Curious Corrosion Compounds Caused by Contact: A Review of Glass-Induced Metal Corrosion on Museum Exhibits (GIMME)

Abstract

Many heritage objects consist of glass in contact with metals. By ion exchange with absorbed water, alkaline aqueous films are formed on the glass surface. They contain sodium and/or potassium, hydroxide, and carbonate (uptake of carbon dioxide) ions. These electrolytes induce corrosion while in contact with metal. Surprisingly, this phenomenon has only been realised by research in Stuttgart in the last two decades. About 350 affected objects were detected in the meantime in a number of heritage collections. Because of the special electrolytes, unusual corrosion products are often formed. The unknown structure and formula of three of them could be determined by modern X-ray powder diffraction data evaluation. One example is the basic potassium lead carbonate, KOH‧2PbCO3, detected on a pewter lid of a glass jug. The sodium analogon of already known structure was found in hollow glass balls mirrored on the inside with molten lead. Chalconatronite, Na2[Cu(CO3)2]‧3H2O, is known as a corrosion product of copper alloys in contact with soda solutions (here: from glass degradation). Exposed to acetic acid emissions (e.g., from wood), it transforms to a sodium copper acetate carbonate of hitherto undetermined structure. The ubiquitous pollutant formaldehyde reacts directly to formate in the alkaline medium provided by glass degradation. On copper alloys in contact with glass, formates are, therefore, frequent: Na4Cu4O(HCOO)8(OH)2‧4H2O in 50% of all cases and in 33% Cu2(HCOO)(OH)3. Zinc (from brass) forms Zn(HCOO)2‧2H2O and Zn4Cu3(Zn1−xCux)6(HCOO)8 (OH)18·6H2O. There are a number of other corrosion products, e.g., containing zinc and carboxylates awaiting further characterisation. Preventive conservation needs to slow down corrosion by dry storage (not lower than 35% rH). Pollutants need to be avoided by careful selection of materials for storage, display, and conservation.