It would be safe to refer to stainless steel as an ubiquitous material, for the prime reason being the fact that it’s used for a manufacturing a great range of products - cooking utensils, jewellery, medical devices, automotive components, and what not! Most people prefer this material simply because it cannot rust. Contrary to popular belief, however, stainless steel products do rust or corrode, which can make a consumer question the authenticity of the items.
But experts dealing with metals are aware of the fact that stainless steel is bound to corrode upon exposure to certain environments like seawater. Normal people would assume seawater naturally has a destructive effect on the metal. The real science behind it though, is the chlorine contained in seawater - it has a corrosive effect on most metallic elements. Other than that, stainless steel can also rust without generating corrosion products for analysis, except rust.
For comprehending what causes the rusting of stainless steel, we need to know what prevents its corrosion in the first place. Raw steel consists of carbon and iron, but its stainless version also contains chromium, manganese and nickel. Now, it’s chromium that renders the material resistant to rust by forming a layer of chromium oxide on the surface. Even if the layer gets removed accidentally due to abrasions, another chromium oxide layer forms for providing protection against corrosion.
According to professionals offering welding corrosion testing services in Melbourne, stainless steel corrodes if particles of raw steel are present on it in sufficient quantity. For instance, if a stainless steel product is cleaned using a wool or wire wheel made of plain steel, tiny particles from the latter settle on the surface of the former. Now in case the the localised concentration of chromium falls below 12 percent, the chromium oxide cannot prevent oxygen from corroding the material. Corrosion occurring on stainless steel in this manner can fortunately be fixed by eliminating the rust entirely and then ridding the surface of the stainless steel off the steel particles using a solvent.
There is still another way in which stainless steel can corrode, and it is more permanent. The method is known as sensitisation and involves exposure to temperatures as high as 850°C. It turns the chromium into carbides and removes the protective layer. The damage caused can still be mitigated using a very complex process of heat treating.
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