Formation of black nickel in leaching solution containing ammonia and chloride

The black nickel formation process in leaching solution containing ammonia and chloride was investigated in terms of cyclic voltammetric and galvanostatic reduction techniques. The structure of black nickel was examined by means of X-ray diffraction technique. The results show that in the scanning region, two oxidization current peaks are observed during the positive sweep, one of which is attributed to a valence state transformation of Ni(OH)(2) to high valence nickel compound (black nickel), and the other is caused by nitrogen evolution. During the formation process of black nickel, gamma-NiOOH probably tends to self-discharge with water to form alpha-Ni(OH)(2). As a result, it is observed that I-pa/I-pc(I-pa-anodic peak current; I-pc-cathodic peak current) maintains at a constant with the scanning rate increasing. Two reduction current peaks in cyclic voltammogram and two potential plateaus in galvanostatic reduction curve for black nickel are ascribed to the reduction of various oxidization states of nickel oxide. The potential plateaus at about 0.75 V reach the maximum in galvanostatic reduction curves. Black nickel presents amorphous structure.