Mineralogical-Geochemical Study of Corroded Iron-Based Metals from a Salt Mine Environment

Metal fragments (pipe, chain, valves), at advanced stages of corrosion, were collected underground in the Wieliczka salt mine. Macroscopically distinct zones of corroded material, as well as black blisters on the surface of different metal fragments, were studied using scanning electron microscopy with energy dispersive spectrometry (SEM-EDS), x-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and Mössbauer spectroscopy (MS). SEM-EDS studies showed various morphological forms with different chemical compositions. The original outer zone of the iron artifacts is mainly composed of aggregates of needle-shaped goethite crystals with idiomorphic crystals of halite. A subsequent zone, toward the inner margin, is composed mainly of fine granular aggregates of magnetite. Goethite (α-FeOOH) and akaganeite (β-FeOOH) form spherical, fibrous, and structureless aggregates in the next internal zone. Forms of aggregates seem to depend on the chloride content, 1 wt% to 3.5 wt% Cl in the structureless aggregates and 5 wt% to 9 wt% Cl in the regular ones. In addition, in the internal zone crystals of lepidocrocite form rosettes. Blisters are built of the acicular akaganeite crystals, which form fibrous aggregates in the shell and spherical ones in the interior. The relative concentrations of iron bearing minerals in the studied zones, i.e., akaganeite, hematite, goethite, magnetite, and lepidocrocite are established. Where they dominate, the zones are: black (magnetite), orange (goethite, lepidocrocite, akaganeite), and light brown (goethite).