Understanding in compositional phases of carbon steel rust layer with a long-term atmospheric exposure

•Phases of rust layer on plain carbon steel with 40 years indoor exposure have been identified.•α-FeOOH is also the final phase for rust layer on carbon steel with a long-term exposure.•Fe3O4 and γ-FeOOH only take a small share in the rust layer with a long-term exposure.•Mechanism of unusual presence of α-FeOOH in the rust layer on carbon steel is explored.