Intergranular corrosion in steel
Imagine what happens when austenitic stainless steel is placed in different media at temperatures between 450 ° C and 800 ° C. These media contain nitric acid at a concentration of 50% to 65%, a sulfuric acid solution containing copper and iron oxide, and a hot organic acid.
In fact, this phenomenon also occurs in chrome steel, chrome-nickel steel, and also exists in nickel, copper, aluminum-based alloys - intergranular corrosion.
Nature and cause
Intergranular corrosion is a common phenomenon of local corrosion of metallic materials in special media.
The cause of intergranular corrosion is the non-uniformity of the chemical composition of the metal composition, which is related to the three properties of the substance: corrosion resistance, chemical stability, and surface activity.
1. The temperature range of stainless steel in the range of 450 ° C to 800 ° C is called the sensitized temperature region. During the sensitizing temperature, due to the uneven distribution of chromium, it is rich in chromium and lacks chromium.
Corrosion occurs in the place, and the corrosion phenomenon can be eliminated by prolonging the heating time.
2. There is also a carbon content and thermal activity that also determines the corrosion tendency between stainless steel. After a large number of tests, the carbon content is less than 0.03% and does not cause corrosion.
In order to reduce the corrosion between stainless steel, stainless steel can be avoided with high carbon content in two ways.
One is to directly reduce the carbon content in the steel; to add a substance that stabilizes the C activity; to reduce the activity of the C, for example, adding an element that reduces the activity of C (manganese, molybdenum, tungsten, vanadium, niobium, titanium) hinders the formation of intergranular corrosion.
Second, the quenching process is used in the heat treatment.
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