Comparative study of Cr3C2–NiCr coatings obtained by HVOF and hard chromium coatings

In the present work the corrosion resistance of micro-cracked hard chromium and Cr3C2–NiCr (HVOF) coatings applied on a steel substrate have been compared using open-circuit potential (EOC) measurements, electrochemical impedance spectroscopy (EIS) and polarization curves. The coatings surfaces and cross-section were characterized before and after corrosion tests using optical microscopy (OM) and scanning electron microscopy (SEM). After 18 h of immersion, the open-circuit potential values were around −0.50 and −0.25 V/(Ag∣AgCl∣KClsat) for hard chromium and Cr3C2–NiCr, respectively. The surface analysis done after 12 h of immersion showed iron on the hard chromium surface inside/near surface cracks, while iron was not detected on the Cr3C2–NiCr surface even after 18 h. For longer immersion time hard chromium was more degraded than thermal sprayed coating. For hard chromium coating a total resistance values between 50 and 80 kΩ cm2 were measured and two well-defined time constants were observed, without significant change with the immersion time. For Cr3C2–NiCr coating the total impedance diminished from around 750 to 25 kΩ cm2 as the immersion time increased from 17 up to 132 h and two overlapped time constants were also observed. Polarization curves recorded after 18 h of immersion showed a lower current and higher corrosion potential for Cr3C2–NiCr coating than other samples studied.