Modeling of hydrogen degradation of a metal pipe under pressure

Abstract

Modeling  of hydrogen effect on the strength of a metal structure is an urgent problem. The hazard of detrimental action of hydrogen lies in the fact that it often has no visible external signs and reveals itself only at the instant of structure failure. The effect of hydrogen on the metal changes its mechanical properties and, as a result, leads to a redistribution of stresses inside the structure. This, in turn, affects the diffusion of hydrogen, creating a complex interrelation that must be taken into account when calculating the strength of real structures. The paper presents a mathematical model for computing  the process of degradation of pipe material under pressure of hydrogen-containing medium. Two continuums are considered: a deformable body and freely diffusing hydrogen, which, under certain conditions, attaches to metal atoms and becomes bound hydrogen. In this case, hydrogen absorbs energy from the crystal lattice, which causes deterioration of mechanical properties of the metal. Since the process of interaction between hydrogen and metal proceeds very slowly, it is considered as quasi-stationary. The mutual influence of hydrogen diffusion and stress-strain state of the pipe on the penetration of hydrogen into the metal and degradation of its mechanical properties is modeled numerically. An iterative algorithm has been developed to calculate the  change in the stress-strain state of the pipe weakened by hydrogen. Numerical calculations have been carried out to evaluate mechanical stresses, concentrations of free and bound hydrogen in the pipe, as well as changes in the modulus of elasticity of the metal as a result of damage accumulation.. The calculations show  that some combinations of hydrogen concentration and pressure on the pipe wall can lead to the formation of pores and circumferential cracks in the pipe.