Hydrogen corrosion may occur in ammonia synthesis, hydrogen desulfurization hydrogenation reactions and petroleum refining units. Carbon steel is not suitable for use in high pressure hydrogen installations above 232°C. Hydrogen can diffuse into the steel and react with iron carbide at grain boundaries or in pearlitic zones to produce methane. Methane (gas) cannot diffuse to the outside of the steel and collects, producing white spots and cracks or either of these in the metal.
In order to prevent the production of methane, carburization must be replaced by stable carbides, steel must be added to chromium, vanadium, titanium or drill. It has been documented that increased chromium content permits higher service temperatures and hydrogen partial pressures to form chromium carbide in these steels, and that it is stable against hydrogen. Chromium steels and austenitic stainless steels containing more than 12% chromium are corrosion resistant in all known applications under severe service conditions (temperatures above 593°C).
Most metals and alloys do not react with molecular nitrogen at high temperatures, but atomic nitrogen can react with many steels. and penetrates into the 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 decomposition occurs in ammonia converters, ammonia production heaters and nitriding furnaces operating at 371°C~593°C, one atmosphere~10.5Kg/mm².
In these atmospheres, chromium carbide appears in low chromium steel. It may be corroded by atomic nitrogen and produce chromium nitride, and the release of carbon and hydrogen to generate methane, as mentioned above, which may then produce white spots and cracks, or one of them. However, with chromium contents above 12%, the carbides in these steels are more stable than chromium nitride, so the previous reaction does not occur, so stainless steels are now used in high temperature environments with hot ammonia.
The state of stainless steel in ammonia is determined by temperature, pressure, gas concentration and chromium-nickel content. Field experiments show that the corrosion rate (depth of altered metal or depth of carburization) of ferritic or martensitic stainless steels is higher than that of austenitic stainless steels, which are more resistant to corrosion with higher nickel content. As the content increases the corrosion rate increases.
Austenitic stainless steel in high temperature halogen vapor, corrosion is very serious, fluorine is more corrosive than chlorine. For high Ni-C r stainless steel, the upper limit of the use temperature in dry gas fluorine for 249 ℃, chlorine for 316 ℃.
Post time: May-24-2024