Effect of electrodeposition temperature on texture, strain, grain boundary constitution and corrosion behavior of Ni-P coatings with low phosphorous content

Abstract

Nickel-phosphorus (Ni-P) alloy coatings were electrodeposited over mild steel substrate from a constant current DC power source. Effect of deposition bath temperature (15 ˚C, 20 ˚C, 25 ˚C, 35 ˚C) on the coating corrosion behavior in 3.5 wt.% NaCl solution was investigated. Corrosion analysis was conducted using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The 15 ˚C coating showed lowest polarization resistance (Rp = 2818.52 Ω.cm2) value indicating degraded corrosion resistance, whereas the 25 ˚C coating showed highest polarization resistance (Rp = 6992.285 Ω.cm2) value implying highest corrosion resistance performance amongst all coatings. This was also confirmed by the potentiodynamic polarization measurement where the 15 °C coatings and 25 °C coating showed respectively the highest and lowest corrosion current density (Icorr) values. Icorr values of 15 °C and 25 °C coating was 7.79 μA.cm−2 and 1.534 μA.cm−2, respectively. Analysis of the coating texture, strain and grain boundary constitution by the electron back scatter diffraction (EBSD) technique showed that the predominance of (101) high energy texture, lower fraction of low angle grain boundaries (LAGBs), and higher coating strain yielded high vulnerability to the corrosive attack for 15 ˚C coating, while (001) low energy growth texture with a relatively higher fraction of LAGBs and lower coating strain led to improved corrosion resistance in 25 °C coating.