A study of the electrolytic reduction of corroded lead objects and the application, characterization and testing of a protective lead carboxylate coating

摘要

A study of the electrolytic reduction of corroded lead objects and the application, characterization and testing of a protective lead soap coating Bart SCHOTTE Lead objects exposed to the atmosphere or buried are known to corrode slightly. However, lead corrodes severely in humid environments, and, more specifically, in the presence of organic acids. This accelerated degradation, commonly referred to as active corrosion, takes place e.g. in display cases of museums and on organ pipes in churches: the objects readily loose mechanical stability and their original surface details. If the process is not stabilized, all details and even the entire artefact could be lost through complete disintegration. A possible treatment to stabilize and conserve active corroded lead is an electrochemical reduction. In this work we have tried to shed some light onto the process. First of all, a fundamental study was performed to understand the mechanism of the ongoing reduction and how it affects the profile of the corresponding reduction curve. In a second part, we focused on the evaluation of the treatment, related to the chemical and morphological changes of the lead artefacts. Finally, we studied the efficiency of the treatment on corroded lead alloys. Therefore, lead and lead alloys, including lead-tin, lead-antimony and lead-bismuth were artificially corroded by vapours of formic acid, acetic acid, propionic acid and acetaldehyde at different concentration levels. Consequently, the electrolytic reduction of the corroded coupons was studied to evaluate the effects of the alloying elements. In this work, we studied the application of lead decanoate, deposited by immersion in sodium decanoate solutions. We focused on the lead decanoate layer growth in situ using electrochemical impedance spectroscopy and X-ray diffraction, in order to obtain insights in the protection and growth properties. The preparation of decanoate solutions was investigated and resulted in the establishment of a protocol for a solution that produces rigid coatings. Using this solution, we studied the lead decanoate surface coverage on a basis of solid/liquid contact angle measurements at regular time intervals during treatment. In addition, we monitored the coating resistance during application by electrochemical impedance spectroscopy and studied effects of stirring and of the layer thickness of the liquid phase to determine most efficient application conditions.