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 at temperatures above 232 C. Hydrogen can spread into the stainless steel plate and form methane at the grain boundary or in the pearlite zone with the iron carbide. Methane can not spread to the outside of the steel and gather together to form white spots or cracks in the metal. In order to prevent the formation of methane, cementite must be replaced by stable carbides. Stainless steel plates must be added chromium, vanadium, titanium and other elements.
In fact, the increase of chromium content allows a higher use of temperature and hydrogen pressure to form chromium carbide in these steels, and the hydrogen element it meets is stable. Chromium steel and austenitic stainless steel with chromium content higher than 12% can be corroded in all known applications under harsh conditions (temperature greater than 593 degrees Celsius).
Most metals and alloys do not react with molecular nitrogen at high temperatures, but atomic nitrogen can react with many stainless steels. It penetrates into the surface of stainless steel to produce brittle nitride. 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 converter, ammonia plant heater and ammonia decomposition at 371 -593 C and -10.5Kg/mm2 under an atmospheric pressure. In these atmospheres, chromium carbide is produced in low chromium steel. It may be corroded by atomic nitrogen to form chromium nitride and release carbon and hydrogen to form methane, which, as mentioned above, may form white spots or cracks. However, when the chromium content is greater than 12%, the carbides in these stainless steel are more stable than chromium nitride, so the front reaction will not appear, so the stainless steel plate can now be used in the high temperature environment of the heat ammonia.
The state of stainless steel plates in ammonia depends on temperature, pressure, gas concentration and chromium and nickel content. The results of field experiments show that the corrosion rate of ferrite or martensitic stainless steel (altered metal depth or carburizing depth) is higher than that of austenitic stainless steel. The higher the nickel content is, the better corrosion resistance is, and the corrosion rate increases with the increase of 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-C r stainless steel, the upper limit of temperature in dry gas is 249, and chlorine is 316.
Source: China Stainless Steel Plates Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)