Electrodeposition of sacrificial tin–manganese alloy coatings

Sn–Mn coatings have been electrodeposited on steel substrates from simple ammonium sulfate baths, with or without the addition of citrate, tartrate, EDTA or gluconate additives. The effect of current density and additives on the coating composition, microstructure, crystallography and corrosion resistance of Sn–Mn deposits has been investigated. It is found that ammonium sulfate brings Sn2+ and Mn2+ discharging potentials closer, allowing codeposition of manganese with tin. Sn–Mn coatings obtained from simple ammonium sulfate baths at low current density contain a large amount of oxygen and are microstructurally heterogeneous, while at high current density amorphous, bright and homogenous Sn–Mn coatings can be obtained. The addition of tartrate, EDTA or gluconate can improve coating quality at low current density and suppress Mn2+ reduction, with a corresponding decrease of Mn content in the alloy. Sn–Mn coatings show an anodic potentiodynamic behavior intermediate between that of pure manganese and pure tin, and their electrochemical characteristics can be adjusted by varying alloy composition and structure. Coatings with a high percentage of the intermetallic Mn1.77Sn phase show good sacrificial protection for steel.