A Replacement for Phosphate Conversion Coating Based on Hexafluorozirconic Acid

Surface pretreatments based on dilute hexafluorozirconic acid (FZ) solution were evaluated as replacements for the phosphating process before paint application. The behavior of a FZ coating was compared to those of a modified FZ (MFZ) coating and phosphating treatments. Results of electrochemical tests on painted cold rolled steel (CRS) samples with different conversion coatings show that the MFZ surface treatment in combination with various paints provides corrosion protection performance comparable to phosphate conversion coatings. AFM studies of MFZ coatings on CRS reveal that the coating surface exhibits small features tens of nm in size and clusters of these features that are on the scale of microns. Clusters have lower surface potentials (higher activity). Z-contrast TEM images of MFZ coatings show that the coating is about 20 nm thick, continuous and adherent to the substrate. Major components are Zr, Fe and O; the Fe amount decreases toward the coating surface. The MFZ pretreatment is environment-friendly as the treatment solution is phosphate-free and dilute. Furthermore, the treatment involves simple immersion at room temperature conditions and there is no need of chromic acid sealing after the treatment.