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Should heat tint discolouration in the heat-affected zone of stainless steel welds be removed? In cases where the application involves an ‘aqueous’ corrosion hazard, the local reduction in sub-surface chromium can affect the corrosion resistance of the steel. The removal of weld heat tint from stainless steel fabrications using acid pickling or electrolytic methods not only improves the overall appearance but is vitally important for restoring the full corrosion resistance of the finished product. As a general rule, if you can see a discolouration on the surface due heat tint, this should be removed as part of good post weld cleaning practice for any stainless steel welded joints. (37)
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Practical advice on the “hows” and “whys” of post weld surface treatments. Why removal of weld heat tint should be be the norm for stainless steel.
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Site location is the initial consideration in selection of stainless steels for external applications. These are classified as rural, urban, industrial or marine. Micro-climates can influence the steel type selection. Other factors that influence the selection are surface finish, design, fabrication methods, accessibility for cleaning and maintenance and mechanical and physical properties of the steels. The main steel types 304 and 316 only are considered for UK external applications. (102)
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The appeal of stainless steels for internal applications is based on their corrosion resistance and wide range of finishes and textures available. For most building interiors intended for human occupation, either the ferritic 430, (1.4016), type or the austenitic 304, (1.4301), type are suitable. Grade, surface finish, fabrication and routine cleaning procedures are all important considerations. Fingermarking can be avoided by correct surface treatments. (101)
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Citric acid is a weak organic acid, found in fruits such as lemons, (citrus). Either the 304 or 316 stainless steel types can be considered for most storage and handling applications. Citric acid is also be used for cleaning and passivating stainless steels.
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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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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)
