Environment-Induced Crack Initiation and Early Stages of Crack Growth in Aluminum Alloys, Part 2

Abstract

A second group of ‘state-of-the-art’ contributions [1-4] by invited researchers on the ‘Environment-Induced Crack Initiation and Early Stages of Crack Growth in Aluminum Alloys’ appear in this issue, complementing the papers published in the January 2023 issue of Corrosion [5-8]. The second section, presented herein, includes an overview of the experimental study of the initiation of environment-induced cracking (EIC) in aluminum alloys conducted since the 1950’s [1], showing crack initiation typically dominates the total life experienced during both service-life for structural applications and for smooth tensile test specimens subjected to conventional standard EIC testing. The review also reveals, although research conducted over the last 30 years almost exclusively focus on EIC propagation, there are notable exceptions worthy of re-evaluation and further study using modern in-situ advanced experimental techniques, paying particular attention to the roles of creep, crack-arrest and surface conditions during EIC initiation, paying particular attention the immediate sub-surface layers. The three additional papers in this collection are experimental studies. One investigates the significant differences in EIC initiation observed from fatigue pre-cracks and corrosion-induced fissures under controlled electrochemical conditions in a sensitized AA5456-H116 fracture mechanics test specimen exposed to a marine environment [2]. The final two papers involve testing conducted on high Zn-content Al-Zn-Mg-Cu 7xxx series alloys [3, 4]: one involves two commercial thick pate commercial high Zn content new-generation alloys [3], AA7449 and AA7085 in the T7651 temper and provides a detailed description of EIC initiation and its transition to long-crack growth during exposure to water vapor at 70 °C. Information gleaned from in-situ time-lapse, optical imaging over large areas has enabled the exact initiation sites to be identified and investigated with high-resolution fractographic studies. In the final paper [4] based on the deformation and fracture behavior of a high Zn-content Al-Zn-Mg-Cu 7xxx alloy observed in situ using synchrotron radiation X-ray tomography, it is proposed the dispersion of Mn-based second-phase particles may offer a novel approach for preventing hydrogen embrittlement in 7xxx series aluminum alloys.