Corrosion resistance is affected by the environment. For example, monel alloy is a corrosion resistant metal material, but it is only used for corrosion protection of chloride, seawater and alkali, and it is not used in nitric acid, acid iron salt, tin salt, hydrochloric acid or high concentration sulfuric acid. So what are the corrosion reasons of metal materials?
First of all, the corrosion resistant metal materials, in addition to the above mentioned, the austenitic stainless steel in the alloy and the steel with lower carbon content and the pure metal nickel, titanium and tantalum have better corrosion resistance. Nickel is especially resistant to alkali corrosion, which is relatively stable in high temperature or molten alkali, so it is mainly used in alkali making industry. Nickel is highly stable in seawater and salt solutions and organic media (such as fatty acids, phenols, alcohols, etc.) at room temperature. Relatively resistant to inorganic acid corrosion, but also unstable in acetic acid and formic acid.
Titanium is a pure metal with good corrosion resistance. Especially in the concentration of nitric acid, organic acid, chloride, wet chlorine gas and alkali have strong corrosion resistance. But it is not resistant to pure reducing acid and hydrochloric acid corrosion. Tantalum is a pure metal with high chemical stability. In many corrosive media, such as inorganic acid, wang shui, organic acid, chloride, salt, corrosive gas and so on have strong corrosion resistance. But it is not resistant to hydrofluoric acid, fuming sulfuric acid, free sulfur trioxide, potassium iodide, fluoride ion solution and strong alkali corrosion at high temperature.
According to the analysis of corrosion reasons of metal materials, according to xianjie.com, the causes are divided into chemical corrosion and electrochemical corrosion according to the mechanism of corrosion. Mechanical factor corrosion, biological corrosion. Chemical corrosion is a kind of medium mentioned above, such as hydrochloric acid. Electrochemical corrosion refers to the electrochemical reaction between metal and electrolyte solution (as mentioned above in sea water). In the process of anti-gas, electric current is generated, which is the difference between chemical corrosion and electrochemical corrosion. Corrosion under mechanical factors refers to the accelerated failure of metal structures or components under the combined action of corrosive media and mechanical factors, which is manifested as stress corrosion cracking, corrosion fatigue and friction and wear corrosion. Biological corrosion is the corrosion of certain microorganisms on the metal surface, which is caused by the action of these microbial life products.
