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Stainless steel is selected for applications where their inherent corrosion resistance, strength and aesthetic appeal are required. However, dependent on the service conditions, stainless steel will stain and discolour due to surface deposits and so cannot be assumed to be completely maintenance-free. In order to achieve maximum corrosion resistance and aesthetic appeal, the surface of the stainless steel must be kept clean. Provided the grade of stainless steel and the surface finish are correctly selected, and cleaning schedules carried out on a regular basis, good performance and long service life will result. Factors affecting maintenance Surface contamination and the formation of deposits on the surface of the stainless steel must be prevented. These deposits may be minute particles of iron or rust generated during construction. Industrial and even naturally occurring atmospheric conditions can produce deposits which can be equally corrosive, e.g. salt deposits from marine conditions. Working environments can also provide aggressive conditions such as heat and humidity in swimming pool buildings. These conditions can result in surface discolouration of stainless steel and so maintenance on a more frequent basis may be required. Modern processes use many cleaners, sterilizers and bleaches for hygienic purposes. Proprietary solutions, when used in accordance with makers’ instructions, should be safe but if used incorrectly (e.g. warm or concentrated), may cause discolouration or corrosion on stainless steel. Strong acid solutions are sometimes used to clean masonry and tiling of buildings. These acids should never be used where contact with metals, including stainless steel, is possible. If this happens, the acid solution must be removed immediately, followed by dilution and rinsing with cleaning water. Maintenance programme With care taken during fabrication and installation, cleaning before ‘hand-over’ should not present any problems. More attention may be required if the installation period has been prolonged or hand-over delayed. Where surface contamination is suspected, immediate cleaning after site fixing should avoid problems later. Food handling, pharmaceutical, aerospace and certain nuclear applications may require extremely high levels of cleanliness applicable to each industry. The frequency of cleaning is dependent on the application – a simple rule is: Clean the metal when it is dirty in order to restore its original appearance. This may vary from once to four times a year for external applications, but may be daily for items in ‘hygienic’ applications. Recommendations on cleaning frequencies in architectural applications are shown below. Cleaning frequency in architectural applications Location 430 (1.4016) 304 (1.4301) 316 (1.4401) Internal As required to maintain appearance or design Suburban or rural 6-12 month intervals (as appropriate to location and design) Industrial or urban Stainless Steel Grade not recommended 3-6 months 6-12 months Coastal or marine Stainless Steel Grade not recommended Grade not recommended 6-12 months Source: Zhejiang wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Stainless Steel Tube‘s Surface roughness —also known as surface profile Ra—is a measurement of surface finish — it is topography at a scale that might be considered “texture” on the surface. Surface roughness is a quantitative calculation of the relative roughness of a linear profile or area, expressed as a single numeric parameter (Ra). Stainless Steel Tube’s Surface roughness is the measure if the finer surface irregularities in the surface texture. These are the result of the manufacturing process employed to create the surface. Stainless Steel Tube’s Surface roughness Ra is rated as the arithmetic average deviation of the surface valleys and peaks expressed in micro inches or micro meters. ISO standard use the term CLA (Center Line Average). Both are interpreted identical. Ra micro-meters Ra micro-inches RMS CLA (N) Rt N Cut-Off Length in. mm 0.025 1 1.1 1 0.3 1 0.003 0.08 0.05 2 2.2 2 0.5 2 0.01 0.25 0.1 4 4.4 4 0.8 3 0.01 0.25 0.2 8 8.8 8 1.2 4 0.01 0.25 0.4 16 17.6 16 2.0 5 0.01 0.25 0.8 32 32.5 32 4.0 6 0.03 0.8 1.6 63 64.3 63 8.0 7 0.03 0.8 3.2 125 137.5 125 13 8 0.1 2.5 6.3 250 275 250 25 9 0.1 2.5 12.5 500 550 500 50 10 0.1 2.5 25.0 1000 1100 1000 100 11 0.3 8.0 50.0 2000 2200 2000 200 12 0.3 8.0 Conversions (math): CLA (micro inches) = Multiply Ra(µm) x 40 RMS * (acceptable 1.1 – 1.7 factor) = Multi ply CLA x 1.1 Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

The Kolsterising improves the wear resistance of stainless steel surface, without degrading their corrosion resistance. There are no additions of chemical elements to the steel during the process. It is claimed that austenitic stainless steel which contain molybdenum, such as 1.4401 (316) can also have enhanced corrosion resistance after Kolsterising.The Kolsterising process does not apply a coating on the surface but is a low temperature surface carbon diffusion treatment. Although large quantities of carbon are diffused into the surface visiblechromium carbides are not formed. The resulting surface treated layers can have hardnesses in the range of 1000 to 1200 VPN (approx 72 HRC). The thickness of the hardened layer is dependent on the process conditions used, but includes 22 or 33 micron effective case depths. Complex shapes can be effectively hardened by this process.Surface property enhancements and benefits associated with Kolsterising. The main properties enhancements from the Kolsterising process, claimed by the developers of the process includeImprovement of wear and galling resistance. Improvement of cavitation erosion resistance Increased fatigue strength Good dimensional stability. There should be no degradation of corrosion resistance and the magnetic properties of austenitic stainless steel (low magnetic permeability) are not affected by the process. The pitting and crevice corrosion resistance of molybdenum containing grades like 1.4404 (316) in chloride environments is claimed to be improved by the process. The surface hardened layer also has good toughness properties and there is no risk of delamination or peeling of the surface hardened layer. The improvement in fatigue resistanceis due to the formation of compressive stresses in the treated surface layer, which also helps enhance the Stress Corrosion Cracking SCC resistance of Kolsterised parts. There is little or no change in dimensions or shape when stainless steel are Kolsterised and no change in steel colour (appearance). The process can be applied to finished parts with a high standard of dimensional accuracy. These high levels of dimensional accuracy also mean that seals may not be needed, enhancing the operational temperature ranges of components with possible reductions in maintenance costs.Range of stainless steel that can be Kolsterised. A wide range of austenitic and duplex stainless steel can be treated. These include 1.4301 (304) 1.4307 (304L) 1.4401 (316) 1.4404 (316L) 1.4462 (2205) Nickel based alloys including Hastelloy C22 and C276 and Inconel 625 and 718 can also be Kolsterised.Applications for Kolsterising The fields of application for Kolsterized surfaces includes automotive, valve manufacturing, marine, oil and gas,fastener, food/beverage, pharmaceutical and medical industries.Specifically, valve parts (balls, needles, seats and housings) in petrochemical industries, parts for NACE MR 0175 sulphide stress corrosion cracking sour gas applications and beverage filling machine pistons and seals. