We present a continuum-scale scientific model which attempts to capture some of the factors controlling the spreading of the intergranular corrosion in sensitized AA5083 alloy and stainless steels. Stainless steels become sensitized upon carbide formation and associated Cr/Mo depletion at and near boundaries. AA5083 alloys become susceptible to intergranular corrosion upon precipitation of the β phase (Al₃Mg₂) in the grain boundary region upon natural and artificial aging. The intergranular corrosion is initiated in several ways including pitting and/or active dissolution of the β phase. Spreading occurs across surfaces because the potential and concentration fields, i.e. the Ohmic potential drop and buildup of ions, associated with such initiation sites may increase the probability of corrosion of nearby β phase particles, or Cr-depleted boundaries across an alloy surface. The model is developed based on properties governing the electrochemistry of the grain interiors and boundary zones as well as the metallurgy at the micrometer scale and geometry. The inputs to the model, such as distribution of the grain boundary containing continuous or discrete beta phase particles, the electrochemical properties of the beta phase, as well as the grain shape and size have been obtained from experimental data and literature. Spreading depends on the number of sensitized boundaries (level of sensitization) and other factors that can be exercised in the model such as grain size, orientation, and shape. Experiments are performed on sensitized AA 5083 alloys, sensitized at various conditions, under different environmental conditions to validate the results of the model. The role of factors such as the sensitization level, the arrangement and nearest neighbor distance of beta particles, grain shape and size are discussed. The extension of the model to understand the 3-D penetration of IGC into the metals plates using empirical penetration laws and by incorporating scientific growth laws is also proposed.