•The presence of sulphate ions does not significantly affect the dissolution mechanism of the alloy.•Corrosion products mostly contain oxides and chlorides of Al, Mn and Cu.•The Cualmn alloy undergoes general corrosion in the studied solutions.•The growth dynamic of the spontaneous surface film in the tested solutions is different.•The stability of the surface film increases with the immersion time and the presence of sulphate ions. The electrochemical behaviour of a CuAlMn alloy in a 0.5 M NaCl solution with the addition of different concentrations of Na2SO4 (0.1, 0.3, and 0.5 M) was investigated by observing the change in open circuit potential (EOC) with time, applying polarization methods and electrochemical impedance spectroscopy (EIS). The damage to the metal surface was determined by optical microscopy and SEM/EDS analysis. Although the increase in the content of sulphate ions in the NaCl solution reduces the corrosion current and increases the polarization resistance, their presence does not affect the CuAlMn dissolution mechanism. The anodic reaction of Cu dissolution is controlled by the diffusion of soluble CuCl2− species from the electrode surface into the bulk solution. Surface analysis showed a uniform dissolution of the investigated alloy. The corrosion resistance of CuAlMn at EOC is a consequence of forming a natural surface film. The experimental conditions (immersion time and presence of sulphate ions) increase the stability of the surface layer (the resistance, compactness, and thickness of the surface layer increase). Moreover, in the presence of sulphate ions, a subsequent ordering of the structure of the surface layer takes place during stabilization at the open circuit potential which is a consequence attributed to the formation of adsorbed species such as Cu(SO4)ads and/or Cu(OH)ads on the electrode surface.