Stress corrosion cracking in Al-Zn-Mg-Cu aluminum alloys in saline environments

Copyright 2013 ASM International. This paper was published in Metallurgical and Materials Transactions A, 44A(3), 1230 - 1253, and is made available as an electronic reprint with the permission of ASM International. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplications of any material in this paper for a fee or for commercial purposes, or modification of the content of this paper are prohibited.Stress corrosion cracking of Al-Zn-Mg-Cu (AA7xxx) aluminum alloys exposed to saline environments at temperatures ranging from 293 K to 353 K (20 °C to 80 °C) has been reviewed with particular attention to the influences of alloy composition and temper, and bulk and local environmental conditions. Stress corrosion crack (SCC) growth rates at room temperature for peak- and over-aged tempers in saline environments are minimized for Al-Zn-Mg-Cu alloys containing less than ~8 wt pct Zn when Zn/Mg ratios are ranging from 2 to 3, excess magnesium levels are less than 1 wt pct, and copper content is either less than ~0.2 wt pct or ranging from 1.3 to 2 wt pct. A minimum chloride ion concentration of ~0.01 M is required for crack growth rates to exceed those in distilled water, which insures that the local solution pH in crack-tip regions can be maintained at less than 4. Crack growth rates in saline solution without other additions gradually increase with bulk chloride ion concentrations up to around 0.6 M NaCl, whereas in solutions with sufficiently low dichromate (or chromate), inhibitor additions are insensitive to the bulk chloride concentration and are typically at least double those observed without the additions. DCB specimens, fatigue pre-cracked in air before immersion in a saline environment, show an initial period with no detectible crack growth, followed by crack growth at the distilled water rate, and then transition to a higher crack growth rate typical of region 2 crack growth in the saline environment. Time spent in each stage depends on the type of pre-crack (“pop-in” vs fatigue), applied stress intensity factor, alloy chemistry, bulk environment, and, if applied, the external polarization. Apparent activation energies (E a) for SCC growth in Al-Zn-Mg-Cu alloys exposed to 0.6 M NaCl over the temperatures ranging from 293 K to 353 K (20 °C to 80 °C) for under-, peak-, and over-aged low-copper-containing alloys (~0.8 wt pct), they are typically ranging from 20 to 40 kJ/mol for under- and peak-aged alloys, and based on limited data, around 85 kJ/mol for over-aged tempers. This means that crack propagation in saline environments is most likely to occur by a hydrogen-related process for low-copper-containing Al-Zn-Mg-Cu alloys in under-, peak- and over-aged tempers, and for high-copper alloys in under- and peak-aged tempers. For over-aged high-copper-containing alloys, cracking is most probably under anodic dissolution control. Future stress corrosion studies should focus on understanding the factors that control crack initiation, and insuring that the next generation of higher performance Al-Zn-Mg-Cu alloys has similar longer crack initiation times and crack propagation rates to those of the incumbent alloys in an over-aged condition where crack rates are less than 1 mm/month at a high stress intensity factor

The influence of Ce-based coatings as pretreatments on corrosion stability of top powder polyester coating on AA6060

Cerium-based conversion coatings (CeCCs) are one of the most prospective alternatives to the widely used chromate conversion coatings (CCCs) due to their anticorrosion efficiency, environmentally friendly nature and low cost. In this work, the CeCCs on AA6060 were prepared by immersion into aqueous cerium salt solutions at room temperature, and subsequently post-treated in heated phosphate solution. The effect of counter ion (nitrate and chloride) on the coating properties was studied testing CeCCs as sole or conversion layers for the top polyester coating. Since the 60 mu m thick polyester coating was applied, an artificial defect of 0.8 mm hole was introduced to faster assess the differences between pretreatments. The system with CCC pretreatment was used as reference. Corrosion properties were investigated in 0.5 M NaCl solution by electrochemical impedance spectroscopy while the adhesion strength was measured by NMPR (N-methyl-2-pyrrolidone) and pull-off tests. As shown, the post-treated chloride-based CeCC offered better protection than crack-free thin nitrate-based CeCC, when used as sole coatings. On the other hand, it was brought to evidence that in combination with top powder polyester coating, the CeCC deposited from nitrate solution exhibited better protection compared to protective system pretreated with chloride-based one. Excellent polyester coating adhesion was found independently on aluminium surface pretreatment. (C) 2013 Elsevier B.V. All rights reserved