in the specimens.

Apparatus: 8 welded specimens for examination

Microscope with data capture facility.

Magnifying glass

polishing equipment and etching acids

Procedure To inspect each specimen macroscopically and where necessary microscopically. Produce a pictorial

sketch or photograph of each specimen and identify any observable faults. Produce

Microphotographs where applicable of the weld metal structure or heat affected zone. Briefly describe the

various welding/ brazing process detailing their advantages in joining engineering metals.

The inspection and the laboratory work may contain the investigation results and the logical theories of the wielded parts. This

will be followed with the comments and conclusion of the parts.

1. Specimens Single pass arc weld in plain low carbon steel Preface:

This is a way of joining metals using submerged arc

wielding. This unique technique is the fast and the

shortest time giving high tolerance . (Refer appendix

2)

(Submerged Arc wielding, PT Houldcraft, 1989)

(Figure 1: Top right shows the specimen 1)

(Figure 2: Left, shows the sub merged arc weild)

Naked eye inspection: Carefully inspecting the specimen, the researcher concludes that the wield is poorly done. There has been

the traces of the fused wield flow on the surface (Figure 1)

Comments: Poor fusion of the metals and can be incomplete; degraded quality of the flux can be another factor;

Microscopic View:

Conclusion:

The wield would not last long under extreme mechanical stresses which means there can be stress concentrations too and may

result into the wield fatigue.

This way of wielding can be improved by precision by the wielder.

2. Bronze weld in brass plate.

Preface: The main alloying element in the brasses is zinc (Zn). (Refere Appendix 2) There are three families; brass with zinc

content less than 20%, high zinc alloys with 30-45% zinc and the nickel-silvers that contain 20-45% zinc and 20% nickel. With

the exception of brasses containing lead (Pb) all the brasses are wieldable, the low zinc alloys being the easiest.. Zinc melts at

420°C and boils at 910°C so brazing using an oxy-acetylene torch and a copper-silver filler is a possible alternative to welding,

being capable of providing joints with adequate mechanical properties and without the porosity problems.

The Cu-Si filler metal flows easily (Refer Appendix 2)

(Figure : Left shows the specimen) (Figure : Shows the side view of the specimen.)

Normal Inspection: With the naked eye it is visible that the wielded region has more sluggish area and thick at least or more than

1mm.

Comments: The main problem with welding the alloys is weld metal porosity caused by the zinc boiling off during melting.

Microscopic View:

Figure on top shows the grain structure of Brazz Figure on top shows the big void of bronze.

Conclusion:

Lack of HAZ is clearly seen in the microscopic view

The difference in the porosity of the metals is visible. This can be a possibility that the wield is not properly done.

Another main factor can be the over use of the oxy acetylene or oxygen during the wield.

Chances of the bronze being boiled cannot be avoided and very little use of the bronze may also be a leading factor.

Post weld heat treatment is rarely necessary but can be of benefit if the welded item is to experience very corrosive conditions. In

this case a stress relief operation at 300-350°C may be beneficial, although precise temperatures and times will depend upon the

specific alloy composition, thickness etc. (Bronze wielding, PP Roughman, 1996)

3. Fusion weld between pipe and plate (compensation plate)

Preface: Due to the high-temperature phase transitions

inherent to these processes, a heat-affected zone is created in

the materials. (Although some techniques, like beam

welding, often minimize this effect by introducing

comparatively little heat into the work pieces.)

Types of fusion welding include:

Arc welding

Oxy-fuel welding

Electric resistance welding

Laser beam welding

Electron beam welding

Thermite welding

Normal Inspection: With carefully watching the specimen (Figure 3) it can be noted that the part has traces of cracks and rust on

it.

Even in the traces of the difference in the scratched and improper wields can be seen. (Figure 4 )

(Figure 4: Top , Shows the wielded pipe and plate specimen)

Microscopic view:

Figure on right shows the grain structure of the compensate particle.

Figure on top shows the grain structure of the pipe.

Comments:

It can be seen that the half of the metal has been properly wielded but the rest moves in a deformation

Conclusion:

There is a possibility that the material has undergone high mechanical stress which has resulted into the fracture of the metal

grains. The fracture has resulted into the propagation of the stress corrosion cracking and resulted into the failure of the sample.

Apart the evidence of the rust on the surface of the sample shows that the sample had been under corrosive environment which

may be a prior factor too. Both the SCC and the rust corrosion can be the leading issue of the compensation plate being subjected

to deformation in the wield which is mot 100 % secure.

Other reasons may include the quick cooling of the fusion wielding which may have made the wield region to be brittle.

Some other reasons that may have initiated are tips and weld pool are underneath solid flux cover. Incorrect selection of

consumables and parameters may lead to lower weld toughness. Potential for lack-of-fusion type defects if welding parameters are

incorrect or misalignment occurs. Fume extraction may have been required.

4. Multi pass weld in low carbon steel Preface: This means a multiple passes done on wielding of the metals together. It is usually done to increase the strength of the

metal and reduce the ductility. (Refer Appendix 2)

(Figure 5: Shows the multi pass wield ) (Figure 6 : Shows the opposite surface of the specimen)

Normal inspection: The normal inspection of the specimen shows the layers or the traces of the multipass wield with the

difference in the HAZ. The wielding can be easily seen with naked eye. There is a hole in the centre of the wield too.

Microscopic view:

Figure on top shows the grain structure of the HAZ being hot Figure on top shows austenite and pearlite HAZ

Figure on left shows the void being created.