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Rouging of stainless steel is sometimes found in high purity hot water systems, usually appearing as a thin red or black powdery or ‘slimy’ deposit. The mechanism that causes rouging does not appear to be fully understood, but is connected to destabilization of the passive layer. Measures that improve corrosion resistance can help prevent rouging. Passivation and smoothing of surface finishes by electropolishing have been used to reduce the risk of rouge formation. What is rouging? ‘Rouge’ is belived to be comprised of be iron oxides and hydroxide corrosion products in various oxidation states. The range of colour observed is probably explained by the different ferric ion (Fe3+) oxide and hydroxide corrosion product. The exact cause of rouging does not seem to have been clearly establishe, but is the result of temporary destabilization in the passive layer. One theory attributes this to ionic interactions between the water and the chromium rich passive layer. Temporary breaches in the passive layer resulting in localized corrosion to the underlying steel, before the passive layer has time to reform. Once reformed the short bust of attack is stopped, until conditions revert to those causing the passive layer de-stabilization. Observations that rouging is affected by:  water oxygen content and pH presence of any iron contamination in systems  surface finish (roughness) non-metallic inclusion levels in the steel welding defects, debris, heat tint or improperly cleaned areas could all support this view as these are known to have an influence on the corrosion resisting performance of stainless steels. The relative water solubilities of ferrous (Fe2+) and Ferric (Fe3+) ions are influenced by pH and result in the range of rouge colours formed from the ‘intermittent’ attack mechanism. Fe2+ ions are readily soluble, but when oxidized to Fe3+ ions are insoluble, forming oxides (Fe2O3 and Fe3O4) and hydroxides (Fe2O3.H2O) like the rusting process of carbon steels. These oxidized products have different colours. The near neutral pH of high purity water promotes the formation of insoluble ferric oxidation products and hence rouge formation. The rouge corrosion products can be deposited away from the actual corrosion site, or as an even film giving the impression that corrosion is general. It therefore often difficult to identify why and where the attack has occurred. Avoiding rouge formation Resistance to rouge formation depends on several factors. These include: Steel composition In ambient temperature water, 304 type is normally considered suitable but for high purity, ‘hot’ water conditions where rouging is a hazard, 316 type has been found to be beneficial and is normally selected in pharmaceutical plant systems. The extra nickel in 316 helps improve passive layer stability, while the additional molybdenum improves micro-pitting resistance, both of which appear to be beneficial in resisting rouge formation. For the same reasons, more highly alloyed stainless steel types could be expected to resist rouging better than 316. Surface finish Smooth surface finishes are important in the avoidance of rouging. Electropolishing is widely used to reduce the risk of rouge formation. This may also help improve the stability of the passive layer, so giving a double benefit. Electropolishing of stainless steel Surface imperfections from non-metallic inclusions can also be sites where the passive layer can become disrupted and become corrosion initiation sites. Steel made using good modern steelmaking methods should be better than older, perhaps ‘dirtier’ steels in resisting rouging. Remelted steels could also be considered, but their higher cost may preclude them. Sound, smoothly contoured weld joints, with any heat tint removed, should also help improve the resistance to rouge formation by minimising crevice effects and optimising the inherent corrosion resistance of the steel surfaces. Avoid iron contamination Any source of iron contamination or ‘carry-over’ corrosion product from other non-stainless steel parts of a system could promote rouge formation as the iron corrodes (rusts) relatively easily (compared to the stainless steel) in the process water. This provides a source of iron ‘ions’ (Fe3+) that form the rouge products. Careful post fabrication clean up is most important. Removal of rouge deposits Often rouge formation cannot be avoided and has to be removed during routine plant shut downs. Besides being unacceptable to the plant operators as product contamination could result, build-ups of rouge products can lead to operational problems such as blockages in filters. More severe localized ‘shielding’ (pitting) corrosion could also result under rouge deposits. In some cases rouge deposits can be wiped off, but abrasion should be avoided as this will result in roughening of the surfaces and may reduce the resistance to future rouge formation. Acid treatments involving ‘moderate’ strength nitric, phosphoric, citric and oxalic acids have been used satisfactorily. The passivating action of some of these treatments could be beneficial, but ‘etching’ of surfaces could result in further problems as this could reduce rouge formation resistance, due to the slight roughening of the surfaces. Electropolishing could be beneficial to re-smooth and repassivate the surfaces. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Mechanical finishes for stainless steel are covered as finish codes G, J, K and P in EN 10088-2. Terms grinding, polishing, brushing and buffing even when used along with these codes are not sufficient to accurately define the finish. There can often be confusion about what these terms mean. To precisely specify a finish also requires agreed samples. Contamination rust staining on mechanically polished stainless steel surfaces is often the result of using contaminated finishing media, often via hand tools which have previously been used on carbon steel. Polishing Both grinding and polishing involve the deliberate removal of metal from the surface using an abrasive. The resulting surface will have some directional marks, partially dependent on the grit size of abrasive used. In the case of the very fine abrasives used in polishing it should only be possible to see any ‘directional marks’ under a microscope. Viewed normally 1P/2P finishes should appear non-directional. Satin finishes (2K of EN 10088-2) are an intermediate between dull polished (2J of EN 10088-2) and bright polished (2P of EN 10088-2). These finishes can