Texture and grain boundary engineering in electrodeposited SnCu coatings and its effect on coating corrosion behaviour

Abstract

ABSTRACT SnCu coatings (Cu between 0–33.0 at.%) were electrodeposited over mild steel. Corrosion analysis was performed over mirror polished SnCu coatings to remove surface morphology effect. Highest corrosion resistance was observed for the SnCu coating with ∼21.2 at.% Cu and lowest corrosion resistance was observed for the SnCu coating with ∼9.9 at.% Cu. Sn-rich phase and Cu6Sn5 intermetallic were present in all the coatings. For lower Cu content (∼6.0 at.% Cu), a significant fraction of Cu was found to be incorporated into the Sn rich solid solution phase, thus increasing strain within the grains. As the Cu content increased, the Cu6Sn5 phase formed predominantly at the Sn-rich phase grain boundaries. For the Sn rich phase, a preferred crystallographic orientation along the (001) was observed in SnCu coating with lower Cu content (Cu < 10 at.%). On the other hand, at high Cu content (Cu > 20 at.%), the low surface energy (100) texture was dominant. Addition of Cu decreased the Sn-rich phase grain size. The SnCu coating with ∼21.2 at.% Cu exhibited highest fraction of low angle grain boundaries (LAGBs) and low energy twin boundaries. It was established that the presence of low surface free energy (100) atomic planes exposed to corrosive medium, a high fraction of LAGBs, and low energy twin boundaries yields high corrosion resistance for SnCu coating with 21.2 at.% Cu, while a strong (001) texture and high strain within the grains, increased the corrosion susceptibility of the SnCu coating with ∼9.9 at.% Cu.