Amount of Oxygen on the Surface of Passive Stainless Steel

The amount of oxygen on the surface of 18‐8 stainless steel was obtained by pickling the alloy with , washing with nitrogen‐saturated water, then exposing the metal to water of known dissolved oxygen content. The decrease in dissolved oxygen for a definite period of exposure was a measure of the oxygen taken up by the 18‐8 surface. The take‐up of oxygen was rapid the first few minutes, then proceeded slowly, reaching a maximum, after 6 hr, of 1.1 microgram/apparent cm 2 of metal surface. No further oxygen was consumed up to a maximum exposure of 48 hr. The alloy attained maximum passivity after about 3 hr in aerated water, as shown by potential measurements. When 18‐8 was pickled with , the oxygen consumed was only 0.33 μg/apparent cm 2 of surface. This smaller value is related to the decreased true surface area of 18‐8 so pickled ( ) compared with the pickle ( ). The maximum amount of oxygen on passive 18‐8 based on so‐called true surface area is 0.27 μg/cm 2 . This value is equivalent to a supposed film only 17 Å thick. On the other hand, an equivalent adsorbed oxygen film consists approximately of 1 layer of atoms over which a close‐packed layer of oxygen molecules is adsorbed. The data are not inconsistent, therefore, with the adsorbed film theory of passivity, and, accordingly, the mechanism of passivation by oxygen need not be considered different from that produced by carbon monoxide in hydrochloric acid saturated with this gas, where the adsorption theory heretofore has provided the only plausible explanation. The point of view is proposed that oxide films previously isolated from stainless steels by several investigators were produced by reaction of the adsorbed film with the metal, the reaction being induced by polishing and abrading, presence of halogen ions and water in the stripping solution, or by high temperatures. Such oxide films, however, are considered to be of secondary importance in accounting for passivity as measured by corrosion resistance and by a noble potential.