The relationship between the corrosion resistance of stainless steel pipes and the chromium content
All metals can react with oxygen in the atmosphere to form an oxide film on the surface, while the iron oxide formed on the ordinary carbon steel tube continues to oxidize, causing the corrosion to continue to expand and eventually forming holes. Paint or oxidation-resistant metal can be used for electroplating to protect the surface of carbon steel, but this protective layer is a thin film. If the protective layer is damaged, the steel below will begin to rust again.
Stainless steel tube corrosion resistance is related to the chromium content, when the chromium content in the steel reaches 12%, in the atmosphere, a layer of passivated and dense chromium-rich oxide is formed on the surface of the stainless steel tube to protect the surface and prevent further Oxidation. This oxide layer is extremely thin, and the natural luster of the steel surface can be seen through it, giving the stainless steel a unique surface. If the chromium film is destroyed, the chromium in the steel and the oxygen in the atmosphere will regenerate a passivation film to continue to play a protective role. Under some special environmental conditions, stainless steel pipes will also experience some local corrosion and failure, but stainless steel, unlike carbon steel, will not fail due to uniform corrosion, so the corrosion allowance is meaningless for stainless steel pipes.
Under the action of static tensile stress, the corrosion damage of metal is generally called stress corrosion fracture, and under the action of alternating stress, the damage of metal is called corrosion fatigue. The stress corrosion fracture of austenitic stainless steel tube is a kind of localized corrosion that produces cracks in metal materials under the conditions of chloride ion, tensile stress exceeding the critical value (including internal stress) and high temperature. The appearance is usually unpredictable. This phenomenon is usually controlled by correct operation, although changing the environment and reducing residual stress can sometimes be effective.
Generally speaking, ferritic and duplex stainless steel pipes have better resistance to stress corrosion cracking and are often substitutes. The stress corrosion resistance of the austenitic-iron-nickel-chromium alloy is also improved when the nickel content exceeds 20%. In fact, the 6%～7% molybdenum alloy with chromium content of 17%～23% and nickel content of 17%～26% has very good resistance to stress corrosion cracking. However, if the austenitic alloy is to be truly free from stress corrosion cracking, the nickel content must be above 35%.
