Corrosion Behaviour in Low Concentrated Sulfate Solution of Thermal Spray (HVOF) Zinc Coatings on Steel

Abstract

The aim of this work was to evaluate the corrosion behavior of (HVOF) thermally sprayed zinc coatings on steel as compared to plain zinc and to zinc coatings obtained by other techniques. The study was perfomed in a low conductive dilute solution of potassium sulfate (0.01 M). This electrolyte has been chosen recently to give us a better tool in order to control the rugosity of zinc surfaces after mechanical polishing with emery paper of different sizes and to evaluate the porosity of chromated zinc coatings. The purpose in this case is to determine the porosity using measurements in a wide range of frequencies by the relative variation of the resistivity of unchromated and chromated zinc coatings. The determined impedance parameters have been found to be very different for the thermal spray zinc coating in comparison to the other zinc samples. As the charge transfer resistance R t in inversely proportional to the effective surface, the decrease in its value for the thermal coating can be explained by the increase of the active surface. The depression angle in the Nyquist plot for the high frequency loop has also been determined for all the systems and the difference found may be interpreted in terms of the physical properties of the different surfaces. The sacrificial protective characteristics of these coatings towards steel and the results obtained when aluminium was added to zinc by plasma - HVOF and other techniques in order to fabricate a less effective cathode especially towards oxygen reduction will be discussed.