Comments: The variation of the grain size can be seen on the samples under the

microscope. The grains vary from large size (Figure ), to minute (Figure ) and

irregular surface of the multi pass wields (Figure ) along with the existence of

hollow space.

Conclusion: The reason for the irregularity can be predicated by the use of improper electrode. Variation of the electric current

can be another factor along with the speed of the wielding and the operator. The excessive oxidation can be a reason of the

porosity of the sample too.

Evidences of the stress can also be seen on the other face of the same which means the material may have undergone mechanical

stress but this is not an important factor for corrosion.

5. Submerged Arc weld in large sectioned low carbon steel plate Preface: In submerged arc welding a mineral weld flux layer protects

the welding point and the freezing weld from the influence of the

surrounding atmosphere, (Figure 7). (Refer Appendix 2)

(Figure 7, Shows the submerged arc wielding)

(Figure 8; Shows the working of a Submerged arc wielding)

(Figure 9; Left Shows the sample) (Figure 10;Top shows the other face of the sample)

Normal Inspection: The wielded sides can be seen (Figure 9 & 10 ). Multiple

pass has been used to join the metals. The material looks perfect on naked eye.

Sample has been product of the rolled manufacturing process.

Microscopic view:

Figure on left shows the grain being rolled.

Figure on top shows the weld section with HAZ itself but Figure on top shows the HAZ grain with difference.

Small grains.

Conclusion: The sample shows that during the wield a large area was affected, as the result of the heat generated. Presence of the

large amount of HAZ is one another factor which has resulted in the possible cracks in the centre of the wields. These can be a

prior factor for the rupture of the material.

6. Friction weld in carbon steel Preface: Sample made by friction wielding. (Refer Appendix 2)

Investigation Comments: Carbon steel. Visible fracture on the

wielding area. (figure 12) Lots of heat treatment has been done.

Void can be easily seen.

(Figure 12; shows the sample made up of the friction wield)

Conclusion: Difference in the heat affected zone is responsible for

the cracks and fracture of the wield part. Since the high heat is

generated at the core the rest of the part of the metal is liable for stress concentration and ultimate resulting into fatigue.

Figure on top shows the effect of rolled treatment. Figure top shows the HAZ affected Zone Figure top shows the weld which is shrunk

Fusion Wielding: Refers to a form of welding that relies upon melting to enable the joining of materials of similar compositions

as well as same melting points. It is commonly used in corner construction where the welded materials are pushed together to

form a joint. Forms of fusion wielding include

Oxy-acetylene welding

Shield metal arc welding

Submerged arc welding

Gas tungsten arc welding

Gas metal arc welding Electro slag welding Plasma arc welding Electron beam welding Laser beam welding

Non Fusion Wielding: Two surfaces forcing together so that they

shapes due to plastic deformation that makes their fit in to one another, at the same

time the surface layers are broken up, allowing the intimate contact needed to fuse

the materials without necessarily melting them. This was the principle of the first

way known to weld metals – by hammering the pieces together whilst hot. Example

is brazing. (Figure13 on left)

Solid phase welding: The two metals

to be brought into contact to produce a metallic bond with or without the

application of heat, but apply pressure for the plastic flow of the metal. (Figure 14

below). Some examples of solid phase welding are Friction welding or explosion

welding.

Effects of Poor Welding of joints

Poor welding results in the following defects on joints

Inclusions

Hammer marks and arc strikes

Porosity

Craters

Spatter

Cracking

Misalignment

Overlap

Undercutting

Conclusion:

As welding defects can greatly affect weld performance and longevity, early detection and correction is important to ensure that

Welds can carry out their designed purpose. Detection techniques need to be sensitive enough to detect harmful or reject able

discontinuities but not to the point where all defects are rejected. It is only necessary to repair defects that are considered

detrimental to the structural integrity of the structure. Welds don’t have to be perfect – this is too costly and time consuming to

achieve. They need simply be within the acceptable working limits specified by the quality control code being used during the

weld inspection.

Chapter 3

Defects and Failures This part of the assignment will focus upon the causes of failures and/or defects found in engineering components.

Procedures To Carefully examine each component and capture the image either by electronic means through a

microscope or a photograph of the specimen. The investigation should thoroughly research the cause/s

of failure and suggest remedies to avoid these in future.

Apparatus 11 samples supplied

Metallurgical microscope with image capture facility

Magnifying glass

Magnet

Sample 1: Cast aluminium collet for attached to a steel rope immersed in seawater

(Figure 1 &2, Left to Right: Shows the corroded cast aluminium used on the steel pipe)

History: Cast Aluminium has been used on the steel rope, under the sea which is corrosive environment. (Used as Galvanic

coating, refer Chapter 4). This means the cast aluminium (figure 1 & 2) was used as sacrificial anode.

Comments: The materials are placed in direct contact. This potential difference produces electron flow between them. The less

resistant material here which is Aluminium becomes anodic and the more resistant material becomes cathode steel. Ionized metal

atoms/ electrons leave the anode, going into solution in the conducting seawater. As the electrons go into solution, free electrons

flow in the metal towards the cathode. The driving force for current and corrosion is the potential developed between the two

metals. The potential differences between metals under reversible, non-corroding conditions form the basis for predicting

corrosion tendencies. (Refer Appendix 3)

This has been the reason for failure.

Sample 2: Stabilised Stainless Steel pipe carried Chlorine at 120 Degree C. Failure occurred in 9 months.

