Single event experiment : a powerful approach for studying passivity breakdown pitting corrosion

C04-Passivity and Localized CorrosionInternational audiencePrediction of material lifetime such as stainless steels requires a clear understanding of passivation, corrosion, and pitting mechanisms. Despite its ubiquity and its history, the mechanisms of pitting are poorly understood, mainly because the stochastic behavior of the phenomenon, which results in many individual processes occurring simultaneously on the material surface. It is thus necessary to devise experiments which allow to work on a single pit for a better understanding of mechanisms governing pitting from its initiation to the propagation or its repassivation. Different experimental setup allowing the local injection of aggressive species such as chloride ions, have thus been developed [1-3], including a flow micro-device allowing a single pit to be initiated and then propagated over long time [4-6].In this work, we show that we were able to reproduce, at will, the pitting of 316L stainless steel in order to study its propagation at various stages and to revisit the different parameters involved in the pitting process. Thanks to a statistical analysis performed on identical experiments, it was possible to build some zone diagrams showing the stability of pits as a function of the chloride concentration, the pit dimensions, and the electrode potential. Interestingly, the chemistry inside the pit was also studied as a function of the electrode potential. These experiments were performed with a specific experimental setup in order to measure the pitting current of a single pit simultaneously with the Raman spectrum thus opening new insight in the identification of species inside the pit. It was also possible to link the sulphate concentration (and thus the pH variation) inside the pit to the current as a function of time.[1] K. Fushimi, K. Azumi, M. Seo, Use of a liquid-phase ion gun for local breakdown of the passive film on iron, J. Electrochem. Soc., 147 (2000) 552-557.[2] K. Fushimi, M. Seo, Initiation of a local breakdown of passive film on iron due to chloride ions generated by a liquid-phase ion gun, J. Electrochem. Soc., 148 (2001) B450-B456.[3] C. Gabrielli, S. Joiret, M. Keddam, H. Perrot, N. Portail, P. Rousseau, V. Vivier, Development of a Coupled SECM-EQCM Technique for the Study of Pitting Corrosion on Iron, J. Electrochem. Soc., 153 (2006) B68-B74.[4] N. Aouina, F. Balbaud-Celerier, F. Huet, S. Joiret, H. Perrot, F. Rouillard, V. Vivier, A flow microdevice for studying the initiation and propagation of a single pit, Corros. Sci., 62 (2012) 1-4.[5] S. Heurtault, R. Robin, F. Rouillard, V. Vivier, Initiation and propagation of a single pit on stainless steel using a local probe technique, Faraday Discuss., 180 (2015) 267-282.[6] S. Heurtault, R. Robin, F. Rouillard, V. Vivier, On the Propagation of Open and Covered Pit in 316l Stainless Steel, Electrochim. Acta, 203 (2016) 316-325