Edouard Asselin

The current work investigates the dissolution of chalcopyrite (CuFeS₂) under two different leaching scenarios using scanning electrochemical microscopy (SECM): (1) leaching in the presence of ferric-ferrous ions (“ferric leaching”) and (2) leaching by the application of a potential without ferric-ferrous ions (“potentiostatic leaching”). Ferric leaching processes have reported a solid elemental sulfur product layer that impedes chalcopyrite dissolution. However, potentiostatic leaching tests have reported a copper sulfide layer as the product that forms during dissolution in the absence of ferric-ferrous.In this work, we used the SECM in two ways: (1) to detect the species released during dissolution in the two scenarios using tip CVs and (2) map the conductivity of the surface under the different conditions. Four ferric-ferrous molar ratios in sulfuric acid solution were used in the ferric leaching scenario: 1:1, 10:1, 100:1 and 1000:1, and the chalcopyrite mixed potentials from these tests were applied to a separate sample potentiostatically in pure sulfuric acid. The tip CVs identified Cu²⁺ and Fe²⁺ released in both leaching scenarios. However, in the potentiostatic leaching case, a CuS product was determined to form at the tip, which was not observed in the ferric leaching case. Diagnostic tests determined that the CuS formed from a copper-thiosulfate complex, with the thiosulfate ion being the intermediate soluble sulfur species released during chalcopyrite dissolution. Once, the CuS-type layer forms on chalcopyrite, in the presence of ferric, it is further oxidized to form a sulfur-rich copper-deficient layer.Contact angle measurements show that the chalcopyrite surfaces produced during ferric leaching are relatively more hydrophobic compared to the potentiostatically leached samples. This is because the sulfur-rich layer in the ferric leaching scenarios is hydrophobic in nature. CuS, from the potentiostatic case, has more holes than electrons and is perceived to have a net positive charge on its surface. These holes attract the water dipole with the negatively charged end oriented toward the surface to flatten the droplet, yielding a lower contact angle and enhancing hydrophilicity. These results support the understanding that mechanistic outcomes from amperometric techniques cannot ultimately be used to explain the results from industrial ferric leaching processes.

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The corrosion properties and passive film behaviour of wrought (WR) and electron beam melted (EBM) Ti-6Al-4V (Titanium Grade 5 or TG5) were compared in simulated physiological solution.Microstructural characterization was performed using field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) to correlate the microstructural characteristics to corrosion resistance. The "bimodal" and "basket-weave" morphologies were observed for WR and EBM samples, respectively. Electrochemical measurements were also carried out for the evaluation of corrosion and passive film properties. Open circuit potential (OCP) and potentiodynamic polarization (PDP) measurements showed that the EBM sample has lower corrosion current densities. After choosing three potentials in the passive region from the PDP curves (300, 500, and 700 mVAg/AgCl), a passive film was formed on the EBM and WR TG5 samples using potentiostatic polarization (PS). Electrochemical impedance spectroscopy (EIS) and Mott-Schottky (M-S) analysis were used to assess the passive film properties in Ringer’s physiological solution at 310.15 K (37±1 °C). A more compact and protective passive film developed on the EBM sample at each film formation potential. M-S measurements revealed that the passive film on both WR and EBM samples were n-type semiconductors with fewer donor densities (ND) for the EBM sample. Furthermore, the spontaneously formed passive layers on the surface after immersion for 1 hour (at OCP) were compared. EIS results showed that the passive layers have duplex characteristics with higher corrosion resistance for the EBM sample. Moreover, the naturally formed passive films on the WR and EBM samples showed n-type characteristics, and the donor density was lower for the EBM sample. Immersion tests (IT) were also carried out at a controlled temperature and time to measure the weight loss due to corrosion of both WR and EBM alloys. The EBM sample showed a lower corrosion rate by weight loss after 3 weeks; however, weight gain was observed after 6 and 14 weeks of immersion for both WR and EBM samples. Overall, microstructural characterization and electrochemical measurements confirmed the better corrosion behaviour and passive film properties for the EBM alloy in Ringer’s physiological solution at body temperature (37±1 °C).