(Figure 3 & 4, left to right; Shows the outer cast of the stainless steel pipe, shows the inside fractured view of the S.S. pipe)

History: Pipe carrying chlorine at 120 degree C which is a hot and the environment becomes corrosive.

Comments: Since the chlorine was hot and the pipe is made up of stainless steel (figure 3 &4) which may have been stabilised

there may have been generation of pits and groves inside the pipe. The heat affected zones may have been heat treated too.

Resistant stress on the pipe may have also been the reason for the failure by the corrosive chlorine flowing in it. It can be assumed

by the investigation that the pipe failed because of the corrosive environment & simultaneously affected by the stress corrosion.

Sample 4: Lifting lug made of low carbon steel plate

(Figure 5 &6, left to right: shows the faces of the lifting lug)

History: Used for lifting, so the material is always under tensile stress with the load and compression when absence of load.

Material made up of low carbon steel plate which is a ductile.

Comments: Traces of damage due to excessive tensile stress can be seen on the metal. Visibility of ductile failure is present and

cannot be neglected. Evidence of necking is present on both sides (figure 5&6) which suggest that the material being ductile had

been under elastic region and the moment when the stress went high the plastic region it fractured.

Sample 5: Cast cam shaft. TO BE REPLACED

(Figure 7: Right; shows the sample Cam shaft)

History: Camshaft used in machines/ engines, are always under stress environment. Operating consecutively every second there is

a range of forces that work on them.

Comments: Evidences shows stress concentration (figure 7) when fatigue took place. The material has been brittle. Potentially

stress corrosion generated a crack earlier before fracture and which propagated through cross sectional area of the material

resulting it to break down. It could have been prevented by various applications.

Sample 6: Mazak torque test specimen. Aluminium/Zinc alloy

(Figure 8; left to right; Sample of Mazak torque test)

History: As a test specimen or torque the sample have always been under stress and strain of

twisting motions.

Comments: The fracture can be explained as the alloy had been brittle and the application of the torsion, sample failed under

principal stress at 45 degrees angle which is when the sample had maximum stress. Pure shear (Refer Appendix 3) at 45’ makes

brittle materials fail. (figures 8)

Sample 7: Slave spring for refuse vehicle

(Figure 9 & 10; Left to right; Shows the

broken slave spring of the refuse

vehicle)

History: The material was used a spring for the refuse/ recycling vehicle. The application of the material was to hold the load

under stress and then release.

Comments: It cannot be neglected that the material had been elastic but failed when the stress with time made the material exceed

its elastic region.(figure 9 & 10) Since the material had been under continuous bends, the possibility is the sample had been under

brittle failure. Evidences of the sample being heat treated and quickly cooled is present. Apart the generation of rust suggests that

the material had been under moist environment which has also coupled faster for the failure of the material.

Sample 8: Shear pin for forging press

(Figure 11: fractured shear pin)

History: The pin had been used for the process of forging (Refer

Appendix 3), which means continuous stress of hammering and heat.

Comments: It is clear from the sample (Figure 11) that the material had been under hot environment, under stress which may

have resulted in creep failure. The metal shows evidences of being brittle failure. Possibility of corrosion cannot be neglected. In

nut heat & high stress resulted into failure.

Sample 9: Sheared stainless steel bolt

(Figure 12: right: sheared stainless steel bolt)

Comments: Evidences of the yielding all over the area before the material underwent fracture. Being

jigsaw shaped it had not been reshaped and even being ductile. It is possible too that the sample had been

under high corrosion. (Figure 12)

Sample 10: Splined vehicle drive shaft

(Figure 13: left; example of drive shaft of a vehicle)

Comments: Reason of failure is because of brittle nature and failure at 90 degrees by the action of

torque. It could have been neglected by increasing the ductility of the material. (Figure 13)

Conclusion: From the above research, it is well established that materials and metals undergo failure at a certain time. The failure

of the materials can be slow down with the better understanding of their application, environment they will operate. It is up to the

engineers for the selection of the right material for longevity of the product keeping in mind with the design and consequences

these materials will undergo.

Chapter 4 Corrosion

Corrosion: It is usually defined as the degradation of the properties of material as a result of the chemical

reaction in the environment. Corrosion can be classified under different categories based on the material, environment

(temperature, conditions), or the morphology of the corrosion damage. (Ricker, Stoudt, Dante; Materials Science & Engineering

Laboratory, Corrosion of Metals, online)

Based on the categories the corrosion morphology it can be classified under

eight different modes:

General Corrosion: It is the result of the chemical or electrochemical

reactions which proceed over the entire expose exposed surface. General

corrosion (figure 1) results in the metal becoming thinner and usually alters

the appearance of the surface. It also results in the failure of the mechanical

strengths of the components too.

(Figure 1, right; General corrosion)

Pitting Corrosion: This corrosion is at a high rate in small spot on the surface

of the material. Pitting corrosion (figure 2) also occurs in the metals that resist

corrosion through the formation of native oxide, hydroxide or salt film.

(Figure 2 Left; Pitting corrosion on metal described with the size of coin)

Crevice Corrosion: Material

when exposed to an

environment, usually occluded regions, such as crevices, where the environment in

this region does not freely mix with the bulk environment. Crevice corrosion

(figure 3) is frequently observed in passivized metals and alloys where they are

exposed to environment that contains halide ions. It can result in failure through

leaking joint, mechanical failure of joints, freezing of joints, or initiation of cracks

which can propagate to failure through other mechanisms.

