Localized corrosion and phase transformation of simulated archaeological iron

The corrosion behaviors of simulated, archaeological iron in solution (0.06 mol.L-1 NaCl+0.03 mol.L-1 Na2SO4+0-01 mol.L-1 NaHCO3) simulating soil water composition was investigated by potentialdynamic polarization, constant potential polarization, and simulated occluded cell (O.C.) galvanostatic tests. X-ray diffraction (XRD), energy dispersive spectrometry (EDS), and scanning electron microscope (SEM) were used to study the corrosion morphology and the evolution of corrosion product. The objective was to discover the transformation process of archaeological iron, and determine the distribution of chlorinated corrosion products. The results showed that the presence of crevice, cavities, and channels facilitates the localized corrosion under rusts; the autocatalytic effect increases the concentration of Fe2+, Cl-, and SO42-, and promotes local acidification within the crevices and cavities. Meanwhile, the phase transformation of corrosion products is concluded to proceed by means of two ways. One is that the ferrous ions are transformed into different kinds of FeOOH via the intermediate Fe(II)-Fe(III) hydroxyl-salt (i.e. Green Rusts); the other is that the Fe2+ ions are transformed into FeCl2, FeCl3, and orange powders akaganeite at the crevices and cavities.