It also helps to detect pinholes after welding.

Why Passivation required ?

1. Stainless Steel owes  its corrosion resistance to the formation of a chromium oxide surface layer but this happened due to presence of oxygen , And 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.

2. 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.

3. Iron Contamination
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 like fork lifts being a common cause for 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.