Using graph theory, it is shown that the introduction of 70 atom % of Fe+3 into a -Cr-O-Cr- oxide network serves to disjoin the network, or conversely, introduction of 30% atomic concentration of Cr+3 serves to provide a connected -Cr-O-Cr- network. The results of surface analysis show that this concentration of 30 atom % Cr+3 in the oxide film corresponds to 13 atom % chromium in the Fe-Cr alloy, i.e., the critical concentration of Cr required for passivity in Fe-Cr binary alloys.