Category: Post Weld Cleaning and Finishing
Surface Finishes, Treatments & Cleaning
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Ko Buijs of Van Leeuwen Stainless describes dry ice blasting as a method for removing surface corrosion from stainless steel. Reprinted from Stainless Steel World – April 2006
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Stainless steel is a great material to work with. As with all materials you need to know how to treat it correctly for maximum cost effectiveness. This article summarises the typical pitfalls and remedies.
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A colour chart is shown for guidance on exposure temperatures on 1.4301, (304), type. The factors affecting the degree or depth of colours formed are outlined. These include steel composition, atmosphere, time and surface finish.
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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. The ferroxyl test can be used to detect ‘free’ iron contamination. (108)
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An up to date review of pickling processes for stainless steel including immersion, spray, localised and anodic. It emphasises the benefits of pickling on the corrosion resistance of stainless steel. Author – John Swain of Anopol first published in Stainless Steel Focus Issue 03/2007.
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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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This 12 page paper by Chris Baxter, Group Technical Market Support, AvestaPolarit Ltd. covers the issues of distortion during fabrication and welding, control of stainless steel welding processes and post fabrication cleaning. Buckling distortion can result from either poor design or welding practices. The key factors for good welding are clearly defined welding procedures, (BS EN 1011 and BS EN 287 and BS EN 288 are mentioned), control of heat input, temperature control and cleanliness. Welding electrode selection and methods including TIG, MMA, FCA, plasma, submerged arc and stud welding are covered. Finally, the importance of careful heat tint removal, (temper coloured areas), is discussed by either mechanical abrasion or chemical pickling cleaning methods is discussed. Although this establishes the corrosion resistance for the particular grade welded, care is needed with chemical pickling as it can change, (dull), the aesthetic appearance of the surfaces.
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The importance of good design and surface finish selection for the successful fabrication of stainless steel is outlined along with some of the properties that can affect the fabrication and performance of stainless steels. These include thermal conductivity, thermal expansion coefficient and the risks of galling, (pick-up).The hazard of iron contamination from carbon steel pick-up is mentioned and that welding oxide or heat treatment scale should be removed prior to service.
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