Stainless steel pipe under normal circumstances is not easy to rust, but after exposure to chlorine and other harsh environments, or passivation layer surface is mechanically or chemically damaged, it is easy to rust than usual, so you see rusting stainless steel not surprising. Life is done with stainless steel passivation oxide, chromium oxide film thickness ratio of the surface of the fresh surface naturally formed film thickness, and are much more dense.
All metal and atmospheric oxygen will be our perception of the reaction, the surface oxide film. Unfortunately, the iron oxide in the form of plain carbon steel proceed oxidation, corrosion continues to expand, eventually forming holes. You can use paint or oxidation-resistant metal (for example, zinc, nickel and chromium) plating to protect the steel surface, but, as people know, this protection is only a film. If the protective layer is damaged, the following steel began to rust Chromium is the corrosion resistance of stainless steel to obtain the basic elements, when the steel contains chromium content reaches about 12% chromium and corrosive media in the role of oxygen in the steel surface to form a layer thin oxide film (from the passive film), can prevent further corrosion of the steel substrate. In addition to chromium, the commonly used alloying elements nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., in order to meet the various uses of stainless steel organization and performance requirements.
Increases the corrosion resistance of stainless steel with the carbon content decreases, and therefore, most of the low carbon volume stainless steel, some steel wC even lower than 0.03% (e.g. 00Cr12). Stainless steel is the main alloying elements Cr, only when the Cr content reaches a certain value, only the corrosion resistance of steel. Therefore, stainless steel is generally wCr were more than 13%. Stainless steel further contains Ni, Ti, Mn, N, Nb and other elements.
Stainless steel pipe is usually divided into: martensitic steels, ferritic steels, austenitic steel. In addition, the composition can be divided into: chromium stainless steel, stainless steel and chrome-nickel-chromium manganese nitrogen stainless steel. 1, ferritic stainless steel: chromium 12% to 30%. Its corrosion resistance, toughness and weldability with the increase of chromium content increased resistance to chloride stress corrosion better than other types of stainless steel. Fall into this category are Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28 like. Ferritic stainless steel due to high chromium content, corrosion resistance and oxidation resistance are better, but the mechanical properties and process performance is poor, and more for large discontinuity as acid structure and antioxidant steel. Such steel resistant to corrosive atmosphere, nitric acid and saline solution, and has good high temperature oxidation resistance, thermal expansion coefficient, etc., for the nitric acid plant equipment and food, can also produce parts at high temperatures, such as gas turbine parts, etc.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)