The pitting behavior of a range of iron‐chromium thin film alloys which are sulfide‐free has been studied by electro‐chemical methods coupled with in situ optical microscopy. In 0.5 M HCl, iron‐chromium alloys with > ∼16 atom % chromium do not pit at any potential, whereas alloys with < ∼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 and simultaneous hydrogen evolution.