Effect of porosity of phosphate coating on corrosion resistance of galvanized and phosphated steels Part I: Measurement of porosity of phosphate

摘要

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 electro galvanized steel. The phosphate layer obtained by immersion with a refiner displayed the lowest porosity on the substrate of ungalvanized steel.