(Figure 3, right ; Example of Crevice corrosion)

Intergranular Corrosion: During the solidification process, crystals

nucleate and grow together forming a solid. As a result of the chemical

composition of the & the properties of the region between the crystals can

differ. Certain alloy environment combinations can result in rapid

corrosion of the region between the crystals which is referred as

Intergranular corrosion (figure 4).

(Figure 4: left; shows the Intergranular corrosion of an alloy cylinder)

Galvanic Corrosion: Since different metals have different electrochemical

properties in the same environment, joining of the two dissimilar metals in a

manner which they complete an electrical circuit results in a corrosion of the

more active metal called galvanic corrosion (figure 5)

(Figure 5, right: Shows the galvanic corrosion)

Erosion corrosion: It is the degradation of material surface due to mechanical action, often by

impinging liquid, abrasion by slurry, particles suspended in fast flowing liquid or gas, bubbles

or droplets, cavitation, etc. (Figure 6)

(Figure 6, right: Shows the erosion corrosion of pipe carrying soda transportation)

Dealloying: Selective leaching of an alloying element can result in the surface being

dealloyed (figure 7) which is not only limited to surface causing serious loss of

mechanical strength of the material.

(Figure 7, left; Shows the Dealloying of zinc and copper)

Environmentally Induced Fracture: Material exposed to chemical reactive environs.

& application of stress to the material can result in formation and propagation of crack.

These environment (exposure to aqueous solution, organic solvents etc.) cause these

failures which are not aggressive but results in the loss of the mechanics and failure.

(Figure 8)

(Figure 8, Right; Shows the E.I.F. corrosion of a sound barrier)

Prevention methods of Corrosion

Methods that include preventing corrosion can be classified as following:

Active corrosion protection: (Figure 9) Aiming to influence the reactions

proceeding during the corrosion. Example use of corrosion resistant alloys or

use of corrosion inhibitors (Refer Appendix 4)

(Figure 9, right; shows the active corrosion prevention method)

Passive corrosion protection: Includes mechanical separating the metals from the corrosive agents.

Permanent Corrosion Protection: Aims to provide the protection at the place of use. Methods include

Tin Plating

Galvanization

Coating

Copper plating

Enamelling: It works in preventing the electrochemical connections of the metals. It varies

depending on the kind metal involved and the kind of corrosion prevention needed. (Figure 10

right, ship’s hull coated with paint)

Temporary Corrosion Protection: Includes : (Refer Appendix 4)

Protective coating method (Barrier films)

Desiccant method

VCI method (Volatile Corrosion Inhibitor)

Cathodic Protection

Anodic Protection

Working of Cathodic Protection: Discovered by Sir Humphrey Davy in 1820’s, this is one of the best ways of protection from corrosion

Principal working: Corrosion in an electro-chemical process involves the passage of electrical currents. The change from the

metallic to combined form occurs by an anodic reaction

M M+ + e-

(metal) (Soluble salt) (electron)

Example: Fe Fe++ + 2e-

This reaction produces free electrons which pass within the metal to another side on

the metal surface. Corrosion usually occurs at the anode.

The principal of cathode protection is in connecting an external anode to the metal to

be protected and passing an electricl dc current so that all area of the metal surface

becomes cathode before it corrodes.

This protection (figure 11) is to confer the continual negative electrical on a metal.

This replicates the effects of a sacrificial coating but with more active metal. It is

achieved by two ways:

(Figure 11, right; shows cathodic protection of underground pipelines)

- By the use of galvanic (sacrificial) anode (Refer appendix 4)

- By impressed current

Corrosion Inspection & Detection methods (Refer Appendix4 for definitions)

Visual: • Used for good resolution & can detect material loss and Thickness

Enhanced Visual: • Large area coverage

• Very fast

• Very sensitive to lap joint corrosion

• Multi-layer

Eddy Current • relatively inexpensive

• Good resolution

Ultrasonic: • Good resolution

• Can detect material loss and thickness

Radiography (figure 12, right): • Best resolution (~1%)

• Image interpretation

Thermography (figure 13, right): • Large area scans (Figure 12)

• Relatively high throughput

• “Macro view” of structures

Robotics and Automation: • Potential productivity improvements

(Figure 13)

Stress Corrosion:

This occurs due to the interaction of corrosion and

mechanical stress to result in cracking of the material is

called stress corrosion. Stress Corrosion Cracking aka

Stress corrosion (figure 14) results in the number of

mechanisms, especially as the result of the hydrogen

embrittlement. They are also called as season cracking

(especially for the brass in environment containing

ammonia). It is the insidious form of corrosion, results

in the loss of the mechanical strength with metal loss. It

triggers as a crack & results in fracture of the structure.

The process involves the corrosion along the path higher

than the normal corrosion with the bulk of material

typically being passive. The most active path is the grain

boundary, where segregation of impurity elements can

make it marginally more difficult for passivation to

occur.

(Refer Appendix 4 for how this occurs)

Figure 14, Top; showing a microscopic view of the stress corrosion of a tubing material

Factors that aggravate Stress corrosion:

Materials: inadequate heat treatment resulting in sensitization (depletion at grain boundaries) for example is extremely

detrimental.

Temperature & Chemical properties

Aggressive ions

Stresses on the material: Internal Stress and the external stress on the material, torsional stress or the tensile stress.

Intergranular corrosion cracking

Environmental conditions

Boiler Tubes Stress Corrosion Cracking (Industrial example of Stress corrosion)

Failure of boiler tubes by corrosion attack has been a familiar phenomenon in power plants resulting in unscheduled plant shut

down, in consequence, there are heavy losses in

industrial production and disruptions to civil

amenities.

