Traditional forecasts of steel corrosion in concrete can be markedly inadequate if based on chloride threshold (CT) values that do not evolve with structure age. Previous work that considered the increase in CT due to macrocell-induced steel potential drop around newly corroded steel zones showed that the traditional forecast can be much too conservative at long service ages, while insufficiently so at early ages. In that work, CT was simplified as a single function of steel potential. Here we take initial steps to consider not only that local (intrinsic) effect, but also the (extrinsic) effect from the macrocell electric fields on the distribution of chloride and hydroxide ions. Those factors are implemented in a tentative finite element multiphysics time evolution model, coupled with electrode reactions at the steel surface. Model viability is assessed. The ultimate goal is to produce damage functions to serve as a powerful tool for deciding on alternative corrosion control approaches.