CORROSION OF LEAD AND LEAD ALLOYS - INFLUENCE OF THE ACTIVE MASS AND OF THE POLARIZATION CONDITIONS

The influence of the active mass, as well as of discontinuous polarization (100 h at 6 mA cm(-2); 68 h at open circuit) on the corrosion rate of lead-antimony alloys has been measured with respect to samples without active mass and continuous polarization. During continuous polarizations two time regions have been found for the corrosion rate (v): during the first 75 h, the corrosion rate decreases according to v similar to 1/t(0.5) and then is constant during the remaining second period. An activemass protection factor (PFAM) describes the decrease of the corrosion rate in the presence of active mass; PE(AM) is > 2. Surprisingly, such a protection factor is also found in the presence of electronically isolating alpha-Al2O3. The influence of changes in potential, current density, pH and corrosion structure on the corrosion rate in the presence of active mass is discussed. The main influence comes from changes in the corrosion structure. A model is developed on the basis of diffusion of oxygen species through a dense corrosion layer. This corrosion layer grows to a critical thickness. After reaching this critical thickness, the layer divides into a inner dense and an outer porous zone as a result of relaxation in the internal mechanical stress that is created by the higher volume of the corrosion product. This model explains the time course of the corrosion rate, the action of the active mass and the influence of rest periods during polarization.