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The current work deals with the development of scanning electrochemical microscopy (SECM) as a product selective catalyst screening tool for CO₂ electroreduction to formate reaction (CO2RF) in aqueous media. CO2RF is a proposed route for utilizing CO₂ and surplus electrical energy produced by intermittent renewable sources. In CO2RF devices, CO₂ is electrochemically converted to formic acid or to a solution of formate ions. The resulting product can be either sold on the market, used for H₂ storage, or oxidized back to CO₂ to recover the electricity in direct liquid fuel cells. Our goal is to develop SECM for the simultaneous characterization of multiple CO₂ electroreduction (CO2RR) catalysts, probing their electrocatalytic activity in a product selective manner. To illustrate our method, we studied tin oxide-based catalyst, one of the most promising CO2RF catalyst. We submitted these catalysts to an electroreduction pre-treatment at −1.25 and −3 VAg/AgCl, respectively yielding a smooth oxide rich surface and an oxide poor surface covered in nanoparticles (30 to 70 nm diameter). The creation of spherical nanoparticles by electroreduction of tin oxide at −3 VAg/AgCl in an aqueous carbonate solution has, to our knowledge, never been reported. These two pre-treated catalysts, in conjunction with the un-pretreated surface, were simultaneously characterized by SECM, determining their CO2RF electrocatalytic activity relative to each other. The cyclic voltammetry (CV) tip detection mode provided product selective activity data for three different reaction products: formate, CO and H₂. We demonstrated that pre-electroreduction at −1.25 VAg/AgCl formed a surface with improved selectivity toward CO2RF. We also discussed the effect of SECM experimental parameters on the success of CO2RR catalyst characterization, as well as the challenges and limitations of the method. Our development in SECM opens the door to automated screening of CO2RR catalyst, which dovetails with the recent advances in model-based computational catalyst screening. While high screening throughput can be achieved using these various theoretical methods, they depend on experimental catalyst characterization and screening to validate their models. By automating the slowest step of the process, the experimental bottleneck on catalyst screening can hopefully be alleviated.

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This thesis analyzed and investigated the premature failures of pipes made from type 316L austenitic stainless steels. Multiple leaks were observed in scattered locations of a piping network of around 10 km after only 4 months in service transferring ammonium sulfate solution. The initial investigation indicated that the piping network was constructed 3 years earlier. After the construction, the stainless steel pipes were hydrotested to ensure the joints integrity. However, the piping network was not properly drained and dried after the hydrotest which resulted in water stagnation for the complete idle period between construction and commissioning. Therefore, an electrochemical, chemical, mechanical and metallurgical testing and analyses were conducted to determine the damage mechanism which consequently caused these failures. I have conducted electrochemical tests on a 316L stainless steel electrode in chloridized ammonium sulfate solution to determine its corrosivity. The electrochemical tests showed that the corrosion rates of 316L SS in ammonium sulfate solution is very low. This conclusion was supported by other laboratory studies at higher temperature and by the industrial corrosion tables published online. Also, two spools from the piping network that experienced the failures were analyzed using stereoscope, optical microscope, scanning electron microscopy/energy dispersive spectrometry, X-ray fluorescence and carbon/sulfur analyzer, tensile testing and microhardness testing. The results of these tests indicated that the pipes were leaking at the 6 O’clock position near the weld and heat affected zone areas. The morphology of the attack illustrated a narrow opening with large sub-surface cavity and tunneling initiated from the internal surface of the pipes. The weld joints displayed weld defects in terms of root concavity and lack of penetration. The metallurgical investigation strongly suggests that the pipes failed due to Microbiologically Influenced Corrosion (MIC). During the idle period of 3 years, the stagnant untreated water in the closed system was an appropriate environment for bacterial growth leading to severe damage at the welding joints and the base metal.

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In the field of hydrometallurgy, the industrial uptake of leaching models has been overlooked partially due to the lack of universal models. A model developed for one plant cannot easily be transferred for the application of a different plant without redesigning the leaching kinetics in the code. The Multiple Convolution Integral (MCI)-based model developed in this thesis has the ability to be universally applied by user-controlled inputs. Chemical reactions can be selected while the modeling software calculates the mass and energy balances. Residence times, operating conditions, and the rate-limiting reagents can also be defined to calculate a precise fraction reacted (leach extent) for sulphide minerals. The ability of the using the MCI model for predicting sphalerite leaching is examined in comparison to hydrometallurgical plant data collected from a Canadian pressure leach operation. The results are promising, showing that the model can predict plant Zn extraction data to within an error of 1.5 %. The model is further verified through bench scale pressure leaching experiments where 94 % of the zinc is extracted within 90 minutes using a concentrate sample from the same industrial plant. The effect of temperature is analyzed and the activation energy is calculated to be 40.8 kJ/mol. Interesting discoveries with respect to the reagent concentrations and their effect on the overall fraction reacted are also explored from the model results. In addition, the limitations of the MCI model are explained along with suggestions for improvement.

