The reaction of stainless steel and gas
Hydrogen corrosion may occur in ammonia synthesis, hydrogen desulphurization and oil refining. Carbon steel is not suitable for high pressure hydrogen device for above 232 ℃. The hydrogen diffusion into the steel, and in the grain boundary or pearlite zone and reaction of iron carbide and produce methane, the methane (gas) can’t spread to the outside steel all together, and white spots and cracks in metal or one of them. In order to prevent to produce methane, cementite must replacement into stable carbide, chromium, vanadium, titanium, or must be added into the steel drill. Data points out, improve the chromium content allows the use of higher temperature and hydrogen partial pressure formed in the steel chrome carbide, and it encounters of hydrogen is stable. In the use of harsh conditions (temperature higher than 593 ℃), the chromium content is more than 12% chromium steel and austenitic stainless steel in all known applications are corrosion resistance.
Most metals and alloys do not react to nitrogen at high temperatures, but atomic nitrogen can react with a lot of steel. It also permeates the steel and forms a brittle nitride surface layer. Iron, aluminum, titanium, chromium and other alloy elements may participate in these reactions. The main source of atomic nitrogen is the decomposition of ammonia. Ammonia converter, the system of ammonia plant production of heater and in 371 ℃ ~ 593 ℃, a nitriding furnace under atmospheric pressure ~ 10.5 Kg/was have ammonia decomposition of the operation. In these atmospheres, chromium in low-chrome steel appears. It may be affected by the corrosion of nitrogen atoms to produce chromium nitride, and release of the carbon and hydrogen to generate methane, as above, then possible to generate white spots and cracks, or one of them. But chrome content more than 12%, the carbides in the steel than chromium nitride is more stable, so the response will not appear in front of, so now the stainless steel used in the high temperature of hot ammonia environment.
The status of stainless steel in ammonia is determined by temperature, pressure, gas concentration and chromium nickel content. Field experiment results showed that the ferrite and martensite stainless steel corrosion rate (alteration metal depth or carburized depth) is higher than austenitic stainless steel, the nickel content, the higher the better corrosion resistance. As the content increases the corrosion speed increases.
The austenitic stainless steel is highly corrosive in the high temperature halide vapors and is more corrosive than chlorine. For high Ni – C r stainless steel, used in dry gas temperature limit, fluorine is 249 ℃, chlorine is 316 ℃.
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)