Acetic Acid Vapor Corrosion of Lead-Tin Alloys Containing 3.4 and 15% Tin

Lead-tin alloy pipes in historic organs frequently suffer from damaging atmospheric corrosion. Organic acids emitted from the wood of organ cases had been established previously as the cause of deterioration in pure lead pipes. In the present study, lead-tin alloy samples containing 3.4 and 15 atom % tin were subjected to laboratory exposure experiments under atmospheres of 1100 ppb acetic acid, 350 ppm CO2, and 60 or 95% relative humidity (RH). Wet and dry corrosion mass gains were monitored, and corrosion product compositions and morphologies were characterized by grazing incidence angle X-ray diffraction and scanning electron microscopy. Cross sections were cut through corrosion sites using a focused ion beam milling method, and elemental information was obtained using wavelength dispersive and energy dispersive X-ray analyses. The corrosion products and morphologies observed in the alloys are the same as those reported for pure lead, but the corrosion susceptibilities of the alloys show a much stronger dependence on RH. The presence of 3.4 or 15 atom % Sn provides corrosion protection at moderate humidity, but this protective effect breaks down at high humidity. These findings highlight the importance of humidity control as well as pipe material selection in the conservation of historic and newly constructed organs