Many people believe that stainless steel can’t get rusty and that stainless steel is completely rust-proof. In fact, although stainless steel is an alloy that contains chromium and other elements offering a certain amount of resistance to corrosion the base material is virtually as active as ordinary carbon steel.
The noble nature of stainless is due to an ultra-thin passive Chromium Oxide layer on the surface, but in the absence of this layer the material is rapidly corroded. So good care should be taken of this oxide layer to ensure stainless steel will last a lifetime. In other words, stainless steel is not at all maintenance-free. It can, however, be said to be low maintenance.
A component made from stainless steel can be compared to a healthy apple that keeps for a long time thanks to its peel, which is less than a tenth of a millimeter thick, but no substances are able to escape or enter – until a worm eats its way through the peel, triggering the rotting process. The flesh of the apple will also oxidize quickly when the apple is cut in half.
The ‘skin’ on stainless steel is much thinner than apple peel yet also provides complete enclosure, which is why, normally speaking, no metal ions can escape and no foreign substances can enter. However, if this skin is exposed to an excessive chemical load, it will break down, leading to corrosion. The biggest difference from the apple is the ability of stainless steel to repair its oxide layer by itself. This is particularly applicable when the layer is damaged mechanically and the chromium oxide layer disappears in local areas. Thanks to the oxygen in the air, passivation of the material will occur spontaneously in those areas through the formation of a new layer of chromium oxide. This is known as the ‘self healing effect’ although this mechanism can be badly disrupted in the presence of chlorides.
It is vital that stainless steel products and fabrications are clean when they go into service and kept clean whilst in service. In particular weld areas must be free from discolourisation and contamination plus any mild steel contamination that may have occurred during fabrication must be completely removed.