The key to ensuring that corrosion properties are restored after welding is to remove not only the heat tint oxide but also the chromium depleted layer below this. Chemical pickling has an inherent advantage in that it will preferentially dissolve these regions leaving only the sound metal behind [1] . Abrasive methods such as grinding or brushing perform this task less successfully; brushing, particularly, might act only to polish and/or embed oxide particles within the steel surface [1] , whilst grinding leaves behind roughened and perhaps hardened surface.
All mechanical methods (including blasting techniques) are possible causes of contamination by embedding foreign particles into the surface of the stainless steel [2] ; this may lead to reduced corrosion properties. Hence, when considering bead-blasting techniques, there will be a maximum value to the number of times the beads may be reused [2] . Blasting with metallic shot is not appropriate.
Bead-blasting is universally recognised in the literature to be the best method of mechanically cleaning the surfaces of stainless steels [1-4] . It is also recognised that properly bead-blasting the surface of a heat-tinted component will generally improve its corrosion resistance.
As to whether or not bead blasting alone can produce comparable results to methods involving pickling, the literature is divided. However, the majority of reports (References 1-3) state that it cannot, and only Reference 4 states that it can. References 5 and 6 provide quantitative data comparing pickling, grinding, brushing and grit blasting. For optimum corrosion resistance, mechanical cleaning, either by bead blasting or grinding, followed by pickling is recommended.
References
- How do I remove heat-tint formed during welding or improper heat treatment of stainless steels?
See further information about Materials Corrosion Management.