The passive state of stainless steels

With the advent of surface-sensitive techniques it has become possible to determine the surface composition of alloys. Analyses using electron spectroscopy for chemical analysis of CrNiMo alloyed steels exposed in acids and neutral solutions show that the reaction products formed on the surface consist mainly of chromium oxide and hydroxide. Nickel is not present in the passive films. The molybdenum content in the oxide is of the same order of magnitude as in the alloy. It is suggested that the beneficial effect of molybdenum is not related to the molybdenum content in the oxide. It is shown that the alloy elements nickel, molybdenum and chromium are accumulated in the metal phase underneath the oxide and that the enrichment of chromium seems to be dependent on the molybdenum content of the alloy. The enrichment of these elements is explained by selective dissolution of iron during the active phase preceding passivation. It is postulated that increased surface concentrations of chromium and molybdenum in the metal synergistically lower the dissolution rate in the active phase and provoke the formation of passive films of high chromium content both at first passivation and during repassivation in conditions of pitting and crevice attack.