Formation behavior of Cr-nitride by thermal nitridation in two-phase Cu-Cr alloy for application in metallic separators in PEMFCs

Abstract

Cu-Cr alloy is an attractive alternative material for application in metallic bipolar plates due to its good electrical conductivity, high mechanical strength and other favorable attributes. In order to improve the corrosion resistance, one barrier to its practical use, the formation of a continuous Cr-nitride on the surface of the alloy is necessary. Here, the formation behavior of Cr-nitride in Cu-Cr alloy was investigated in a nitrogen atmosphere in a temperature range of 600 similar to 1,000 degrees C using thermal nitridation. The optimum conditions to form the continuous Cr-nitride on the surface of Cu-Cr alloy were thereupon suggested. Nitridation of Cr started at about 600 degrees C and the only surface product was Cr(2)N. Both CrN and Cr(2)N phases are observed at 700 degrees C, while only CrN phase was formed at and above 800 degrees C. Therefore, it was concluded that Cr-nitride is formed by phase transformation in a sequence of Cr -> Cr(2)N -> CrN. The amount of Cr-nitride formed on the surface increased with an increase of Cr content, annealing time and annealing temperature. The continuous Cr-nitride was formed under the annealing for 12h at 1,000 degrees C, but not under the annealing for 2h or below 900 degrees C. Also it was formed in Cu-5.3(wt.%)Cr and Cu-7.6Cr, but not in Cu-2.4Cr. In Cu-Cr alloy, the continuous Cr-nitride(CrN) was formed with low Cr content in comparison with other alloys. This is attributed to the very low mutual solubility and absence of any intermetallic compound in Cu-Cr alloy.