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Leaching of a limonitic laterite (1.49% Ni, 0.33 % Co, 2.29% Mn, 51.8% Fe) using ferrous sulphate as reductant was studied in ammoniacal solutions. Studied parameters included temperature, agitation, ferrous sulphate concentration, solids content, ammonia concentration and a comparison between ammonium sulphate and chloride. Tests were performed in a batch cell with temperatures between 20 and 80ºC at atmospheric pressure. Leaching kinetics for nickel were different to cobalt and manganese. Nickel was favoured by temperature in ammonium sulphate reaching 66% extraction in 3 hours, whereas in ammonium chloride it showed slower but steady kinetics with an extraction of 75% in 24 hours. Cobalt extraction in ammonium sulphate was low at high temperature and solids content; however, at low solids content (4% w/w) and high temperature (80°C) cobalt extraction reached 80% in 4 hours. With 11% solids at the same temperature, extraction decreased to 20%. In presence of ammonium chloride, temperature had a positive impact on cobalt reaching 80% in 8 hours at 23% w/w solids. Cobalt appeared to re-precipitate in presence of ammonium sulphate. Manganese behaved similarly to cobalt, however, in presence of ammonium sulphate, it only reached 60% extraction after 8 hours at 80°C at low solids. In presence of ammonium chloride, manganese reached 65% in 3 hours while increasing solids was not detrimental. Manganese suffered adsorption and/or co-precipitation in ammonium sulphate and chloride. This decrease in extraction exhibited by cobalt and manganese was attributed to co-precipitation and/or adsorption in iron oxides/hydroxides. Nickel extraction appeared aided by ferrous sulphate. Cobalt and manganese improved in presence of higher ammonia/ammonium, agreeing with the observation that cobalt and manganese are more prone to co-precipitation/adsorption; for which they prefer lower ferrous sulphate and higher ammonia/ammonium in order to stabilize their ammines. Feed and solid residue analyses using chemical assays, x-ray diffraction, scanning electron microscopy and mineralogy, provided better understanding of limonite reduction. The main phase in the residue was magnetite. Differences in behaviour between nickel and cobalt/manganese suggest a two-stage process, and chloride solutions seem promising due to better stability of cobalt and manganese.

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Aluminium alloy B206 is one of the strongest and toughest alloys in the cast aluminium family. Although it is light and has excellent low cycle fatigue strength, AA B206 has been known to perform adversely due to its poor corrosion resistance. Thus corrosion has been identified as one of the major issues that jeopardizes the long-term use and performance of B206. The corrosion behaviour of B206 in seawater is studied through immersion testing and electrochemical techniques such as Potentiodynmaic Polarization, Potentiostatic Polarization, Cyclic Potentiodynmic Polarization and Linear Sweep Thermmametry in two different solutions, namely natural seawater and simulated seawater, at various temperatures. Techniques like Optical Microscopy, Energy Dispersive X-ray Spectroscopy and Scanning Electron Microscopy have been used to investigate the microstructure and surface morphology before and after the electrochemical tests. Heat treatment has been performed on the as-received samples using RRA and T7 heat treatment techniques to compare the corrosion behaviour of the former with the latter using electrochemical techniques and image analysis. Lastly, hardness tests have been performed on various heat treated and as-cast samples to establish a comparison in mechanical properties. This study shows that the extent of B206 corrosion depends on the oxidizing nature of the seawater environment i.e. low or high redox potential rather than on the temperature of the seawater. Natural seawater is more aggressive than simulated seawater. Also, heat treatment improved the corrosion resistance as compared to as-cast B206 which was determined by the values of corrosion current density and surface analysis. Furthermore, heat treatment has led to better mechanical properties as determined by hardness tests.

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Electrodeposition of metallic molybdenum from aqueous electrolyte has in most cases previously yielded poor results due to the extremely high rate of the secondary hydrogen evolution reaction occurring at the cathode. This results in low current efficiencies and thin brittle films. The use of a highly concentrated aqueous-acetate based electrolyte containing molybdate ions has been used to deposit thick (~50 μm) adhered, mirror like metallic molybdenum coatings. Plating variables were investigated to determine the optimum deposition conditions; it was seen that current density was the most influential factor for the successful deposition of the refractory metal. The coating surface was analysed using SEM and EDX. XRD analysis confirmed the deposits were amorphous in nature with broad peaks in the (110) orientation. The deposition mechanisms were studied through electrochemical techniques such as PDP and CV. It was concluded that metallic molybdenum is deposited in a two-step reduction process, with the formation of an intermediate coating of molybdenum oxide, requiring hydrogen gas to fully reduce. Corrosion studies have shown the coatings stability in a chlorinated environment however active uniform corrosion in alkaline conditions resulted in film failure. Exposure to strong acidic conditions result in oxidation and delamination of the coating.Up-scaling of the process was seen to be successful and large deposits of well adhered and uniform metallic molybdenum were formed under high applied currents.

