The main difference between the passivation of iron and chromium is that the rust layer on iron (a mixture of iron oxides and hydroxides) is permeable to water and air, while the chromium oxide layer on chromium or alloys containing a sufficient amount of chromium is dense and impermeable. For this reason, under normal conditions, the chromium oxide layer on stainless steel is only a few nanometers, but this is enough to isolate the metal from a corrosive environment (water plus air). On the contrary, a layer of rust grows and grows on iron, but because of its friability, it cannot isolate the metal from the corrosive environment.

Dear Vadim,

many thanks for the reply,

Would you please let me know about the criteria determine the permeability of the layer?

I was using Pilling-Bedworth ratio (https://en.wikipedia.org/wiki/Pilling%E2%80%93Bedworth_ratio). If the 1

Dear Ahmad,

The Pilling-Bedworth ratio provides information only on the volume of the growing oxide layer, but says nothing about its chemical composition and properties. Judging by this ratio, the Fe2O3 oxide layer growing on iron should be denser than the FeO layer, but in reality the opposite is true: if high-temperature black scale (FeO) still has some density and strength, then the brown corrosion product of iron in water (Fe2O3) nH2O + FeO(OH), Fe(OH)3) is a loose mass, perfectly permeable to water.