CMSX-4 and IN738LC, industrial gas turbine (IGT) aerofoil alloys with low (6.5 wt%) and high (16 wt%) chromium contents respectively, were exposed to type II hot corrosion conditions (700 °C, SOX in air, alkali sulphate/chloride deposits) for up to 1000 h. By comparing detailed pre- and post-exposure dimensional metrology from the samples, the fraction of each sample's surface undergoing the incubation stage (lower levels of sample damage due to the presence of a protective scale) or the propagation stage (higher levels of sample damage due to the direct attack of the base alloy) of type II hot corrosion damage was determined. Corrosion pit development under type II conditions may be described as an extreme event. As such, the transition from incubation to propagation may be modelled using Weibull statistics, which were found to give a good fit to the spread of incubation lifetimes in the exposed IN738LC samples. Much shorter incubation lifetimes were found for the lower chromium content alloy (CMSX-4). Propagation rates for CMSX-4 were found not to be constant over time. Instead, when exposed to the higher deposition fluxes, the propagation rates fell after long exposure times as the resultant deposit/corrosion product inhibited the transportation of reactive species. This improved, quantitative understanding of the transition from incubation to propagation damage under type II hot corrosion conditions will assist in the development of quantitative hot corrosion damage models, enhancing IGT component lifetime prediction.