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Trace amounts of electrolyte cobalt during zinc electrowinning (EW) significantly decrease the current efficiency of the EW process by accelerating the parasitic hydrogen evolution reaction. The maximum tolerable level of cobalt in zinc EW can be as low as 0.1- 0.3 mg/L. The typical method to remove cobalt from zinc electrolyte, which is based on cementation onto zinc dust at approximately 85°C, is not an efficient process. It suffers from long retention times (2-3 hours) and high consumption of reagents; especially zinc dust. The aim of the present research was to study cobalt cementation at high temperature and high pressure (HT/HP) to accelerate the rate of cobalt removal and reduce the consumption of the reagents (zinc dust and activators). Experimental variables included temperature (85-150°C), pressure (0-100 psig), zinc dust dosage, zinc dust particle size, and activators (copper and antimony).Based on this research, the following results were obtained:1. Increasing temperature had a significant effect on the rate of cobalt removal. The optimum temperature was found to be 125°C - temperature at which the target level of cobalt (0.1 mg/L) could be met in 20 min.2. At 125°C and in the presence of 2.5 mg/L Sb and 45 mg/L Cu, 3.5 g/L zinc dust was found as the optimum zinc dust addition to lower cobalt concentration from the initial level of 15 mg/L to below 0.1 mg/L.3. Smaller zinc particles showed better cobalt removal results, but the cement redissolution was also more severe with these particles.4. The role of Sb in the activation system was more important than Cu. However, the best result in terms of the rate and extent of cobalt removal was achieved when both of the activators were added to the solution together.5. As expected, increasing the overhead pressure of N₂ (tested at 85°C) did not alter the cobalt removal profile greatly. Also, the effect of increasing the partial pressure of H₂ (tested at 125°C, and above the amount generated in situ by the reaction) on cobalt removal was negligible.

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Residues of the CESL Cu and Vale Ni sulphide medium temperature pressure leach contain elevated levels of the interest Cu and Ni metals. Metal losses are disproportionately found within a poorly formed amorphous and semi-crystalline iron oxide phase analogous to ferrihydrite. This phase results from incomplete ferric hydrolysis and precipitation that occurs simultaneously with the leaching of the sulphide minerals. An investigation was conducted by ageing the residues at 95°C in water to assess the extraction of lost Cu and Ni and the transformation of ferrihydrite to the more crystalline goethite and hematite species. It was found that 22% of the Cu could be removed from the CESL residue in 24 hours. Repeated ageing was only capable of extracting 38% of the Cu. Extractions of 22% of the Ni and 6% of the Cu were achieved in 24 hours from the Vale residue. Recovery of the lost Cu and Ni was mainly attributed to the washing of entrained and adsorbed Cu and Ni sulphates evenly distributed throughout the residues. A small amount of Cu and Ni was gained from ferrihydrite dissolution during the ageing process. Further investigations by SEM-EDS revealed the presence of unleached Cu and Ni sulphides that also contribute notable metal losses. EMP analysis showed that all hematite precipitated in the Vale residue contained Cu, Ni and SO₄²−. The average hematite particle consisted of 66.1% Fe, 0.85% Ni, 0.73% Cu and 1.19% SO₄²− for an equivalent of 30% Ni and 25% Cu losses to the residue. QXRD analysis of the aged residue showed little evidence of ferrihydrite transformation though ageing stabilised the residues by reducing the amount of X-ray amorphous material present. Improved stability was confirmed by a reduction in mass loss following treatment by a sequential extraction procedure utilising acidified hydroxylamine hydrochloride. This treatment dissolves the ferrihydrite present in the residues and is used as a proxy to assess residue stability and losses of Cu and Ni to the non-crystalline iron oxide phase.