The failure of boiler tubes appears in the form of

bending, bulging, cracking, wearing or rupture,

causing leakage of the tubes. (Figure 15)

Figure 15, top ; Shows the crack in the boiler tube

Cause of failure can be caused by one or more modes such as overheating,

stress corrosion cracking (SCC), hydrogen embrittlement, creep, flame

impingement, Sulphide attack, weld attack, dew point corrosion, etc.

Figure 16 showing the breakdown of the Hinkley point power station boiler

tube failure in UK 1969.

Boiler tube failure example: At Hinkley point power station the failure was initiated by the SCC growing in the condensed water

at the keyway in the bore disc. The steel has a low fractured toughness as the result of the temper brittlement. The disc ultimately

failed by fast facture and crack. The failure was so serious that a massive chunk of steel rotating at 3000 rpm was flying as debris.

Two new effective ways of corrosion control: Hot Dip Galvanising (Figure 17) Advantages

1. Competitive First Cost As a highly mechanised, closely controlled process, Hot Dip

Galvanising can be carried out very economically in large batches.

The alternative - painting - is highly labour intensive.

2. Lowest Lifetime Cost

Figure 17 Hot dip galvanizing

3. Long Life

4. Reliability The process is simple, straightforward and closely controlled. The coating thicknesses are regular, predictable and easily

specified

5. Speed of Application A fully galvanised protective coating can be applied in a few hours. A proper four coat paint system requires

approximately one week.

6. Complete Coverage

7. Coating Toughness Hot Dip Galvanising is unique - the coating bonds metallurgically with the steel giving a much greater resistance to

damage than other coatings.

Areas of SCC and failure

8. Three Way Protection 1. It weathers at a slow rate giving a long and predictable life.

2. The coating sacrifices itself to any small areas exposed through drilling, cutting or accidental damage.

3. If large areas get damaged it prevents the sideways creep of rust.

9. Ease of Inspection Coating thickness can be checked easily with the use of a magnetic probe (Elcometer).

10. Faster Construction (Sperin Galvanisers, Online)

Enameling: (Figure 18)

1. Its resistance to chemicals

2. resistance to high temperatures and heat reflection properties

3. aesthetically pleasing, durable and easy to clean

4. Water resistant

5. Multiplicity and stability of colour

6. Thermal shock resistance

7. Mechanical strength of the enamelled surface

(ArcelorMittal Steel, Online)

Figure 18, right ; Enameled steel in daily use)

Old corrosion control demerits:

Electroplating: Disadvantages of electroplating might be that its time consuming, it may not be uniform, and the coating may be

brittle. Examples include copper plating

Painting: Deformation in paint results into the corrosion of the metal and is quiet hard to detect even a small deformation.

Tin plating

Copper plating

Temporary coating.

(Refer appendix for more Old corrosion techniques)

End of Chapter 4

References: Arcelormittal Steel. (2008). Advantages of Enamel. Retrieved April 19, 2014, from Enamelled Street:

http://www.constructalia.com/repository/transfer/en/05586089ENLACE_PDF.pdf

BUSHMAN & Associates, Inc. (2010). Corrosion and Cathodic Protection Theory. Retrieved April 23, 2014, from C O R

R O S I O N C O N S U L T A N T S: http://www.bushman.cc/pdf/corrosion_theory.pdf

Encocam . (2012). Retrieved April 23, 2014, from Encocam: http://www.encocam.com/

Houldcroft, P. T. (1988). Welding Process Technology. In Welding Process Technology (pp. 15,23, 83,123). Cambridge

University Press.

Materials & Process. (2010). Retrieved April 10, 2014, from All subjects for you:

http://www.allsubjects4you.com/Welding.htm

National Physical Labratory - Sereco Group plc. (n.d.). Stress Corrosion Cracking. Retrieved from The National

Corrosion Service: http://www.npl.co.uk/upload/pdf/stress.pdf

National Physical Labratory. (2010). Basics of Corrosion . Retrieved April 18, 2014, from Corrosion & Protection :

http://www.npl.co.uk/upload/pdf/basics_of_corrosion_control.pdf

Prof. J.S Colton. (2013). Manufacturing processes and systems. Retrieved April 24, 2014, from Georgia Institute of

Technology: http://www-old.me.gatech.edu/jonathan.colton/me4210/fusionanal.pdf

Sigma-Aldrich Corporation. (2010). Aldrich Chemistry. Retrieved April 15, 2014, from Sigma Aldrich:

https://www.sigmaaldrich.com/content/dam/sigma-aldrich/materials-science/material-

matters/material_matters_v2n4.pdf

Sperrin Galvanisers. (2010). Hot Dip Galvanising . Retrieved April 2, 2014, from Sperrin Galvanisers:

http://sperringalvanisers.com/

Stress Corrosion Cracking. (2010). Retrieved April 30, 2014, from Corrosion Doctors: http://corrosion-

doctors.org/Forms-SCC/scc.htm

Superior Consumables. (2008). Retrieved April 12, 2014, from The robotic choice:

http://www.subarcwelding.com/sub-arc-article.asp

The North American Technology and Industrial based Organization. (1998). Corrosion Detection technologies.

Retrieved April 7, 2014, from http://www.acq.osd.mil/mibp/natibo/docs/cdt_ss.pdf

Transport Information Service - Ministry of Germany. (2014). Transport Information Service. Retrieved April 30, 2014,

from Die Deutschen Versicherer : http://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htm

Truck lite Global. (2013). Retrieved April 20, 2014, from Truck Lite: http://www.truck-lite.eu.com/

Appendices

Appendix 1 – Industry Visit.

