Stress Corrosion Cracking of 18 Ni Maraging Steel in Chloride Solutions

Abstract

A variety of techniques involving electrochemical, hydrogen permeation and acoustic emission measurements have been used in studying the stress corrosion cracking of initially smooth and pre-cracked specimens of a maraging steel in different chloride-containing solutions. In solutions having pH's in excess of 2, cracking of smooth specimens occurred in two regimes of potential separated by a region in which cracking did not occur, although cracking was induced in this latter region if the specimens were precracked or pre-pitted, or pitting was facilitated by non-metallic inclusions emerging at the test specimen surfaces by stressing the specimens transversely to the rolling direction. It is considered that such geometrical discontinuities are more important in facilitating cracking by fostering local changes in solution chemistry than because of their effect in terms of stress intensification. However, the evidence, viewed in its entirety, does not support the hypothesis that failure invariably results from the ingress of hydrogen into the steel following the creation of acidic conditions within an initiating pit or pre-crack. Rather does it support crack growth by dissolution at high potentials, and by a hydrogen induced process at low potentials, with possibly both processes involved at some intermediate potentials, including the free corrosion potential. This conclusion is supported by various electrochemical measurements, which show good correlation with the potential dependence of cracking and with the effects upon cracking of additions of chloroplatinic acid, sodium arsenate or thiourea to the chlQride solutions, with or without applied polarisation. Thus, the effects of these additives upon the hydrogen and dissolution reactions are in agreement with their influence upon stress corrosion cracking, as are the effects of applied polarisation. Hydrogen permeation measurements under conditions of cathodic polarisation confirmed the effects of these additions upon the uptake of hydrogen by the steel, but when permeation membranes were subjected to anodic polarisation they were perforated by dissolution and the apparent hydrogen permeation was due to the passage of solution through the membrane. Further support for hydrogen induced cracking dominating at lower potentials and dissolution being controlling at higher potentials was derived from acoustic emission experiments, which showed emissions to be enhanced by cathodic but not by anodic polarisation as crack growth occurred, implying different mechanisms of growth in the different potential regimes.