Phosphate Coatings Porosity: Review of New Approaches

The value of a phosphate coating's porosity varies between 0.1 and 25%, depending on process details and the method of determination. In the case of electrochemical methods, low amplitude or other low invasive techniques are recommended. The assumption that the mechanism of electrochemical/chemical reaction in pores of the phosphate coating and bare metal is the same is often not verified. The free surface at the bottom of the pores was found to be modified in pretreatment and post-treatment operations and in some cases was found to be more active than the clean bare metal surface. The reference sample should be at least treated in diluted phosphoric acid that has the same free and total acidity as the phosphating bath.