HYDROGEN ATTACK ON STEEL FOR HIGH-PRESSURE HYDROGEN SERVICE AS A RESULT OF THE ALTERATION OF CARBIDES BY AMMONIA-SYNTHESIS GAS

Certain ammonia synthesis conditions lead to nitriding of a relatively thin surface layer of low-alloyed ''hydrogen-resistant'' steels. For reasons of thermodynamic stability of carbides and nitrides this cause s a modification of the first, releasing carbon which can effuse from the steel and can react with diffusing hydrogen to form methane. Depen ding on the level and the direction of the total component stresses, s mall separations of grains, parallel to the surface, or cracks across the wall may slowly develop. Thus, the Nelson diagram has only limited validity in the case of materials selection for NH3-containing synthe sis gases.