Most metals and alloys at high temperatures can not react with nitrogen at high temperatures, but atomic nitrogen can react with many steel. And penetrate into steel to form a brittle nitride surface layer. Iron, aluminum, titanium, chromium and other alloying elements may be involved in these reactions. The main source of atomic nitrogen is the decomposition of ammonia. Ammonia converter, ammonia plant production heater and in 371 degrees ~593 DEG C, the nitriding furnace of operation under a high pressure ~10.5Kg/mm2 are ammonia decomposition. In the atmosphere, appearance of chromium carbide in low chromium steels.
It may subject to corrosion of nitrogen atoms and chromium nitride, and the release of carbon and hydrogen methane generation, as mentioned above, this may generate white and crack, or one of them. But the chromium content of more than 12%, the carbide of these steels is more stable than chromium nitride, so the reaction in front will not appear, so the stainless steel tube is now used in the high temperature environment of hot ammonia.
Hydrogen corrosion may occur in ammonia synthesis, hydrogen desulfurization and hydrogenation reaction and petroleum refining unit. Carbon steel is not suitable for high pressure hydrogen device with 232 degree centigrade. Hydrogen diffusion into the steel, and the grain boundary or zone of pearlite and cementite reaction and produce methane, methane (gas) would not spread to outside the steel and gather together, white and crack or one of them in the metal. To prevent the generation of methane, cementite must be replaced by stable carbide and steel must be added to the chromium, vanadium, titanium or drill. Data indicate that increasing chromium content allows higher operating temperature and hydrogen partial pressure of formation of chromium carbide in the steel, and it meets the hydrogen is stable. Under harsh operating conditions (temperature above 593 DEG C) and chromium content greater than 12% chromium steel and austenitic stainless steel pipe in the known all application are resistant to stress corrosion.
The state of the stainless steel tube in ammonia is determined by the temperature, pressure, gas concentration and the content of chromium and nickel. The experimental results show that the corrosion rate of ferrite or martensite stainless steel tube is higher than that of austenitic stainless steel, and the corrosion resistance of the latter is better. Corrosion rate increases with increasing content.
Austenitic stainless steel tube in high temperature halogen vapor, corrosion is very serious, the corrosion of fluorine than chlorine. For high R Ni-C stainless steel pipe, the upper limit of the temperature in the dry gas, the fluorine is 249, and the chlorine is 316.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)