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Alkaline sulfide leaching (ASL) at approximately 100 ºC has been used to selectively extract arsenic and antimony from enargite and tetrahedrite concentrates. Sodium thio-arsenate has been postulated to crystallize from alkaline sulfide leaching solutions upon cooling. However, literature data on the solubility of sodium thio-arsenate as well as proof of its crystallization from ASL solutions is scant. In this thesis, the solubility of leach-produced and synthetic sodium thio-arsenate is studied.To determine arsenic solubility in ASL solutions, sodium thio-arsenate and sodium arsenic oxide sulfide complexes are synthesized by various means and characterized by EDX, QXRD, and ICP. The synthesis of amorphous As₂S₃, sodium arsenic oxy-sulfide complexes, and sodium thio-arsenate is first presented. For amorphous As₂S₃ synthesis, the effect of concentration of sodium sulfide (0.1 M) and hydrochloric acid (1 M), temperature (40 ~ 60 ºC), and aging time (48 hours) was optimized. The solubility of synthetic sodium arsenic oxy-sulfide complexes and sodium thio-arsenate in ASL solutions increases significantly as temperature is increased to 95 ºC. More importantly, the solubility of sodium thio-arsenate at certain temperatures is significantly affected by the concentration of sodium hydroxide and sulfide in solution. Due to the common ion effect, if NaOH and HS- concentrations are very high, the solubility of sodium thio-arsenate decreases. Enargite leaching tests were done to characterize the precipitate that occurred upon cooling and to verify the arsenic saturation point, which should be between 38.5 ~ 58 g/L (0.51 M ~ 0.78 M) As depending on the NaOH and HS- concentration. Comparison with solubility experiments of pure sodium thio-arsenate shows that arsenic solubility in ASL solutions is supersaturated. However, direct comparison of saturation in ASL solutions and the solubility as obtained by the synthetic solutions/crystallites prepared here is not possible given the complex nature of the ASL crystallites that appear not to contain the often discussed “sodium thio-arsenate”.

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The Caron process was developed in the 1920s. In this process, the ore is dried and milled, then roasted in a reducing atmosphere to convert nickel and cobalt to their metallic form. Nickel and cobalt are then leached in ammoniacal solution but some losses occur due to adsorption and co-precipitation with iron oxide or hydroxide precipitates. The effects of various parameters on iron precipitation in the Caron process leach were studied. The parameters studied include pH, temperature, contact time with iron and its precipitates and initial ferrous concentration. The adsorption of otherwise soluble Co onto freshly precipitated ferric hydroxide was measured at four different temperatures between 20ºC and 35ºC. The percentage of cobalt adsorbed onto iron precipitates was directly related to the initial iron concentration but the effect of temperature was less clear. The amount of cobalt adsorbed depended on the pH. Solution contact time with iron precipitates had a very small effect on cobalt adsorption (2-3% difference). The highest adsorption of cobalt (99%) was obtained with an initial concentration of 0.2 M ferrous sulfate at pH 7 after 2 hours of contact time. The main iron precipitate was ferrihydrite. X-ray analysis revealed the characteristic “2 line” ferrihydrite (Fe₅HO₈4H₂O) with no goethite being observed in any of the precipitates. TEM revealed an amorphous structure, which is also indicative of ferrihydrite. SEM showed a preponderance of poorly resolved precipitates, which did not appear to be crystalline. Traces of cobalt were measured in the ferrihydrite particulate by EDX.

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The electrochemical passivation of pure Fe in ammoniacal solution was investigated to determine the stability of both Fe-oxides and Fe-ammines during anodic polarization. The potentiodynamic experiments were done in 6M total NH₃[3M NH₄OH and 1.5M (NH₄4)₂CO₃] solution. Different experimental parameters such as temperature (15°, 25°, 35°, 45° and 55°C) and pH (6, 8, 10 and 12) were used. Pure oxygen and argon gas was sparged during the experiment to oxygenate or de-oxygenate the solution with no stirring in either case. Polarization plots show that both active anodic dissolution and passive regions are present for pure iron in ammoniacal solution. It also shows that as the temperature increased the dissolution rate increased in both anodic and passive regimes. At the same time, the active region for iron dissolution is present across a wider potential range. For pH 10 the highest dissolution rate is around 0.025A/cm² or 260 g/m²hr¹ at 55°C and passivation of iron generally occurs at ca. -0.36 V (SHE) irrespective of the temperature. The peak anodic dissolution rate (0.2 A.cm₋² or 2080 g/m²hr¹) surprisingly occurs at pH 6. Potentiostatic experiments were done at different fixed potentials at pH 9 and 25°C. The highest current density was registered at -0.6 V. The peak dissolution current observed for the potentiostatic tests is roughly two orders of magnitude lower than that observed in the potentiodynamic test for pH 9 and at 25°C. Solution and morphological analyses were done by ICP and SEM, respectively for pH 6 and 9 solutions. The current efficiency for pH 6 is far lower than at pH 9 which implies that the current registered at pH 6 is used for the formation of a product film. Speciation calculations indicate that this film may be siderite (FeCO₃) at low potential. From XPS analyses, it is believed that the passive layer formed at higher potentials (more than 0.40V) is Fe₂O₃. Speciation diagrams point to the stability of iron tetra-ammines at pH 10. It was shown that metastable Eh-pH diagrams for Fe/NH₃/CO₃/H₂O system can be generated through potentiodynamic measurements aimed at active and passive behavior of iron.

