Passivation of zinc coatings in molybdate solutions

Abstract

The present work is concerned to the study of the process of passivation of zinc and its alloys in ecologically safe molybdate-containing solutions in order to replace highly toxic solutions of black chromate passivation. The concentration range of the solution components was determined, in which black coatings are deposited, corresponding to a score of 10 on a ten-point color scale. It was found that the optimal pH values of the solution are in the range from 4,8 to 5,2 units. At lower pH values, adhesion deteriorates and the black color of coatings degrades, at higher pH values, coatings do not form at all. High-quality deep black coatings (10 points) are formed at temperatures of 45–85 oC. The color of the coatings changes from rainbow to black depending on the duration of their formation; the coatings of deep black color are formed with a process duration of at least 5 minutes. Spectral studies have shown that with an increase in the duration of the process, the content of molybdenum (V) oxide increases, molybdenum (VI) oxide decreases, and the content of molybde num (IV) oxide remains practically unchanged. Has been developed the technological process of passivation of zinc surfaces in a solution containing in g/l: 14–20 (NH4)6Mo7O24; 10–18 CH3COONa; 1–5 NiSO4·6H2O, which allows the formation of black molybdenum-containing coatings at t = 45–85 oC and τ = 7–10 min, comparable in protective and optical characteristics with black chromate coatings.This research was funded by D. Mendeleev University of Chemical Technology of Russia. Project No. X-2020-027.