In the environment of high temperature (usually high pressure) hydrogen and ammonia, localized metal etching copper corrosion is caused by hydrogen entering the material.
The mechanism is that carbon in steel, mainly carbon in cementite, reacts with hydrogen to produce methane CH4. The reaction formula is as follows: Fe3C+2H2≒3Fe+CH4. Methane cannot escape through diffusion in α-iron. CH4 bubbles are formed at grain boundaries and inclusions such as MnS and MnO. As CH4 continues to enter, the bubble pressure continues to rise, causing the bubbles to grow and connect with each other. Finally, hydrogen corrosion cracks along the grain boundaries occur, accompanied by severe Surface decarburization.
The degree of hydrogen corrosion is related to temperature, hydrogen pressure, applied stress and alloy composition. It generally becomes more severe as temperature, pressure and work stress increase. The hydrogen corrosion resistance of steel is greatly improved when alloying elements are added, especially the carbide forming elements molybdenum, chromium, vanadium, niobium and laser forming technology of titanium alloy structural parts for aviation.
The famous Nelson curve, summarized in 1949 and later revised several times based on the experience of various countries, stipulates the safe limit for various steels to be used at different temperatures and hydrogen pressures no matter how long they are used.