Linking electrochemical corrosion behaviour and corrosion mechanisms of thermal spray cermet coatings (WC–CrNi and WC/CrC–CoCr)

In this experimental study the corrosion characteristics (rates and mechanisms) of two thermally-sprayed cermet (ceramic/metal composite) coatings have been examined in a static saline environment. The coatings have complex microstructures where the ceramic hard-phase constituents are embedded in a metallic matrix. Their electrochemical response during accelerated corrosion DC polarisation tests reflects this complexity. A test protocol involving potentiostatic and potentiodynamic tests has been devised which enables the mechanisms of corrosion occurring on the WC/CrC–CoCr cermet and the WC–CrNi cermet to be compared and contrasted. A ‘critical’ temperature has been defined for the WC–CrNi coating similar to the critical pitting temperature defined for stainless steels. Electrochemical behaviour, which can not be simply described as truly ‘passive’ behavior, is displayed by the WC/CrC–CoCr coating and such a well-defined ‘critical’ temperature is not apparent. The contrasting mechanisms of corrosion on the two coatings, identified during post-test examination, can be correlated to the contrasting electrochemical response.