be enhanced by a final brushing operation.Mill finishes ‘1’ and ‘2’ are described in the article Surface Finish for Stainless Steel Sheet Plate Coil Strip There is no accepted definition of an abrasive grain or grit size that differentiates grinding from polishing. As a guide, but not a definition, grit sizes of 80 and coarser would be associated with grinding, whereas grit sizes of 120 and finer are used in preparing polished finishes. Like polishing, which often involves using successively finer abrasive grit sizes to obtain the desired final finish, grinding can also involve more than one abrasion stage. The final grit size used in both grinding and polishing does not fully define the finish and must not be used in an attempt to specify a ground or polished finish on stainless steel. Other parameters such as abrasive pressure, contact time, material feed rate and whether the operations are done dry or wet all affect the character of the finish produced. Mechanical finishes merely described as ‘satin’, ‘polished’, ‘dull’, ‘bright’ or ‘mirror’ can vary quite significantly between mechanical finishing contractors. The correct choice of stainless steel grade is also important when considering bright polished finishes.Grades such as 1.4541/321 and1.4571/316Ti, which contain small amounts of titanium to improve their “intercystalline” corrosion resistance, cannot be expected to be capable of completely defect-free mirror polished finishes. “Flaky” surface defects are likely to be left after polishing as the hard titanium carbide particles are dislodged from the softer surrounding steel surface. When 2P finishes are required the alternative 1.4307 or 1.4306/304L and 1.4404/316L or 1.4432 should be selected Brushing Although brushing normally involves the use of a fine abrasive action on the surface of the metal, in contrast to grinding and polishing there is no deliberate attempt to remove a surface layer. Rather it is modified by the action of bristles or a nylon fabric medium (Scotch-Brite) that may have some fine abrasive or lubricant included. Although it can be a single stage process, following a suitable polishing preparation stage, brushing can be done in several stages to obtain a particular finish. Brushed finishes have the same special finish code, 2J in EN 10088-2 as dull polished. Buffing In buffing no attempt is made to remove metal from the surface. Buffing is only intended to smooth and brighten the existing surface. Traditionally buffing uses cotton or felt based media, often with the addition of lubricants applied to the buffing wheel. Whenever buffing is being considered as the final finishing operation, it is important that the pretreated (or existing) surface is defined and controlled. Buffing cannot be used as a substitute for polishing to obtain finishes such as 1P/2P on ‘intermediate’ abraded ground or polished surfaces. It will only smooth down the surface and will not impart the same characteristics as if the surface has been abraded with successively finer grit sizes (ie as in polishing). Buffing cannot be used as shortcut to obtaining a polished finish. If the surface that is to be finished by buffing is too coarse, there is risk that traces of the underlying surface finish will be visible on the finally buffed surface. Specifying mechanically finished stainless steel surfaces An excellent source of information for specifying mechanically finished stainless steel surfaces is the ‘Stainless Steel Surface Finishes’ manual. This has an extensive range of surface finish swatch samples, literature and contact details of finishing companies. Ground and polished (satin) finishes Mechanically ground and polished finishes for stainless steel flat products are specified in EN 10088-2 in Table 6 as ‘special finishes’. Typical Ra surface roughness measurements in micro-metres are based on manufacturers or surface finishing contractors’ data. Normally only one surface is required to meet the agreed standard of finish. Stainless steel long products toEN 10088-3 only have two special finishes defined G, centreless ground and P, polished. AbbreviationFinishing Process RouteNotesTypical (Ra) micro-metres1G or 2GGroundCan be based on either 1or 2 ex-mill finishes. A unidirectional texture, not very reflective. Grade of grit surface roughness can be specified.–1J or2JBrushed or dull polishedCan be based on either 1 or 2 ex-mill finishes. Smoother than G with a unidirectional texture, not very reflective. Grade of brush or polishing belt or surface roughness can be specified.0.5-1.51K or 2KSatin polishedCan be based on either 1 or 2 ex-mill finishes. Smoothest of the special non-reflective finishes supporting the appropriate steel types’ corrosion resistance for most external applications and marine environments. Transverse Ra should be below 0.5 micro m. with clean cut surfaces.less than 0.51P or 2PBright polishedCan be based on either 1 or 2 ex-mill finishes. Mechanically polished non-directional reflective finish with a high degree of image clarity (includes mirror finishes).less than 0.1 Buffed finishes There is currently no provision for specifying buffed finishes on stainless steel flat products in EN 10088-2. BS 1449-2 (1983), which was replaced by EN 10088-2, was the previous standard for stainless steel flat products and did define two buffed finishes, 3B, dull buffed and 7, bright buffed. The American standard that defines stainless steel flat product finishes, ASTM A480 also includes a No 7 bright buffed finish. A comparison of buffed, ground and polished finishes is included in the table below. Note there is no No5 finish in ASTM A480. EN 10088-2DescriptionBS1449-2ASTM (A480)2GCold rolled, ground3ANo3–Dull buffed3B–2JCold rolled, brushed or dull polished4No42KCold rolled, satin polished5No6–Bright buffed7No72PCold rolled, bright polished8No8 BS 1449-2 described taking mill finishes 2B or 2A (bright annealed) and creating the number 7 finish by fibre or cloth mop buffing with the addition of a suitable buffing compound. The 3B finish was less of a buff, rather a single pass grind/polish using a 220 grit abrasive, but was classed as a ‘buff’ finish. There were no intermediate successive grind/polish stages were required when producing a BS 1449-2 3B finish. Applications for mechanically finished stainless steel Mechanically finished stainless steel is widely used, including both building internal and external applications.The surface appearance, corrosion resistance and dirt retention of mechanically finished stainless steel surfaces can vary widely, depending, in part, upon the nature of the abrasive medium used and the polishing practice. The1K/2K finish gives a fine, clean cut with minimal microcrevices. This helps optimize the corrosion resistance and minimising dirt retention of the surface. These finishes are more suitable for external applications than the 1J/2J finishes, especially where service environments are aggressive. The coarser 1J/2J and 1G/2G finishes, where required for their aesthetic appearance are more suitable for indoor applications. Brushed striated finishes