Experimental-size specimens of two different compositions of nickel-aluminum bronze were cast, subjected to five different heat treatments and were exposed to seawater. Both alloys were in compliance with UNS C95800 and MIL-B -244 80, except that the aluminum content of one alloy, at 8.1%, was below the lower limit of the specified compositional range. Their microstructures were characterized and correlated with corrosion resistance. Selective phase attack was seen in both alloys, although it was much more pronounced in the alloy with the lower aluminum content, to the point where entire grains fell out. It was observed that the higher aluminum-containing alloy had markedly superior corrosion resistance. Thus it is important to maintain the aluminum level in alloy UNS C95800 at or above the lower limit, and within the specified range of 8.5-9.5%. Aluminum Bronze, Nickel-Aluminum Bronze, Seawater, Microstructure, Martensite, Bainite, Eutectoid, Alpha, Widmanstatten, Phase, Corrosion, Localized Corrosion, Selective Phase Corrosion, Selective Attack, Depth of Attack, Depth of Penetration, UNS C95800, MIL-B-24480.
The alloys in the aluminum bronze family typically contain 5% to 11% aluminum. In addition, some of the alloys contain iron, nickel, manganese or silicon. They are available in both wrought and cast product forms, and offer good combinations of mechanical properties and corrosion resistance. Consequently, they have been used for decades in a wide variety of marine applications including valves and fittings, ship propellers, pumps, pump shafts, valve stems and heat exchanger waterboxes ~1). Aluminum bronze has many attributes, but can occasionally exhibit deficiencies under certain circumstances ~2>. As reported by Meigh (3~, Sperry ~2~ stated, over ninety years ago, "After much good and bad experience with it, I will frankly say that it is bronze without peer, and the early worshippers of it did not overate it by any means." In this instance ~2~, the bad experience referred to problems encountered in producing sound billets and castings due to dross and shrinkage. The subject of this paper touches upon another problem area, the seawater corrosive attack of high-strength nickel- aluminum bronze (4-8), which is sporadically experienced in service.
Nickel-aluminum bronzes are generally two-phase, duplex alloys containing 5% to 11% aluminum as well as additions of iron and nickel for strength (). Increasing the aluminum content results in higher strength, which is attributable to a hard, body-centered-cubic phase, which enhances properties of castings as well as hot working in wrought alloys (9). The other alloying elements also improve properties and alter microstructure. Specifically, nickel improves corrosion resistance, while iron acts as a grain refiner and increases tensile strength. Nickel also raises yield strength, and both - o ) nickel and manganese act as microstructure stabilizers . Nickel-aluminum bronzes are metallurgically complex alloys in which small variations in composition can result in the development of markedly different microstructures (3, 10, 11~, which can, in turn, result in wide variations in seawater corrosion resistance. Those microstructures, which result in optimum corrosion resistance, are obtained by control of composition and heat treatment (12). The subject of this paper is the evaluation of the effect of a small change in aluminum content, as well as heat treatment, on microstructure and of their influence on seawater corrosion resistance.
EXPERIMENTAL PROCEDURE
The present study explores the effect of variations in aluminum content on the seawater resistance of cast nickel-aluminum bronze. The two alloys designate
