Effect of Phosphorous Content on the Corrosion Behavior of Trivalent Chromium Coatings

Effect of hypophosphite anion on morphology, deposition rate, composition, and corrosion behavior of trivalent chromium coatings, as a complexing agent in trivalent chromium electrolyte, was investigated. For this purpose, the concentration of hypophosphite anion in trivalent chromium electrolyte was changed from 0-0.75 M. Morphology and composition of the coatings were evaluated by using scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). Corrosion behavior of the coatings was investigated using Tafel extrapolation method and electrochemical impedance spectrometry (EIS) technique in 3.5 wt% NaCl solution. Results show that the deposition rate was decreased by addition of hypophosphite anion to the bath. However, further increasing of its concentration to 0.525 M, results in increased deposition rate. Moreover, the incorporation of phosphorous in the trivalent chromium coatings caused higher micro cracks density and increase in porosity, and thus the corrosion resistances of these coatings were decreased.