An approach for the time dependent evolution of the local chemical environment during the initial stage of localised corrosion for aluminium alloys has been developed. The approach was applied to study anodic dissolution of S phase in sodium chloride, where both metallic species, Al3+ and Mg2+, were taken into consideration. The developed model includes hydrolysis products of aluminium and species obtained as a result of homogeneous reactions between chloride and Al3+ and Mg2+ ions and Al hydrolysis products. The model does not assume the equilibrium state in solution: all terms in the homogeneous reactions are treated explicitly using kinetic constants either taken from the literature or derived using the Eigen mechanism for complex formation. It was shown that for an active dissolution process, the time needed for chemical species to achieve their steady state concentrations ranges from minutes (for solution potential and the pH value) to hours or even days. Thus, modelling predictions obtained with the steady state approximation can give inaccurate values for species concentrations. Two modelling examples are discussed: the dissolution of a large S phase and small isolated S phase particle in Al alloys. Input data for the models are supplied by microcapillary measurements.