Injection Welding: Within minutes, epoxy can be injected 9 feet deep into cracks as small as 0.002 inch wide. Within hours, this

same epoxy will surpass the compressive and tensile strengths of the surrounding concrete. Such effectiveand easily attained

results make epoxy injection one of the most common ways of repairing narrow crack s. It has been used to repair cracks in

buildings, bridges and dams. To achieve such good results, though, requires proper materials and injection techniques and an

Understanding of what epoxy injection can and cannot do.

Appendix 2 – Welding & Joints.

Arc Welding: An electric arc is formed at the tip of the welding rod when a current passes across an air gap and continues through the

grounded metal which is being welded. Welding machine. This is the term used to describe the machine which converts 120-240 volt AC

electricity to welding voltage, typically 40-70 volts AC, but also a range of DC voltages. It generally consists of a large, heavy transformer, a

voltage regulator circuit, an internal cooling fan, and an amperage range selector.The term welder applies to the person doing the welding. A

welding machine requires a welder to operate it.

Leads, or Welding leads. These are the insulated copper conductors which carry the high amperage, low voltage electricity to the work piece that

is being welded.

Rod holder, or stinger is the device on the end of the lead that holds the electrode, which the person welding uses to accomplish the welding

task.

Ground and ground clamp. This is the lead that grounds, or completes the electrical circuit, and specifically, the clamp that is attached to the

work to allow the electricity to pass through the metal being welded.

Amperage, or amps. This is an electrical term, used to describe the electrical current supplied to the electrode.

DC and reverse polarity. This is a different configuration used in welding with an arc/electrode system, which offers more versatility, especially

in overhead welding applications and for use welding certain alloys that do not weld easily with AC voltages. The welding machine that

produces this current has a rectifier circuit or has the current supplied by a generator, and is much more expensive than a typical AC welder.

Electrodes. There are many specialized welding electrodes, used for specific alloys and types of metals, such as cast or malleable iron, stainless

or chromolly steel, aluminum, and tempered or high carbon steels. A typical electrode consists of the wire rod in the center covered with a

special coating (flux)which burns as the arc is maintained, consuming oxygen and producing carbon dioxide in the weld area to prevent the base

metal from oxidizing or burning away in the arc flame during the welding process.

Brazing: Brazing is a joining process wherein metals are bonded together using a filler metal with a melting (liquidus) temperature greater than

450 °C (840 °F), but lower than the melting temperature of the base metal. Filler metals are generally alloys of silver (Ag), aluminum (Al), gold

(Au), copper (Cu), cobalt (Co) or nickel (Ni).

Heat Sources for Brazing

Torch Brazing A heating source supplied by a fuel gas flame. Gases include acetylene, hydrogen or propane. A typical application is to braze a

tube into a fitting using copper or silver brazing filler metals. Induction Brazing Electric coils, which are designed for specific joint geometries,

are used to heat the part and the brazing filler metal until the liquid metal flows via capillary attraction into the joint. This process is primarily

used for brazing with copper and silver alloys. A typical application is a tube to tube assembly.

Continuous Furnace Conveyor belts transport the pre-alloyed components through preheating, heating and post-heating zones

where the braze alloy reaches temperature, then resolidifies during cooling. Silver and copper based brazing filler metals are most commonly

used in these processes.

Retort or Batch Furnacec The furnace used can be refractory lined and heated by gas, oil or electricity. Atmospheres can be either a generated

gas (endothermic or exothermic) or an inert gas such as argon or nitrogen. Hydrogen gas is also used for brazing filler metals that oxidize in

other atmospheres. Copper, silver, nickel and gold based brazing filler metals can be brazed successfully in these types of furnaces.

Vacuum Furnace A furnace with electrically heated elements that surround the workload and heat the brazing filler metal to the liquidus state

so flow and capillary attraction are achieved. To permit brazing of alloys that are sensitive to oxidation at high temperatures, a pumping system

is employed that removes oxygen. Gold, copper, nickel, cobalt, titanium and ceramic based filler metals are successfully vacuum brazed

Appendix 3 – Defects and failure.

Aluminum as galvanic corrosion: Corrosion between anodized aluminium and steel

Aluminium is a reactive metal (un-noble), which should have a low corrosion resistance according to thermodynamics. The high corrosion

resistance found on aluminium nevertheless, is due to the presence of a thin, compact film of adherent aluminium oxide on the surface.

Whenever a fresh aluminium surface is created and exposed to either air or water, a surface film of aluminium oxide forms at once.

This aluminium oxide dissolves in some chemicals, notably strong acids and alkaline solutions. When the oxide film is removed, the metal

corrodes rapidly by uniform dissolution. In general, the oxide film is stable over a pH range of about 4.0 to 9.0, but there are exceptions.

One of these exceptions is in environments where the surface film is insoluble, but weak spots in the oxide film leads to localized corrosion.

Local corrosion can only be found when aluminium is passive, covered by an oxide layer as the one formed by anodizing.

Localized corrosion has an electrochemical nature and is caused by a difference in corrosion potential in a local cell formed by differences in or

on the metal surface.

Galvanic corrosion is due to an electrical contact with a more noble metal or a non-metallic conductor in a conductive environment. The

galvanic corrosion is very dependent of the cathode reaction and which metals are in contact which each other.

