Passive rust layer on low-alloy steel exposed in atmospheric environment

The passive rust layer formed on a low-alloy steel after a long-term exposure to an atmospheric environment has been examined. It was shown that the passive rust layer was composed of densely aggregated fine-particles of α-ferric-oxyhydroxide containing a considerable amount of Cr. The characteristic feature of synthetic α-ferric-oxyhydroxides containing various amounts of Cr suggests that the coexistence of Cr in the passive rust crucially affects the protective ability of the passive rust against atmospheric corrosives. It was also found that the nature of the Cr-containing oxyhydroxide was dependent upon the Cr content in the rust matter.