Poly-phosphated Galvannealed Steel Sheets for Auto Application

Abstract A hydroxylated ester based phosphate (i.e. poly-phosphate) coating has been developed and addressed in this paper. Poly-phosphate coating obtained on galvannealed steel sheet surface was found dense, uniform with coating weight up to 1.2 to 1.7 g/m2. The porosity found in the coating was 0.5%. The stretcheability and corrosion resistant properties of the poly-phosphate coated material were found better than that of rust preventive oiled (RPO) steel sheet of surface oil content 250 mg/m2. Polyphosphate coating showing better corrosion resistance compared to RPO in Humidity cycle test. AC impedance results revealed that poly-phosphate coating is resisting diffusion of water/ions into the substrate through coating. It was evidenced by showing constant coating capacitance from Bode plot. In case of RPO, coating capacitance increases at the elapse of immersion time indicating that coating is degraded. Poly-phosphate coating could be employed as a temporary corrosion resistant layer instead of rust preventive oil to the galvannealed steel sheets during transportation and storage. The major advantage of polyphosphate coated galvannealed steel sheets is that it can be rephosphated and painted at automotive industries without degreasing. The rephosphatability and paintability results of polyphosphated steel sheets are found to be excellent at automotive industries.