Abstract
Passivity determines corrosion resistance and stability of highly-alloyed stainless steels, and passivity breakdown is commonly believed to occur at a fixed potential due to formation and dissolution of Cr(VI) species. In this work, the study of a 25Cr–7Ni super duplex stainless steel in 1 M NaCl solution revealed that the passivity breakdown is a continuous degradation progress of the passive film over a potential range, associated with enhanced Fe dissolution before rapid Cr dissolution and removal of the oxide. The breakdown involves structural and compositional changes of the passive film and the underlying alloy surface layer, as well as selective metal dissolution depending on the anodic potential. The onset of passivity breakdown occurred at 1000 mV/Ag/AgCl, and Fe dissolved more on the ferrite than the austenite phase. With increasing potential, the passive film became thicker but less dense, while the underlying alloy surface layer became denser indicating Ni and Mo enrichment. Rapid Cr dissolution occurred at ≥1300 mV/Ag/AgCl.
Highlights
Passivity has vital importance to our civilization because corrosion resistance of most of metallic materials used today is due to spontaneous formation of a passive film on the surface, preventing the materials from rapid degradation.[1,2] The electrochemical characteristics of passivating metals are reflected by their current-potential behavior showing a transition from an active to passive state
For low and medium-alloyed stainless steels, an abrupt increase of current density in the polarization curves occurs at low potentials indicating passivity breakdown, known as the breakdown/pitting npj Materials Degradation (2019) 22
In the PDM, the prediction of transpassive breakdown potential is based on the Cr3+ → Cr6+ reaction.[8,81,82]
Summary
Passivity has vital importance to our civilization because corrosion resistance of most of metallic materials used today is due to spontaneous formation of a passive film on the surface, preventing the materials from rapid degradation.[1,2] The electrochemical characteristics of passivating metals are reflected by their current-potential behavior showing a transition from an active (high current) to passive (low current) state. The presence of the passive film decreases the corrosion rate by orders of magnitude.[3,4,5,6,7] passivity breakdown can occur in corrosive environments, leading to a sharp increase of the anodic current at the so-called breakdown potential When this happens, either the passive film breaks locally triggering localized corrosion, or it vanishes allowing rapid dissolution leading to material failure with serious consequences.[1,2,3,8,9,10,11] The passivity and breakdown behavior of metals have been studied extensively using electrochemical methods in corrosive aqueous electrolytes, often combined with surface analyses. Many advanced alloys nowadays contain multi-elements and consist of multi-phase microstructures, where different kinds of precipitates and phase/grain boundaries may trigger localized corrosion.[16,17,18,19,20,21,22,23] Further studies are necessary to better understand the passivity and its breakdown of such alloys
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