A model for corrosion crack nucleation and growth is presented, where the corrosion forms the geometry of the crack tip, thus creating the conditions for strain concentration. The interaction between electro- chemical processes and the deformation of the crack tip region is incorporated in a continuum mechanical theory. No crack growth criterion is used. The formation of a crack from a surface depression via a pit is studied. Low frequency cyclic load is considered. At the end of a load cycle a metal oxide compound is growing on the crack surface. It is assumed that there is sufficient time for the chemical process to form a protective film that fully covers the crack surface. This temporarily interrupts the corrosion process. During the application of next load cycle the stretch of the surface breaks the protective film. This creates gaps in the film, which allow dissolution of the uncovered metal. The chemical environment of the crack tip is assumed to be constant and unaffected by the changing geometry as the crack is developing. This leads to a linear relationship between strain and corrosion rate, in the sense of removed material per unit of area during each load cycle. The model simulates how pits evolve to become cracks and how cracks then propagate in one continuous process. Mathematical and finite element analyses of stationary cracks with appropriate geometry arc involved to explain the behaviour predicted by the model.