High temperature hydrogen attack is a damage mechanism that threatens the integrity of critical steel components in petrochemical plants and refineries when the hydrogen diffuses into the steel and reacts with the carbides within to produce pores containing methane. With the motivation of understanding the role of carbide stability on the reaction with hydrogen, samples of a classic 2.25Cr-1Mo steel were subjected to a variety of heat treatments that generate a variety of carbides, prior to exposure to high-pressure hydrogen in an autoclave. Using quantitative carbide and microstructural characterisation, it has been possible to demonstrate the roles of four variables: (a) the non-equilibrium chemical composition of carbide; (b) the fraction of the carbide that is closest to the thermodynamic equilibrium state; (c) the location of where the depletion preferentially occurs.
Steel components in refineries and petrochemical plants are exposed to conditions of temperatures higher than 200°C with high pressures of hydrogen. Such conditions avail the driving force needed for the hydrogen to dissociate and penetrate the steel surface. Once the atomic or nascent hydrogen is within the steel microstructure, it can react with the carbon present, usually in carbides, to form methane gas within the steel structure as suggested by the below reactions:
(equation)
where α refers to ferrite.
(equation)
for the case where cementite is the principal carbide.
The reaction of the carbide with hydrogen depletes the carbon in the ferrite, which dissolves more carbides to replenish equilibrium and because hydrogen can move rapidly in ferrite, the attack can occurs to a larger extent in the vicinity of the carbide, where methane gas that is too large to diffuse out hence, methane gas creates cavities where carbides previously existed, i.e. creation of voids. Therefore, if the carbon is stable enough in the carbides for the reaction not to occur, the attack is then assumed to be mitigated [1–14].
