Effect of sulphide inclusions on hydrogen diffusion in steels

Abstract

Elongated sulphide inclusions may affect hydrogen permeation through steel membranes by acting both as traps and as short-circuit diffusion paths. By considering steady-state permeation only, the two effects have been separated and possible short-circuit diffusion investigated. By treating a steel containing elongated sulphide inclusions as an assembly of composite elements, an electrical analogue has been used to calculate the effective diffusion coefficient at steady state as a function of metallurgical variables. Experimental permeation measurements on membranes of mild steels containing 0·046 and 0·24% sulphur at 2 and 50°C failed to reveal any significant effect of short-circuit diffusion on steady-state permeation rates. However, the possibility of local short-circuit diffusion on the scale of individual inclusions could not be excluded. On the basis of the experimental scatter, an upper limit for the interfacial diffusion coefficient in the range ∼1 – 10 cm² s⁻¹ has been deduced. The effects of trapping and possible local short-circuit diffusion on hydrogen cracking susceptibility are discussed.