Article: Treatments of corroded lead artefacts (2006)

Title: Treatments of corroded lead artefacts - An overview
Authors: Bart Schotte and Annemie Adriaens
Reference: Studies in conservation 56 (4), 2006, 297-304
Link: http://hdl.handle.net/... (pdf, 589 Kb)

Abstract:

In the past, corroded lead objects have been treated in many different ways. Some treatment methods are based on cleaning the surface, and rely on the dissolution of the stable corrosion layer. Other methods involve the conversion of the corrosion products into metallic lead, and have been used to preserve the design, which is still present in the corrosion layer and which contains information of the original surface. Such techniques work particularly well on lead objects that have been exposed in a humid environment containing organic acid vapours, where an active corrosion crust is likely to develop.The volume of the corrosion products in an active corrosion crust is greater than that of the original metal, resulting in cracking over time, thereby increasing the access of pollutants to the metal below.This overview describes the different treatment methods applied to lead objects; it is not intended to provide practical guidelines with regard to the conservation of lead objects.

Thesis: electrolytic reduction of corroded lead objects (2007)

Title: A study of the electrolytic reduction of corroded lead objects and the application, characterization and testing of a protective lead carboxylate coating
Author: Bart Schotte
University: Ghent University
Year: 2007
Pages: 199
Type of document: PhD thesis
Link: http://hdl.handle.net/... (pdf, 14.6 Mb)

Note: Resource kindly indicated by Goran Budija

Abstract:
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.


Table of contents (short version):

I. General introduction, corrosion and conservation, applied techniques
1. General introduction on lead
2. Experimental methods

II. Study of the electrolytic reduction of actively corroded lead objects
3. A fundamental study of the electrochemical reduction treatment
4. Chemical and morphological changes
5. Reduction of artificially corroded samples

III. Lead soaps
6. Lead carboxylate coatings: General introduction
7. Real time spectroelectrochemistry
8. Optimization of the Pb(C10)2 coating process
9. Growth and corrosion monitoring of PbC10 layers

Summary and conclusions
Appendicea