| Volta Potential Evolution of Aluminium Alloys under Chloride-Contaminated Thin-Film Electrolyte Yanhan Liew1; Cem Örnek2; Jinshan Pan3; Dominique Thierry4; Sudesh Wijesinghe5; Daniel John Blackwood1; 1NATIONAL UNIVERSITY OF SINGAPORE, Singapore, Singapore; 2ISTANBUL TECHNICAL UNIVERSITY, Istanbul, Turkey; 3KTH ROYAL INSTITUTE OF TECHNOLOGY, Stockholm, Sweden; 4FRENCH CORROSION INSTITUTE, Brest, France; 5SINGAPORE INSTITUTE OF MANUFACTURING TECHNOLOGY (SIMTECH), Singapore, Singapore; PAPER: 192/Corrosion/Regular (Oral) SCHEDULED: 14:50/Mon. 28 Nov. 2022/Andaman 2 ABSTRACT: Corrosion is an issue that has led to widespread cost and even danger. Thus, it is of importance to investigate the nature of corrosion initiation and/or propagation mechanisms; one such way is via analysing the nobility and electrochemical changes of the metal microstructure through their Volta potential (Ψ), which is the potential difference between a point just outside the surface of the metal and a point infinitely far away from the surface [1]. The Ψ gives information about the surface electrons, and hence can be used as a gauge of a material’s tendency towards physicochemical reactions such as corrosion [2]. Using a recently developed Scanning Kelvin Probe Force Microscopy (SKPFM) setup that incorporates an ability to vary the relative humidity [3,4], the earliest stages of localised corrosion on aluminium alloys, such as AA5083, under chloride-contaminated thin-film electrolytes were investigated at ambient temperature over the humidity range 20%-85% RH. The in-situ time-lapse SKPFM investigation elucidated magnesium silicide particle regions in AA5083, which initially showed positive Volta potential (vs matrix), suffering from severe, sharp nobility loss due to the ongoing dissolution of magnesium and the formation of rod-like corrosion products [5]. The corrosion product growth, most likely a form of nesquehonite, was highly favoured when exposed to humidities >80% RH. Transient events that occurred during the corrosion were also captured by the in-situ time-lapse SKPFM method, further demonstrating that it is necessary to measure the Volta potential during corrosion to reflect the true relationship between the Volta potential and corrosion potential or breakdown potential of a material. Furthermore, the phenomenon of nobility adoption is discussed, detailing instances when local sites adopted the nobility of their neighbouring region. Nobility adoption seemingly occurs at elevated humidity (>80% RH) and within close proximity of a particle with large Volta potential difference relative to the matrix. Different behaviours of nobility adoption were also seen in the analysis, when the aluminium alloys were exposed to thin-film electrolyte over extended period, suggesting that the localised corrosion mechanism evolves over time. References: [1] J.O. Bockris, A.K.N. Reddy, M. Gamboa-Aldeco, Modern Electrochemistry 2A, Kluwer Academic Publishers, Boston, 2002. doi:10.1007/b113922. [2] C. Örnek, C. Leygraf, J. Pan, On the Volta potential measured by SKPFM–fundamental and practical aspects with relevance to corrosion science, Corros. Eng. Sci. Technol. 54 (2019) 185–198. doi:10.1080/1478422X.2019.1583436. [3] C. Örnek, C. Leygraf, J. Pan, Real-Time Corrosion Monitoring of Aluminum Alloy Using Scanning Kelvin Probe Force Microscopy, J. Electrochem. Soc. 167 (2020) 081502. doi:10.1149/1945-7111/ab8826. [4] C. Örnek, D.L. Engelberg, SKPFM measured Volta potential correlated with strain localisation in microstructure to understand corrosion susceptibility of cold-rolled grade 2205 duplex stainless steel, Corros. Sci. 99 (2015) 164–171. doi:10.1016/j.corsci.2015.06.035. [5] Y. Liew, C. Örnek, J. Pan, D. Thierry, S. Wijesinghe, D.J. Blackwood, In-Situ Time-Lapse SKPFM Investigation of Sensitized AA5083 Aluminum Alloy to Understand Localized Corrosion, J. Electrochem. Soc. 167 (2020) 141502. doi:10.1149/1945-7111/abc30d. |
