Kinetics inversion in isothermal oxidation of uncoated WC-based carbides between 450 and 800 °C

Oxidation of uncoated WC-based carbides (hard metals) has been investigated between 450 and 800 °C. An anomalous change in the oxidation behaviour has been found in the temperature range between 528 and 630 °C. Instead of the normal increase of the oxidation rates when the temperature increases, a kinetics inversion is produced: an anomalous decrease of the oxidation rates has been observed in this temperature range. Furthermore, this anomalous behaviour produces an important change in the activation energy. At temperatures below the anomaly (below 528 °C), an activation energy of 119 ± 8 kJ/mol has been found, whereas above the anomaly (above 630 °C), an activation energy of 208 ± 8 kJ/mol has been found. This anomalous temperature dependence of the oxidation behaviour can be related to a higher amount of complex oxide CoWO4 formed on the oxide scale, and to the effect that this oxide produces in the oxidation kinetics of the other oxides.