Effect of Zr addition on corrosion behavior of Cu-6Ni-2Mn-2Sn-2Al alloy
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O, Graduate Student, and SANGSHIK KIM, Associate Professor, are with Division of Materials Science and Engineering, Engineering Research Institute, Gyeongsang National University, Chinju, Korea. Contact e-mail: [email protected] SEUNGZEON HAN, Senior Researcher, and CHANGJOO KIM, Principal Researcher, are with Materials Engineering Department, Korea Institute of Machinery and Materials, Changwon, Korea. Manuscript submitted August 29, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Potentiostatic test results for Cu-6Ni-2Mn-2Sn-2Al-xZr alloys, either as-cast or 60 pct swaged, in 3.5 pct NaCl aqueous solution.
pct NaCl aqueous solution. For each condition, the current density of the non-Zr added specimen was substantially higher than the Zr containing counterpart, suggesting that the resistance to corrosion was significantly improved with the Zr addition. Figure 2 shows the SEM micrographs of both as-cast and as-60 pct swaged Cu-6Ni-2Mn-2Sn-2Al alloys, with and without Zr addition, after potentiostatic testing in 3.5 pct NaCl solution for 1800 seconds. This figure clearly demonstrates that the addition of a small amount of Zr drastically changes the corrosion behavior of Cu-6Ni2Mn-2Sn-2Al alloy. In its as-cast condition, Cu-6Ni-2Mn2Sn-2Al alloy exhibited a typical interdendritic corrosion mode, while the Zr containing specimen showed a uniform corrosion mode with a small degree of intergranular corrosion. The overall amount of corrosion was significantly reduced with Zr addition. Compared to the as-cast specimens, the overall resistance to corrosion for the swaged specimens appeared to be substantially improved, regardless of Zr addition. In the as-swaged condition, both specimens showed a pitting type of corrosion. However, the number of the pits was significantly reduced with Zr addition. The average depth of these pits was also greatly reduced with Zr addition, as demonstrated in Figure 3, where both specimens after potentiostatic tests were sectioned and documented with SEM. The change in general corrosion behavior of Cu-6Ni2Mn-2Sn-2Al alloys with Zr addition also affects the stress corrosion cracking behavior. Table I demonstrates the change in tensile properties, including tensile strength and percent elongation, of Cu-6Ni-2Mn-2Sn-2Al-xZr alloys after exposure in 3.5 pct NaCl solution for 20 days with an applied stress of 80 pct of the yield strength. Regardless of Zr addition, the tensile strength of the present alloy was not affected by the 20 day exposure. The Zr free specimens, however, showed 9 to 11 pct decreases in tensile elongation after exposure. In Cu-6Ni-2Mn-2Sn-2Al-0.1Zr alloy, on the other hand, any reduction in tensile elongation with 20 days of exposure was not noticed. In the present study, the addition of a small amount of Zr to Cu-6Ni-2Mn-2Sn-2Al alloy was found to be beneficial for improving the corrosion resistance in NaCl aqueous environment. As demonstrated in Figure 2, a typical interdendritic corrosion was observed in as-cast Cu-6Ni-2Mn-2Sn2Al alloy. It has been previously report
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