Black protective molybdate coatings obtained on galvanized steel

Abstract

This paper looks at the passivation of zinc in environmentally safe molybdate solutions with the aim to replace highly toxic solutions used in black chromate passivation. It is shown that higher current density and process duration make black coatings more saturated. However, this affects the adhesion of the deposited layers. The optimum pH of the solution was found to be in the range from 4.0 to 4.3 units. At the same time, poor quality coatings are obtained outside the above range. It was also found that the process duration can be reduced from 3–6 to 2–5 min by lowering the pH of the solution from 5 to 4 units. The colour of the coatings changes from the colour of the rainbow to black depending on the process duration, current density and the temperature of the working solution. The coatings of deep black colour are formed at the process time of 2 to 5 min, current density of 0.7 to 0.8 A/dm2 and temperature of 50 to 65 oC. Spectral studies showed that molybdate coatings that were produced in optimal conditions consist of molybdenum(VI) oxide, molybdenum(IV) oxide, molybdenum(IV) oxide-hydroxide, zinc oxide, as well as zinc and nickel hydroxides. It should be noted that molybdenum is predominantly integrated in the coating in the form of MoO2. The authors have developed an electrochemical process for the deposition of protective black molybdate coatings on galvanized surfaces in the following solution, g/L: 15–19 (NH4)6Mo7O24; 16 CH3COONa; 4 NiSO4·6H2O. This process helps obtain compact coatings corresponding to 10 on a ten-point black colour scale in the following conditions: i = 0.7–0.8 A/dm2, t = 50–65 оC, = 7–10 min and pH = 4.0–4.3.The authors would like to thank the Mendeleev Shared Knowledge Centre for support of this research.