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Leaching of enargite samples containing approximately 12 % As, 0.5 % Sb and 38 % Cu was studied in alkaline sodium sulphide solutions. Samples were leached in the presence of sodium hydroxide and sodium sulphide, which is expected to hydrolyse and form sodium hydrosulphide. Kinetic parameters studied included temperature, particle size, reagent concentration and stoichiometry in high pulp density tests. Leaching behaviour of arsenic and antimony was very similar; it was enhanced as temperature and reagent dosage was increased and/or particle size decreased. Copper, iron, zinc, and silver were not extracted during the leaching procedure. Through chemical analysis, X-Ray Diffraction and Scanning Electron Microscopy leach solutions and residues were characterised. Arsenic and antimony were efficiently removed, leaving copper-sulphur compounds such as digenite, bornite and sodium copper sulphide (NaCu₅S₃). Some of the leaching results differ from those found in the literature, especially in regards to the nature of the solid residue and the leaching reaction given.Removal of arsenic from solution was analysed by acidification and crystallization. Acidification removed arsenic and antimony from solution to produce a mixture of oxides and sulphides; however, sulphide was removed from solution most likely as hydrogen sulphide, which would need to be scrubbed in a sodium hydroxide solution. Finally acid consumptions over arsenic plus antimony ratios were too large for a practical application. Crystallization on the other hand is a simpler alternative. The main requirement is to have high concentrations of arsenic and antimony in solution. In this case part of the arsenic and antimony would be recirculated to the leaching stage. Other aspects included behaviour of chalcopyrite and pyrite in alkaline solutions and the possibility of producing sulphide ions in situ. Unfortunately no considerable amounts of aqueous sulphide were produced. Also, the behaviour of arsenic and antimony (III) in sodium sulphide alkaline solutions was analysed using arsenic and antimony trioxide. These results are in an early phase of study and could be a relevant topic for further research. In both cases a black precipitate formed containing elemental antimony and oxides. However, no crystallization of thio-compounds seemed to have occurred.

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As aqueous processing moves to higher temperatures and pressures to take advantage of increased kinetics, there is a need to develop and test appropriate reactor materials to ensure that corrosion is minimized. Corrosion testing often requires an electrochemical approach for a comprehensive understanding of the range of behaviors exhibited from a corroding metal or alloy in different environments. Prior art of designs for electrodes, associated pressure vessels and sealing technology is presented. The development of an apparatus and methods for high temperature and high pressure electrochemical corrosion testing are discussed. The final flow-through electrochemical cell design, the Flow-Through External Pressure-Balanced Reference Electrode (FTEPBRE) design, working/counter electrode and other components, which were developed for temperatures and pressures in excess of 500ºC and 5000 PSI is presented. A two-electrode electrochemical testing method is presented, using Stainless Steel (SS 316) as both Quasi Reference Electrode (QRE) and Counter Electrode (CE), and Alloy 625 (Ni-062.8%, Cr-21.8%, Mo-7.35, Fe-3.97%, Nb-2.7%) as the Working Electrode (WE). The effects of pressure, and its combination with temperature on OCP and corrosion rate of alloy 625 (WE) in both naturally aerated and de-oxygenated environments in 0.1 M sodium sulphate (Na₂SO₄) solution with a flow rate of 7 mL/min were investigated and discussed. The effect of pressure represented as a change in activation volume and reaction volume for the homogenous and heterogeneous phases is also presented. The corrosion rate was observed to increase with both temperature and pressure: higher for naturally aerated conditions than the corresponding de-aerated ones. Results also show that the instability of the QRE affected the result and direction of the OCP tests. A reduction in the corrosion current was observed above 207 bar (3000 PSI) in the polarization tests and was attributed to the increasing stability of the passive film formed on the surface of the alloys.

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