The corrosion behaviour of the plasma sprayed AlCoCrFeNi HEA coating was analysed using the potentiodynamic-polarization method in filtered sea water. The polarization curve demonstrates critical information such as corrosion potential(Ecorr), current density(corr) and pitting potential(Epit). which are important parameters for investigating the corrosion behaviour of a material.The polarization curves of the AlCoCrFeNi coating and SS316L substrate were measured at 1 mV/s scan rate in sea water at room temperature, Fig.1. Neither the HEA coating, nor the substrate established an active to passive transition zone, and the curves transformed directly from the Tafel region to the passive region.This behaviour could be due to the high scan rate; however, this characteristic may also be attributed to the instantaneous formation of a protective film on the sample surface at the corrosion potential. Shi et al. reported a similar polarization curve characteristic while analysing the electrochemical performance of cast AlxCoCrFeNi HEAs in 3.5 wt% NaCI solution.
The quantification of resistance to pitting/localized corrosion is given by the critical pitting potential(Eoit), which is the potential corresponding to a transition in the current density from steady toa rapid increment. Beyond Eoit. faster diffusion of CI ions takes place on the coating surface through defects or corrosion prone phases and reduces the effect of the passivation film; resulting in localized corrosion. APS AICoCrFeNi HEA coating exhibited a gradual increase in current density without any indication of a critical potential.However, current fluctuations were noticed in the HEA coating, shown by blue arrow in Fig.1, which might be ascribed to the development and repassivation of metastable pits or selective corrosion . On the contrary,a rapid increase in current density for SS316L substrate was observed, which represents the stabilization of pitting.
Fig.1. Potentiodynamic polarization curves of both AlCoCrFeNi HEA coating and S5316L substrate with scan rate 1 mV/s in the sea water at room temperature. Green box represents the Tafel area and current fluctuation within HEA coating is shown byblue arrow.(For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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