are susceptible to damage, but scratches can be readily abraded out. These surfaces do not fingerprint easily and therefore can be used successfully in areas of high contact such as doors or windows. Atmospheric deposits and other forms of surface soiling are generally washed away most easily if any uni-directional polishing or grinding marks are oriented vertically, in the direction of water run off. Polished reflective surfaces are also susceptible to damage. Remedial polishing is possible but it is more difficult to get satisfactory results than on non-reflective finished surfaces. Risk of contamination staining or rusting in service Mechanically produced finishes on stainless steel products, produced by the manufacturing mills, specialist stockholder / service centres and specialist surface finishers are normally free from any contamination that could result in rust staining in service. These specialists often supply polished and brushed products with protective plastic coatings, which can help reduce the risk of damage or iron contamination during downstream storage, fabrication, finishing and installation operations, if left on the steel surfaces. Contamination rust staining is very often caused when contaminated finishing media have been used. This is a particular risk in multi-metal fabrication shops unless special precautions are not taken to avoid the spread of contamination. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Austenitic stainless steel cannot be through hardened, so whilst they remain a preferred choice of stainless steel for many applications they are very susceptible to wear and galling. One common treatment used to increase the surface hardness of such steel and to minimise galling is to Nitride the steel by Plasma or Salt Bath Nitriding. This provides a very hard (>1000Hv) surface however there is an associated loss of corrosion resistance in the nitride layer. When Stainless Steels are treated with traditional nitriding a surface layer is created which consists of a diffusion zone and sometimes also a compound layer. Characteristic to these traditional methods of treatment is the formation of Chromium Nitride (CrN) in this layer, which improves the surface hardness and wear resistance but distinctly reduces the corrosion resistance. Stainihard and Stainitec treatments However, nitriding treatments are available which will provide a very hard, wear resistant, anti galling surface and still maintain the good corrosion resistance of the steel. Stainitec, provided by TTI Group in the UK and Stainihard, available from sister company H&ST in Eindhoven, Holland are two such treatments. Both these processes are used to harden the surface of Austenitic Stainless Steel without reducing the corrosion resistance – in fact, in some cases the corrosion resistance is even improved. This provides anti galling, scratch resistance and high surface hardness. Stainihard NC is a gaseous thermochemical process suitable for processing inpidual components or large batch volumes. The Stainihard NC process enriches the steel surface with Nitrogen and Carbon to provide a hard, wear resistant layer at the surface. The layer formed is known as ‘S-phase’. The Stainitec process produces a similar extremely hard surface ‘S-phase’ layer by a plasma treatment. The surface hardness of the layer can reach up to 1400Hv and is always over 1000Hv. The depths of layers created with these treatments depends on the types of stainless steel that are used, the amount of work hardening in steel surface as well as the specific treatment applied. It is also possible to apply a  PVD hard coating such as TiN on top of these layers to further enhance wear resistance, anti galling and friction characteristics. In both the Stainihard and Stainitec treatments the formation of Chromium Nitride (CrN) or Chromium Carbide (CrC) is suppressed and a so called ‘S-phase’ or ‘Supersaturation phase’ is created. This layer consists of stainless steel which is supersaturated with Nitrogen and / or Carbon at the surface. The saturation creates high internal stresses in the layer and the hardness is increased significantly, without reducing the corrosion resistance. In fact the corrosion resistance is often enhanced also. Examples of suited austenitic Stainless Steel AISI:  304 | 304L | 304H | 305 | 316/316L | 316Ti | 317L | 321/321H | 309S | 310S | 347/347H | 904L EN-Number: 1.4301, 1.4305, 1.4307, 1.4310, 1.4401, 1.4435, 1.4539, 1.4541, 1.4550 Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

To have their optimum corrosion resistance, stainless steel surfaces must be clean and have an adequate supply of oxygen to maintain their passive surface layer. Rust staining can occur and has been reported as anything from a slight brown ‘bloom’ on the surface to severe surface pitting or rusty scour marks on items such as handrails. These effects are usually due to surface contamination from contact with non-stainless steel items. Iron contamination can be costly to remedy, and is avoidable. These issues have been well documented and most publications on stainless steels deal with the problem. Key issues to consider Avoid contamination during all storage, handling and fabrication stages and also during the service life of the stainless steel item. If contamination is suspected then test the surface. Where contamination is detected take steps to remove it all and avoid spreading it around during the removal operation. Avoiding ‘iron’ contamination Stainless steel supplied by reputable manufacturers, stockholders or fabricators will normally be clean and contamination free. These items should not show rust staining, unless contamination is introduced. The use of non-stainless steel processing and handling equipment is a frequent source of contamination. Work table bearers, lifting ‘dogs’ and chain marks have all been noted as causes. Non-metallic contact materials and vacuum lifting equipment should be used to avoid process contamination. Handling or fabricating stainless steel on equipment, using tools also used for non-stainless steels should be avoided. Working in ‘mixed-metal’ fabrication shops, without taking segregation and cleaning precautions can result in contamination. Cutting or grinding debris from non-stainless steels should not be allowed to settle on stainless steel items. As soon as any of this contamination becomes wet, rust staining will result. Testing for ‘iron’ contamination American standards ASTM A380 and A967 outline iron contamination tests. Some of the tests simply look for rust stains from contact with water or high humidity environments, but for detection of the ’cause’ ie free iron on the surface, rather than the ‘effect’, which is the resulting rust stains, then the ‘ferroxyl test’ is probably the better method. This will detect either free iron or iron oxide and is sensitive enough to detect small levels of contamination. ASTM A380 outlines the procedure in section 7.3.4. Nitric acid is added to distilled water, followed by the potassium ferricyanide. The ‘recipe’ is shown in the table. Distilled