Review of Dealloying of Cast Aluminum Bronze and Nickel-Aluminum Bronze Alloys in Seawater Service

Abstract A review of dealloying of aluminum-bronze (AB) and nickel-aluminum bronze (NAB) castings used by the U.S. Navy is provided. Relationships among dealloying, alloy composition and microstructure are discussed. The use of an eddy-current test method to determine dealloying depth is discussed and an assessment of an existing nondestructive inspection procedure is provided. The results of 6-months and 24-months seawater corrosion tests on four cast NAB alloys are reported. All four alloys displayed varying degrees of dealloying in the as-cast condition depending on alloy composition and microstructure. Heat treatment at 700°C (1300°F) for 8 hours practically eliminated susceptibility to dealloying in all four alloys and changed the predominant mode of corrosion attack to pitting or surface wastage. Dealloying depth measurements obtained from numerous castings with varying lengths of service time were compiled. Results indicate that dealloying in NAB castings containing a minimum of 4-percent nickel is limited to a penetration depth of 6mm (1/4 in.) for service times of up to fifteen years. Dealloying in AB castings however was a more serious problem with penetration depths in excess of 13mm (1/2 in.) being observed.