The corrosion resistance of phosphated and painted steels is associated with the integrity of phosphate and paint layers. In this work, a methodology to measure the porosity of phosphated coatings based on cathodic polarization technique with an electrolyte of a sodium sulfate aqueous solution was developed. The morphology and mass of phosphate layers were determined. Phosphating was performed using the spraying and immersion techniques, varying the refiner content of bath in the immersion operation. The phosphate coatings obtained by immersion with a refiner showed the lowest values of porosities among the samples studied. The phosphate layers, obtained by spraying, immersion without a refiner, and immersion with a half content of refiner, showed the lowest porosity on electrogalvanized steel. The phosphate layer obtained by immersion with a refiner displayed the lowest porosity on the substrate of ungalvanized steel.
The effect of porosity of phosphate coatings on the corrosion resistance of ungalvanized (UG), electrogalvanized, and hot dip galvanized steels is evaluated in this study. The corrosion resistance of phosphatized and painted steel is related to the integrity and continuity of phosphate and paint layers, and pores in the phosphate layer affect the corrosion resistance of material. The porosity of the phosphate coating was evaluated by using the cathodic polarization electrochemical test. To evaluate the corrosion resistance of the phosphatized and painted steels, they were submitted to accelerated corrosion tests. As was expected, the creepback from the scribe increased with the increase in porosity. This behavior was evident for UG steel, but less evident for galvanized steels due to cathodic protection and/or barrier effect of the zinc coating.
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