Another Base, Another Solvent? Desalinating Iron Finds with Tetramethylammonium Hydroxide Solution

The desalination processes commonly applied to improve the corrosion stability of archaeological iron artifacts are based on immersion treatments in aqueous sodium hydroxide solutions. Faster and more efficient chloride extraction in solutions based on organic solvents with a lower surface tension can be expected. Furthermore, the danger of new corrosion forming during the subsequent washing out of residual chemicals from the desalinating solution could be minimised, if organic solvents would replace the water, commonly used for this process. Only alkali metal hydroxide (LiOH and NaOH) solutions in organic solvents have been tested so far. Their comparatively low chloride extraction efficiency was ascribed to the low solubility of the alkali metal hydroxides and the corresponding chlorides in the solvents used. Tetramethylammonium hydroxide (TMAH) is readily soluble in alcohols and has been tested in aqueous and/or methanolic solutions as an alternative. Neither improved overall chloride extraction efficiency, nor a higher chloride extraction rate could be observed using methanolic solutions. However, aqueous TMAH showed a trend towards higher overall chloride extraction efficiency than the common alkaline treatments. These results could be explained by the different solubility of corrosion products, in particular akaganéite, β-FeO(OH), in the tested solutions.