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Nitric acid is strongly oxidising and promotes the resistance of stainless steel to corrosion. Generally stainless steels are resistant to corrosion in nitric acid. Nitric acid is used in the chemical ‘passivation’ of stainless steels. The 304 types are preferable to 316 types. Localised attack at grain boundaries can occur in hot concentrated nitric acid.
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Phosphoric, (orthophosphoric acid), is a weak acid. Austenitic stainless steels have good corrosion resistance to chemically pure phosphoric acid. Wet process phosphoric acid, (WPA), can be aggressive.
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Sodium Hydroxide, (Caustic Soda), is a strong base, used widely for cleaning metals. Stainless steels types 304 and 316 can be considered resistant below 80 degC, up to the limit of solubility. There can be a risk of stress corrosion cracking, (SCC), attack at higher temperatures.
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Bead and shot-blasted finishes produced by the impact of a hard, inert medium onto the steel surface result in non-directional, uniform matt surfaces with low reflectivity. Blast media include glass, ceramic or lead bead, silicon carbide, aluminium oxide, stainless steel shot and ground quartz. A comparison of blasting and peening is made and the properties of bead blasted finishes briefly described. The steps involved in specifying bead blasted finishes for stainless steel are covered.
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Although it is possible to colour fabricated components, colouring of stainless steel is normally restricted to sheet products. This article describes the specification and applications for chemically coloured stainless steel flat products. The sequence of colours formed as the film grows in thickness ranges through bronze, blue, black, charcoal, gold, red-violet and green. With care, intermediate colours are possible. A comparison with anodised, painted and oxidised surfaces is made. The steps involved in specifying these 2L finishes to BS EN 10088-2 is outlined.
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Surface finishes for stainless steel sheet, (coil, strip), and plates, are specified in BS EN 10088-2:2005. Ex-mill hot and cold rolled finishes and special finishes are tabulated. These include mill finishes 1D, (heat treated), 1E, 2B, (skin passed), 2C, 2D, (pickled), 2E, (mechanically descaled), 2H, (work hardened), 2R, (bright annealed, reflective) and 2Q, (hardened and tempered). Special finishes include 1G, 2G, (ground), 1J, 2J, brushed or dull polished, 1K, 2K, satin polished, 1P, 2P, bright polished, 1M, 2M, patterned, 2W, corrugated, 2L, coloured, and 1S, 2S, surface coated. Electropolished, shot and bead blasted and peened finishes are not covered by BS EN 10088-2. Where available representative Ra surface roughness values are given for these finishes. A comparison of BS 1449-2 DIN and ASTM A480 surface finish symbols to those now used in BS EN 10088-2 is also shown. (100)
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Mechanical finishes for stainless steels are covered as finish codes G, J, K and P in BS EN 10088-2 and G and P in BS EN 10088-3. Terms grinding, (ground), polishing, (polished), brushing, (brushed), satin and buffing, (buffed), are discussed and compared with codes used previously in BS 1449-2 and currently in ASTM A480 for flat products. Contamination rust staining on mechanically polished stainless steel surfaces is often the result of using contaminated finishing media.
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Patterned stainless steel finishes are usually taken as relief or three-dimensional effects formed by either texture rolling or pressing (embossing). Specifying these finishes as 1M, (chequer or durbar plate), bright cold rolled 2M or non-reflective matt rolled 2F to BSEN 10088-2 is outlined. The importance of matching the finish on large cladding areas is noted.
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Guidelines for the grade selection, design, fabrication, maintenance, cleaning and inspection of stainless steels items for use in swimming pool buildings. Avoidance of stress corrosion cracking, (SCC), of safety-critical, load-bearing components. (57)
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This article summarises the proprietary surface hardening processes Stainihard and Stainitec
