Hydrogen corrosion may occur in ammonia synthesis, hydrogen desulfurization and hydrogenation and petroleum refining units. Carbon steel is not suitable for high pressure hydrogen installations above 232 degrees. Hydrogen can diffuse into steel and react with ferric carbide at grain boundaries or pearlite zones to produce methane. Methane (gas) can not diffuse to the outside of steel and aggregate together to produce white spots and cracks in the metal or one of them. In order to prevent the formation of methane, cementite must be replaced with stable carbide, and chromium, vanadium, titanium or drill must be added to the steel. It has been reported that increasing chromium content allows for higher operating temperature and hydrogen partial pressure to form chromium carbide in these steels, and it is stable when encountering hydrogen. Chromium steels and austenitic stainless steels containing more than 12% chromium are corrosion resistant in all known applications under harsh service conditions (temperatures above 593 C).
Most metals and alloys do not react with molecular nitrogen at high temperatures, but atomic nitrogen reacts with many steels. And penetrated into the steel to form a brittle nitride surface layer. Iron, aluminum, titanium, chromium and other alloying elements may participate in these reactions. The main source of atomic nitrogen is the decomposition of ammonia. Ammonia decomposition occurs in ammonia converters, ammonia plant production heaters, and a nitriding furnace operating at 371 ~593 ~10.5 Kg/mm2 atmospheric pressure. In these atmospheres, chromium carbide appears in low chromium steel. It may be corroded by atomic nitrogen to produce chromium nitride and release carbon that interacts with hydrogen to form methane, which, as mentioned above, may produce white spots and cracks, or one of them. But when the chromium content exceeds 12%, the carbides in these steels are more stable than chromium nitride, so the preceding reaction does not occur, so stainless steels are now used in hot ammonia environments.
The state of stainless steel in ammonia depends on temperature, pressure, gas concentration and chromium and nickel content. Field test results show that the corrosion rate of ferritic or martensitic stainless steel is higher than that of austenitic stainless steel, and the higher the nickel content, the better the corrosion resistance. The corrosion rate increases with the increase of the content.
The corrosion of austenitic stainless steel in high temperature halogen vapor is very serious, and the corrosion of fluorine is more serious than chlorine. For high Ni-Cr stainless steel, the upper limit of temperature in dry gas is 249, and chlorine is 316.
