Novel smart coatings for corrosion protection

Abstract

There is a desire in defence departments throughout the world to decrease the costs associated with corrosion ($2.3 billion in the US alone in 2012-2013) especially in the naval sectors. Using smart coatings that are able to self-heal autonomously is one way of reducing costs. Self-healing capabilities can be induced through the incorporation of healing agents that have been encapsulated in microcapsules which rupture when the coating is damaged, releasing the healing agent and healing the damage to the polymer. If a porous network can be incorporated into the coating then better dispersion of microcapsules can take place.
Polydopamine (PoDA) was identified as a potential coating as a result of its facile auto-oxidative deposition and novelty. PoDA films are highly adherent, can be deposited onto a multitude of substrates and have been used for applications as diverse as DNA patterning and surface functionalization for the production of self-assembled monolayers.
The PoDA coatings were deposited onto structural steel from an alkaline buffered phosphate buffer solution and their corrosion performance after immersion in 3.5% NaCl solution was analysed using electrochemical impedance spectroscopy. The results are presented in the form of Nyquist and Bode plots. The deposition and polymerisation parameters such as temperature, monomer concentration and molar ratio of oxidant to dopamine were examined and optimised. Once the best combination of deposition parameters had been identified the resultant coating was subjected to further testing including X-Ray Photoelectron Spectroscopy and Scanning Kelvin Probe in order to better understand its properties and corrosion protection mechanism.
The EIS results showed that a 0.5:1 molar ratio of p-benzoquinone (p-BQ) : dopamine monomer with a 2 mg mL^-1 dopamine monomer concentration and a deposition temperature of 25°C produced the coating that offered the best protection against corrosion. In addition, when the coating was purposefully damaged, EIS testing showed the presence of an adsorption process and maintained corrosion protection for the p-BQ oxidized PoDA coating when compared to the autoxidated coating meaning the presence of p-BQ offers some intrinsic protection against corrosion. XPS demonstrated differences in composition between the autoxidated and p-BQ oxidized PoDA coatings and indicted that p-BQ acts as a more efficient oxidant than atmospheric oxygen. However, the similarities in functional groups between autoxidated and p-BQ oxidized PoDA mean that it is not possible to determine what the fate of p-BQ oxidant is and so further analysis is required. SKP results revealed that the protection mechanism was found to be different to that of established conducting polymers such as PANi. Overall the results indicate that p-BQ itself is acting as a corrosion inhibitor. However the coatings were too thin, even after optimisation of the deposition conditions and so polyaniline deposited from an oxalic acid solution of aniline monomer was identified as a suitable alternative. A cubic liquid crystal template was developed following examination of mesophases by polarised light microscopy and viscosity measurements. The mesophase was formed from a 50/50 wt% mix of Brij S100 and this was used as the basis of the templating of polyaniline. The presence of the template changed the requirements for the deposition of polyaniline, primarily because the oxidation of aniline monomer is a diffusion controlled reaction. Experiments were undertaken to determine optimum conditions for the deposition of PANi. Aniline monomer was encapsulated into polydopamine capsules using a procedure published by Cui et al [1] . The filled capsules were then incorporated into the templated polyaniline coating and an epoxy topcoat was applied. The self-healing capabilities of the coating system were evaluated by deliberately inducing a defect into the coating using a scalpel blade. The corrosion behaviour of the coating system in 3.5% NaCl was evaluated using EIS and the coating was found to self-heal. The results of this project represent the formation of a novel-self healing coating which protects steel against corrosion in seawater.

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ORCID for Julian Wharton: orcid.org/0000-0002-3439-017X

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