Monitoring corrosion and corrosion protection in bare and coated aluminium by optical-electrochemical hyphenated tools
Electrochemical Impedance spectroscopy (EIS) is proven to be a useful technique to monitor corrosion processes and evaluate the properties of polymer coatings during the exposure to corrosive environments. Despite the ever-increasing literature on EIS analysis, data interpretation is still a major limitation as it requires a high level of user-experience and skills . In this work we propose a new approach that reduces subjectivity and increases reliability of electrochemical signals interpretation (e.g. EIS) and their relation to the responsible macroscopic (surface) phenomena. For this, we hyphenated inexpensive ordinary optical microscopes to highly used electrochemical setups. The optical microscopy images are taken continuously during EIS measurements with the use of a specially designed opto-electrochemical setup as shown in Figure 1.
The optical images are acquired with a USB microscope and processed using a bimodal threshold algorithm. The software is then used to obtain characteristic parameters related to the evolution of on-going processes such as pitting corrosion, and in the case of coatings, delamination and underfilm corrosion-related phenomena. Besides this, the time-evolution and variation of the Open Circuit Potential (OCP) and low frequency impedance were used to monitor electrochemical processes. The setup and approach was validated by monitoring the corrosion behaviour of AA2024 in sodium chloride solutions. In this process it was possible to identify similar phenomena previously reported in time-consuming electrochemical  and post-mortem optical inspection . The setup has also been used to evaluate the inhibiting efficiency of promising chromate replacing corrosion inhibitors for AA2024, i.e. Cerium Nitrate, Lithium Carbonate, Sodium diethyldithiocarbamate trihydrate, 2,5-dimercapto-1,3,4-thiadiazolate, 2-mercaptobenzothiazole, and 8-hydroxyquinoline. Furthermore, the setup has been proven as very effective to evaluate newly developed anticorrosive coatings using nanonetworks and algae exoskeleton carriers [4,5]. The hyphenation with in-situ optical analysis resulted in a better understanding of the corrosion inhibition and degradation processes revealing, in some cases, new information that could not be obtained by electrochemistry alone. Although the used data-treatment is currently still time-consuming, further progress on automated user-friendly software should make the live-interpretation of opto-electrochemical results possible for a broader audience outside the scientific community. W. Tait, K. Handrich, S. Tait, J. Martin, in:, J. Scully, D. Silverman, M. Kendig (Eds.), Electrochem. Impedance Anal. Interpret., ASTM International, West Conshohocken (1993) 428–437.
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 P.J. Denissen, S.J. Garcia, Cerium-loaded algae exoskeletons for active corrosion protection of coated AA2024-T3, Corrosion Science, 128 (2017) 164-175.
 C.D. Dieleman, P.J. Denissen, S.J. Garcia, Inhibiting Nano-networks for Long-term Active Corrosion Protection of Damaged Coated Metals, to be submitted.