All metals react with oxygen in the atmosphere and form oxide films on the surface. Unfortunately, iron oxide formed on ordinary carbon steel continues to oxidize, causing corrosion to expand and eventually form holes. Electroplating can be used to ensure the surface of carbon steel using paint or oxidation resistant metals such as zinc, nickel and chromium, but, as it is known, this protection is only a kind of film. If the protective layer is damaged, the steel below will rust.
The corrosion resistance of stainless steel depends on chromium, but because chromium is one of the components of steel, the protection methods are different.
When the chromium content reaches 10.5%, the atmospheric corrosion resistance of the steel increases significantly. However, when the chromium content is higher, although the corrosion resistance can be increased, it is not obvious. The reason is that when chromium is used for alloying steel, the type of surface oxide is changed into a surface oxide similar to that formed on pure chromium metal. This tightly adhered chromium rich oxide protects the surface from further oxidation. The oxide layer is very thin, through which it can see the natural luster of the steel surface and make the stainless steel have a unique surface. Moreover, if the surface is damaged, the exposed steel surface will repair itself with the atmospheric reaction, reformed the "passivation film" and continue to play a protective role. Therefore, all stainless steels have a common characteristic, that is, chromium content is above 10.5%.
Experience shows that the degree of atmospheric corrosion varies from region to region. For ease of explanation, it is suggested that the region be divided into four categories, namely, rural areas, cities, industrial areas and coastal areas.
The countryside is basically a pollution-free area. The area has a low population density and only pollution-free industries.
The city is a typical residential, commercial and light industrial area with mild pollution, such as traffic pollution.
Industrial areas create heavy air pollution in heavy industries. Pollution may be caused by fuel gas, such as oxides of sulfur and nitrogen, or other gases released by chemical plants or processing plants. Particles suspended in the air, such as the deposition of dust or iron oxide in the process of iron and steel production, also increase corrosion.
Coastal areas usually refer to areas within a mile from the coast. However, the marine atmosphere can spread deep inland, especially on islands. The prevailing wind is from the oceans and the weather is bad. For example, the climate conditions in the UK are so, so the whole country belongs to the coastal area. If the wind is mixed with the sea fog, especially the evaporation caused the accumulation of salt, and the rain is less, not often washed by the rain, the conditions of the coastal area are more disadvantageous. If there is industrial pollution, it will be more corrosive.
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