Abstract
Bipolar electrochemistry has been applied to Type 420 ferritic stainless steel in order to determine the full spectrum of anodic-to-cathodic polarisation behaviour. The occurrence of crevice corrosion, pitting corrosion in combination with general corrosion, pitting corrosion only, general corrosion only, followed by a cathodic region has been observed. Instances of pitting corrosion initiated near chromium-rich carbides with Cr23C6, Cr3C2, and Cr7C3 identified as pit nucleation sites. The observed pit growth kinetics were independent of the electrochemical over-potential. Characterisation of the pit size distributions supports the presence of a critical dissolved volume for the transition of metastable to stable pit growth and pit coalescence.
Highlights
The application of bipolar electrochemistry provides access to the full spectrum of anodic-to-cathodic reactions along a bipolar electrode (BPE) [1,2,3,4]
The work reported in this paper provides insight into the pitting corrosion behaviour of annealed Type 420 ferritic stainless steel via the application of a bipolar electrochemistry approach
For the electron backscatter diffraction (EBSD) and angle selective backscatter (AsB) microstructure analyses, the samples were ground to 4000 grit and polished to a 0.25 μm diamond paste finish, followed by fine polishing with oxide polishing suspensions (OPS) of colloidal silica
Summary
The application of bipolar electrochemistry provides access to the full spectrum of anodic-to-cathodic reactions along a bipolar electrode (BPE) [1,2,3,4]. Pitting corrosion is a rapid metal dissolution process, affected by the applied electrochemical potential, concentration of halides, microstructure, and electrolyte temperature [8,9]. Changes in exposure temperature typically do not affect pit morphology, but the applied potential, microstructure condition, and Cl− concentration can influence the resulting pit shape [1,9]. The final microstructure contains chromium carbides, and the corrosion behaviour of this microstructure condition is dependent on austenitisation temperature, quench rate, and tempering treatments [19,20]. The work reported in this paper provides insight into the pitting corrosion behaviour of annealed Type 420 ferritic stainless steel via the application of a bipolar electrochemistry approach. The aim is to obtain information about the relationship between the electrochemical potential and the corrosion behaviour of this alloy, with a focus on determining pit nucleation sites and the associated growth kinetics
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