The pitting behavior of a range of iron-chromium thin film alloys which are sulfide-free has been studied by electrochemical methods coupled with in situ optical microscopy. In 0.5 M HCl, iron-chromium alloys with > similar to 16 atom % chromium do not pit at any potential, whereas alloys with < similar to 16 atom % chromium pit at approximately +500 mV standard calomel electrode. The sharp transition is believed to be associated with the existence of a critical size of iron cluster which act as pit nucleation sites. This is modeled by the achievement of a high density percolation condition for iron in the body-centered cubic alloy. The pits propagate under remnants of the passive film as two-dimensional disks, with current densities up to 80 A cm(-2) and simultaneous hydrogen evolution.