A surface science study on the influence of manganese and copper on the atmospheric corrosion of Fe-Mn-Al-C alloys under simulated SO2 conditions

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Four distinct compositions of Fe-Mn-Al-C steels, covering high- and low-Mn contents (austenitic and ferritic alloys) and either containing or lacking Cu as a minor alloying element, were selected to evaluate the influence of Mn and Cu on their corrosion behaviour. For this purpose, the samples were subjected to SO₂-induced corrosion tests under humid/dry cyclic conditions. The selected Al concentration lies within the range reported to be necessary for ensuring good corrosion resistance. The study was conducted using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES, including Ar⁺ depth profiling), ⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The native oxide film formed upon exposure of freshly polished alloy surfaces to the laboratory atmosphere was also characterized. This film was found to contain Al³⁺, Mn²⁺, and Fe³⁺, with Al and Mn concentrations exceeding those in the bulk alloy, while the Fe concentration was lower. Under SO₂-polluted atmospheres, the initial corrosion rate of each alloy strongly depends on the nature and thickness of this naturally formed oxide film. The thicker film observed on the austenitic samples can be attributed to the higher Mn content at their surfaces, since Mn is more reactive toward atmospheric oxygen. The very low corrosion rates exhibited by these alloys, compared with pure iron and weathering steel, are associated with the formation of corrosion products composed of Al- and Mn-based compounds (Al₂O₃, MnSO₄). These compounds form a uniform layer that effectively blocks the access of acidic electrolytes to the metallic surface. The austenitic sample microalloyed with Cu shows the lowest corrosion rate among all tested materials. This behaviour is attributed to the combined effect of higher concentrations of amorphous and/or finely divided Fe-hydroxides together with Mn–Al compounds within the corrosion layer.