The Influence of Solution pH on Microstructure of Electrodeposited Cobalt

In this study we have attempted to elucidate the influence of p H on microstructures of electrodeposited cobalt using transmission electron microscopy. It is observed that at low p H (∼1.6), the microstructure mostly consists of highly faulted fcc regions, and electron diffraction patterns are very complex. On the other hand, at high p H (∼5.7) hcp phase is observed, and the density of faults is greatly reduced. From these observations together with results of chemical analysis, it is proposed that at low p H, the formation of fcc cobalt, probably in the form of metastable cobalt hydride, is greatly facilitated by the adsorption and subsequent incorporation of atomic hydrogen into the electrodeposits. However, later on these hydrogen atoms rapidly diffuse out of the film, thereby the resulting "fcc cobalt" (cobalt hydride) becomes unstable at room temperature and transforms martensitically into hcp cobalt.