Welding / Fabrication
When stainless steel is welded, a scale is formed and on either side of the weld is a heat affected zone that varies in colour from straw to blue-black, depending on welding conditions. This area is rich in metal oxides that possess very little corrosion resistance and will readily allow corrosion to commence. This effect extends beyond the areas affected by colour change. This is a very reactive area that will readily rust in a very short time. Because this is little understood, it is common to find badly corroded areas where stainless steel is found on site. This is most unfortunate since the method of avoiding such corrosion is simple and relatively inexpensive.
Descaling, pickling and passivating are separate processes that are often confused.
Pickling removes a thin surface layer from the stainless, using an acid solution, which is usually a hydrofluoric and nitric acid mixture. For the pickling to be effective the surface should be clean and free of greases and oils. Pickling will remove embedded iron, surface iron contamination, heat tint and weld scale. Pickling will produce a dull matt uniform grey finish.
A major cause of corrosion in service is iron contamination which can arise from a variety of sources. These include using tools made of mild or constructional steels (fork lifts being a common cause) abrasives that contain iron and any tool that has previously been used on non stainless steel. Cross contamination can also arise from grindings and debris created whilst working close by on steel, either in the workshop or on site. Iron contamination is not always obvious, if it is suspected it can be detected using the ASTM A380, ferroxyl test. This is a rapid test in which a solution turns blue in the presence of iron. Pickling is a very effective method of removing this iron contamination from the surface.
Heat Tint and weld scale
The heat tint produced by welding is not only unsightly but the thicker oxide layer includes chromium from the surface of the metal, lowering its corrosion resistance. Pickling both removes the oxide layer causing the colour tints and a thin layer of the underlying metal to restore the original properties.
Stainless Steel owes its corrosion resistance to the formation of a chromium oxide surface layer and is then referred to as being passive. This occurs naturally and spontaneously provided sufficient oxygen is available. Even aerated water provides enough oxygen for this process to occur. Material supplied by the producing mills is fully passivated and further passivation is rarely required. However, if the oxide has been stripped, perhaps by pickling, then the oxide layer takes a short time to reach its full thickness and this can be accelerated by passivation. This treatment also builds up the layer in crevices, for example joints, where oxygen could have limited access.
Passivation is a separate process to pickling. It can be carried out as an independent process or as part of pickling involving nitric acid in the solution. In this case passivation occurs sequentially and not simultaneously. Nitric acid is the usual strong oxidising agent used for chemical passivation. On its own it will only remove free iron contamination from the stainless steel surface and is not an effective acid for pickling stainless steel.
Pickling after the construction phase will remove the chromium depleted zone and free iron, fully renews the corrosion resistance and brings it back entirely to the specifications of the original raw material.