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A Study of the Effects of Microstructure on the Seawater Jet Impingement Resistance of Nickel Aluminum Bronze
Nickel-aluminum bronzes are metallurgically complex alloys. Small variations in composition can result in markedly different microstructures. In a previous study, the microstructures of selected alloy compositions were characterized and correlated with seawater corrosion resistance. The present study discusses the results from seawater jet impingement tests conducted on these same alloy compositions.
Specimens prepared from two different experimental heats of nickel-aluminum bronze were subjected to various heat treatments prior to jet impingement testing in seawater. As-cast commercial nickel-aluminum bronze was also tested, as an experimental control. Both experimental heats and the experimental control are in compliance with UNS C95800 and MIL-B-24480, except that one of the experimental heats, at 8.1% Al, was intentionally melted to be below the lower limit of the specified aluminum compositional range of 8.5 to 9.5%. The microstructures of both experimental compositions were characterized and compared with seawater jet impingement test results.
Keywords: Aluminum Bronze, Nickel-Aluminum Bronze, Seawater, Microstructure, Martensite, Bainite, Eutectoid, Alpha, Widmanstatten, Phase, Jet Impingement, Impingement, Erosion, Corrosion, Erosion-Corrosion, Cavitation, Localized Corrosion, Selective Phase Attack, Intergranular, Crevice, Maximum Depth of Attack, Dealloying. UNS C95800, MIL-B-24480.
Aluminum bronze alloys 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. This paper touches upon another problem area, performance of nickel-aluminum bronze in seawater (4-8), where corrosion and erosion failures are 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 (9). An increase in the aluminum content results in higher strength, which is attributable to a hard, body-centered-cubic phase. This 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 nickel and manganese act as microstructural stabilizers (9). Nickel-aluminum bronzes are metallurgically complex alloys in which small variations in composition can result in the development of markedly different microstructures (3, 10 - 12). This, in turn, may result in wide variations in seawater corrosion and erosion-corrosion resistance. Those microstructures, which result in optimum performance in seawater, are obtained by control of composition and heat treatment (13). 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 resistance to jet impingement by seawater. This paper is a continuation of a previous reported study, in which the same materials were exposed to quiescent seawater over six-months (14).