Study of conversion coatings obtained from tungstate-phosphoric acid solutions

Conversion coatings on zinc surfaces (Zn and electrogalvanized steel) were obtained with tungstate baths acidified by phosphoric acid under different experimental conditions. Open circuit potential and local pHs were monitored during the conversion processes. The conversion layers were submitted to immersion tests monitored by open circuit potential and electrochemical impedance spectroscopy. Scanning electron microscopy and energy dispersive X-ray complemented the electrochemical measures. The local pH measurements have evidenced that the conversing process occurs at a specific value of pH. Electrochemical impedance has detected similarities between the deterioration process of tungstate and chromate conversed zinc surfaces.