Optimización del tratamiento de fosfatado como méto do de protección frente a la corrosión

This paper evaluates the phosphate treatment as a chemical conversion treatment (CCT) free of chromium, from the basis of the study of a commercial treatment applied in phosphoric solution at different concentrations (100% and dilutions 1:2, 1:5 and 1:10). The optimum immersion time for the phosphate conversion treatment was established from the evolution over time of the open circuit potential. Protection against corrosion of the conversion layers was evaluated by polarisation resistance and electrochemical impedance measurements. The adhesion to the substrate was evaluated by pull-off tests. Results have shown that the application of an organic coating on the substrate with a previous phosphate treatment in 1:5 dilution (10 min.) increases the corrosion resistance in, at least, one order of magnitude with respect to the non phosphate-coated metal and improves the adhesion to the substrate, since the conversion layer obtained is part of the substrate and the surface generated increases the number and area of active sites.