Apparatus for Studying the Corrosion Resistance of Alloys in High Temperature Geothermal Well Environment

Abstract

An apparatus and experimental procedures for studying the corrosion behaviour of commercial corrosion resistant alloys (CRAs) are described. Harsh environment in high temperature geothermal wells was tried simulated with a field-relevant test fluid at low pH, temperature 350 °C and pressure 170 bars. Variations in pH during isochoric heating of 200 g·L-1 H2O with HCl are shown. When starting with water at room temperature and pH 3, pH decreases slightly with increasing temperature before reaching a minimum close to 335 °C. The reason behind this behaviour is the competition between water dissociation, HCl dissociation and HCl partitioning (fractioning). HCl – which is commonly known as a strong acid – behaves as a weak acid at high temperature, and with increasing temperature the vapour phase becomes more acidic. The transition area of subcritical two-phase and supercritical water/superheated steam environment was of special interest since the corrosivity is expected to become more severe when liquid phase is present. The corrosion performance of materials at reducing conditions was investigated using rectangular, rod and C-ring samples. A 300-hour exposure test of Hastelloy C-276 (UNS N10276), Inconel 625 (UNS N06625) and Titanium grade 29 (UNS R56404) specimens are reported. Only the first two materials were available as C-rings test specimen. The samples were visually evaluated, and microstructure and corrosion products were further studied using SEM/EDS. The examination of specimens showed little or no corrosion. There were no cracks visible after C-ring testing of Hastelloy C-276 and Inconel 625. Plans for improved equipment design, characterization methods, and test procedure to obtain even more representative geothermal conditions in the future were outlined.