Corrosion behaviour of nickel coating obtained by ball milling

Nanocrystalline materials have received much attention as a result of their unique physical, chemical and mechanical properties. However, little work has been performed to establish the corrosion behaviour of this materials. Thorpe, Ramaswami and Aust [1] studied the corrosion behaviour of nanocrystalline Fe32Ni36Cr14P12B6 alloy obtained by crystallization of the melt-spun amorphous ribbon. They determined that the corrosion resistance was significantly greater than that of its amorphous counterpart. The authors attributed this improved corrosion resistance to the observed greater Cr enrichment of the electrochemical surface film via rapid interphase boundary diffusion [1]. Similarly, Braganolo et al [2] determined enhanced corrosion resistance with ''nanocrystallized'' Fe17Si10B15Cr3 metallic glass wires. In this latter study, the beneficial effects of nanocrystalline processing for corrosion resistance were not evident with a non-passivating Fe17Si10B15 alloy composition. Inturi and Szklarska-Smialowska [3] have observed superior localized corrosion resistance in HCl for sputter deposited nanocrystalline 304 stainless steel in comparison with conventional material; attributed to the fine grain size and homogeneity of the nanocrystalline material. Rofagha et al [4, 5] studied nanocrystalline Ni and Ni-P produced by electrodeposition technique. The results of corrosion measurements in acid media 0.1 M indicate that nanocrystalline processing of Ni may catalyze hydrogen reduction processes, reduces the kinetics of passivation and compromises passive film passivity. The observed behaviour is considered consistent with substantial contributions to the bulk electrochemical behaviour from the intercrystalline regions ( i.e., grain boundaries and triple junctions) of these materials. The nanocrystalline Ni-P alloys were non-passiving in 0.1 M H2SO4. Their enhanced dissolution rates over those of conventional polycrystalline nickel was explained as due to the presence of P and the high volume fraction of grain boundaries and triple junctions. Further, anodic polarization of the nanocrystalline material resulted in the formation of non-protective surface films [5]. The objective of this study is to assess the corrosion behaviour of ball-milled nanocrystalline Ni coatings (12 H and 24 H duration) in 0.1M H2SO4 media. Potentiodynamic anodic polarization are presented and compared to polarization curves obtained for conventional nickel.