Corrosion and dealloying of cast lead-free copper alloy–graphite composites

Abstract

Plumbing components fabricated from copper base alloys containing lead, pose health and environmental problems, and there is a need to find suitable alternatives to lead which can impart comparable machinability to cast copper alloys. Bismuth and selenium are two potential substitutes for lead, but their cost is high and their toxicity is in question. In the present work, graphite particles have been substituted for lead in a cast copper alloy. Graphite is low in cost, non-toxic, and our previous work showed that it imparts machinability to cast copper alloys similar to lead. However, very limited information is available on the corrosion behavior of cast copper alloys containing graphite particles. In this study, C90300 copper alloy containing titanium as the wetting agent and graphite particles were cast using a stir casting technique. The cast microstructure of this alloy has α-dendrites with graphite and a titanium rich intermetallic phase in the interdendritic regions; the graphite is presumably changed to titanium carbide formed on the surface of graphite. The corrosion and dealloying of (a) the base C90300 copper alloy, (b) the base copper alloy containing only titanium, and (c) C90300 copper alloy containing graphite particles and titanium were studied in aggressive aqueous solutions. Two types of corrosion tests have been carried out: (1) 17-day Australian test, and (2) 1-year Rockwell test. In both the 17-day Australian test and the 1-year Rockwell test, the copper alloys containing both graphite and titanium, as well as the copper alloy containing only titanium gained weight. Dealloying penetration of the graphite containing copper alloy is observed to be only slightly higher than that of the base copper alloy, but microhardness at the dealloyed layer of the composite containing graphite and titanium is higher than the base alloy. Electrochemical measurements suggest that the addition of graphite to the copper base alloy may not significantly alter the corrosion characteristics of the base alloy.