Size matching effect between anion vacancies and halide ions in passive film breakdown on copper

Different types of anion vacancies were optimized in passive films on copper via first-principles calculations based on density functional theory, and numerous potentiodynamic polarizations were performed to evaluate the size matching effect between anion vacancies and halide ions in passive film breakdown on copper. The size of the anion vacancies decreased to some extent compared with the corresponding atom due to the surface relaxation effect, and electrochemical experiments showed that the pitting potential for an oxide passive film on copper followed the order F− < Cl− < Br−, while this order was Cl− < F− < Br− for a sulfide passive film, which was attributed to the integrated effect of the energy of anion vacancy expansion and the Gibbs energy of dehydration. The pitting potential orders were the same on pre-oxidized or pre-sulfided copper with that of fresh copper in the same solution. Meanwhile, iodide ions were not compared due to the formation of poorly soluble copper iodides. The ratio of pit depth to pit mouth diameter on copper decreased with increasing pit volume, resulting in shallow pits with large mouth; thus, the catalytic-occluded cell effect was not obvious for copper.