Direct observation of metal dissolution during anodization of niobium

•An in situ investigation of metal dissolution during anodisation of Nb was performed.•A constant dissolution rate during potentiodynamic Nb oxide growth was observed.•A time-dependent dissolution was identified during potentiostatic anodisation of Nb.•The Nb corrosion rate is dependent on potential in the potentiostatic oxide growth regime.

Direct in situ Nb release measurements were performed by ICP-MS during Nb anodisation. In the potentiodynamic regime (0-8 VSHE, 10 mV s− 1), constant electrochemical current (45 μA cm− 2) and Nb dissolution rate (70 pg s− 1 cm− 2) confirm the high field oxide growth. The valve metal character of Nb is demonstrated during the reverse scan when both electrochemical current and dissolution rate decrease, indicating that Nb release is solely triggered by the high field conditions. It was shown that an extremely small fraction of the current (0.75%) was responsible for Nb dissolution. During 1000 s of potentiostatic anodisations up to 9 VSHE the dissolution rates decreased from 100 pg s− 1 cm− 2 to 10 pg s− 1 cm− 2 and were slightly potential dependent, with higher dissolution rates at higher anodisation potentials. The electrochemical current transients decayed faster than the dissolution rates, leading to an overall increase in the dissolution current fractions with time (up to 4.5% at 3 V). This fraction was also found to be slightly potential dependent. Interestingly, higher anodisation potentials resulted in lower dissolution fractions during potentiostatic oxide formation. Overall, the potentiostatic dissolution fractions were at least twice as high as in the potentiodynamic case.

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