Structures and characteristics of Cr(III)-based conversion coatings on electrogalvanized steels

Trivalent chromium (Cr(III)) conversion coatings have been made on electrogalvanized steels in the solutions containing Cr(III) sulfate, nitrate, and chloride among which the sulfate, nitrate, and chloride anions function as passivating, oxidizing, and pitting agents, respectively. The structure and properties of the Cr(III) conversion coatings were examined by electrochemical methods and surface morphology observations. Experimental results showed that the Cr(III) coating formed in the sulfate bath displayed a thin, compact, single-layered structure, before and after baking. The conversion coating formed in the nitrate bath consisted of two porous layers for both the as-deposited and baked states. Moreover, the coating prepared in the chloride bath exhibited a porous, single-layered structure regardless of the baking treatment. Electrochemical impedance analyses further showed that the anions in the bath strongly influenced the structure and corrosion resistances of the Cr(III) coatings.