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. © 2002 Elsevier Science B.V. All rights reserved.