Investigation of the natural plant extract Emblica officinalis for chloride release from βFeOOH

摘要

Controlling post-excavation corrosion of chloride (Cl~-) infested archaeological iron presents a major storage problem for museum professionals. This corrosion is stimulated by Cl~- containing corrosion products, such as akaganeite (βFeOOH), whose hygroscopicity and mobile surface adsorbed Cl~- corrode iron at low relative humidity (RH). Renewed interest in inhibitors to control corrosion of archaeological iron has centred on low toxicity eco-friendly compounds. Recent study suggests that the extract of Emblica officinalis (Indian gooseberry) has inhibitive properties for iron. This is attributed to the high level of the strongly hydrolysable gallic (C_6H_2(OH)_3COOH) acid in E. officinalis extract dissolving βFeOOH to produce Fe~(2+), which it complexes to form magnetite (Fe_3O_4) covered in iron gallate. This transformation releases Cl~- ions from βFeOOH into solution, thereby removing electrolyte ions that drive iron corrosion and interfere with the effectiveness of many inhibitors. Simply cold washing βFeOOH removes most of its Cl~- and significantly slows its ability to corrode iron at high RH. The study reported here examines quantitatively the impact of washing βFeOOH with various aqueous concentrations of E. officinalis, over differing time periods, to assess the ability of βFeOOH to corrode iron. βFeOOH transformation was investigated using FTIR and Micro-Raman spectroscopy.