The efficiency of this Cathodic reaction will determine the corrosion rate. The most

common examples of galvanic corrosion of aluminium alloys are when they are joined

to steel or copper and exposed to a wet saline environment.

The galvanic corrosion of aluminium is usually mild, except in highly conductive media

such as i.e. slated slush from road de-icing salts, sea water and other electrolytes. The

contact area must be wetted by an aqueous liquid in order to ensure ionic conduction.

Otherwise there will be no possibility of galvanic corrosion.

The galvanic series for metals shows that aluminium will be the anode of the galvanic cell in contact with almost all other metals and hence the

one which suffer from galvanic corrosion.

The galvanic corrosion performance of the different aluminium alloys is quite similar, so that changing alloys cannot solve the problem.

There has to be an electrical contact either by direct contact between the two metals or by a connection such as a bolt.

The dissolution rate depends on the surface ratio between the two metals:

The most favourable case is a very large anodic surface area and a small Cathodic surface area. The galvanic corrosion is a local corrosion, and

is therefore limited to the contact zone.

It is unusual to see galvanic corrosion on aluminium in contact with stainless steel (passive). In contrast contact between copper, bronze, brass

and different kinds of steel alloys (passive and active) and aluminium can cause severe corrosion so it is advisable to provide insulation between

the two metals.

The reason for the confusion of the galvanic corrosion between aluminium and steel is that stainless steel can be found as passive or active, and

if the environment contains chloride. This will change the corrosion effect on aluminium substantially. Generally, the closer one metal is to

another in the series, the more compatible they will be, i.e., the galvanic effects will be minimal. Conversely, the farther one metal is from

another, the greater the corrosion will be.

Though prediction of galvanic corrosion is not easy. For contact between common metals, in particular steel and stainless steel, experience show

that laboratory testing always leads to more severe results than what is actually observed under conditions of weathering.

The photo shows galvanic corrosion after 1000 hours salt spray test in laboratory where stainless steel bolts have been screwed into an anodized

aluminium block.

Normally the galvanic coupling with stainless steel works very well but when there is even the slightest trace of chloride in the environment a

galvanic corrosion will take place.

Due to the cracking of the anodized layer when mounting, a very little area of un-noble metal (the aluminium underneath) will be in contact with

a very big area of the more noble metal (the stainless steel). This will cause galvanic corrosion which can increase tremendously dependent on

the area of the aluminium.

Appendix 4 – Corrosion. Active Corrosion Prevention: The aim of active corrosion protection is to influence the reactions which proceed during corrosion, it being

possible to control not only the package contents and the corrosive agent but also the reaction itself in such a manner that corrosion is avoided.

Examples of such an approach are the development of corrosion-resistant alloys and the addition of inhibitors to the aggressive medium.

1. Protective coating method

The protective coating method is a passive corrosion protection method. The protective coating isolates the metallic surfaces from the aggressive

media, such as moisture, salts, acids etc..

The following corrosion protection agents are used:

Solvent-based anticorrosion agents

Very high quality protective films are obtained.

Once the anticorrosion agent has been applied, the solvent must vaporize so that the necessary protective film

is formed.

Depending upon the nature of the solvent and film thickness, this drying process may take as long as several

hours. The thicker the film, the longer the drying time. If the drying process is artificially accelerated, there

may be problems with adhesion between the protective film and the metal surface.

Since protective films are very thin and soft, attention must always be paid to the dropping point as there is a

risk at elevated temperatures that the protective film will run off, especially from vertical surfaces.

Since solvent-based corrosion protection agents are often highly flammable, they may only be used in closed

systems for reasons of occupational safety.

Water-based anticorrosion agents

Water-based anticorrosion agents contain no solvents and thus do not require closed systems.

Drying times are shorter than for solvent-based anticorrosion agents.

Due to their elevated water content, water-based anticorrosion agents are highly temperature-dependent (risk

of freezing or increased viscosity).

The advantage of this method is that the protective film is readily removed, but the elevated water content,

which may increase relative humidity in packaging areas, is disadvantageous.

Corrosion-protective oils without solvent

Corrosion-protective oils without solvent produce only poor quality protective films. Good quality protection

is achieved by adding inhibitors. Since these corrosion-protective oils are frequently high quality lubricating

oils, they are primarily used for providing corrosion protection in closed systems (engines etc.).

Dipping waxes

The protective layer is applied by dipping the item to be packaged into hot wax. Depending upon the type of

wax, the temperature may have to be in excess of 100°C. Removal of the protective film is relatively simple

as no solid bond is formed between the wax and metal surface. Since application of dipping waxes is

relatively complex, its use is limited to a few isolated applications.

2. Desiccant method

Introduction

According to DIN 55 473, the purpose of using desiccants is as follows: "desiccant bags are intended to protect the package contents from

humidity during transport and storage in order to prevent corrosion, mold growth and the like."

The desiccant bags contain desiccants which absorb water vapor, are insoluble in water and are chemically inert, such as silica gel, aluminum

silicate, alumina, blue gel, bentonite, molecular sieves etc.. Due to the absorbency of the desiccants, humidity in the atmosphere of the package

may be reduced, so eliminating the risk of corrosion. Since absorbency is finite, this method is only possible if the package contents are enclosed

in a heat sealed barrier layer which is impermeable to water vapor. This is known as a climate-controlled or sealed package. If the barrier layer is

not impermeable to water vapor, further water vapor may enter from outside such that the desiccant bags are relatively quickly saturated, without

the relative humidity in the package being reduced.