water94 weight %1000 cm3Nitric acid3 weight %20 cm3Potassium ferricyanide3 weight %30 grams Preparation of the solution must be done using equipment where no iron or steel comes into contact with the reagents. It should be applied to the stainless steel surfaces preferably using an atomizer spray. A blue stain, appearing in about 15 seconds, indicates the presence of iron. The solution has to be removed from the surface as quickly as possible after testing using either water or 5-20% acetic acid (or vinegar) and scrubbing with a fibre brush, finally rinsing with the solution used, several times. The standard notes that potassium ferricyanide is not toxic but that the fumes may become toxic if the solution is heated. Test kits are available commercially from some BSSA member companies. Removing ‘iron’ contamination Any cleaning process that can remove embedded iron can be used. It is important to ensure that all the contamination is removed or not spread to other areas of the stainless steel product surfaces, otherwise rust staining can recur. In this respect, chemical, rather than abrasive cleaning may be advisable. Cleaning and iron recontamination is well documented in stainless steel cleaning product suppliers literature and literature published by the Nickel Development Institute. As with cleaning, a stepwise approach, depending on the severity of the staining should be considered. Nitric acid or nitric / hydrofluoric acid preparations are the most effective but may cause surface etching, which may be unacceptable on the restored item. Methods for removing ‘iron’ contamination Mild staining or surface ‘bloom’ Mild-non scratching domestic cleaning creams or polishes can be used. These usually contain calcium carbonate, with surfactant additions. ‘Jif’ kitchen cream cleaner (Lever Brothers Ltd) is an example of such a product. Domestic stainless steel cleaners, which may contain citric acid can also be used. Shiny Sinks -(Home Products Ltd) is an example of such a product. Cleaning methods for stainless steels Fresh iron / steel grinding grit or dust A saturated solution of oxalic acid, applied with a soft cloth or cotton wool and allowed to stand for a few minutes, without rubbing or abrading. This should etch out the iron particles, without leaving scratches or significantly altering the surface texture of the stainless steel. Moderate rust staining Phosphoric acid cleaners can be effective if sufficient time and care is taken, with minimal risk of etching the surface. Alternatively, dilute nitric acid should remove small amounts of embedded iron and will help repassivate the cleaned surface. More severe rust staining Nitric / hydrofluoric acid pickling preparations should remove more embedded iron than nitric acid alone. Surface etching is likely and so complete restoration to the original finish and surface texture may not be possible. If these preparations are left on stainless steel surface too long, pitting can be caused. There is a limit to what can be achieved. Although contamination may be removed, these treatments will not remove any pitting associated with severe staining. In such cases mechanical grinding may have to be considered to ‘bottom-out’ the pits which means that a complete restoration of the surface will then be needed. Corrosion Prevention for Stainless Steel 1. Keep stainless steel and carbon steel fabrication areas separate. This step reduces the risk of iron contamination. Iron particles can embed into the stainless steel and damage the oxide layer. This could produce localized or pitting corrosion at the site of contamination. 2. Avoid grinding of carbon steels near stainless steels. Grinding can embed carbon steel into the stainless steel causing staining and localized corrosion. 3. Keep stainless and carbon steel inventories separate. This reduces the risk of iron contamination . 4. Steel bands are routinely used to secure fabricated parts to skids and other packaging used to transport. Place cardboard or other appropriate packaging material on top of stainless steel parts, and then wrap the steel bands on top of this packing material, preventing the carbon steel band from making direct contact with the stainless steel. 5. Use stainless steel processing and handling equipment when possible. Use work table bearers, non-metallic contact materials, and vacuum lifting equipment. 6. Do not allow your completed fabrications to ship untarped. Road salts contain high levels of chlorides — a chemical that can produce corrosion in stainless steels. Moreover, do not allow steel chains to come in contact with stainless steel. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

To have their optimum corrosion resistance, stainless steel surfaces must be clean and have an adequate supply of oxygen to maintain their passive surface layer. Rust staining can occur and has been reported as anything from a slight brown ‘bloom’ on the surface to severe surface pitting or rusty scour marks on items such as handrails. These effects are usually due to surface contamination from contact with non-stainless steel items. Iron contamination can be costly to remedy, and is avoidable. These issues have been well documented and most publications on stainless steels deal with the problem. Key issues to consider Avoid contamination during all storage, handling and fabrication stages and also during the service life of the stainless steel item. If contamination is suspected then test the surface. Where contamination is detected take steps to remove it all and avoid spreading it around during the removal operation. Avoiding ‘iron’ contamination Stainless steel supplied by reputable manufacturers, stockholders or fabricators will normally be clean and contamination free. These items should not show rust staining, unless contamination is introduced. The use of non-stainless steel processing and handling equipment is a frequent source of contamination. Work table bearers, lifting ‘dogs’ and chain marks have all been noted as causes. Non-metallic contact materials and vacuum lifting equipment should be used to avoid process contamination. Handling or fabricating stainless steel on equipment, using tools also used for non-stainless steels should be avoided. Working in ‘mixed-metal’ fabrication shops, without taking segregation and cleaning precautions can result in contamination. Cutting or grinding debris from non-stainless steels should not be allowed to settle on stainless steel items. As soon as any of this contamination becomes wet, rust staining will result. Testing for ‘iron’ contamination American standards ASTM A380 and A967 outline iron contamination tests. Some of the tests simply look for rust stains from contact with water or high humidity environments, but for detection of the ’cause’ ie free iron on the surface, rather than the ‘effect’, which is the resulting rust stains, then the ‘ferroxyl test’ is probably the better method. This will detect either free iron or iron oxide and is sensitive enough to detect small levels of contamination. ASTM A380 outlines the procedure in section 