Polarized light was used to measure the thickness and optical properties of passive films as they were forming while simultaneous electrochemical measurements were being made. The film formation studies showed that a change in film thickness of 15–20Aå was associated with the onset of a passive potential. A direct, logarithmic‐type law governed the growth of a semiconductor film with crystallographic orientation of the substrate having little effect. When iron surfaces prepared in an ultrahigh vacuum (a vacuum of the 10 −9 Torr order of magnitude), were passivated a 20Aå film formed instantaneously on the introduction of an air‐saturated 0.1 N NaNO 2 . This film then dissolved and a new film commenced to grow. A passive potential coincided with a film 15Aå thick. No such initial dissolution occurred when the solution contained only dissolved oxygen, the instantaneously formed film continuing to grow and a passive potential occurring after the growth of around 40Aå. When the solution contained no dissolved gases, the instantaneous film did not form. Film breakdown studies revealed that only the 20–30Aå film next to the metal was responsible for passivity even if thicker films were initially present. Studies of the sites of film breakdown showed that the number of sites was highest for films on {110} iron surfaces and that these sites were not related to points where dislocations intersected the surface. These studies support the oxide film theory of passivity.
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J. Krüger (1963) studied this question.