Mechanism Of Intergranular Corrosion Of Stainless Steel Seamless Tubes

Intergranular corrosion of stainless seamless steel pipe is a common type of partial corrosion. Corrosion is carried out along the metal or alloy grain gap or its close area, and the grain corrosion is very fine. This type of corrosion is called intergranular corrosion. Such corrosion greatly weakens the bonding force between the grains. Severe intergranular corrosion can cause the metal to lose strength and ductility and break under normal load. Modern intergranular corrosion theory, mainly chromium deficiency theory and grain boundary impurity selection dissolution theory.

According to the theory of lean chromium, the commonly used austenitic stainless steel has intergranular corrosion in oxidizing or weakly oxidizing medium, and 80% is caused by improper heating during processing or application. Improper heating means that the steel is heated or slowly cooled through the temperature range of 450 to 850°C, and the steel is sensitive to intergranular corrosion. So this temperature is a dangerous temperature for austenitic stainless steels. The stainless steel material is now solution treated at the factory.

The so-called solution treatment is to quench the steel after heating to 1050 ~ 1150°C, in order to obtain a homogeneous solid solution. Austenitic steels contain a small amount of carbon, and the solid solubility of carbon in austenite decreases with decreasing temperature. For example, 0Cr18Ni9Ti has a solid solubility of about 0.2% at 1100 °C and about 0.02% at 500 to 700 °C. Therefore, carbon is supersaturated in solution treated steel.

When the steel is heated or cooled for 450 to 850°C, carbon can be formed (Fe, Cr) 23C6 is separated from the austenite and distributed on the grain boundary. (Fe,Cr)23C6 has a much higher chromium content than the austenite matrix, and its natural consumption consumes a lot of chromium near the grain boundary, and the consumed chromium cannot be compensated in time from the grain.

Since the chromium dispersing rate is very slow, the chromium content near the grain boundary is lower than the necessary amount of passivation (ie 12% Cr), which constitutes a chromium-depleted region, and thus the passive state is destroyed, and the potential near the grain boundary is lowered. The grain itself remains in a passive state, the potential is high, and the grain and the grain boundary constitute a living state--a passive micro-electrode cell, and the battery has a large cathode and a small anode area ratio, thus causing a grain boundary region. corrosion.

In production practice, we also learned that austenitic stainless steel tubes can also undergo intergranular corrosion in strongly oxidizing media (such as concentrated nitric acid), but the corrosion is different from that in oxidizing or weakly oxidizing media. Generally occurs on solid solution treated steel, and the sensitized steel generally does not occur. When the solid solution contains phosphorus as an impurity of 100 ppm or a silicon impurity of 1000 - 2000 ppm, they are segregated on the grain boundary.

These impurities dissolve under the action of a strong oxidizing medium, resulting in intergranular corrosion. When the steel is sensitized, the carbon can form (MP) 23C6 with phosphorus, or the primary segregation of carbon constrains the diffusion of phosphorus to the grain boundary. In both cases, the segregation of impurities at the grain boundary is eliminated or reduced. Or weakened the sensitivity of steel to intergranular corrosion.

The above two theories describing the intergranular corrosion mechanism are each applicable to the organization of a certain alloy and a certain medium, which are not mutually displaced but compensate each other. The most common intergranular corrosion of stainless steel in production practice occurs in weakly oxidizing or oxidizing media, so most corrosion examples can be explained by the theory of lean chromium.