Investigation of Carbon Steel Passivation Behavior in Deionized Water by Ellipsometry

Corrosion and passivation behavior of carbon steel in deionized water with or without dissolved oxygen (DO) was investigated by ellipsometry. It was found that a 4 to 5 nm thick passive film, whose refractive index was N2=3.0−0.4i, was formed on the surface of carbon steel in oxygen containing flowing water. The film thickness was smaller with higher DO concentration and larger flowing velocity. In stagnant DO containing water, the passive film was not stable and small corrosion pits occurred. In deaerated water, the passive film disappeared in a relatively short time and another type of film grew. The characteristics of the film formed in deaerated system was different from that of the passive film. It's refractive index was N2=2.0−0.2i and the thickness was 15 to 30 nm or more. This film was considered to be a corrosion product film which grew as corrosion proceeded. The film thickness was especially large in stagnant system.