Study of a chromium-free treatment on Hot-Dip Galvanized steel: Electrochemical behaviour and performance in a saline medium

This paper focuses on the physicochemical characterization and on the electrochemical behaviour of a Cr-free treatment applied on Hot-Dip Galvanized (HDG) steel. The treatment is composed of fluorotitanic acid, manganese phosphate, phosphoric acid and an organic compound. Effects of the two major constituents of the solution (i.e. the organic phase and manganese phosphate) on the homogeneity, defects, electrochemical behaviour and composition of the conversion layer were investigated. For this purpose, electrochemical tests were performed in a 3.5% NaCl medium for HDG samples treated with the standard version of the Cr-free treatment or with derivative versions without either manganese phosphate or the organic compound. The conversion coating obtained with the standard Cr-free solution leads to a significant decrease in the corrosion current density (icorr). On the basis of polarization curves and electrochemical impedance spectroscopy (EIS) analyses, it was shown that it also limits oxygen diffusion to the HDG substrate. In the absence of either manganese phosphate or the organic compound, the surface morphology is noticeably different. Depth profiling analyses using Glow Discharge Optical Emission Spectroscopy (GDOES) indicate the formation of thinner layers when compared with the standard-treated sample. As a consequence, the performance of the conversion layer is significantly affected.