The role of âFeOOH in the corrosion of archaeological iron

The chloride bearing corrosion product akaganéite (βFeOOH) can form during postexcavation corrosion of chloride infested archaeological iron and is able to corrode iron in contact with it. Its action on iron is examined using βFeOOH synthesized from ferrous chloride and iron powder. Using weight measurements the hygroscopicity of βFeOOH is established. The influence of relative humidity on the corrosion of iron powder mixed with βFeOOH is examined by dynamic mass change within a climatic chamber. At 20°C and 12% relative humidity, iron in contact with βFeOOH did not corrode. At 15% relative humidity slight iron corrosion was detected after 160 hours, but at 35% relative humidity corrosion occurred after a few hours. Surface adsorbed chloride was removed from βFeOOH by aqueous washing and this reduced its hygroscopicity. The reported metastability of βFeOOH was examined via XRD of a 23 year old sample, which was found to be still entirely composed of βFeOOH. These results provide better understanding of βFeOOH corrosion of iron, corrosion control of chloride infested iron using dry storage and the effect of aqueous washing on archaeological iron