Revelation of solid solubility limit Fe/Ni = 1/12 in corrosion resistant Cu-Ni alloys and relevant cluster model
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Minor Fe additions are necessary to enhance the corrosion resistance of commercial Cu-Ni alloys. The present paper aims at optimizing the Fe content in three alloy series Cu90(Ni,Fe)10, Cu80(Ni,Fe)20, and Cu70(Ni,Fe)30 (at.%) from the viewpoint of their corrosion performance in a 3.5% NaCl solution. An Fe/Ni = 1/12 solid solubility limit line was revealed in the Cu-Ni-Fe phase diagram. Three Fe/Ni = 1/12 alloys, Cu90Ni9.23Fe0.77 (at.%) = Cu-8.6Ni-0.7Fe (wt.%), Cu80Ni18.46Fe1.54 = Cu-17.3Ni-1.4Fe, and Cu70Ni27.7Fe2.3 = Cu-26.2Ni-2.1Fe, show the best corrosion performances in their respective alloy series. The Fe/Ni = 1/12 solubility limit is explained by assuming isolated Fe-centered FeNi12 cuboctahedral clusters embedded in a Cu matrix. The three Fe/Ni = 1/12 alloys can be respectively described by cluster formulas [Fe1Ni12]Cu117, [Fe1Ni12]Cu52, and [Fe1Ni12]Cu30.3. The Fe/Ni = 1/12 rule may serve an important guideline in the industrial Cu-Ni alloy selection because above this limit, easy precipitation would negate the corrosion properties of the Cu-Ni-based alloys.
I. INTRODUCTION
Cu-Ni alloys are widely used as tube and vessel materials due to their excellent resistance to seawater corrosion.1 The base binary alloys are often alloyed with small amounts of Fe, such as the case with UNS C70600 and C71500 alloys, which contain 0.5–2.3 wt% Fe. The Fe additions were proved necessary to maintain the corrosion resistance of the base Cu-Ni alloys and the increased corrosion resistance was attributed to the formation of a protective film containing iron oxides.2 A synergistic effect of Ni and Fe on seawater corrosion behavior was also found3 and alloys with visible Fe-Ni discontinuous precipitates made the corrosion performance worse when the Fe content exceeded 2% in a Cu-10Ni alloy (10wt% Ni).4,5 It was demonstrated that the corrosion rate decreased with a further increase in Fe content.6 Although much research has been devoted to the corrosion behavior of Cu-Ni alloys in seawater, the Fe modification mechanism and its optimum content in Cu-Ni alloys remain unresolved and in practice Fe-modified Cu-Ni alloys often suffer from degraded corrosion and processing problems due to Fe-induced precipitation. The key issue here is the optimum Fe content in different Cu-Ni alloys. It can be seen from phase diagrams that Cu and Ni can form a continuous FCC solid solution phase while Fe and Cu are largely immiscible even at
high temperatures.7 In contrast, Cu-Ni together can dissolve small amounts of Fe and the solubility of Fe in CuNi alloys increases with increasing Ni content, as shown in an isothermal section at 600 C (Fig. 1).8,9 Therefore, the Ni content should be a controlling factor to improve Fe solubility in Cu-Ni alloys. Generally, single-phase homogeneous solid-solution alloys are desirable for good corrosion resistance performance. From the Cu-Ni-Fe alloy phase diagram, we can see that the FCC solid solubility boundary at 600 C is essentially coincident with a straight composition line of a constant Fe/Ni = 1/12
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