Atmospheric Corrosion of Historic Lead Organ Pipes

Doctoral thesis, 2007

The atmospheric corrosion of lead is investigated, with the aim to improve the understanding of the environmental degradation of historical organ pipes. Fieldwork is combined with laboratory investigations. The field studies included characterization of the organ environment and exposures of lead coupons. The environment inside the organ wind system of instruments suffering corrosion was mapped and compared with instruments unaffected by corrosion, in five countries in Europe. In the heavily corroded organs, relatively high concentrations of acetic and formic acid vapours were present, and large amounts of carboxylates accumulated on the lead coupons. The organic acids are emitted from woodwork in the organs. The effect of acetic and formic acid vapours in the ppb range of the atmospheric corrosion of pure lead was investigated under well controlled conditions in the laboratory. The combined effect of acetic and formic acid vapours was also examined. The influences of temperature, relative humidity, inorganic air pollutants (NO2 and SO2) and particulates (NaCl) were studied as well. Corrosion rate was measured gravimetrically and the corrosion products were analysed by XRD, ESEM, FIB, ion chromatography and quantitative carbonate analysis. The mechanism of corrosion attack is addressed. Traces of acetic acid vapour strongly accelerate lead corrosion. Mass gain is linear with time and depends linearly on the acetic acid concentration. Corrosion showed a relatively weak dependence on relative humidity in the range 50 - 95%. Lead corrosion was inversely related to temperature; mass gain was 50% higher at 4.0 ºC than at 22.0 ºC. The corrosion rate increased with decreasing temperature in the range 22.0 - 4.0 ºC. It is proposed that lead suffers electrochemical corrosion in the presence of traces of acetic acid vapour. The corrosion products were unevenly distributed. Lead acetate oxide hydrate (Pb(CH3COO)2∙2PbO∙H2O), plumbonacrite (Pb10O(OH)6(CO3)6), litharge (a-PbO) and massicot (b-PbO) were identified by X-ray diffraction. Formic acid vapour is also corrosive towards lead, although somewhat less so than acetic acid. In this instance the corrosion products, consisting of plumbonacrite and lead formate hydroxide (Pb(HCOO)(OH)), are evenly distributed. The combination of acetic and formic acid has a slightly synergistic effect on lead corrosion, which resulted in a more localized attack than for the acetic and formic acid exposures. The results show that acetic and formic acid vapours emitted from woodwork in the organs are decisive in the corrosion of historical organs. Methods for mitigating organ corrosion are discussed. The work in this thesis is a basis for formulating conservation strategies for historical organs.