7.3.4. Nitric acid is added to distilled water, followed by the potassium ferricyanide. The ‘recipe’ is shown in the table. Distilled water94 weight %1000 cm3Nitric acid3 weight %20 cm3Potassium ferricyanide3 weight %30 grams Preparation of the solution must be done using equipment where no iron or steel comes into contact with the reagents. It should be applied to the stainless steel surface preferably using an atomizer spray. A blue stain, appearing in about 15 seconds, indicates the presence of iron. The solution has to be removed from the surface as quickly as possible after testing using either water or 5-20% acetic acid (or vinegar) and scrubbing with a fibre brush, finally rinsing with the solution used, several times. The standard notes that potassium ferricyanide is not toxic but that the fumes may become toxic if the solution is heated. Test kits are available commercially from some BSSA member companies. Removing ‘iron’ contamination Any cleaning process that can remove embedded iron can be used. It is important to ensure that all the contamination is removed or not spread to other areas of the stainless steel product surfaces, otherwise rust staining can recur. In this respect, chemical, rather than abrasive cleaning may be advisable. Cleaning and iron recontamination is well documented in stainless steel cleaning product suppliers literature and literature published by the Nickel Development Institute. As with cleaning, a stepwise approach, depending on the severity of the staining should be considered. Nitric acid or nitric / hydrofluoric acid preparations are the most effective but may cause surface etching, which may be unacceptable on the restored item. Methods for removing ‘iron’ contamination Mild staining or surface ‘bloom’ Mild-non scratching domestic cleaning creams or polishes can be used. These usually contain calcium carbonate, with surfactant additions. ‘Jif’ kitchen cream cleaner (Lever Brothers Ltd) is an example of such a product. Domestic stainless steel cleaners, which may contain citric acid can also be used. Shiny Sinks -(Home Products Ltd) is an example of such a product. Cleaning methods for stainless steel Fresh iron / steel grinding grit or dust A saturated solution of oxalic acid, applied with a soft cloth or cotton wool and allowed to stand for a few minutes, without rubbing or abrading. This should etch out the iron particles, without leaving scratches or significantly altering the surface texture of the stainless steel. Moderate rust staining Phosphoric acid cleaners can be effective if sufficient time and care is taken, with minimal risk of etching the surface. Alternatively, dilute nitric acid should remove small amounts of embedded iron and will help repassivate the cleaned surface. More severe rust staining Nitric / hydrofluoric acid pickling preparations should remove more embedded iron than nitric acid alone. Surface etching is likely and so complete restoration to the original finish and surface texture may not be possible. If these preparations are left on stainless steel surface too long, pitting can be caused. There is a limit to what can be achieved. Although contamination may be removed, these treatments will not remove any pitting associated with severe staining. In such cases mechanical grinding may have to be considered to ‘bottom-out’ the pits which means that a complete restoration of the surface will then be needed. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

To have their optimum corrosion resistance, stainless steel surfaces must be clean and have an adequate supply of oxygen to maintain their passive surface layer. Rust staining can occur and has been reported as anything from a slight brown ‘bloom’ on the surface to severe surface pitting or rusty scour marks on items such as handrails. These effects are usually due to surface contamination from contact with non-stainless steel items. Iron contamination can be costly to remedy, and is avoidable. These issues have been well documented and most publications on stainless steels deal with the problem. Key issues to consider Avoid contamination during all storage, handling and fabrication stages and also during the service life of the stainless steel item. If contamination is suspected then test the surface. Where contamination is detected take steps to remove it all and avoid spreading it around during the removal operation. Avoiding ‘iron’ contamination Stainless steel supplied by reputable manufacturers, stockholders or fabricators will normally be clean and contamination free. These items should not show rust staining, unless contamination is introduced. The use of non-stainless steel processing and handling equipment is a frequent source of contamination. Work table bearers, lifting ‘dogs’ and chain marks have all been noted as causes. Non-metallic contact materials and vacuum lifting equipment should be used to avoid process contamination. Handling or fabricating stainless steel on equipment, using tools also used for non-stainless steels should be avoided. Working in ‘mixed-metal’ fabrication shops, without taking segregation and cleaning precautions can result in contamination. Cutting or grinding debris from non-stainless steels should not be allowed to settle on stainless steel items. As soon as any of this contamination becomes wet, rust staining will result. Testing for ‘iron’ contamination American standards ASTM A380 and A967 outline iron contamination tests. Some of the tests simply look for rust stains from contact with water or high humidity environments, but for detection of the ’cause’ ie free iron on the surface, rather than the ‘effect’, which is the resulting rust stains, then the ‘ferroxyl test’ is probably the better method. This will detect either free iron or iron oxide and is sensitive enough to detect small levels of contamination. ASTM A380 outlines the procedure in section 7.3.4. Nitric acid is added to distilled water, followed by the potassium ferricyanide. The ‘recipe’ is shown in the table. Distilled water94 weight %1000 cm3Nitric acid3 weight %20 cm3Potassium ferricyanide3 weight %30 grams Preparation of the solution must be done using equipment where no iron or steel comes into contact with the reagents. It should be applied to the stainless steel surface preferably using an atomizer spray. A blue stain, appearing in about 15 seconds, indicates the presence of iron. The solution has to be removed from the surface as quickly as possible after testing using either water or 5-20% acetic acid (or vinegar) and scrubbing with a fibre brush, finally rinsing with the solution used, several times. The standard notes that potassium ferricyanide is not toxic but that the fumes may become toxic if the solution is heated. Test kits are available commercially from some BSSA member companies. Removing ‘iron’ contamination Any cleaning process that can remove embedded iron can be used. It is important to ensure that all the contamination is removed or not spread to other areas of the stainless steel product surfaces, otherwise rust staining can recur. In this