Incipient processes in the corrosion of mild steel in 90°C hypersaline geothermal brine

Abstract

Mild steel (AISI 1009 and 1018) specimens were exposed for ca. 20 h to Salton Sea, hypersaline, geothermal brine at ca. 90°C. The incipient growth of the precipitated scale and iron corrosion products were analyzed by scanning and transmission electron microscopy, X-ray fluorescence, X-ray diffraction, electron diffraction and electron microprobe techniques. In brine at its natural pH of 5.7, several distinct phases appear on the steel surfaces, the most important being iron-rich amorphous silica. The silica deposit grows in irregular patches, and corrosion of the substrate occurs between the patches. Subsequent acidification of the brine for short periods does not disturb the protective layers of silica but greatly increases the rate of corrosion of surface areas not covered by silica. Oxidation of mild steel in acidified brine is quite different. Rather than iron compounds, deposits of both metallic lead and PhCO 3, the latter in unusual crystalline needles, and an unidentified copper compound dominate the surface coverage at low pH.