Possible Causes Analysis for Stainless Steel Rust

When brown rusty spots (points) apprear on the surface of stainless steel products, people will feel extremely surprised:"Stainless steel must be stainless, and it must be quality problem if it rusts."
Actually, this is a unilateral wrong opinion of lack the understanding of stainless steel. Stainless steel will also rust under certain condition. Stainless only have the capability of resist oxidation, this is why called stainless; also it can be durable in acid, alkali, salt areas, this is why called resist corrosion. But this capability of resist oxidation and corrosion can be various due to the factors such as chemical composition of the steel itself, mutual condition, outer using conditions and environmental medium type.
As we know that, 304 steel tube can be absolutely excellent corrosion resistance in dry and clean areas. But once be moved to coastal ares which contains a lot of salt fog particles, it will soon be rusty, while this is not gonna happens on 316 steel. So not all kinds of stainless steel can resist corrosion in any condition.
The reason why stainless steel resists corrosion is that there is a layer of thin and strong film which mainly contains chromium oxide that prevents oxygen atoms continuing go inside. Once the film is destroyed, oxygen atoms in the air will get touch to the material or iron atoms will precipitated and be changed to ferric oxide, then it will be continuous rusted. There are many ways may cause film destoryed, usually as follows:
1. When other metal particles attached to the steel surface, and if exposed in damp area, then it will function as a battery and electrochemical reaction triggered;
2. Organic fluids remained on the surface, if with oxygen and water it will turn to acid, which can slowly rust the steel;
3. The surface adhered with acid, alkali, salt substance, which can cause part corrosion;
So stainless steel is not actually stainless, unless we use it in the proper ways.
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