A study on the effect of solution pH and Cl⁻ ion concentration on the electrochemical behavior of FeCo alloy using the WBE technique

Near-equiatomic FeCo soft magnetic alloys are widely used in industrial applications due to their excellent magnetic properties. Numerous studies have focused on balancing its magnetic and mechanical properties, but the theoretical framework of corrosion in different environments remains incomplete. This study employs wire beam electrode (WBE) technology, combined with EIS and PDP testing, to investigate the corrosion behavior of FeCo alloy under different pH values and Cl⁻ concentrations. The results show that at pH 3, no oxide film is formed on the alloy surface, exhibiting uniform corrosion. As the pH value increases, a Co oxide layer begins to form on the surface, and its corrosion resistance gradually improves. When the pH is between 5 and 9, due to the de-iron reaction and the instability of the Co oxide layer, the corrosion behavior gradually shifts from initial localized corrosion to uniform corrosion. XPS results indicate that at pH 11, the Co(II) oxide film on the alloy surface further oxidizes into the more stable Co(III) oxide, resulting in pitting corrosion. With increasing Cl⁻ concentration, steady-state growth of the pits occurs, and the pit size gradually enlarges.