Solving the corrosion behaviour and cobalt content correlation anomaly in electrodeposited Ni–Co coatings by analysis of coating micro-texture, strain and grain boundary constitution

Ni–Co alloy coatings exhibit excellent tribological properties. However, these coatings are vulnerable to corrosion due to the lower corrosion potential of Co when compared to Ni. The decrease in the corrosion resistance of Ni–Co alloy with increasing Co content is not monotonous. At an optimum Co content, very high corrosion resistance behaviour is observed. This anomaly is investigated here. Ni–Co coatings (Ni, Ni–9 wt% Co and Ni–20 wt% Co) were electrodeposited over mild steel. Potentiodynamic polarisation illustrated Ni coatings highest corrosion resistance, whereas, between the Ni–Co coatings, Ni–20 wt% Co coating exhibited higher corrosion resistance in 3.5 wt% NaCl medium. Higher corrosion resistance in Ni–20 wt% Co coating was primarily due to uniform distribution of low energy grain boundaries like Σ3 coincidence site lattices (CSLs) in the coating. Higher corrosion rate for Ni–9 wt% Co coatings was due to strained grains and non-uniform distribution of low energy grain boundaries.