A simple testing method for quality control of phosphate coatings based on impedance measurements

A simple technique for quick quality control of phosphate layers based on impedance measurement has been developed. The coverage of iron surfaces by insulating phosphate crystals is the most important measure of the corrosion protection ability of the layers. From the detailed analysis of the measured impedance we were able to determine the surface coverage which was found to be in very good agreement with the results of SEM (scanning electron microscopy) and WDS (wavelength dispersive spectroscopy) investigations. The results provided by the impedance analysis are reliable but for industrial quality control simpler methods are needed. Our purpose was to provide a fast, objective procedure characterizing the corrosion protection ability that is easily applicable in industry. In the present paper we propose a method based on electrode impedance measurements of phosphated steel surfaces but, instead of measuring the impedance in a wide frequency range, we only use one appropriate frequency and the imaginary part of the impedance is recorded.