Electrochemical properties and Morphology evaluation of the Lanthanum-based Conversion Coating on Cold rolled steel

In this research, conversion coating was applied on cold rolled steel by immersing specimens in a solution including La(NO3)3, and the effect of immersion timehas been investigated. Electrochemical properties of the formed oxide layer were assessed by Electrochemical Impedance Spectroscopy (EIS) and DC Polarization measurements. The morphology of the surface of cold rolled steel treated by conversion coating at different immersion times was studied by Field Emission Scanning Electron Microscopy (FE-SEM). The EIS and SEM results showed that better corrosion resistance (2.5 kΩ.cm2) along with a more uniform surface profile was achieved after 5 min immersion in the conversion coating bath. On the other hand, the coatings, which were formed by immersion in rare-earth salt containing solution, significantly reduced the dissolution of La oxides and deteriorated the substrate after 50 min. Adhesion of epoxy organic coating to the untreated and La conversion-coated CRS was evaluated by pull-off method and results revealed the positive performance of La conversion coating.