The embrittlement of Al-Zn-Mg and Al-Mg alloys by water vapor

Abstract

Al4.5Zn1.5Mg and Al5Mg were reacted in water-vapor saturated air (WVSA) at 120°C and tensile tested. After an initial loss of ductility with exposure time, probably caused by hydrogen embrittlement of the grain boundaries, between 15 hours and 25 hours exposure the mechanical properties of Al4.5Zn1.5Mg improved, this effect being due both to a reduced corrosion activity of the grain boundaries in producing embrittling hydrogen at the external surface and to grain boundary MgZn2 precipitates acting as hydrogen traps. After 25 hours exposure water was shown to penetrate the grain boundaries, and a layered corrosion product identified as the aluminum hydroxides boehmite and diaspore was formed. This resulted in a marked fall of ductility. Re-solution heat treatment and reaging partially recovered the mechanical properties of Al4.5Zn1.5Mg if the exposure time was less than 50 hours, and would not recover properties for longer exposure times. Small additions (0.1 pct) of iron and nickel to Al4.5Zn1.5Mg lessened the grain boundaries’ sensitivity to corrosive attack whereas the addition of 0.1 pct copper did not. Also, the former two additions did not cause the relative ductility increase during 15 to 25 hours exposure in WVSA at 120°C shown by Al4.5Zn1.5Mg. It is proposed that these elements alter the magnesium segregation levels at the grain boundaries which in turn affects their electrochemical attack.