Direct Measurement of the Dissolution of Valve Metals Under High Field Anodisation Conditions

Abstract

Valve metals such as Ti, Nb and Ta react spontaneously with water or oxygen forming dense oxide layers on their surface. These thin passive layers are protecting the metals from further corrosion. Anodisation can be used to increase the thickness of the passive layers for an improved corrosion and wear resistance. Oxide growth during anodisation occurs via ion transport through the oxide film under high field conditions. The oxide growth process during anodisation of valve metals has been investigated in detail in the past. Most widely used for the description of the oxide growth mechanism during anodisation is the high field model [1]. However, there is one parameter that cannot be described by the high field model or any other existing model that is the metal dissolution during oxide growth. To date the metal dissolution during anodisation of Nb, Ta and Ti has also never been experimentally observed or quantified. In an attempt to study this phenomenon a flow-type scanning droplet cell microscope (FT-SDCM) [2] which is directly coupled to inductively coupled plasma mass spectrometry (ICP-MS) [3] was used. By means of this technique it was possible to investigate the time and potential dependent metal dissolution during anodisation. Additionally the influence of the pH value of the used electrolyte was investigated. All experiments showed an overshoot in metal dissolution during the initial stage of oxide growth. The dissolution rate stabilized into a constant plateau (stationary dissolution) after this initial period for all investigated materials. In acidic electrolytes a constant increase of the overall amount of dissolved metal was observed when increasing the anodisation potential. For neutral electrolytes on the other hand the exact opposite was observed. Higher formation potentials were leading to lower dissolution rates. Furthermore, a direct potential - independent proportionality between the amount of charge carriers crossing the electrolyte-oxide interface and the amount of dissolved metal during anodisation was observed. The measured dissolution rates were highest for Ti and lowest for Ta under all investigated pH values. Interestingly almost exactly one order of magnitude difference in dissolution rate between each of the three investigated materials was found. For a better understanding of the observed dissolution behaviour electrochemical impedance spectroscopy was performed under oxide formation conditions for each investigated material. Differences were significant in the film resistance depending on the pH value of the electrolyte. They could be used to rationalise the observed dissolution behaviour. ____________ [1] M. M. Lohrengel, Mater. Sci. Eng. R. 11 (1993) 243 [2] J. P. Kollender, A. I. Mardare, A. W. Hassel et al., J. Electroanal. Chem. 740 (2015) 53 [3] A. I. Mardare, J. P. Kollender, A. W. Hassel et al., Electrochem. Comm. 59 (2015) 5