Abstract
The characterisation of passive oxide films on heterogeneous microstructures is needed to assess local degradation (corrosion, cracking) in aggressive environments. The Volta potential is a surface-sensitive parameter which can be used to assess the surface nobility and hence passive films. In this work, it is shown that the Volta potential, measured on super duplex stainless steel by scanning Kelvin probe force microscopy, correlates with the electrochemical properties of the passive film, measured by electrochemical impedance spectroscopy and potentiodynamic polarisation. Natural oxidation by ageing in ambient air as well as artificial oxidation by immersion in concentrated nitric acid improved the nobility, both reflected by increased Volta potentials and electrochemical parameters. Passivation was associated with vanishing of the inherent Volta potential difference between the ferrite and austenite, thereby reducing the galvanic coupling and hence improving the corrosion resistance of the material. Hydrogen-passive film interactions, triggered by cathodic polarisation, however, largely increased the Volta potential difference between the phases, resulting in loss of electrochemical nobility, with the ferrite being more affected than the austenite. A correlative approach of using the Volta potential in conjunction with electrochemical data has been introduced to characterise the nobility of passive films in global and local scale.
Highlights
The nobility of metals determines their propensity to undergo reactions with a corrosive environment which is strongly related to the work function and, to the Volta potential of the metal.[1,2] Less noble metals with strong passive films can outperform nobler metals with less protective surface oxides in corrosive media
Chromia has a p-type semiconductor property, and a high work function of ≈5 eV was reported for Cr2O3(0001).[26]
The passive film of stainless steels is known to be composed of an outer iron-rich
Summary
The nobility of metals determines their propensity to undergo reactions with a corrosive environment which is strongly related to the work function and, to the Volta potential of the metal.[1,2] Less noble metals with strong passive films can outperform nobler metals with less protective surface oxides in corrosive media. This between calculated Volta potentials by density functional theory suggests that the passive films of both ferrite and austenite were and experimentally measured Volta potentials in the presence of of different character but became similar or identical due to nitric oxides as well as adhering monolayers of water, demonstrating its usefulness in assessing the nobility of complex microstructures.[12] acid treatment as the Volta potential contrast vanished and both phases assumed similar values. Laurent et al.[24] reported a chromium-rich oxide, in the form of chromia, that became further enriched in the passive film upon passivation in nitric acid, with the thickness, not been majorly altered This may explain the reason why the Volta potential of ferrite and austenite of the DSS were similar after immersion in nitric acid. The chromia in the passive film of the investigated DSS has been earlier demonstrated to be nanocrystalline and to be composed of a metal oxide/hydroxide mixture, consisting mainly of Fe and Cr and some Mo and Mn species.[6]
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