Optimization of conversion coatings based on zinc phosphate on high strength steels, with enhanced barrier properties

•Improved performance of zinc phosphate coatings deposited on high strength steel.•Ultrasound stirring produces more homogeneous and protective phosphate layers.•EIS helps in understanding the phosphate growth process.•Electronic conduction dominates over ionic conduction during the phosphating process.

This paper aims investigating the corrosion protective properties of zinc phosphate coatings deposited on high strength steel (HSS) under different operating conditions. An in-situ electrochemical analysis by impedance spectroscopy was performed during the phosphate layer formation in order to obtain a full understanding of the process and the effect of the different parameters involved. Scanning Electronic Microscopy (SEM) was employed for surface microstructure analysis. The activation time prior coating is a parameter that shall be carefully controlled to avoid the development of low Zn content in the phosphate. Moreover, the use of ultrasonic vibration during the phosphating process promotes modifications in the phosphate crystal growth and, consequently, in the characteristics of the coating. The modified coatings result less porous, with improved corrosion resistance.