Comparison of test methods used to analyze stress corrosion cracking of differently tempered 7xxx alloys

Abstract

This paper compares and critically analyzes various test methods used to assess stress corrosion cracking susceptibility of the alloy EN AW-7075 (Al-Zn-Mg-Cu) in various temper conditions. Constant load test in an alternating immersion environment was used as the standard reference test. Constant displacement tests in the form of U-bend specimens and four-point loaded bend specimens was conducted in a neutral and acidic salt-spray fog test environment, an alternating saltspray fog environment and alternating immersion environment. A novel incremental step load test was also carried out, in a continuous immersion environment. It was observed that the susceptibility ranking from U-bend test and the standard constant load test correlated well, while the ranking obtained by the 4-point loaded bend test, showed lower correlation. The same susceptibility ranking was observed in all the test environments. It was shown that the neutral salt-spray fog test environments could potentially substitute the alternating immersion test for testing SCC susceptibility.The T4 treated material showed maximum SCC susceptibility followed by T6 treated and the overaged T7 showed least susceptibility. K E Y W O R D S7000 aluminium, Al-Zn-Mg-Cu, automotive corrosion testing, bend test, stress corrosion cracking

Investigating the effects of pre-deformation and aging conditions on mechanical properties and SCC resistance of aluminium alloy 7075 via slow strain rate testing and electrochemical polarisation

“…Hence, the T4 specimen is considered as the least corrosion-resistant specimen in this study. Naturally aged 7075 alloys are already known to exhibit high SCC susceptibility [1 3]. Severe IGC can be observed in the T4 specimen from both cross-section and exposed surface, as shown in Figure 6(c,d).…”

“…Introduction 7075 Al-Zn-Mg-(Cu) aluminium alloy is widely used in industrial applications due to its high strength-to-weight ratio and toughness achieved by precipitation hardening. However, the high susceptibility of 7075 alloy to SCC at peak aged condition limits the application of 7075 alloy in corrosive environments, and continues to remain a great concern [1][2][3][4][5]. Among various corrosion mechanisms, SCC is particularly hazardous.…”

“…Literatures have shown that the anodic dissolution of the Mg and Zn rich η phase GBPs is responsible for the continuous dissolution of the GBs of 7075 alloy [2]. The naturally aged 7075-T4 is known to hold high susceptibility to IGSCC, as the susceptibility gradually decreases with increasing artificial aging [1][2][3]. T7 over-aging treatment sacrifices the material strength by 10-15% for better SCC resistance [24].…”

Stress Corrosion Cracking in an Extruded Cu-Free Al-Zn-Mg Alloy

“…Other testing methods are also possible and must be considered in future works. Some test methods have been reviewed for EN AW-7075 by Magaji et al [55].…”