Electrochemical Phase Diagrams for the Fe/S/H₂O System under Geothermal Conditions

Abstract

Previously published electrochemical phase (potential-pH) diagrams for the Fe/S/H₂O system at elevated temperatures are assessed. It is concluded that the potential-pH equations derived by Biernat and Robins¹ can be used to provide reliable diagrams over the temperature range 298-573 K. These equations were used to derive a set of diagrams relevant to geothermal fluids in general terms, so that the corrosion products of iron and ferrous alloys immersed in these fluids may be predicted. In addition, diagrams are presented for geothermal fluids in the Broadlands and Wairakei fields in New Zealand. The significant changes in the E-pH diagrams as the temperature increases over the range 298-573 K are that the region of stability for Fe(OH)₃^- widens and the regions of stability for FeS₂ and Fe₃O₄ and, at higher temperatures, FeS, shrink in size.