Chemistry and Art Conservation
Polyaniline as Anticorrosive Coating on Iron
This research is focused on corrosion and corrosion protection/prevention of historical iron artefacts and metallic works of art using new techniques of mitigation. We are currently testing newly developed coatings on iron, brass, and copper samples.
The iron samples are coated with various types of polyaniline that are doped and dedoped in the conventional and nanofiber form synthesized by the research group led by Ric Kaner combined with various barrier coatings frequently used in art preservation such as wax, Acryloid B-72, and Incralac. Corrosion of iron occurs when the surface of the metal oxidizes on exposure to water and oxygen forming ferrous and ferric oxides. Iron will readily undergo a redox reaction in the presence of hydronium ions. These ferrous ions form unstable ferrous salts and ferrous oxide (FeO), which then react with oxygen to form more stable ferric oxides (Fe2O3, Fe3O4). The oxidation then continues causing the formation of pits. The corrosion process can be inhibited by surface coatings that either protect the surface from the water and oxygen in the environment like paint, or interact chemically with the iron surface like zinc. The chemically reactive coatings either form a passive oxide, or have a lower reduction potential to thus be oxidized preferentially to the iron surface. The polyaniline coating should act like a chemically reactive coating helping form a stable iron oxide that should prevent further corrosion.
The purpose of this experiment is to determine the effectiveness of polyaniline nanofibers as corrosion inhibitors alone or with additional over-coatings.
Figure 1. Images of the iron samples first placed on roof (left) and after three months (right).
Degradation of Museum Shell Collection
Our interests are in the characterization and prevention of deterioration of calcareous materials such as shells in museum collections. Over time a large number of the shells and fossils begin to grow crystals known as efflorescence. Eventually the efflorescence causes deterioration of the shells and the loss of many collections as seen in figure 2. This growth on shells is known today as Byne’s disease. We are currently working with the Los Angeles County Museum of Natural History to investigate aspects of this deterioration. The main interest of this is to design new techniques/ materials to prevent the loss of museum objects.
Figure 2. Shell collection in the Natural History Museum, Los Angeles 2006. Some shells have Byne’s disease, leading to total deterioration of the shell.
For many years and in most museums today, shell collections are stored in wooden cases. It is now known that wood such as oak, pine, spruce, plywood, and medium density fiberboard outgas formaldehyde, acetaldehyde, formic acid, and acetic acid. Even paints, wood coatings, and adhesives are known to outgas these harmful chemicals.The efflorescent salts have been identified as calcium acetate formate hydrate Ca(CH3COO)(CHOO)•H2O (mixed salt), calcium acetate hemihydrate Ca(CH3COO)2•˝ H2O, calcium acetate hydrate Ca(CH3COO)•H2O, calcium formate Ca(CHCOO)2, and calclacite Ca(CH3COO)Cl•5H2O.
Some suggestions have been made to alter the environment around the collections to prevent efflorescence. The major suggestions have been to use sorbents such as activated charcoal, molecular sieves, and zeolites in the storage cabinets; increase ventilation to reduce concentrations of pollutants; and store items in acid-free paper boxes, or polycarbonate plastic boxes. Some museums use activated charcoal but it must be replaced after a specific amount of time. Currently, the majority of museums store collections in acid-free paper boxes likes the ones shown in figure 2.
Completely metal or polycarbonate plastic cabinets would help reduce the occurrence of efflorescence, but replacing the existing wooden cabinets would costs a substantial amount of money, which most museums do not have. Experiments need to be done to prevent efflorescence in the museums’ current conditions. A few things must be understood before a preventative treatment can be tested. Will there be a preference of crystal formed in presence of acetic and formic acid? Is there accurate identification of the possible salts? Can we setup reliable conditions in which efflorescence will happen to then test treatments?
Previously, Brokerhof and Bommel determined how fast efflorescence happens on shells at specific concentrations of acetic acid and relative humidity. However, no one has reported to have grown crystals using a solution of acetic and formic acid, nor has anyone attempted to determine a pretreatment for new and old collections that will prevent efflorescence from happening with current storage conditions.
The focus of this experiment is to grow crystals on fresh shells with acetic acid and formic acid vapor at 54% RH in a desiccator. The results from this experiment will help determine which salt is more likely to form and how long it takes to grow crystals under the give conditions. If different salts form, there can be an assessment of which salt causes more damage or faster deterioration. The results of this experiment will lay down the foundation to eventual determination of a pretreatment to prevent efflorescence. In addition to simulating efflorescence, samples of shells and fossils that have efflorescence were taken from the Natural History Museum, Los Angeles to identify. The identification of these samples by X-ray diffraction and ATR/FT-IR spectroscopy will confirm current ideas of efflorescence or bring new types of efflorescence to light.