Evolution of texture and strain in Sn coating with Cr addition and its effect on the coating corrosion behavior

Sn-Cr coatings with different amounts of Cr (similar to 1.3-5.5 wt%) were electrodeposited over mild steel substrate. Addition of Cr altered the coating morphology from less compact and columnar for pristine Sn coating to relatively more compact and globular for Sn-Cr coatings. Incorporation of Cr led to reduction in crystallite sizes, increase in coating strain, and enhancement in Sn crystal growth along {011} and {112} planes. Microstructural characterization revealed the presence of Cr at the grain boundaries and formation of Sn and Sn-Cr grains in the coating microstructure. Corrosion measurements conducted using the Tafel polarization and electrochemical impedance spectroscopy revealed that incorporation of minor amounts of Cr leads to significant enhancement in the corrosion resistance property of the coatings when compared to the pristine Sn coating. When compared to the pristine Sn coating, the Sn-1.3 wt% Cr coating exhibited a 40% reduction in the corrosion current density value. The corrosion resistance properties however deteriorated for Sn-Cr coatings with higher Cr content (3.3 and 5.4 wt%). Reduction in the corrosion rate for lower Cr additions was attributed to enhancement in the grain boundary fraction and segregation of Cr to the grain boundaries. Enhancement in the corrosion rate for higher Cr addition was attributed mainly to increase in the coating strain.