Cerium loaded algae exoskeletons for active corrosion protection of coated AA2024-T3
In recent times the use of synthetically produced nano-carriers doped with environmentally friendly corrosion inhibitors has been proposed as an alternative to existing corrosion protective concepts using highly efficient but toxic corrosion inhibitors based on Cr VI. Despite the promising results using these nanocarriers, limitations related to their versatility, manufacturing, embedding in coating matrices, and efficient and time-sustained protection of damages makes the search for carrier alternatives a necessary step for a successful industrial implementation of the concept. In our work we explore the potential use of environmentally friendly and biobased diatom exoskeletons for the active corrosion protection of metallic structures.
The proof-of-concept is demonstrated using Aulacoseira diatom exoskeleton doped with the corrosion inhibitor cerium nitrate for the active protection of aerospace aluminium alloy AA2024. Time-based release from the exoskeletons was confirmed and monitored in real immersion time by a modified UV-Vis spectrometer. Corrosion protection was evaluated by a home-built hyphenated opto-electrochemical setup. This technique allowed for real-time monitoring of both electrochemical response and optical variations at damaged sites and facilitated the interpretation of the inhibition and degradation process of the coating systems. Detailed Raman analysis at the scribe confirmed the fast cerium release and preferential deposition at Cu-rich sites in the case of the cerium loaded refined diatomaceous earth (DE) systems.
The work demonstrates that the use of diatom exoskeletons as carriers for active corrosion inhibition in coatings is feasible and very promising. The study shows the feasibility of developing protective systems based on fast release and inhibition at damaged sites followed by a sustained release of corrosion inhibitors supplied at a sufficient concentration to ensure the long-term protection. The use of inhibitor loaded algae exoskeleton particles for sustained corrosion inhibition here presented is not restricted to cerium and epoxy coatings on aluminium substrates but should be regarded as a generic concept with high potential for active corrosion protection in coated metals.
Hyphenated opto-electrochemistry for the development of self-healing coatings
Organic coatings are generally applied on metal surfaces to provide a long-lasting barrier against corrosive species. Regulatory changes on conventional Cr VI based coatings due to their health and environmental impact drive the search for new environmentally friendly anti-corrosion coatings. In the last years several promising intrinsic and extrinsic self-healing coatings have been presented in literature. While their active corrosion protection at damaged sites has mainly been analysed by traditional Electrochemical Impedance Spectroscopy (EIS) and complex local electrochemical techniques (eg SVET, SECM), direct correlation of the electrochemical signals to physical phenomena remains a difficult task. For this, post-mortem analysis is often necessary requiring multiple samples. This results in difficult correlations due to the potential differences in scratch area, coating thickness, formulation and porosity between the samples affecting the interpretation and relative variation of the impedance data.
In this work we explore the use of an in-situ hyphenated opto-electrochemical set-up as an improved method to simplify the evaluation/interpretation of the corrosion protection processes at exposed metal surfaces and relatively big damages in coated metals. The technique allowed for real-time acquisition of optical and electrochemical information at the damaged or exposed site and was validated by the evaluation of corrosion inhibiting species and self-healing coatings. Several signals obtained during the test were used for the dedicated analysis. Time variations of the Open circuit potential (OCP) and low frequency impedance rather than time evolution were used to monitor electrochemical processes at the metallic surface. The optical images were processed using a bimodal threshold method to subtract the background and calculate the degradation area over time (variations at and around the damaged site). From this, the type of degradation could be established and coupled to the electrochemical signal variations (e.g. coating delamination, the formation of corrosion products, local corrosion sites, and active corrosion protection). From the data analysis the kinetics of the degradation process itself (on-set of degradation and kinetics of degradation) were obtained thereby identifying two critical parameters for the design and comparative evaluation of self-healing and active corrosion protective systems.