A stainless steel surface should appear clean, smooth and faultless. This is obvious when the steel is used for such purposes as façades or in applications with
stringent hygienic requirements, but a fine surface finish is also crucial to corrosion resistance
Stainless steel is protected from corrosion by a thin, impervious, invisible surface layer - the passive layer - that consists mainly of chromium oxide. The oxygen content of the atmosphere or aerated aqueous solutions is normally sufficient to create and maintain this passive layer. Unfortunately, surface defects and imperfections introduced during manufacturing operations may drastically disturb this "self-healing" process and reduce resistance to several types of local corrosion. This means that a final cleaning process will often be required to restore an acceptable surface quality with regard to hygiene and corrosion. The extent of and methods for post-manufacture treatment will be determined by the corrosivity of the environment, the corrosion resistance of the steel grade, hygienic requirements (e.g. in the pharmaceutical and food industries) or by purely aesthetic considerations. Consideration must also be paid to local environmental requirements. Both chemical and mechanical cleaning methods are available. Good design, planning and methods of manufacture can reduce the need for finishing work and thus reduce costs. The influence of defects, and ultimately their removal, must be considered when manufacturing to specifications that relate to certain surface quality requirements.
Mechanical methods for post weld cleaning of stainless steels
Grinding
Grinding is normally the only method that can be used to remove defects and deep scratches. A grinding disc is usually adequate for treating defects of this type. The grinding methods used should never be rougher than necessary, and a flapper wheel is often sufficient for removing weld tint or surface contamination.
The following points must always be considered:
- Use the correct grinding tools - self-sharpening, iron-free discs should always be used for stainless steel - and never use discs that have previously been used for grinding low alloy steels.
- Avoid producing a surface that is too rough. Rough grinding with a 40-60 grit disc should always be followed by fine grinding using, for example, a higher grip mop or belt to obtain a surface finish corresponding to grit 180 or better. If surface requirements are very exacting, polishing may be necessary.
- Do not overheat the surface. Apply less pressure when grinding in order to avoid creating further heat tint.
- Always check that the entire defect has been removed.
Blasting
Sand and grit blasting (peening) can be used to remove high temperature oxide as well as iron contamination. However, care must be taken to ensure that the sand (preferably of olivine type) or grit is perfectly clean. The blasting material must therefore not have been previously used for carbon steel; not should the sand or grit be too old, since it becomes increasingly polluted, even if it has only been used for blasting contaminated stainless steel surfaces. The surface roughness is the limiting factor for these methods. Using low pressure and a small angle of approach, a satisfactory result can be achieved for most applications. For the removal of heat tint, shot peening using smooth glass beads produces a good surface finish and introduces compressive stresses which improve stress corrosion cracking resistance and resistance to fatigue.
Brushing
For the removal of heat tint, brushing using stainless steel or nylon brushes usually provides a satisfactory result. These methods do not cause any serious roughening of the surface, but do not guarantee complete removal of the chromium-depleted zone. As regards the other mechanical methods, the risk of contamination is high, and it is therefore important that clean tools that have not been used for processing carbon steels are used.
Pickling
Chemical methods can remove high temperature oxide and iron contamination without damaging the surface finish.
Electropolishing may improve the surface finish. Since they remove the surface layer by controlled corrosion, chemicals will also selectively remove the least corrosion-resistant areas such as the chromium-depleted zones.
Pickling is the most common chemical procedure used to remove oxides and iron contamination. Thorough rinsing with clean tap water must follow pickling. The water quality requirements, including acceptable chloride content, increase with the surface requirements. Pickling normally involves using an acid mixture containing 8-20 vol% nitric acid (HNO3) and 0.5-5 vol% hydrofluoric acid (HF). Chloridecontaining agents such as hydrochloric acid (HCl) should be avoided, since there is an obvious risk of pitting corrosion.