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Corrosion of Lead Alloys for Organ Pipes

Constituent Materials, Microstructure and Inner Structure

University of Bologna Team Leader: Martini Carla - Assistant Prof. - Dep. of Sciences of Metals, Electrochemistry and Chemical Techniques.

University of Bologna Research Group: Carla Martini, Cristina Chiavari, Daria Prandstraller (from Sept. 2006 at Ravenna Campus, University of Bologna, Ravenna, Italy).

Partner/Collaborations:

  • University of Gothenburg, Göteborg Organ Art Center, Sweden;
  • Ev.-Luth. Kirchengemeinde St. Jakobi, Lübeck, Germany;
  • Marcussen & Søn, Orgelbyggeri A/S, Denmark;
  • Department of Environmental Inorganic Chemistry, Chalmers University of Technology, Sweden;
  • Department of Metals Science, Electrochemistry and Chemical Techniques, University of Bologna, Italy.

Context and objectives

The European heritage of the organ is preserved in numerous historical instruments. However, one major threat to this heritage is the indoor atmospheric corrosion of lead and lead-tin alloys of organ metal pipes, constituting the central sounding part of the organ. The COLLAPSE project objectives were to define relevant methods and products as well as to create conservation strategies in order to combat the corrosion of lead and lead-tin alloy organ pipes.

The aims of the University of Bologna Research Group were the following: (i) Microstructural and chemical analysis of corroded pipes and (ii) Development of methods for pipe surface protection treatments.

Methodologies and equipment

The phase composition, morphology and topography of corrosion products on the surface of corroded pipes was non-destructively analysed by x-ray diffractometry (XRD), micro-FTIR, micro-Raman, SEM/EDS and stylus profilometry or atomic force microscopy (AFM), respectively. Micro-samples were subsequently mounted and polished according to metallographic techniques specifically optimised for lead and lead alloys, and their microstructure was observed by optical microscope (OM) and SEM/EDS. After the complete documentation of the microstructure, a few milligrams from the uncorroded (metallic) part of the samples was analysed by flame atomic absorption spectroscopy (FAAS). The corrosion behaviour of the treated samples was evaluated by accelerated corrosion tests performed by exposing the treated coupons to acetic acid in gas phase (about 7 ppm). Corrosion products were analysed by XRD and SEM/EDS. Mass variation was evaluated by gravimetric measurement as a function of exposure time.

Results

The corrosive environment inside the pipe foot is created by the emissions of organic acids (especially acetic acid) from the wooden parts in the organ. The organic acids enter the pipe foot through the toe hole and a corrosive environment is created inside the pipe foot. For a corrosive environment to create conditions of corrosion in the pipes will mainly be dependent on the pipe metal alloy, the temperature, and the relative humidity in the organ. The pipe metal is normally a lead-tin alloy. The composition of the pipe metal is a very important factor in influencing corrosion. In a corrosive environment, metal containing less than about 1% tin and especially pure lead is very sensitive to corrosion but a few percents of tin in the alloy makes the metal more corrosion-resistant. Tin, however, has a protecting effect at lower humidity but at higher humidity the protecting effect gradually disappears.

The results from the surface protection tests showed that not one of the tested surface treatments could guarantee a protection from corrosion in a corrosive environment in a long time perspective, therefore none of the candidate treatments should be used on lead organ pipes.

Considering the reasons for the corrosion, the only sustainable way to solve a pipe corrosion problem in a long time perspective is to change the environment in order to decrease the concentration of organic acids in the pipe foot. It is also important not to create a corrosive environment when repairing or restoring an instrument.

More details can be found in:

  • C. Chiavari, C. Martini, D. Prandstraller, A. Niklasson, L-G.Johansson, J-E. Svensson, A. Åslund, C.J. Bergsten, Corrosion Science 50 (2008) 2444-2455.