Determination of parameters for local electrolytic treatment of corroded lead and lead-tin alloys

A new electrolytic pencil (composed of a pseudo-electrode and a counter electrode contained in a nozzle enclosed by a microporous sponge pad in contact with a metallic object) was tested on its capacity to reduce the lead carbonates, which develop on lead and lead-tin objects exposed to corrosive organic acid vapors. Pure lead and two lead-tin alloys (5 and 50% of tin) were considered. Treatment parameters were defined by an electrochemical study on artificially corroded samples. Lead carbonate reduction is usually carried out at −1.3 V versus a mercury saturated sulfate electrode (SSE). In the case of lead-tin alloys, a second reaction exists in the cathodic field corresponding to tin oxide (SnO2) reduction, which starts at −1.7 V/SSE. Thus, it is possible to realize a selective treatment, reducing only lead carbonates. The potential value depends on the reference electrode and the ohmic drop induced by the configuration of the electrolytic pencil, the Pleco. The pseudo-reference electrode can be a glassy carbon electrode (GC) or a tungsten electrode (W). Treatment parameters were determined with the Pleco using the two above-mentioned reference electrodes: lead carbonate reduction is at −1.5 V/GC or −1.4 V/W. Reduction of lead carbonates was done successfully with Pleco: overall thickness of lead carbonates was reduced without affecting their original shape, thereby ensuring the preservation of surface details and decorations. This local treatment was also tested on the interface between the metal and cord parts of seals attached to parchment: the cords are better protected while the other parts of the seals will undergo electrolytic treatment by immersion. Corrosion products were reduced without degrading the threads. Finally, a reduction treatment was performed on a lead seal belonging to the Departmental Archives of Loire-Atlantique (France).