Entwicklung eines vereinheitlichen mechanistischen Verständnisses für die Einflüsse der Umgebung auf die Degradation von Mg-Al-Legierungen
Final Report Abstract
The primary remit of this project was to develop a unified, mechanistic understanding of the influence of microstructure, especially the concentration and morphology of the β phase, on the susceptibility of Mg-Al alloys to environmentally-assisted degradation. Mg-Al alloys are susceptible to all modes of environmentallyassisted degradation, i.e. corrosion, stress corrosion cracking (SCC), hydrogen embrittlement (HE) and creep. There is considerable evidence in the literature to suggest that the concentration and morphology of the β phase are key factors affecting the susceptibility of Mg-Al alloys to all forms of environmentally-assisted degradation. The influence of the β phase on the susceptibility of Mg-Al alloys to these degradation modes is complex and may have opposing consequences on durability. This poses a considerable challenge to the development of Mg-Al alloys with greater durability in service environments where they may be subject to multiple degradation mechanisms. Model materials, including binary Mg-Al alloys with 3, 6 and 9wt%Al and ternary alloys containing 9wt%Al as well as 1wt.%Ca, 2wt.%Sr, 1wt.%Pb, 3wt%Bi or 1wt.%Sb, were specially prepared by permanent mould chill casting. The binary alloys were also heat-treated into various tempers in order to investigate the influence of the concentration and morphology of the β phase on the mechanisms for environmentally-assisted degradation. The susceptibility of the test materials to corrosion was evaluated using electrochemical noise (EN) analysis. The steady-state corrosion rates for the as-cast Mg3%Al and Mg6%Al alloys in 0.5 M NaCl at pH 12 were relatively consistent, but higher than that of as-cast Mg9%Al alloy. The corrosion rate of Mg9%Al alloy increased with gradual homogenisation of the microstructure by solution annealing at 420 °C for up to 24 hours. Artificial aging of the Mg9%Al alloy after solution annealing resulted in a recovery of its low corrosion rate. Solution annealing of the Mg3%Al and Mg6%Al alloys for 24 hours did not result in a significant change in their corrosion rates. None of the tertiary alloying elements resulted in a reduction in the corrosion rate relative to that of the as-cast Mg9%Al alloy. The addition of Sr and Bi resulted in a slight increase in the corrosion rate, whereas the addition of Ca, Sb and Pb resulted in a significant increase in the corrosion rate; the corrosion rate of the Ca-containing alloy was the highest of all materials tested. For the Mg-Al alloys, steady-state corrosion was highly localised, usually occurring at a single point on one of the sample surfaces. For the Mg3%Al alloy in the as-cast and solution-treated conditions, pitting was preceeded by an initial period of general corrosion. For the pure Mg, only general corrosion occurred. The resistance of the Mg6%Al and Mg9%Al alloys to general corrosion may be due to the effect of Al on the thickness and stability of the surface film. The pit morphology for the Mg3%Al and Mg6%Al materials was characteristic of an autocatalytic mechanism, in which the Al-depleted α-Mg dendrites were preferentially dissolved. In contrast, the pit morphology for the Mg9%Al materials was characteristic of a metastable mechanism, in which corrosion was halted at Al-enriched interdendritic α-Mg and grain boundaries. The results are consistent with a strong galvanic coupling between Al-depleted and -enriched zones of the α phase. The SCC susceptibilities of the test materials in the as-cast condition were evaluated using constant extension rate tensile (CERT) tests in distilled water under a constant displacement rate of 5µm/hr. Most materials suffered a considerable reduction in elongation-to-failure, εmax, and UTS due to SCC in distilled water. The exceptions were the Ca and Sb-containing alloys, which were generally immune from SCC under the test conditions. This was inconsistent with the results of the corrosion tests, which showed that the Ca and Sb-containing alloys had the lowest corrosion resistance of all the ternary alloys tested. This suggests that the susceptibility of the materials to corrosion is a poor indicator of their susceptibility to SCC, and vice versa. SEM analysis of the fracture surfaces of the SCC samples indicated that the mechanism for SCC propagation involved HE. Moreover, εmax and UTS for the Mg-Al-Ca and Mg-Al-Sb samples tested in air were relatively low. Indeed, the absolute values of εmax and UTS for the Mg9%Al and ternary alloy samples (with the exception of the Mg-Al-Bi alloy) tested in distilled water were relatively consistent. Thus, it may be that the apparent immunity of the Mg-Al-Ca and Mg-Al-Sb to SCC is a consequence of their relatively low strength, i.e. that they are unable to withstand stresses at which SCC occurs. This implies that the addition of tertiary alloying elements has little influence on the SCC susceptibility of Mg-Al alloys. The compositions of oxide films formed in aqueous environments on pure Mg and the binary Mg-Al alloys in the as-cast condition were characterised using X-ray photoelectron spectroscopy (XPS). Al-enrichment of the oxide layer was not detected for any of the test materials. The oxide layer formed on the Mg9%Al alloy was markedly thicker than those formed on the Mg3%Al and Mg6%Al alloys. This is broadly consistent with the corrosion measurements.
Publications
- Development of a Generalised Understanding of Environmentally‐Assisted Degradation of Magnesium‐Aluminium Alloys, in: M. Alderman, M.V. Manuel, N. Hort, N.R. Neelameggham (Eds.), Magnesium Technology 2014, Wiley, Hoboken, 2014, pp. 365-370
N. Winzer, P. Casajus, H. Höpfel
- Electrochemical noise analysis of the corrosion of high-purity Mg-Al alloys. Corrosion Science Volume 94, May 2015, Pages 316-326
P. Casajus, N. Winzer
(See online at https://doi.org/10.1016/j.corsci.2015.02.014)
