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/dm2. X-ray diffraction results revealed that the deposited Ni–Cr–P coatings had an amorphous structure due to the high P content.