Atmospheric Corrosion of Lead. The Influence of Organic acid Vapours and Inorganic Air Pollutants. A contribution Towards Understanding the Environmental Degradation of Historic Organ Pipes
This thesis presents different perspectives on the atmospheric corrosion of lead, aiming at a better understanding of the environmental degradation of historical organ pipes. Corroded organ pipes from the field have been analyzed and the environment inside the organ wind system has been mapped. Comparably high concentrations of organic acid vapours are present in heavily corroded organs. In addition, acetaldehyde and formaldehyde are present in smaller amounts.
To verify the connection between environmental parameters and corrosion effects, the field work is combined with laboratory investigations. The effect of sub-ppm concentrations of acetic and formic acid vapours on lead corrosion was investigated. The effect of NO2 and SO2 was also studied. The samples were exposed to synthetic air with careful control of relative humidity, temperature, concentration of air pollutants and flow conditions. Exposure times were between one and four weeks. Corrosion rate was measured gravimetrically and the corrosion products were analysed by X-ray diffraction, environmental scanning electron microscopy, ion chromatography and quantitative carbonate analysis.
The atmospheric corrosion of lead is strongly accelerated by traces of acetic acid vapour. Mass gain is linear with time and depends linearly on the acetic acid concentration. It is suggested that the corrosion of lead in the presence of traces of acetic acid vapour is electrochemical in nature. The corrosion products detected by
X-ray diffraction were plumbonacrite, Pb10O(OH)6(CO3)6, lead acetate oxide hydrate, Pb(CH3COO)2∙2PbO∙H2O, and lead oxide, PbO. Formic acid is slightly less corrosive than acetic acid. In this case, the main corrosion product were lead formate hydroxide, Pb(HCOO)(OH) and plumbonacrite.
In comparison to the organic acids, NO2 and SO2 are far less corrosive towards lead. This implies that emissions of NO2 and SO2 from traffic and combustion of fossil fuels are of minor importance for the atmospheric corrosion of lead.
The results imply that acetic and formic acid vapours emitted from woodwork in the organs are very important corrosive agents for lead pipes in historical organs.
Keywords: Atmospheric corrosion, lead, organ pipe, acetic acid, formic acid, lead acetate, lead formate, NaCl