Stress Corrosion Cracking in Metal Alloys Exposed to Corrosive Geothermal Fluids

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dc.contributor.advisor Zarrouk, SJ en Dacillo, Katrina en 2016-12-13T20:50:35Z en 2016 en
dc.identifier.uri en
dc.description Full text is available to authenticated members of The University of Auckland only. en
dc.description.abstract 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. en
dc.publisher ResearchSpace@Auckland en
dc.relation.ispartof Masters Thesis - University of Auckland en
dc.relation.isreferencedby UoA99264918607202091 en
dc.rights Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. Previously published items are made available in accordance with the copyright policy of the publisher. en
dc.rights Restricted Item. Available to authenticated members of The University of Auckland. en
dc.rights.uri en
dc.rights.uri en
dc.title Stress Corrosion Cracking in Metal Alloys Exposed to Corrosive Geothermal Fluids en
dc.type Thesis en Engineering Science en The University of Auckland en Masters en
dc.rights.holder Copyright: The author en
pubs.elements-id 554664 en
dc.relation.isnodouble 594671 *
pubs.record-created-at-source-date 2016-12-14 en

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