Heat treatment effect on the microstructure and the corrosion resistance of manganese-aluminum bronzes

Abstract

The analyses have shown that the corrosion behavior of manganese-aluminum bronzes (MAB) can be improved by keeping the fraction of β phase low. As opposed to the cast structure, the sample annealed at 850 °C and water quenched does not only contain coarse α and β phase but also a fine α/β phase mixture. The κ phase is coarsened. After annealing at 600 °C, not only isolated coarse α phase but also a fine α/β phase mixture is present. The κ phase has a globular appearance. Since, at an Al content of 7 wt. %, the alloy is located in a three-phase region consisting of α, β, and κ phase, the β phase is preserved even at slow cooling. In both corrosive media, SFW and SSW, predominantly selective corrosion takes place after which, similar to the dezincification of brass, metallic copper remains (so-called dealloying). The heat treatments show different effects on the corrosion behavior in fresh water or sea water. In fresh water, both heat treatments result in a deterioration of the corrosion resistance. Here, the α-β phase mixture, the β phase, and the κ phase were attacked. Measurements in sea water revealed a deterioration of the durability after annealing at 850 °C, but an improvement after annealing at 600 °C. Since no coarse β phase is observed after slow cooling, the MAB corrosion resistance, especially to sea water, can thus be improved.