Corrosion behavior of lanthanum-based conversion coating modified with citric acid on hot dip galvanized steel in aerated 1 M NaCl solution

Rare earth conversion coating is one of the most promising substitutes to the toxic chromate coating. The corrosion resistance of lanthanum conversion coating modified with citric acid on hot dip galvanized (HDG) steel was investigated in aerated 1 M NaCl solution by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. Equivalent circuits were subsequently developed from the measurements to elucidate the corrosion behavior of the coating. The surface morphology of the lanthanum conversion coating was observed by scanning electron microscopy (SEM), and the chemical composition of the coating was characterized by energy dispersive spectroscopy (EDS). The results showed that the corrosion process of the modified lanthanum conversion coating consisted of three stages. The overall corrosion resistance of the coating was excellent.

Research highlights
► The corrosion behavior of lanthanum conversion coating modified with citric acid on hot dip galvanized (HDG) steel was investigated in aerated 1 M NaCl solution.
► The La salt conversion coatings inhibit both the anodic and cathodic responses of the system and hence prevent the HDG substrate from corrosion.
► The corrosion process of the coating can be divided into three stages. The outer layer of the coating was destroyed at the initial stage of immersion (<18 h), then the inner layer was damaged at the second stage (18–144 h), and the corrosion of the substrate played a dominant role at the finial stage (>144 h).