Zn-phosphate conversion coatings developed on high-strength steels at reduced processing temperature

Abstract

The effect of process time, ultrasonic stirring, and the addition of $H_2{\mathrm{PO}}_4^{-}$ on the morphology and corrosion resistance of phosphate conversion coatings was investigated by Scanning Electron Microscopy and electrochemical tests. The phosphating treatments were performed at 50 °C, lower than traditional processes, and pH 2.9 and 2.4. An enhanced film of bigger mass and reduced corrosion rate was produced when the processing time was extended from 6 to 30  min. at pH 2.9. No important changes were identified in the appearance other than a slight crystal coarsening and Zn enrichment. Both the ultrasonic stirring and the addition of $H_2{\mathrm{PO}}_4^{-}$ (pH 2.9, 6 min.) accelerate the coating formation rate and assist in the formation of more uniform layers with a greater Zn concentration. An important crystal refinement was also obtained in the ultrasound-induced treatment, whereas no relevant changes were found for the corrosion behaviour. The addition of $H_2{\mathrm{PO}}_4^{-}$ in the amount of 10 g/L (pH 2.9, 6  min.) facilitates the formation of a layer with a quality like that found in the longest treatment (30 min.). None of the tested variables provided an effective modification to improve the performance of the conversion layers developed for the baths considered in this study at pH 2.4.