Operational challenges and types of corrosion
The interaction between brine and metallic elements in the installations at sites commonly results in corrosion, whose reaction products may cause operational difficulties. Corrosion can occur with a variety of oxidants. Corrosion of Fe(0), a major constituent of many alloys, is particularly rapid through redox reaction with oxygen, which often cause the formation of Fe(III) oxides (rust), e.g.:
4 Fe0 + 3 O2 + n H2O= Fe2O3 * n H2O
In the absence of O2, corrosion occurs anaerobically at much slower rate using water as the oxidant:
Fe0 + H2O = Fe2+ + H2 + 2 OH–
This reaction produces hydrogen gas (H2). It also increases pH locally so that the solubility of ferrous hydroxide may be exceeded. At such places, the net reaction becomes:
Fe0 + 2H2O = Fe(OH)2 + H2
Ferrous hydroxide is not very stable thermodynamically and usually dissolves if pH is not above 7.5 or so. Given that the reported pH of waters in our database typically is lower than 7.5, it is unlikely that ferrous hydroxide can migrate from the site of formation at temperatures above ~75 °C. However, ferrous hydroxide can itself reduce the protons in water at appreciable rates, leading to the formation of the mixed valent iron oxide magnetite, Fe(II)Fe(III)2O4, which is thermodynamically very stable.
Thus, formation of smaller amounts of ferrous, mixed valent, or ferric oxides, hydroxides, or oxyhydroxides from corrosion of infrastructure with metallic iron is unavoidable. If temperatures are sufficient or oxidation occurs with O2, the Fe oxides are likely to be thermodynamically stable. The formed Fe oxides are typically nanoparticulate, aggregated to variable degree, and they may migrate with the flow, which could cause clogging of the injection well. Typically, however, they form surface coatings that protect the metallic installations from further corrosion. Thus, the slow anaerobic corrosion with water as the oxidant generally causes little harm. The ingress of oxygen, however, can dramatically increase corrosion rates. Hence, the geothermal operations are typically operated at significant overpressure to avoid influx of O2 (and minimize formation of CO2). Figure 1 presents an overview of corrosion types that can occur for both unalloyed steel and corrosion resistant alloys (CRA). A brief description of each corrosion type is outlined below.