I would start from examining scientific articles. There are several papers on this topic where different approaches to pitting corrosion modeling are used. Most authors simply model already developed pits as holes in the specimen. Then they evaluate the reduction in strength, stiffness or fatigue life due to these holes. Some however introduce complex damage models via user material subroutines to account for actual growth of pits and perform fracture mechanics simulations.
To extract stresses and strains you can either request them as history output for specified set or export in postprocessing (using Report —> Field Output or Query —> Probe Values).