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Characterization of zinc phosphate coatings: influence of the pH and temperature inmorphology and corrosion resistance

Zinc phosphate coatings are commonly used to protect high-strength steel rods and improve their corrosion protection ability. The temperature and the pH of the phosphating bath are important parameters that affect the film’s appearance, porosity, and composition. In this work, several variations in the phosphating solution have been analyzed. In particular, the bath temperature was modified in the range of 50-75 °C, while the bath pH kept at 2.4, 2.8, and 3. Phosphate coatings were investigated via Scanning Electron Microscopy and Energy Dispersive X-ray that allow a complete surface analysis, including both the morphology and the composition. The coatings mass was determined by the gravimetric method. For the corrosion resistance of the film, the linear polarization curves obtained in 0.1 M Na2SO4 solution were analyzed. The results showed that the phosphating baths at 60-65ºC and pH 2.4 produced a thicker film, with the highest amount of Zn and improved corrosion resistance.

Keywords: Keywords: phosphate; corrosion; SEM