Variation with time of the chemically active three-phase boundary, due to the spontaneous sintering of metal particles, is simulated with the help of a correlated percolation model. Different modes of variation are obtained depending on the relative composition and porosity and the conditions of sintering. They range from a moderate degradation (when the three-phase boundary still penetrates through the whole volume of the anode) to a catastrophic degradation (when the three-phase boundary is localized within a thin layer near the current collector). More specific situations, including non-monotonic variation, are also revealed, as well as a rare case where the three-phase boundary increases when the sintering leads to the opening of new pores. Optimization of composition for a stable anode performance is discussed.