Corrosion creep of magnesium-based alloys

Abstract

In recent years special attention has been concentrated on Mg alloys as the lightest structural metal. However, their insufficient corrosion resistance and creep resistance limit Mg alloys application in automotive and aerospace industry. Until now these problems have been treated in many works separately as corrosion behavior in non-stressed state and creep in noncorrosive conditions. However, under real operational and environmental conditions mechanical and corrosion processes occur simultaneously, accelerating each other. This inevitably leads to the appearance of new synergistic effects that significantly reduce the lifetime of metals. For example, the failure of AZ91D alloy (Mg-9%Al-1%) exposed to a rural atmosphere while loaded in constant tension of 102 MPa (near 60% of yield strength) occurred within 10–100 days [1]. In the present work the effect of corrosive medium on the creep of some die-cast Mg alloys has been studied for the first time, and a phenomenon named “corrosion creep” has been found. Specimens (5.9 mm in diameter, gauge length of 75 mm) of Mg alloys AZ91D (Mg-8.4% Al, 0.85% Zn, 0.17% Mn), AM50 (Mg-5.1% Al, 0.15% Zn, 0.57% Mn) and AS21 (Mg-2.3% Al, 0.23% Mn, 1.10% Si)) were produced on a die-cast machine with the locking force of 2000 KN. To study the surface layer effect on corrosion creep, a surface layer 0.5 mm thick was removed by mechanical treatment from the surface of some specimens. Creep tension tests were carried out on Model 3 Satec machine at 25 ± 2 ◦C in air and in 0.1N Na2B4O7 buffer solution (pH 9.3) at stress values of 0.85 and 0.89 of tensile yield strength (TYS) for each alloy (Table I). Creep specimen 1 in a glass electrochemical cell 3 was placed into an averaging high-temperature