Laboratory HCl acid condensate testing of geothermal casing steels

Abstract

The geothermal steam discharged from superheated geothermal wells can contain a range of corrosive gases: mainly hydrogen sulphide (H2S), carbon dioxide (CO2) and ammonia. Occasionally hydrogen chloride (HCl) and boric acid may also be present. At the temperatures encountered in geothermal wells the superheated “dry” steam is rarely corrosive. However, dew point corrosion can be significant even with relatively low concentrations of HCl gas leading to corrosion of steam pipelines, wellheads and at times well casings. HCl contaminated dry steam reaching the turbine can lead to corrosion as well as Stress Corrosion Cracking of the turbine components. This paper is aimed to the evaluation of the corrosion behavior of K55 and new casing steels in an acid steam condensate derived from high temperature HCl gas contaminated steam condensing at atmospheric pressure and a temperature of the order of 97 °C. A Steam Condensate Test rig was developed for testing of the comparative resistance of the casing steels. This work used a miniature probe design. A three electrode system was used with Electrochemical Impedance Spectroscopy (EIS) to compare the performance of K55 alloy and the new casing steels, 17Cr and Cr13U. This paper will summarise the results from this study (a NEDO commissioned project) including measured corrosion rates and pH limits identified for examined casing steels in HCl contaminated steam condensate.