The Corrosion of Mn Coatings Electrodeposited from a Sulphate Bath with Te(VI) Additive and Influence of Phosphate Post-Treatment on Corrosion Resistance

Abstract

Manganese coatings are excellent for the cathodic protection of steel against corrosion. Although manganese is more electrochemically active than widely used protective coatings of zinc, the exceptional resistance of manganese coatings in neutral and basic media is determined by the film of insoluble corrosion products, which forms on the surface of manganese and greatly suppresses its further corrosion. It is known that the electrodeposition process of Mn coatings from sulphate electrolytes is positively affected by some additives of chalcogenide (S, Se and Te) compounds in the electrolyte. However, a more detailed study on the corrosion properties of Mn coatings electrodeposited from sulphate bath with Te(VI) additive is lacking. In this work, the measurements of free corrosion potential and potentiodynamic polarization in a neutral NaCl solution, as well as the corrosion resistance properties of obtained Mn coatings, were evaluated in a salt spray chamber. It was obtained that the best corrosion resistance was shown by Mn coatings, electrodeposited at the cathodic current density of 15 A⋅dm−2 and at higher temperatures (60 and 80 °C). Meanwhile, the corrosion resistance of phosphated Mn coatings, obtained from a room temperature bath, increased about 5 times and reached up to 1000 h until corrosion of the steel substrate occurred.