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Abstract
The influences of mechanical strain on hydrogen entry and transport are reported for a high strength steel alloy (AISI 4340) under cathodic polarization in aqueous sodium chloride solution. The overall rate of hydrogen permeation and the quantity of hydrogen absorbed are shown to be particularly low in alkaline chloride at low cathodic current densities if the charging surface has been contaminated by slight corrosion. Evidence is presented that illustrates the role of mechanical strain in promoting enhanced hydrogen absorption as a result of film rupture. In addition to the conventional Devanathan‐Stachurski method, results from plastic straining of a cylindrical annular Devanathan‐Stachurski sample confirm the film rupture concept. Film removal is shown to enhance the overall rate of hydrogen entry by increasing the hydrogen surface coverage and possibly the adsorption‐absorption rate constant on the bare surfaces formed. This increases the mobile hydrogen concentration in the metal lattice. The presence of the intact metastable film modifies the kinetics of the hydrogen evolution reaction, lowering the cathodic hydrogen overpotential at a given cathodic current density. This, consequently, is shown to lower hydrogen absorption for a given cathodic current density. The net effect of film rupture is to promote an increase in the hydrogen permeation rate independent of changes in the diffusion coefficient. Additional information indicates that long range enhanced diffusion of hydrogen by dislocation transport is not observed. Further evidence suggests that for sharp cracks, the rate of hydrogen accumulation at a position a distance from the crack tip where the crack initiates is an absorption controlled transport process. These findings provide additional insight on the rate limiting steps controlling the “time dependent” hydrogen assisted failure process of high strength steels in sodium chloride solution.
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