Stress Corrosion Cracking in Metal Alloys Exposed to Acid-SO42- Chloride Type Geothermal Fluids

Abstract

Stress corrosion cracking (SCC) is a materials failure caused by the simultaneous existence of stress, susceptible material and corrosive environment. Conventional geothermal wellbores producing high temperature (T > 180 oC) acid-SO42- chloride fluids (pHave = 3.5) may be at risk of this damage especially when subjected to stressed condition such as intermittent shut-down and discharge activities. This study evaluated the SCC resistance in acid-SO42- chloride geothermal fluids of six (6) types of metal alloys (i.e. J55, L80 Type-1, N06059, R53400, S32750, N08904) for potential use as casing material in geothermal wellbores. The field exposure test was conducted for 35 days using plastically deformed U-bend coupons. SCC susceptibility was measured through the appearance of a recognizable crack. Other SCC influencing factors like integrity of passive scales, surface discontinuity manifestations, test fluids chemistry and alloy composition were also considered. The SCC damage was diagnosed through gravimetric analysis, light microscopy, coupled environmental scanning electron microscopy – energy dispersive spectroscopy (ESEM-EDS), powder X-ray diffraction (XRD) and metallographic analysis. Field test results showed all alloys covered with passive scales of sphalerite (ZnS) possibly co-existing with galena (PbS), bornite (Cu5FeS4), magnetite (Fe2+Fe23+O4) and pyrite (FeS2). These were adherent, porous and likely precipitated from the geothermal fluids due to absence of Zn in the original alloy composition. J55 and L80 Type 1 carbon steels had no cracks but underwent piting and appeared the most corroded. Only S32750 and N08094 corrosion resistant alloys did not develop cracks or any surface discontinuities. Nickel base alloy N06059 acquired a crack from secondary carbide precipitation and crack growth from surrounding pits. The titanium base R53400 sustained a crack that grew either from surrounding pits or through cyclic process of film rupture, dissolution and film repair based on growth of sphalerite scales into the crack. The carbon steels and R53400 were also considered at risk of hydrogen induced cracking due to H2S presence and production of hydrogen and chloride ions during sphalerite scale formation.