Electrochemical corrosion behavior of chromium–phosphorus coatings electrodeposited from trivalent chromium baths

Chromium–phosphorus (Cr–P) coatings are electrodeposited from trivalent Cr (Cr(III)) baths containing hypophosphite. The electrochemical corrosion behavior of Cr–P coatings, traditional Cr coatings deposited in hexavalent Cr (Cr(VI)) baths, and chromium–carbon (Cr–C) coatings deposited in Cr(III) baths containing formate are studied by measuring potentiodynamic polarization curves in a 10 wt% HCl solution. The composition and morphology of the coating surface layers are investigated by X-ray photoelectron spectrometry (XPS) and scanning electron microscopy (SEM), respectively. The results of electrochemical tests show that Cr–P coatings exhibit better corrosion resistance than traditional Cr and Cr–C coatings, which is characterized by a lower critical current density, lower passive current density, and lager passive potential range. XPS and SEM analyses confirm that the excellent corrosion resistance of Cr–P coatings is attributed to the formation of a phosphide passive film, which has high stability and self-repairing ability, and can act as a “buffer” to reject the penetration of chloride ions.