Polymer coatings may damage iron artifacts

Conservators use polymer coatings to preserve iron artifacts by keeping out moisture and oxygen to prevent corrosion. But a recent study suggests that as they age, such coatings may trigger a cycle of corrosion, leading to further aging of the coating itself (ACS Cent. Sci. 2024, DOI: 10.1021/acscentsci.5c00067).

It’s already known that polymer coatings age, potentially generating reactive oxygen radicals such as carboxyls. But researchers have not studied the effect of the aging process on the underlying iron artifacts. To fill this knowledge gap and better understand the reactions occurring at the polymer-metal interface, a team of chemists from the Beijing University of Chemical Technology and Peking University created artificially aged samples and used 3D fluorescence imaging to monitor changes in the amount of carboxyl groups both at the coating surface and at the interface between the polymer and the metal.

The researchers coated the iron samples with B72, a copolymer of methyl acrylate and ethyl methacrylate commonly used in conservation. They simulated aging by exposing the samples to ultraviolet irradiation and an elevated temperature of 60 °C for between 3 and 30 h. To image the resulting changes, the team used an amino-fluorescein (AF) probe, a technique borrowed from molecular biology for labeling proteins.

Fluorescence levels indicated that as samples aged, more carboxyl groups were generated at the polymer surface and, in even greater quantities, at the interface between the surface and iron. Additionally, X-ray photoelectron spectroscopy (XPS) of polymer-coated iron showed that more Fe(II) and Fe(III) species were present after aging than before. This finding provides evidence that, as the polymer aged, an increasingly acidic and oxidizing microenvironment emerged, causing further corrosion.

When doing the same experiments with B72-coated quartz glass, the researchers found polymer aging at the coating surface, but there was no deterioration of the polymer at the interface of the two materials. They concluded that, because of the catalytic action of the metal ions, the iron-polymer system demonstrated a unique cycle of polymer aging followed by accelerated corrosion, which then caused further polymer chain breakages.

The team also analyzed several archaeological samples, including iron debris recovered from the Nanhai No. 1 shipwreck, dating from the Southern Song dynasty (1127–1279). These samples included rust layers under the polymer coating. The samples showed that the damage after 5 h of artificial aging was even more pronounced than in the nonarchaeological samples. XPS again showed there was a higher proportion of Fe(III) ions present after aging, which provides evidence of active corrosion.

Valentin Boissonnas, a metals conservation researcher at Haute Ecole Arc, says via email that, in his experience, corrosion under polymer coatings occurs mainly with outdoor iron objects. He says that before becoming alarmist, we need to understand how the accelerated UV exposure levels in the simulated aging process correspond to real-life exposure outdoors or indoors. The team has not yet modeled how its accelerated aging process would compare with aging in real time.

Coauthor Rui Tian, a chemist from Beijing University of Chemical Technology, says via email that at this point, they are not recommending conservators stop using polymer coatings. “Instead, we recommend stricter conditions for conserving artifacts, such as avoiding UV light, heat, and oxygen,” Tian says.

Boissonnas says the type of solvent used to apply the coating and the time between coating and testing could also be factors because remaining solvents can influence corrosion behavior. But ultimately, these polymers are stabler than alternative nitrocellulose coatings. “There are no other transparent coatings that could replace it [B72] at the moment should it turn out to be problematic.”

Tian says one solution could be developing coatings that contain antiaging additives, including UV absorbers such as zinc or titanium oxides; radical quenching agents to decay possible hydroxyl, peroxyl, and alkoxyl radicals; and additives with the ability to decrease carboxylic acid moieties in aging polymers.