Nickel anodic behaviour in formic acid solutions

The anodic behaviour of nickel in formic acid solutions is strongly dependent on its concentrations. In solutions of 2, 97 and 99% HCOOH, the anodic current density is limited at values ≤ 1 mA/cm 2 , and the electrode surface remained unchanged after polarisation. In the solutions of mean concentration on the anodic curves current peaks appear, situated in the different potential region in function of the formic acid concentration. At concentrations of 5 and 10% HCOOH the peak potentials are 0.16 and 0.24 V/SCE, respectively. The current increase is attributed to the anodic dissolution of Ni, probably in pitting (corrosion type observed on electrode surface after polarisation), and the decrease of the current is due to the pitting passivation. In the solutions of 20-80% HCOOH the peak potential shifts by more than 0.5 V in the positive direction, and it corresponds to the oxidation of formic acid. Although the corrosion pitting is not observed, the nickel surface was oxidised. The results of this study show that nickel is not corrosion resistant in formic acid solutions with the exception of very diluted solutions (≤ 2%) or of those very concentrated (≥ 97% HCOOH).