<p>A series of measurements including electrochemical measurements, atomic force microscope and immersion test were employed to investigate the impact of SO₄²⁻ concentration over a range of 0 to 20&#xa0;g/L on the anti-corrosion performance of FeCoNi MEA in a simulated ocean environment. With the increase in the concentration of&#xa0;SO₄²⁻, the corrosion resistance of the MEA is severely damaged, which is demonstrated by the significant increase of i_p value, metastable pitting number and corrosion rate, as well as the occurrence of fused corrosion pit. The negative influence of&#xa0;SO₄²⁻&#xa0;on the anti-corrosion property results from its cyclic acidification reaction, which generates unstable iron compound that is unfavorable for protecting the substrate. The addition of SO₄²⁻ increases the flaw amount of surface passivation film, which provides the increased pathways for the infiltration of corrosive&#xa0;Cl⁻ and H⁺, weakening the protective capability. Metal elements in the substrate are comprehensively dissolved, resulting in the production of pit.</p>