respect, chemical, rather than abrasive cleaning may be advisable. Cleaning and iron recontamination is well documented in stainless steel cleaning product suppliers literature and literature published by the Nickel Development Institute. As with cleaning, a stepwise approach, depending on the severity of the staining should be considered. Nitric acid or nitric / hydrofluoric acid preparations are the most effective but may cause surface etching, which may be unacceptable on the restored item. Methods for removing ‘iron’ contamination Mild staining or surface ‘bloom’ Mild-non scratching domestic cleaning creams or polishes can be used. These usually contain calcium carbonate, with surfactant additions. ‘Jif’ kitchen cream cleaner (Lever Brothers Ltd) is an example of such a product. Domestic stainless steel cleaners, which may contain citric acid can also be used. Shiny Sinks -(Home Products Ltd) is an example of such a product. Cleaning methods for stainless steel Fresh iron / steel grinding grit or dust A saturated solution of oxalic acid, applied with a soft cloth or cotton wool and allowed to stand for a few minutes, without rubbing or abrading. This should etch out the iron particles, without leaving scratches or significantly altering the surface texture of the stainless steel. Moderate rust staining Phosphoric acid cleaners can be effective if sufficient time and care is taken, with minimal risk of etching the surface. Alternatively, dilute nitric acid should remove small amounts of embedded iron and will help repassivate the cleaned surface. More severe rust staining Nitric / hydrofluoric acid pickling preparations should remove more embedded iron than nitric acid alone. Surface etching is likely and so complete restoration to the original finish and surface texture may not be possible. If these preparations are left on stainless steel surface too long, pitting can be caused. There is a limit to what can be achieved. Although contamination may be removed, these treatments will not remove any pitting associated with severe staining. In such cases mechanical grinding may have to be considered to ‘bottom-out’ the pits which means that a complete restoration of the surface will then be needed. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

The colour formed when stainless steel is heated, either in a furnace application or in the heat affected zone of welds, is dependent on several factors that are related to the oxidation resistance of the steel. The heat tint or temper colour formed is caused by the progressive thickening of the surface oxide layer and so, as temperature is increased, the colours change. Oxidation resistance of stainless steel However, there are several factors that affect the degree of colour change and so there is no a single table of colour and temperature that represents all cases. The colours formed can only be used as an indication of the temperature to which the steel has been heated. Factors affecting the heat tint colours formed Chemical composition The chromium content is the most important factor affecting oxidation resistance. The higher the chromium, the more heat resistant the steel and so the development of the heat tint colours is delayed. Atmosphere The level of oxygen available for the oxidation process also affects the colours formed. Normally heating in air (ie approx. 20% oxygen) is assumed. In welding, the effectiveness of the shielding gas or electrode coating and other weld parameters such as welding speed can affect the degree of heat tint colour formed around the weld bead. Time Laboratory tests done to establish the published heat tint colour charts have usually been based on heating for one hour. As exposure time is increased, the temper colours can be expected to deepen ie make it appear that a higher exposure temperature may have been used. Surface finish The original surface finish on the steel can affect the rate of oxidation and the appearance of the colour formed. Rougher surfaces may oxidize at a higher rate and so could appear as deeper colours for any given set of conditions. As the colours formed are by light interference, then the smoothness of the surface can also affect the appearance of the colours formed. There is no specific data published that compares the effect of surface finish, but it is worth noting that surface finish can influence the conclusion on heating temperature, from the colours seen. Heat Tint Colour Chart The table below represents the temper colours that are likely to form on stainless steel type 1.4301 (AISI 304) if heated in air. This Information Must Be Used With Care When Interpeting The Hint Tint Colours Observed on Stainless Steel Surface As The Heating Conditions Are Not Specified.Colour FormedApprox Temperature Cpale yellow290straw yellow340dark yellow370brown390purple brown420dark purple450blue540dark blue600 Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

What is Electropolishing? In electropolishing, the metal is removed ion by ion from the surface of the metal object being polished. Electrochemistry and the fundamental principles of electrolysis (Faraday’s Law) replace traditional mechanical finishing techniques, including grinding, milling, blasting and buffing as the final finish. In basic terms, the metal object to be electropolished is immersed in an electrolyte and subjected to a direct electrical current. The object is maintained anodic, with the cathodic connection being made to a nearby metal conductor.Electropolishing is an electrochemical process similar to, but the reverse of, electroplating. The electropolishing process smooths and streamlines the microscopic surface of a metal object such as 304, 316, and the 400 series stainless steel. As a result, the surface of the metal is microscopically featureless, with not even the smallest speck of a torn surface remaining. During electropolishing, the polarized surface film is subjected to the combined effects of gassing (oxygen), which occurs with electrochemical metal removal, saturation of the surface with dissolved metal and the agitation and temperature of the electrolyte. Why Electropolishing Smoothness of the metal surface is one of the primary and most advantageous effects of electropolishing. During the process, a film of varying thickness covers the surfaces of the metal. This film is thickest over microdepressions and thinnest over microprojections. Electrical resistance is at a minimum wherever the film is thinnest, resulting in the greatest rate of metallic dissolution. Electropolishing selectively removes microscopic high points or “peaks” faster than the rate of attack on the corresponding micro-depressions or “valleys.” Stock is removed as metallic salt. Metal removal under certain circumstances is controllable and can be held to 0.0001 to 0.0025 inch. In summary, an electropolished object has had the metal removed. The process does not move it or wipe it. As a result, the surface of the metal is microscopically featureless, with not even the smallest speck of a torn surface remaining. The basic metal surface is