An investigation of Stress corrosion cracking in MgAZ61 alloy in 3.5% NaCl + 2% K₂CrO₄ aqueous solution at room temperature

Abstract

Effect of specimen orientation, heat treatment and applied potential on the stress corrosion susceptibility of magnesium AZ61 (Mg‐6.3% Al‐0.5% Zn‐0.20% Mn) alloy in an aqueous 3.5% NaCl + 2% K2CrO4 solution at room temperature was investigated. Stress corrosion times to failure were measured at different values of initial stress intensities using single edge (pre) cracked sheet tensile specimens and a modified tensometer. It was observed that while the specimen orientation has a significant effect on the measured values of stress corrosion threshold stress intensity, KIscc, the effect of varying the quench rate during heat treatment was minimal. Polarization measurements both in stressed and unstressed conditions, failed to reveal any significant effect of the applied stress intensity on the anodic polarization behavior of the alloy. However, measurements made under four different potentiostatic conditions showed a considerable increase in stress corrosion times to failure of the alloy. The results, together with fractographic observations of fractured specimen are discussed in terms of the mechanisms of stress corrosion cracking, in magnesium alloys.

Mechanisms for corrosion fatigue crack propagation

“…An increase of the yield strength of both alloys by ageing at 220 °C for 24 h, carried out to increase their susceptibility to HIC, did not change the retarding effect of cathodic polarization on crack growth rates in NaCl solutions. The observed effect of polarization on corrosion FCG rates was consistent with that for SCC of magnesium alloys 22,27 , 29,30 …”

“…Others provide evidence that SAD is the dominant mechanism for corrosion FCG in high‐strength steels 16–18 and titanium alloys 19, 20 . In contrast to the emphasis placed on high‐strength steels and titanium alloys, there is very little information on the corrosion FCG behaviour of materials such as magnesium alloys which have high strength‐to‐density ratios, 21 although much more effort has been devoted to studying SCC in magnesium alloys 22–31 . The mechanism of SCC of magnesium alloys has been ascribed to either continuous crack propagation as a result of SAD, 22,23 , 27 or discontinuous crack propagation as a result of a series of hydrogen‐induced fractures at the crack tip 26,28–31 .…”

“…In contrast to the emphasis placed on high‐strength steels and titanium alloys, there is very little information on the corrosion FCG behaviour of materials such as magnesium alloys which have high strength‐to‐density ratios, 21 although much more effort has been devoted to studying SCC in magnesium alloys 22–31 . The mechanism of SCC of magnesium alloys has been ascribed to either continuous crack propagation as a result of SAD, 22,23 , 27 or discontinuous crack propagation as a result of a series of hydrogen‐induced fractures at the crack tip 26,28–31 . Furthermore, SCC of magnesium alloys can involve the repeated cycles of a slow stage of SAD, followed by a fast stage of mechanical fracture 24,25 .…”

“…Furthermore, SCC of magnesium alloys can involve the repeated cycles of a slow stage of SAD, followed by a fast stage of mechanical fracture 24,25 . One of the main arguments in favour of the SAD mechanism is that SCC of magnesium alloys can be stopped by cathodic polarization and restarted after polarization is removed 22,27 …”

A Critical Review of the Stress Corrosion Cracking (SCC) of Magnesium Alloys

“…Moccari and Shastry [93] compared the SCC characteristics for rolled AZ61, as received and after two heat treatments. The two heat treatment consisted of annealing at 475 C for 90 min, quenching in boiling water or iced brine, and aging at 200 C for 48 h. The heat treatment did not significantly improve the SCC resistance.…”