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Stress corrosion cracking (SCC) is caused by the simultaneous existence of stress, susceptible material and a corrosive environment. Geothermal wellbores producing corrosive geothermal fluids are at risk of SCC damage when subjected to stressed conditions such as intermittent shut-down and discharge activities. This study evaluates the SCC resistance in corrosive geothermal fluids of carbon steels, stainless steels, nickel-base alloys and titanium-base alloys with potential use as casing materials in geothermal wells. Field tests were conducted using plastically deformed U-bend coupons exposed to the following geothermal fluid conditions: (1) two-phase acid-SO42− chloride fluids at pHave = 3.5, temperature of 185±2°C and pressure of 1.14-1.16 MPa from geothermal well XM-02 for 35 days, and (b) almost superheated steam (h = 2745 kJ/kg, Tsat = 307°C) with 13.8% by weight non condensable gases (NCG) from geothermal well HX-01 for 95 days. SCC susceptibility was measured through the appearance of a recognizable crack, integrity of passive scales, surface discontinuity manifestations, test fluid chemistry and alloy composition. 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 show that the two types of corrosive geothermal fluids promote passivation of the metals. Exposure to acid-SO42− fluids resulted in deposition of sphalerite (ZnS) scales on the metal surface while contact with the high NCG steam caused iron sulfide (troilite, pyrrhotite and mackinawite) scales to form. The scales were adherent but had a porous texture. The carbon steel samples did not sustain cracks but had pits that could serve as initiation sites for crack growth. No recognizable cracks were found on most of the corrosion resistant alloys (CRAs) except for the Ti-Gr 12 titanium-base alloy and the 5923 HMO nickel-base alloy exposed to acid-SO42− chloride fluids. An irregular surface defect was observed on the I625 nickel base alloys exposed to high NCG steam. Follow-up tests on the CRA samples to validate SCC resistance using other types of statically loaded specimens and to evaluate susceptibility to tribo-corrosion are recommended. Geochemical and corrosion modeling to predict stable phases at wellbore conditions and in-depth studies on the thermodynamic stability, kinetics and breakdown of passive scales formed on the alloys should also be considered.
Corrosion is a huge issue for materials, mechanical, civil and petrochemical engineers. With comprehensive coverage of the principles of corrosion engineering, this book is a one-stop text and reference for students and practicing corrosion engineers. Highly illustrated, with worked examples and definitions, it covers basic corrosion principles, and more advanced information for postgraduate students and professionals. Basic principles of electrochemistry and chemical thermodynamics are incorporated to make the book accessible for students and engineers who do not have prior knowledge of this area. Each form of corrosion covered in the book has a definition, description, mechanism, examples and preventative methods. Case histories of failure are cited for each form. End of chapter questions are accompanied by an online solutions manual. * Comprehensively covers the principles of corrosion engineering, methods of corrosion protection and corrosion processes and control in selected engineering environments* Structured for corrosion science and engineering classes at senior undergraduate and graduate level, and is an ideal reference that readers will want to use in their professional work* Worked examples, extensive end of chapter exercises and accompanying online solutions and written by an expert from a key pretochemical university
Volume 65 of Reviews in Mineralogy and Geochemistry attempts to fill this gap and to explicitly focus on the role that co-existing fluids play in the diverse geologic environments. It brings together the previously somewhat detached literature on fluid–fluid interactions in continental, volcanic, submarine and subduction zone environments. It emphasizes that fluid mixing and unmixing are widespread processes that may occur in all geologic environments of the entire crust and upper mantle. Despite different P-T conditions, the fundamental processes are analogous in the different settings.