The evaluation of corrosion inhibitors for application to copper and copper alloy archaeological artefacts

Abstract

This thesis concerns corrosion inhibiting compounds which slow the deterioration of archaeological copper artefacts. Benzotriazole (BTA) and 2-Amino- 5-mercapto-1,3,4-thiadiazole (AMT) have been applied as corrosion inhibitors in archaeological conservation. A search was conducted for similar compounds that could be applied in the conservation of copper and copper alloys. According to a list of requirements specific to archaeological conservation, six new inhibitors were tested. 2-Aminopyrimidine (AP), 5,6-Dimethylbenzimidazole (DB), 2-Mercaptobenzimidazole (MBI), 2-Mercaptobenzoxazole (MBO), 2-Mercaptobenzothiazole (MBT), 2-Mercaptopyrimidine (MP), were tested for potential archaeological conservation applications. The literature 1ists only one corrosion test for copper chloride corrosion in archaeological conservation, with intact surface corrosion products. Industrial corrosion tests were checked in American Society for Testing Materials (ASTM), British Standards Institution (BSI), Deutsche Industry Norm (DIN), International Standards Organisation (ISO). Chlorides are known to be a major factor in ongoing deterioration of copper artefacts in the museum environment. The mechanisms and types of chloride corrosion are discussed, including past and recent literature on the subject. No corrosion test was found entirely suitable for the evaluation of corrosion inhibitors against corroded metallic surfaces in conservation, so an appropriate corrosion test had to be developed. Copper coupons were immersed into cupric chloride solutions to produce cuprous chloride corrosion, the structure of which is similar to corrosion found directly against remaining metal of artefacts. These coupons were subsequently treated with corrosion inhibitor solutions, and exposed to elevated relative humidity to assess corrosion inhibition, by monitoring the weight changes during the experiments. The inhibitor solutions were analysed regarding the depletion of copper in copper corrosion products, using Atomic Absorption Photospectroscopy (AAS). The colour changes of copper corrosion products treated with inhibitor solutions were determined using a chroma meter. According to these findings AMT, and MBT were selected for more extensive performance tests against BTA on archaeological metal artefacts