Effects of hydrogen recombination poisons on stress corrosion crack initiation and propagation in Al-Zn-Mg alloys

Abstract

Stress corrosion cracking (SCC) and corrosion behaviour of Al-Zn-Mg alloys have been studied in deaerated 3·5 wt-%NaCl solution at pH 3·5 with and without the hydrogen recombination poisons (HRPs) Na₂S.9H₂O or Ca₃P₂. SCC tests were carried out in the range of applied potential from −2150 to −400 mV(SHE) using constant load apparatus equipped with a linear variable differential transformer. The stress corrosion (SC) crack initiation and propagation periods were differentiated by measuring the extension of the gauge length with time. When the NaCl solution contained HRP the anodic dissolution rate decreased, probably due to the adsorption of hydride and/or its reduced phase, and the hydrogen reduction rate decreased due to the inhibition of hydrogen recombination. SC crack initiation and propagation times decreased with increasing HRP concentration, probably due to enhanced hydrogen permeation into the crack tip region. Incipient SCC was initiated preferentially at matrix/intermetallic inclusion interfaces. The results support the view that SC crack initiation and propagation in Al-Zn-Mg alloys are caused by hydrogen embrittlement.