Chemical corrosion is a process in which metal is oxidized and lost by direct oxidation-reduction reaction with the contacted substance. For example, the chloride ion of chemical raw materials reacts with iron to form ferrous chloride: Cl2 + Fe → FeCl2.
Electrochemical corrosion, when the metal is placed in aqueous solution or humid atmosphere, the metal surface will form a kind of micro battery, also known as corrosion battery (its electrode is usually called cathode and anode). The anode of the corrosion battery is iron, while the cathode is impurity. Iron has been sacrificed.
Secondly, we will talk about the way of metal corrosion. There are mainly five types:
Surface corrosion: most or all of the common chemical or electrochemical reactions on the metal surface are called uniform corrosion, also known as comprehensive corrosion.
Intergranular corrosion: corrosion along the boundary between the grains of a metal facing internal growth. It is mainly due to the difference of chemical composition between the surface and the interior of the grain and the existence of impurities or internal stress in the grain boundary.
Pitting corrosion: it is short for pitting corrosion, which usually refers to a kind of corrosion on the surface of metal or alloy with easy passivation characteristics in the solution environment containing corrosive anions such as Cl ions and oxidants.
Stress corrosion: refers to the damage caused by metal in corrosive medium under the action of tensile stress.
Corrosion fatigue: the combination of alternating load and corrosive environment can significantly reduce the fatigue performance of components.
The composition comparison of the four grades of 304304L and 316316316l is shown in the table below. It can be seen that the composition of stainless steel and carbon steel is quite different in terms of trace elements, and C, Cr and Ni are the main differences. 316 series contains more Mo and Ni than 304 series, and the carbon content of low carbon stainless steel is only 0.03.
Common methods of corrosion resistance of stainless steel
Alloy trace elements such as Cr, Mo, Ni are added to stainless steel. The data show that when the content of Cr is more than 12.5%, the surface oxide similar to Fe Cr is formed when Fe is alloyed by C. This tightly adherent chromium rich oxide protects the metal surface from further oxidation. Moreover, even if the surface layer is damaged, the exposed steel surface will react with the atmosphere for self repair, forming the oxide "passivation film" again, and continue to play a protective role. When Mo is added to 316 series, Mo and Cr cooperate. Mo oxide and solid solution form coexist with Cr hydroxide in passivation film, which further improves pitting resistance of stainless steel. This is also the reason why 316 is more resistant to pitting (including Cl ions) than 304.
The electrode potential of stainless steel was changed by adding alloying elements such as Ni, Cr and mo. Or Cr ion, when the number of chromium atoms in steel is more than 12.5%, the electrode potential of Fe will change suddenly, from negative potential to positive electrode potential, making the electrons in metal crystal more difficult to lose, thus effectively preventing electrochemical corrosion. 316 stainless steel has more mo than 304 stainless steel, which further increases the electrode potential, so 316 series is more corrosion-resistant than 304 series.
The carbon content should be as low as possible. Because the diffusion rate of carbon in stainless steel grain is faster than that of CR. In high temperature such as welding state, the excess carbon will continuously diffuse to the austenite grain boundary, and form chromium carbide compound with Cr in the intergranular. When the mass fraction of chromium in the grain boundary is less than 12%, the so-called "chromium poor area" will be formed. The chromium poor area will lose the corrosion resistance, and intergranular corrosion will occur. Once it is stressed, it will break along the grain boundary, with almost the strength Completely disappeared. Intergranular corrosion can occur in the heat affected zone, weld or fusion line of welded joint. This also explains that many stainless steel products always crack at the weld.
Because of the addition of Mo, the corrosion resistance of 316 (L) series stainless steel is better than that of 304 (L);
300L low carbon stainless steel usually has obvious corrosion resistance than 300 series only under special circumstances such as high temperature or welding, but the corrosion resistance of pure base metal in solid solution state is not different.