ELECTRODEPOSITION OF ZN-NI ALLOYS IN SULFATE ELECTROLYTES .2. REACTION MODELING

A reaction model has been developed to account for the mechanism of Zn -Ni alloys deposition from acidic sulfate electrolytes. The heterogene ous reactions imply the presence of four adsorbates: (i) ZnNiaddivided by which catalyzes nickel deposition, (ii) the intermediate H-ad invo lved in hydrogen evolution which occurs at low polarizations on nickel -rich deposits, (iii) the intermediate adion Zn-ad(+) acting as a cata lyst for the deposition of the zinc-rich delta-phase which prevails at high polarizations, and (iv) the anionic species Zn/NiSO4,ad2- which inhibits the formation of the delta-phase. In the transition region of intermediate polarizations, an increase in the interfacial pH facilit ates the chemical incorporation of S in deposits, which is catalyzed b y the anionic adsorbate. The model validity has been confirmed by comp arison with experimental polarization curves, electrode impedance meas urements, and analyses of the deposit structures and compositions.