Reactions of Co based and Fe based superalloys with a molten Zn – Al alloy

Abstract

Static corrosion tests of Co based and Fe based superalloys were carried out at 470°C in a molten Zn galvanising alloy containing about 0.22 wt.-%Al and saturated with Fe for various lengths of time up to 168 h (one week). The superalloys readily reacted with the galvanising alloy and CoAl or Fe₂Al₅ formed initially on the sample surfaces. The reaction front moved towards the depth of the test pieces with increasing dipping time, leaving behind a Zn rich reaction zone. At the same time, other intermetallic phases formed in the Zn rich zone and on the surfaces of the samples. These intermetallic phases are complex in nature and vary with the alloys. In Stellite 6, Fe aluminide was prominently present in the vicinity of surviving carbide particles in the Zn rich zone of the sample tested for 168 h. Some areas of iron aluminide were intimately connected to Fe₂Al₅ particles building up on the sample surface. This observation suggests that the iron aluminide formed in the reaction zone provided ideal sites for the attachment of Fe₂Al₅ particles pre-existing in the melt onto the sample surfaces. It also served as nucleation and growth sites for the Fe₂Al₅ compound on the sample surface. This is one possible mechanism of dross buildup on submerged hardware made of this type of Co based superalloy in continuous galvanising.