1. Experiments of Francis and Lees
Recently, Francis and Lees [1] reported on the growth of iron oxide at 890 Β°C on pure iron disks held rigidly during the oxidation. In one type of experiment, a thin (1.7 mm) disk specimen was oxidized on one face under 200 Torr 02 (in argon) gas while the back face of the coupon was protected from oxidation by exposure to an Ar-H mixture. In this arrangement, the growth of a single (normal three-layered) oxide scale resulted in a movement of the unoxidized back face of the coupon toward the oxidized face, after the scale had reached some given thickness. The interpretation of Francis and Lees for this advance of the back face toward the oxidized face was that, during oxidation, vacancies diffusing in a counter direction to the cation flux in the scale are injected into the metal at the metal-scale interface. They proposed that these vacancies then diffuse down a gradient in vacancy supersaturation to the opposite metal-gas interface and vacancy annihilation then occurs at this free surface. This suggested mechanism is identical to that previously proposed by Dunnington et al. [2], Tylecote and Mitchell [3], and others cited in ref.
- According to this mechanism, the oxide-metal interface is not a good sink for vacancies, which causes the vacancy injection into the metal. However, when such an experiment was performed for an iron disk with an initial thickness greater than 2.9 ram, no displacement of the back face was detected. Supposedly, for the thicker disk, the vacancy sink at the free surface was too