Desiccants are commercially available in desiccant units. According to DIN 55 473:

"A desiccant unit is the quantity of desiccant which, at equilibrium with air at 23 ± 2°C, adsorbs the following quantities of water vapor:

min. 3.0 g at 20% relative humidity

min. 6.0 g at 40% relative humidity

The number of desiccant units is a measure of the adsorption capacity of the desiccant bag."

Desiccants are supplied in bags of 1/6, 1/3, 1/2, 1, 2, 4, 8, 16, 32 or 80 units. They are available in low-dusting and dust-tight forms. The latter

are used if the package contents have particular requirements in this respect.

Calculation of required number of desiccant units

The number of desiccant units required is determined by the volume of the package, the actual and desired relative humidity within the package,

the water content of any hygroscopic packaging aids, the nature of the barrier film (water vapor permeability).

Formula for calculating the number of desiccant units in a package (DIN 55 474):

n = (1/a) × (V × b + m × c + A × e × WVP × t)

3. VCI (Volatile Corrosion Inhibitor) method

Mode of action and use

Inhibitors are substances capable of inhibiting or suppressing chemical reactions. They may be considered the opposite to catalysts, which enable

or accelerate certain reactions.

Unlike the protective coating method, the VCI method is an active corrosion protection method, as chemical corrosion processes are actively

influenced by inhibitors.

In simple terms, the mode of action (see Figure 1) is as follows: due to its evaporation properties, the VCI substance (applied onto paper,

cardboard, film or foam supports or in a powder, spray or oil formulation) passes relatively continuously into the gas phase and is deposited as a

film onto the item to be protected (metal surfaces). This change of state proceeds largely independently of ordinary temperatures or humidity

levels. Its attraction to metal surfaces is stronger than that of water molecules, resulting in the formation of a continuous protective layer between

the metal surface and the surrounding atmosphere which means that the water vapor in the atmosphere is kept away from the metal surface, so

preventing any corrosion. VCI molecules are, however, also capable of passing through pre-existing films of water on metal surfaces, so

displacing water from the surface. The presence of the VCI inhibits the electrochemical processes which result in corrosion, suppressing either

the anodic or cathodic half-reactions. Under certain circumstances, the period of action may extend to two years.

Mode of action of VCI

The mode of action dictates how VCI materials are used. At item to be protected is, for example, wrapped in VCI paper. The metallic surfaces of

the item should be as clean as possible to ensure the effectiveness of the method. The VCI material should be no further than 30 cm away from

the item to be protected. Approximately 40 g of active substances should be allowed per 1 m³ of air volume. It is advisable to secure this volume

in such a manner that the gas is not continuously removed from the package due to air movement. This can be achieved by ensuring that the

container is as well sealed as possible, but airtight heat sealing, as in the desiccant method, is not required.

The VCI method is primarily used for articles made from carbon steel, stainless steel, cast iron, galvanized steel, nickel, chromium, aluminium

and copper. The protective action provided and compatibility issues must be checked with the manufacturer.

N.B.: The use of water-miscible, water-mixed and water-immiscible corrosion protection agents, corrosion protection greases and waxes,

volatile corrosion inhibitors (VCI) and materials from which volatile corrosion inhibitors may be released (e.g. VCI paper, VCI films, VCI foam,

VCI powder, VCI packaging, VCI oils) is governed by the German Technical Regulations for Hazardous Substances, TRGS 615 "Restrictions

on the use of corrosion protection agents which may give rise to N-nitrosamines during use".

Galvanic Series:

Occurrence of Steel Corrosion Cracking

Steels in ‘passivating’ environments

Carbon and low alloy steels can suffer from SCC in a wide range of environments that tend to form a protective passivating film of oxide or

other species. Cracking will not normally occur when there is a significant corrosion rate (note that this is not the case for hydrogen

embrittlement). A wide range of environments have been found to cause SCC, including strong caustic solutions, phosphates, nitrates,

carbonates, and hot water. The problems are important for both economic and safety reasons. Caustic cracking of steam-generating boilers was a

serious problem in the late 19th century (the necessary strong caustic solution was produced by evaporation of the very dilute solution inside the

boiler as it escaped through leaks in the riveted seams) and boiler explosions led to significant loss of life. More recently gas transmission

pipelines have cracked in carbonate solutions produced under protective coatings as a result of cathodic protection systems. In this case the crack

runs along the length of the pipe, and may propagate for very

long distances by fast fracture. If the gas cloud that is released ignites, the resultant fireball is devastating.

Hydrogen embrittlement of high strength steels

All steels are affected by hydrogen, as is evidenced by the influence of hydrogen on corrosion fatigue crack growth, and the occurrence of

hydrogen-induced cracking5 under the influence of very high hydrogen concentrations. However, hydrogen embrittlement under static load is

only experienced in steels of relatively high strength. There is no hard-and-fast limit for the strength level above which problems will be

experienced, as this will be a function of the amount of hydrogen in the steel, the applied stress, the severity of the stress concentration and the

composition and microstructure of the steel. As a rough guide hydrogen embrittlement is unlikely for modern steels with yield strengths below

600 MPa, and is likely to become a major problem above 1000 MPa.

The hydrogen may be introduced into the steel by a number of routes, including welding, pickling, electroplating, exposure to hydrogen-

containing gases and corrosion in service. The effects of hydrogen introduced into components prior to service may be reduced by baking for a

few hours at around 200 °C. this allows some of the hydrogen to diffuse out of the steel while another fraction becomes bound to relatively

harmless sites in the microstructure.