Corrosion of Fe-Ni alloys by Cl-containing akaganeite (beta -FeOOH); the Antarctic meteorite case

Abstract

A suite of 12 iron and 8 stony meteorites from various environments and locations in Antarctica was studied, representing a range of compositions, metallographic structures, and degrees of weathering. Polished sections of meteoritic metal with adhering corrosion products were examined microscopically, with the electron microprobe and SEM, and were used as samples for X-ray diffraction examination. The major corrosion products are Cl-containing akaganeite (up to 5wtper thousand Cl) and goethite, with minor amounts of lepidocrocite and maghemite. Cl is not native to the meteorites but, owing to the electrochemical nature of the corrosion of Fe-Ni alloys, is attracted from the snow and ice, or rocky soil, environment that contains low amounts of Cl. Akaganeite precipitates near the reaction front, incorporating Cl - ions into ion-exchange sites, where they may be retained, made available for further depassivating, corrosive action, or become flushed from the system. With time and distance from the reaction surface, akaganeite ages and transforms to mainly goethite and maghemite. It is again argued that lawrencite (FeCl 2) is not a mineral in meteorites. In the stormy Antarctic, wind-blown terrestrial particles may become attached to the weathering products on the meteorite and in time themselves decompose, delivering terrestrial ions to the meteorite corrosion system. The ions may move a significant distance from the surface, and in particular Cl may be found along almost invisible cracks in the deep interior. Trace-element work and age determinations ( 36 Cl) should take this problem into account.