Megan Creamer and Emy Kim
Waxes are a common method of barrier coating outdoor metal sculpture in conservation, although several studies have identified instances where waxes can promote corrosion. Plant extracts have been identified in industrial applications as having distinct corrosion inhibition effects, particularly for iron alloys, but have been minimally explored for conservation applications. Oils and waxes that do not crosslink and are retreatable remain appealing conservation coatings. Therefore, the goal of this research was to broadly explore the application parameters and corrosion inhibition potential of select plant extracts in combinations with waxes for outdoor steel sculpture.
The oil extracts of tea tree, clove, and a complex mixture of plant extracts including tea tree, rosemary and others in an almond carrier oil were tested with and without Trewax paste wax or Cosmolloid 80H microcrystalline wax on mild steel coupons. Multiple experiments were done to characterize the corrosion inhibition properties including variables such as the presence of chlorides, hot versus cold waxing, and performance in different outdoor seasonal environments. Test coupons were compared to a variety of controls with sample replicates of three or more for each test. The steel coupons were previously corroded and then cleaned to simulate the complex surface topography and processes used for corrosion reduction prior to coating outdoor sculpture.
Results from this study indicate that the selected plant extracts combined with waxes could reduce corrosion of outdoor steel objects compared to samples without any coatings. Iterative experiments identified some initial parameters that impacted the corrosion inhibition efficacy of these coatings: different oil extracts had variable performance, select oils performed differently with the two waxes, and the temperature of wax application affected the corrosion inhibition efficacy of the combined plant oil and wax coating. The coatings of these oil-based plant extracts and waxes maintained their solubility profiles, allowing for removal with conventional methods without visible discoloration over the limited duration of the tests.
Literature from different fields of study suggest that the active components of plant extracts driving corrosion inhibition effects include carboxylic acids and flavonoids. Suggested mechanisms of action from the literature include adsorption of organic compounds to the steel where they perform a mixed anodic and cathodic inhibition; the chelation of ferric and ferrous ions and free radical scavenging abilities; the reduction of available iron oxides through tannic acid components; biocidal activities of organic molecules, and the possibility of an improved wax film due to interactions between non-drying oils and the wax pore matrix.
Both the literature from other fields and the data from this exploratory study of practical application indicate that these materials should continue to be researched and tested in both analytical and practical settings for their corrosion inhibition potential in the conservation of iron alloys. This research provides accessible materials and methodologies and adds to our knowledge of retreatable corrosion coatings for the conservation of outdoor iron-alloy cultural heritage.