Skip to main content

New strategies for corrosion inhibition coatings for lead and its characterization by in-situ spectroelectrochemical studies

Final Report Summary - PBCOATINGS (New strategies for corrosion inhibition coatings for lead and its characterization by in-situ spectroelectrochemical studies)

Corrosion is the major problem in the degradation of heritage metal objects, and any remedial measures are subject to a strong ethic that favours conservation as opposed to restoration. Accordingly, major scientific challenges exist for developing appropriate treatment methods to stabilize and protect artefacts. Because inappropriate treatments can cause irreversible damage to unique and irreplaceable objects, it is crucial that the chemical processes involved are fully understood and characterized before any preservation work is undertaken. Besides being of fundamental interest, study of the corrosion behaviour is essential to the development of adequate conservation and preservation processes.
Lead is susceptible to corrosion in the presence of organic acids and humidity. This accelerated degradation seriously affects cultural heritage objects, it takes place in display cases in museums and is has become a serious issue on organ pipes in churches or concert halls. The pipes of ancient organs are made from lead and the organic acids are emitted from the wooden parts in the organ (the windtrunks and the windchests). The aim of this project was the development of corrosion protective coatings for lead heritage objects which are stable, reversible, easy to apply and to remove, and aesthetically justified.
Long chain monocarboxylic acids coatings have shown very promising results, forming a lead dicarboxylate layer upon reaction with lead. However, the effectiveness of such a treatment was proved to be highly dependent on carbon chain length, carboxylate concentration in the solution and exposure time. Consequently, at the beginning of this project, carboxylate molecules with chain lengths longer than C12 had hardly been studied. This project aimed at tackling the key issue of the low solubility of monocarboxylate molecules, in particular long C chain.
Two different coating formation strategies were studied in this project. The corrosion resistance of all coatings was fully characterized in the host lab by different electrochemical techniques, along with X-ray diffraction, FT-IR spectroscopy, SEM microscopy, and other surface analyses methods. For the second and most successful strategy, these studies were complemented by synchrotron spectroelectrochemical studies (the simultaneous collection of synchrotron spectroscopy and electrochemistry data) at the European Synchrotron Radiation Facility in Grenoble, France.
The first deposition strategy consisted of the preparation of monocarboxylate solutions in the form of direct (oil-in-water) microemulsions, i.e. stable, isotropic liquid mixtures of the monocarboxylate molecule of interest, water, surfactant, and co-surfactant. In contrast to ordinary emulsions, microemulsions form upon simple mixing of the components and do not require the high shear conditions generally used in the formation of ordinary emulsions. The microemulsions, allowing for a considerable increase in monocarboxylate solubility, act as nanocompartmentalized systems, i.e. nanocontainers acting as “drug delivery devices”, i.e. delivery of molecules to the lead surface. The experimental procedure to treat lead objects will be their immersion in the microemulsion solution.
Within this coating strategy, our first task was to search for suitable combinations of surfactants and co-surfactants to prepare stable microemulsion solutions that effectively solubilize our molecules of interest (long chain monocarboxyc acids). Within the wide range of surfactants and co-surfactants tested, the best results were obtained with TWEEN® 80 (with butanol as co-surfactant), PLURONIC® F127 (with ethanol as co-surfactant), and SHELLAC® (forming colloids, not microemulsions in the presence of ethanol). The search for a successful combination of components to effectively solubilize carboxylic acids was harder than initially forecasted. We looked for a bottom line concentration of carboxylic acid of 10mM, and ideally a concentration above 20mM. Results show that for all the combinations of surfactant/cosurfactant tested, the desired concentrations could only be achieved with very high surfactant concentrations (in molarity), at least 5 times more concentrated in the best case.
The coatings produced by reaction of lead electrodes with such microemulsions or colloidal solutions were proved to be protecting against corrosion. However, the anticorrosion efficiency only showed relatively minor improvements when compared to other coating formation strategies. And therefore new coating formation strategies were sought for.
The second coating formation strategy consisted in the production of lead carboxylate coatings by immersing lead samples in pure melted carboxylic acids. Carboxylic acids have relatively low melting points (46oC, 54 oC, 62.5 oC, 72 oC, 76 oC, for C12 to C20 carboxylic acids with even numbers, respectively). The experimental procedure for coating formation is very simple and consists of immersing the lead sample in the previously melted acid for 24hours, maintaining the temperature in the oven 5oC above the melting point of the acid. Then the lead sample is immersed in pure ethanol at room temperature to dissolve the unreacted acid in excess (pure acids are soluble in ethanol, but lead carboxylate is not).
Coatings prepared in this way were proved by both FT-IR spectroscopy and X-ray diffraction analyses to be only composed of lead carboxylate. These experiments have been performed for C12, C14, C16 and C18 carboxylic acids.
Electrochemical impedance spectroscopy experiments have shown excellent results. Unprecedented impedance magnitude values and phase angle shifts for such a system attest to the protective nature of such coatings (extremely high logZ values at low frequencies and phase angle shifts very close to -90oC at high and middle frequencies).
Moreover, time lapse Electrochemical Impedance Spectroscopy and X-Ray diffraction experiments have shown the excellent capabilities of these coatings towards corrosion resistance. The shape and magnitude of the Bode plots was almost unaltered during 53 days of continuous exposure (immersion) to an aqueous corrosive media. In addition, the XRD patterns remained mostly unchanged during 59 days of exposure to a humid and oak vapour rich atmosphere (imitating natural exposure conditions of organ pipes or objects in oak museum cases).
The low cost and simplicity of the coating method makes it an interesting candidate to test its applicability on real heritage objects. The coating formation method is extremely simple, inexpensive and non-contaminant. The melted carboxylic acid sample use for coating formation can be reused many times.
This coating formation strategy was further studied in a synchrotron beamline. These experiments allowed us to study in real-time the coating formation mechanism. X-ray diffractograms were measured regularly every 10 minutes while a lead sample was continuously immersed in a melted carboxylic acid solution. The large dataset produced during these experiments is currently been analysed. Our plan is to compare the time-lapse diffractograms showing the evolution profile of the growing lead carboxylate layer with SEM micrographs and mass-gain experiments on lead coupons exposed to melted carboxylates for different time periods. We believe that these results, will help us determine the best exposure time to create coatings with optimal anticorrosion efficiency, while avoiding some undesirable effects of keeping the lead objects at temperatures above ambient temperature for unnecessary long time periods.