Focused ion beam and electron microscopy analysis of corrosion of lead-tin alloys: Applications to conservation of organ pipes

Across Europe, lead-tin alloy organ pipes are suffering from atmospheric corrosion. This deterioration can eventually lead to cracks and holes, preventing the pipes from producing sound. Organ pipes are found in compositions ranging from >99% Pb to >99% Sn. For very lead-rich (>99% Pb) pipes, organic acids emitted from the wood of organ cases have previously been identified as significant corrosive agents. In order to study the role of alloy composition in the susceptibility of pipes to organic acid attack, lead-tin alloys containing 1.2-15 at.% Sn were exposed to acetic acid vapors in laboratory exposure studies. Corrosion rates were monitored gravimetrically, and corrosion product phases were identified using grazing incidence angle X-ray diffraction. In a new method, focused-ion beam (FIB) cross sections were cut through corrosion sites, and SEM and WDX were used to obtain detailed information about the morphology and chemical composition of the corrosion layers. The combination of FIB and SEM has made it possible to obtain depth information about these micron-scale layers, providing insight into the influence of acetic acid on alloys in the 1.2-15 at.% Sn range