As long as it is a material, it has a service life. In the process of use, it will be directly or indirectly damaged more or less. There are many kinds of damage to materials. Among them, corrosion is the most common form of damage. The damage phenomenon caused by the chemical, electrochemical and physical effects of materials in environmental media is called corrosion, especially commonly used metal materials. Corrosion problems involve all areas of life, and corrosion problems are inevitable as long as materials are used. Corrosion will not only cause some accidents, but also bring huge economic losses.
According to statistics, 10% to 20% of metals are lost due to corrosion every year. If 1 billion tons of steel are produced annually, it means that 100 million tons of steel are wasted due to corrosion. Therefore, how to prevent corrosion is an urgent problem. The most basic ways to protect against corrosion are usually the following six: (1) Improve the corrosion resistance of the material itself; (2) Change the environment: (3) Use coating and surface modification; (4) Separate the material from the corrosive medium: (5) Use electrochemical protection; (6) Correct material selection and reasonable design. Ferritic stainless steels can also undergo intergranular corrosion (IGC) similar to that often seen in austenitic stainless steels. Due to the existence of interstitial elements such as C and N in Fe-17Cr ferritic stainless steel, the intergranular corrosion sensitivity of stainless steel is increased.
C tends to segregate to grain boundaries, while Cr has a strong affinity for C in stainless steel. Therefore, Cr is attracted to the grain boundary and combines with C to form Cr carbides such as Crz3C6o Crz3C6. The precipitation leads to the appearance of a poor Ming area near the grain boundary, causing IGC to occur in the poor Ming area of the grain boundary. In addition, ferritic stainless steel is more susceptible to IGC than austenitic stainless steel. Due to the lower solubility of C in ferritic stainless steel, Cr carbides in steel are more likely to precipitate. In this case, the following measures are usually taken to prevent the occurrence of intergranular corrosion: (1) Tempering at 700-800 °C is used; (2) Adding stabilizing alloying elements, such as Ti, Nb, etc.; (3) Reducing the C and N content in the steel.
In addition, pitting corrosion is also a common corrosion phenomenon. Pitting corrosion is the formation of small holes or rust spots with a certain depth on the surface of stainless steel parts. Epit in the polarization curve of electrochemical test is usually used to characterize the pitting performance of the sample. Electrochemical corrosion refers to the corrosion caused by the electrochemical reaction caused by the contact between metal and electrolyte aqueous solution. Electrochemical corrosion means that there are two relatively independent cathodic and anodic reactions during the corrosion reaction, accompanied by the occurrence of electric current. Electrochemical corrosion is basically a redox process, while the anodic reaction is the dissolution of metal and the cathodic reaction is reduction by a depolarizing agent in solution. In addition to characterizing pitting performance by testing polarization curves, electrochemical impedance spectroscopy (EIS) can also be used to characterize.
EIS is mainly used to characterize the strength of the passivation film formed on the surface of the material by measuring the resistance and capacitance. Also, the effective resistance and capacitance data are plotted, ie Nyquist plot and Bode plot. On this basis, the equivalent circuit model was used to properly fit the impedance data to obtain the required results. Mahato et al. studied the pitting behavior of food-grade Fe-17Cr ferritic stainless steel in 20% acetic acid aqueous solution containing 0.5mol/L NaCI, and found that it is an autocatalytic process. It also showed that chloride ions and acetate ions both play an important role in pitting corrosion. Yin et al. found that the addition of hydride ions in NaCl solution would increase the pitting sensitivity of Fe-17Cr ferritic stainless steel due to the reduction of metal oxide content in its passive film and the additional hydroxide reaction associated with permeated hydrogen. Yu et al. found that the addition of an appropriate amount of Ce can improve the pitting behavior of Fe-17Cr ferritic stainless steel. This is because Ce helps to produce dispersed and fine rare earth inclusions and inclusions, but excessive Ce will lead to an increase in the size and number of inclusions, thereby reducing pitting corrosion resistance. Lee et al. found that the addition of Cu would lead to an increase in Cr-containing inclusions, thereby inhibiting the passivation performance of Fe-17Cr ferritic stainless steel.
