Preparation And Corrosion Behavior Of A Novel Fine-grained Ni₃Al Coating

A novel fine-grained Ni3AI coating was fabricated by a two-step process: co-electrodeposition of Ni with Al particles (average particle size:1μm) on Ni3Al substrate and subsequent annealing in vacuum at 600℃for 2 h. The corrosion behavior in air at high temperature and in aqueous solution at ambient temperature of the Ni3Al coating with fine-grain and the cast Ni3AI alloy with coarse-grain was comparatively investigated.High temperature oxidation tests were conducted in air at 900℃and 1000℃. Results showed that the growth of NiO on the coating was inhibited, owing to the fast formation of an Al2O3 scale. The reason was that the fine-grained structure increased the density of surface nucleation sites and simultaneously increased the flux of Al to the oxidation front through an abundance of grain boundaries. Moreover, the scale formed on the coating exhibited better adhesion than that on the alloy.The cyclic oxidation resistance of both the coarse-grained alloy and the fine-grained coating were investigated at 900℃. It was found that the cyclic oxidation resistance of the coating was greatly increased compared with that of the alloy, due to the formation of a continuous and adherent Al2O3 scale. The main reason for the increased scale adhesion lay in the fact that the fine-grained structure prevented the formation of large-sized voids at the Al2O3 coating interface.The potentiodynamic polarization tests of both the coarse-grained alloy and the fine-grained coating were investigated in a 3.5% NaCl aqueous solution at 30℃. Results showed that the corrosion resistance of the coating was worse than that of the alloy, because intrinsic pores at the surface of the coating increased its susceptibility to pitting corrosion. However, the behavior was reversed when the samples were pre-oxidized in vacuum at 1050℃for 5 h to form a thin Al2O3 surface layer. The more compact and stable Al2O3 film formed on the fine-grained coating became an effective barrier to Cl- attack. It showed superior corrosion resistance by reducing the corrosion potential and enlarging the passive region.