STRESS CORROSION CRACKING OF 18 Ni MARAGING STEEL IN CHLORIDE SOLUTIONS.

A variety of techniques involving electrochemical, hydrogen permeation and acoustic emission measurements have been used in studying initially smooth and pre-cracked specimens. 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 surfaces by stressing the specimens transversely to the rolling direction. The evidence does not support the hypothesis that failure invariably results from the ingress of hydrogen following the creation of acidic conditions within an initiating pit or pre-crack. Rather it supports crack growth by dissolution at high potentials, and a hydrogen induced process at low potential. This conclusion is supported by electrochemical measurements and the effects of additions of chloroplatinic acid, sodium arsenate or thiourea to the chloride solutions, with or without applied polarization. Hydrogen permeation measurements under conditions of cathodic polarization confirmed the effects of these additions upon the uptake of hydrogen but when permeation membranes were subjected to anodic polarization 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 emission to be enhanced by cathodic but not by anodic polarization as crack growth occurred.