The corrosion resistance of stainless steel is derived from a chromium-rich passive film formed on the steel surface. When the chromium content reaches 10.5% or above, a dense protective film spontaneously forms when the steel contacts oxygen. This film isolates the substrate from water, air and corrosive media, and features self-repairing properties when damaged, which constitutes the fundamental anti-corrosion principle of stainless steel.
Nevertheless, the corrosion resistance of stainless steel is not absolute, and different grades present significant performance differences:
200 Series It only provides basic rust resistance for indoor environments. Adopting a low-nickel and high-manganese formula without sufficient nickel and molybdenum, it delivers poor resistance to chloride ions, salt spray, acid and alkali. It is prone to pitting corrosion and rusting in coastal, humid and chemical environments, and is only applicable to dry indoor working conditions.
304 / 304L Series Featuring balanced and universal corrosion resistance with standard chromium and nickel ratios, it resists corrosion in normal atmospheric, fresh water and food-grade environments. However, it is susceptible to pitting corrosion in high-chloride seawater and salt spray environments, making it unsuitable for marine and heavy chemical industrial conditions. The low-carbon 304L reduces carbon content to improve intergranular corrosion resistance, ideal for welded components.
316 / 316L Series The addition of molybdenum greatly enhances resistance to chloride ions, pitting corrosion and crevice corrosion. It adapts to seawater, coastal salt spray and moderate acid and alkali chemical environments. The low-carbon 316L effectively prevents intergranular corrosion after welding.
400 Series (430 Ferritic) Nickel-free 430 ferritic stainless steel offers acceptable atmospheric rust resistance but poor resistance to acid, alkali and chloride ions. Martensitic stainless steel provides high hardness through quenching treatment yet has overall limited corrosion resistance.
Duplex Stainless Steel With a sophisticated combined ratio of chromium, molybdenum and nitrogen, duplex stainless steel integrates high mechanical strength with excellent resistance to stress corrosion and pitting corrosion. It withstands seawater and high-salinity chemical media, and is widely used for severe corrosive working conditions.
Key Influencing Factors
The corrosion resistance is determined by alloy composition (Cr, Ni, Mo, N), carbon content, surface condition and service environment (chloride ions, humidity, acid-alkali level and temperature). Even premium stainless steel raw materials may suffer corrosion under high-chloride and strong acid or alkali environments.