Annealing temperature effect on the corrosion parameters of autocatalytically produced Ni–P and Ni–P–Al2O3 coatings in artificial seawater

Ni–P and Ni–P–Al2O3 amorphous alloy coatings with 9.3 and 8.3 wt.% P respectively were obtained by autocatalytic deposition at 90 °C on carbon steel substrates. The effect of annealing temperature (100, 200, 300, 400 and 500 °C) upon the corrosion parameters of the coatings in artificial seawater with pH 5.0 and 8.1 at room temperature was evaluated by potentiodynamic polarisation and electrochemical impedance spectroscopy. It was found that deposits annealed at 400 and 500 °C presented an increase of the charge transfer resistance and negligible changes on samples annealed at lower temperature. Polarisation tests showed a charge transfer controlled anodic kinetics on both Ni–P and Ni–P–Al2O3 deposits and diffusion controlled cathodic reaction in artificial seawater at pH 5.0 and 8.1. The coatings did not present passive behaviour in the electrolytes and impedance measurements showed a single time constant for all cases with the lowest double layer capacitance (Cdl) for samples annealed at 400 and 500 °C. The best corrosion parameters were observed on Ni–P and Ni–P–Al2O3 coatings annealed at temperatures higher than 400 °C, which is the temperature where crystallisation of this kind of coatings takes place.