Mechanistic study of Ni-Cr-P alloy electrodeposition and characterization of deposits

For the first time, the electrochemical reduction mechanism of Ni-Cr-P alloys was studied by means of linear sweep voltammetry and cyclic voltammetry. The experimental results evidence that low pH conditions are more favorable for Cr(III) electrodeposition in a glycine bath. The electroreduction of Cr(III) complexes to metallic chromium is performed via two steps in which the [Cr(H2O)(4)(Gly)](+) complex act as the electroactive intermediate. The first step is controlled by diffusion. In Ni-Cr alloy electrodeposition, Ni initially deposits at the cathode surface, which acts as a catalyst to induce a significant positive shift in the onset reduction potential of Cr(II) ions, thus satisfying the potential difference (approximately -180 mV) for Ni-Cr codeposition. The onset reduction potential of Cr(II) ions in Ni-Cr-P alloy electrodeposition was negatively shifted compared to that in Ni-Cr alloy electrodeposition, which can be attributed to the delayed deposition of Ni. Nevertheless, the actual potential difference between Ni and Cr is the same as in the case of Ni-Cr codeposition, thus ensuring the occurrence of Ni-Cr-P codeposition. In addition, it was experimentally demonstrated that P tends to codeposit with Cr during Ni-Cr-P electrodeposition, which can be explained by the difference in the electrodeposition behavior of Ni-P and Cr-P alloys. Scanning electron microscope images showed that smooth and compact Ni-Cr-P coating was obtained at a current density below 15 A/dm(2). X-ray diffraction results revealed that the deposited Ni-Cr-P coatings had an amorphous structure due to the high P content.