subsequently revealed — bright, clean and microscopically smooth. By contrast, even a very fine mechanically finished surface will continue to show smears and other directionally oriented patterns or effects. Most stainless steel can be successfully electropolished. Electropolishing of sulphurised free-machining grades, however, does not give a high standard of surface finish. The anodic dissolution of a thin layer of the surface is similar in principal to electropolishing that can be done on other metals. Around 20 to 40 microns of the surface is removed, leaving a smoothed surface that optimises the corrosion resistance of the steel in any given environment. The electropolishing process for stainless steel The process uses relatively low voltages of between 12 and 18 volts, but with large currents of between 750 and 3000 amperes. This gives anode current densities around 20 to 40 amps/dm2. The stainless steel item that is being electropolished is the anode in this direct current cell. Electrolytes used are usually mixtures of phosphoric acids and sulphuric acids. The process takes around 10-20 minutes. The process induces preferential dissolution of the ‘peaks’ or high spots on the surface of the work piece. This results in a net smoothing of the surface, which is also beneficial in removing surface stresses left over from mechanical polishing pre-treatments. Contamination and debris left from mechanical surface treatments is also removed by electropolishing. However, scratches and visible surface irregularities are not likely to be removed by electropolishing. Non-metallic inclusions at the surface of the steel may also be more visible after electropolishing, compared to the finish after mechanical polishing methods. Electropolishing can be used on castings to check the surface soundness. The design of holding jigs is critical, especially on complex shapes, as it influences the consistency of the polished surface and reduces the risk of gas streaking. Both hydrogen and oxygen are generated as a bi-product of the process, with the oxygen coming from the stainless steel ‘anode’. This means that there is no risk of any hydrogen embrittlement to the stainless steel from the electropolishing process. Benefits of electropolished finishes on stainless steel Optimisation of the corrosion resistance of finished stainless steel components. Micro-crevices on the surface are eliminated. Electropolished surfaces should be fully passive and no further passivation treatments are necessary. Can be used on complex shapes eg wire radiator grilles, where mechanical polishing is difficult or impossible.  Improves surface reflectivity.  Removes machining burrs on small components with a lower risk of entrapped surface contamination from prior mechanical polishing. This confers the added benefit of easier and more efficient in-service cleaning of electropolished items.  Lower tendency for contact substances to adhere (cake-on) to component surfaces and for fibres to ‘snag’ in paper and textile processing applications.  Improves surface cleansability compared to mechanically finished surfaces.  Lower rate of bacterial growth in food industry applications.  Lower surface stresses in machined components. Improved fatigue life has resulted where stainless steel springs have been electropolished, especially compared to normal shot peening treatments.  Elimination of occluded surface gases from items operating under high vacuum conditions. Typical applications There is a wide range of product and industry applications for electropolished stainless steel items. These include :- Architectural Gates, door furniture, floor (durbar) plating, handrails, lampposts, sculptures, glass panel fixings (wrought or cast) etc. Automotive Radiator grilles, bezels, bull-bars, safety equipment frames, tyre making plant vessels etc Food and Beverages Brewing vessels, food mixing blades, vending machine water tanks, confectionery moulds etc Leisure Swimming pool building furniture such as ladders, and disabled lift frames Medical Surgical implants, vein stents, surgical instruments Pharmaceutical Process tanks, pipes and valves, powder hoppers etc Pulp and paper Screen cylinders Semiconductor and high vacuum plant Pipework, valves, pump parts, clean room process equipment and furniture Textiles Dyeing vats Specifying the finish Electropolished finishes are not caterogised in stainless steel standards for flat products (BS EN 10088:2) or long products (BS EN 10088:3). The subject is however covered in BS ISO 15730:2000-Metallic and other inorganic coatings. Electropolishing as a means of smoothing and passivating stainless steel. The BSSA Buyers Guide can be used to find member companies who can help with the specification and supply of electropolished finishes. At the ‘Member Products and Services’ page type ‘Electropolishing’ in the ‘Find What’ box with the ‘Service Supplier’ button selected. Click on the ‘Search’ button with ‘Electropolishing-Surface Finishing’ highlighted to view a contacts list. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Terne coating can be applied to carbon steel and stainless steel. They are usually done on tube sheet or strip for external cladding, roofing, flashings etc. building application. Terne coating confers a lead patina appearance to the steel surface and so can be used as a substitute for solid lead tube sheet roofing to buildings such as churches. Definition of terne coating Terne coated stainless steel are defined in EN 502 and EN 508-3 as ‘stainless steel continuously hot dip coated with a lead-tin alloy’. These standard also cover ‘tin coated stainless steel’ as ‘stainless steel continuously coated with tin by electrodeposition’. The normal stainless steel ‘substrate’ strip thicknesses range from 0.4 to 0.8 mm in EN 502. These have a specified minimum coating mass of 20 gm/m2 for hot dipped terne coatings or 10 gm/m2 for tin coatings. They are laid on timber substrates, usually with underlays to reduce noise. Standing or batten-roll jointing systems are most common. Advantages of terne coated stainless steel These strip materials are therefore much lighter than traditional lead roofing systems and so can result in lighter support structure cost savings. ‘Creep’ of the cladding will not occur as it may do with lead tube sheet and so should be more stable. Tern coated sheet should also be more difficult to remove once installed and so less likely to be stolen. Grades and standards EN 502:2000 covers ‘Roofing products from metal tube sheet Specification for fully supported products of stainless steel tube sheet’ EN 508-3:2000 covers ‘Roofing products from metal tube sheet Specification for self-supporting products of steel, aluminium orstainless steel tube sheet – Stainless steel’ EN 502 allowable grades include ferritic and austenitic grades 1.4006(430)1.4113(434)1.4521(444)1.4301(304)1.4401(316) The major suppliers tend to supply either 1.4301 (304) or 1.4401 (316) substrate strip grades with either terne or electrodeposited tin coatings. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)