Characterization of the electrochemical process responsible for the free radical release in hard metals

WC–Co mixtures, widely used in industry, have been intensively investigated because of their noxious effects on human health when the powder is inhaled. A particular chemical reaction, taking place in the presence of both WC and Co powders has been claimed to contribute to the development of the lung diseases. Activated oxygen species (AOS) are produced through the reduction of oxygen in presence of the WC–Co particles. It has been demonstrated that this spontaneous process involves the oxidation of Co and the reduction of oxygen catalysed by WC via anelectrochemicalredoxreaction,beingCotheanodeandWCthecathode.Theelectrolyteforthisprocess,takingplaceinthedistalzoneofthe lungs, is a wet layer distributed on the powder surface by the humid ambient of the lungs. Being the electrodes short-circuited spontaneously, the Co oxidizes and the oxygen dissolved in the wet layer reduces at a high rate being catalysed by WC. In our investigation, we have focussed our attention to the electrochemical characteristics of the process and pointed out that the spontaneous process may take place at an appreciable rate only when the WC surface is covered by a thin layer of electrolyte, so maximising the diffusion of O2 through the solution. A voltaic cell corresponding to the overall spontaneous electrochemical process has also been assembled and discharged at constant current. It hasbeenexperimentallydeterminedthatinthesamevoltaiccell,duringdischarge,speciesareproducedwhichcanreactwithgluthatione(GSH), a component of the antioxidant system protecting cells, in the same way as the AOS produced by the industrial WC–Co mixture.