Passivity of Nickel electrodes in presence of the inhibitor PAB

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Abstract
The passivity of nickel electrodes is investigated at high potentials in absence and presence of the inhibitor 2‐pentyl‐aminobenzimidazole (PAB). At high potentials, the electrode is covered by a thick film of NiOOH. Various states of the 3‐valent oxide film can be distinguished: Without inhibitor (3o), with an outer layer of inhibitor (3out), with an almost constant concentration profile in the total film (3tot) and a concentration gradient of inhibitor in the film caused by diffusion (3diff). Under steady state conditions, the films 3o, 3out and 3tot can be distinguished by the rate of anodic oxygen evolution, and the electrode capacity. Moreover, the reduction and reoxidation capacity of the electrode decreases from 3o to 3out and 3tot. If the film 3o is reduced and reoxidized in presence of PAB, the redox capacity decreases simultaneously with the electrode capacity due to the formation of 3diff. The role of the inhibitor is due to the formation of a thick polymer film on top of the oxide as well as to the formation of a nickel hydroxide/PAB complex. The outer polymer film has a good proton conductivity and presumably a good electron conductivity. It is concluded that the anodic oxygen evolution takes place at the inner boundary between the oxide and PAB film. The inhibition of local corrosion is probably due to formation of the outer polymer film. XPS measurements verify the expected concentration profiles of PAB in the oxide and on top of it. The PAB‐film contains a lot of water, and within the passive film PAB causes a substitution of O2− by OH−.