Surface spreading of intergranular corrosion (IGC) on Al-4.4Mg alloys in NaCl solution was modeled at the micrometer scale, using a continuum granular scale model as a function of the degree of sensitization (DoS). The model utilizes electrochemical, metallurgical, and physical inputs. Al-4.4Mg alloys become susceptible to IGC after precipitation of highly anodic beta-phase precipitates along the grain boundaries. In addition, cathodic constituent particles form a galvanic couple with the matrix. Local corrosion is triggered at anodic beta-phase precipitates proximate to the constituent particles, which subsequently triggers the corrosion of un-attacked beta-phase precipitates often aligned on grain boundaries. The fraction of surface intergranular sites corroded and intergranular surface cluster length increased sharply at a certain degree of sensitization. Specifically, IGC spreading was extensive when the DoS level was greater than 24 mg/cm(2) and was linked to a decrease in the spacing between the nearest beta-phase precipitates below a threshold distance. (C) 2013 Elsevier Ltd. All rights reserved.