Materials and Process - Industry visit, Corrosion & material weld analysis
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Materials and Process - Industry visit, Corrosion & material weld analysis



Materials and Process, weld analysis, corrosion

Materials and Process, weld analysis, corrosion



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Materials and Process - Industry visit, Corrosion & material weld analysis Materials and Process - Industry visit, Corrosion & material weld analysis Document Transcript

  • 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: & 2
  • 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) 3
  •  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 4
  • 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) 5
  •  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) 6
  • 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. 7
  • 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 8
  • 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 9
  • • 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 10
  • 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. 11
  • 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. 12
  • Chapter 2 13
  • 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: 14
  • 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. 15
  • 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 16
  • 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 17
  • 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. 18
  • 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 19
  • 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. 20
  • Chapter 3 21
  • 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. 22
  • 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) 23
  • 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. 24
  • Chapter 4 Corrosion 25
  • 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) 26
  • 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- 27
  • 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 28
  • 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 29
  • 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 30
  • References: Arcelormittal Steel. (2008). Advantages of Enamel. Retrieved April 19, 2014, from Enamelled Street: 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: Encocam . (2012). Retrieved April 23, 2014, from Encocam: 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: National Physical Labratory - Sereco Group plc. (n.d.). Stress Corrosion Cracking. Retrieved from The National Corrosion Service: National Physical Labratory. (2010). Basics of Corrosion . Retrieved April 18, 2014, from Corrosion & Protection : Prof. J.S Colton. (2013). Manufacturing processes and systems. Retrieved April 24, 2014, from Georgia Institute of Technology: Sigma-Aldrich Corporation. (2010). Aldrich Chemistry. Retrieved April 15, 2014, from Sigma Aldrich: matters/material_matters_v2n4.pdf Sperrin Galvanisers. (2010). Hot Dip Galvanising . Retrieved April 2, 2014, from Sperrin Galvanisers: Stress Corrosion Cracking. (2010). Retrieved April 30, 2014, from Corrosion Doctors: http://corrosion- Superior Consumables. (2008). Retrieved April 12, 2014, from The robotic choice: The North American Technology and Industrial based Organization. (1998). Corrosion Detection technologies. Retrieved April 7, 2014, from Transport Information Service - Ministry of Germany. (2014). Transport Information Service. Retrieved April 30, 2014, from Die Deutschen Versicherer : Truck lite Global. (2013). Retrieved April 20, 2014, from Truck Lite: 31
  • 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 32
  • 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. 33
  • 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 34
  • 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. 35
  • 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: 36
  • 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. 37