Edouard Asselin

Influence of Cupric, Ferric, and Chloride on the Corrosion of Titanium in Sulfuric Acid Solutions Up to 85°C

Titanium is chosen as the construction material of autoclaves for pressure acid leaching of metal ores. The corrosion behavior of titanium was studied in sulfuric acid solutions with different addi...

Effect of Oxygen on the Corrosion Behavior of Alloy 625 from 25 to 200 ° C

Electrochemical polarization and impedance experiments were conducted on alloy 625 in ammoniacal solution between 298 and 473 K and at various oxygen partial pressures. Increased oxygen concentrations (as a result of higher temperatures or autoclave oxygen loading) resulted in increased corrosion potentials and corrosion rates. The inversion temperature (onset transpassive corrosion) was found to be ∼ 373 K at P TOTAL > 40 bar. A binary barrier layer/ outer layer film configuration was assumed and it was found that the outer layer dominated the corrosion process as temperatures and pressures were increased such that oxygen diffusion control was rate limiting. An oxygen solubility model for supercritical water oxidation (SCWO) conditions was generated using the Redlich-Kwong-Soave equation of state. Results of the model experimental work confirm many of the observations from SCWO reactors which suggest that corrosion reaches a maximum at temperatures just short of the critical point. Exchange current densities for oxygen reduction and effective diffusion layer thicknesses have also been calculated.

In Situ Electrochemical Analysis of Surface Layers on a Pyrrhotite Electrode in Hydrochloric Acid Solution

The present study considers the electrochemical dissolution of pyrrhotite electrodes in 1 M HCl solution. Conventional electrochemical techniques as well as electrochemical impedance spectroscopy (EIS) and in situ Mott-Schottky analysis have been applied for surface studies during the anodic dissolution of the electrode. EIS results showed the formation of two distinct surface layers on the electrode. The first layer forms during sample preparation and is stable up to around 600 mV vs saturated calomel electrode (SCE). The second layer starts to form at high anodic potentials (>670 mV vs SCE). Electrochemically active dissolution of the pyrrhotite occurs between the formation potential of the two surface layers (600-670 mV vs SCE). Mott-Schottky plots showed that both of the layers are n-type semiconductors with quite different donor densities. Moreover, the second surface layer is less conductive than the first and thus hinders the dissolution current more effectively. Three different equivalent electrochemical circuits were modeled for different dissolution potential ranges and the model regression results was compared to the experimental results of the EIS.

Thermodynamics of the Corrosion of Alloy 625 Supercritical Water Oxidation Reactor Tubing in Ammoniacal Sulfate Solution

During the process of supercritical water oxidation (SCWO), organic chemical streams are oxidized at temperatures and pressures in excess of 647 K and 221 bar. Due to high operat- ing temperatures and pressures, severely corrosive environ- ments often ensue and eventually lead to SCWO reactor tube failures. This case study looked at one such failure of Alloy 625 (61Ni-21.5Cr-9Mo (UNS N06625)) tubing, which occurred at the UBC SCWO pilot plant while treating a feed of waste- water containing ~0.1 m 2,4 dinitrophenolate (C6H4N2O5), ~0.25 m ammonium sulfate ((NH4)2SO4), and ~1.3 m free am- monia (NH3). The feed pH was approximately 10 and therefore not expected to be corrosive; in fact, the tube failed when ex- posed to this feed (with oxygen) for a period of about 1 h at two sections subjected to temperatures in the 650 K to 655 K range. Through a detailed thermodynamic examination of the Ni-H2O-NH3 system at 373, 473, 573, and 653 K, as well as scanning electron microscopy (SEM) and energy-dispersive x-ray (EDX) analysis of the ruptured portions of tubing, it was found that the addition of ammonia and sulfate lead to solu- tion acidifi cation and NiO instability, which resulted in little protection for the alloy when subjected to the oxidative high- temperature medium. The alloy ultimately failed by transpas- sive dissolution.

A Polarization Study of Alloy 625, Nickel, Chromium, and Molybdenum in Ammoniated Sulfate Solutions

Abstract A polarization study of Alloy 625 (UNS N06625), nickel, chromium, and molybdenum, in a solution of ammonia (NH3) and sulfuric acid (H2SO4) at pH 10, is presented. The effect of air, oxygen, hydrogen peroxide (H2O2), and variation in pH is examined. Pourbaix diagrams for the Ni-NH3-H2O and Cr-H2O systems are presented. The formation of metastable oxides is suggested by the observed passivation of both the alloy and Ni outside their thermodynamic fields of stability. The alloys polarization curves are similar to those obtained for Cr. Ni is seen to have the highest passive current density while Mo does not passivate. Selective dissolution of Ni from the surface regions of the alloy is predicted. Corrosion rates, which are low (1.2 × 10−3 mmpy at pH 10), increase with decreasing pH. Passive current densities are found to be higher in solutions of sulfate alone. Highly oxygenated solutions do not result in an increase in corrosion rate. However, the addition of H2O2 to the ammoniated solution result...

An Investigation on the Effects of Organic Additives on Zinc Electrowinning from Industrial Electrolyte

The influence of Saponin, Licorice, Tennafroth 250, and Dowfroth 250 at different concentration levels (0, 5, 10, and 15 ppm) were investigated during zinc electrowinning from industrially supplied spent electrolyte. Acid mist suppression efficiency, polarization behavior, current efficiency and energy consumption were investigated for a current density of 500 A.m. Surface tension measurements were used to find a correlation between the concentration of the different additives and the acid mist suppression efficiencies. Morphological and crystallographic orientation studies indicated dense and well defined crystalline grains with homogeneous distribution mostly at the 5 ppm level of suppressant. Electrowinning in the presence of Saponin and Tennafroth was found to have high current efficiencies of 90% each with low energy consumptions of 2641 and 2675 kWh/t, respectively and the deposited crystals were found to be oriented preferably in the [101], [102], [103] and [112] planes for both.

Electrochemical and corrosion behaviour of stainless steels 316L and 317L in chloridised ammonium sulphate solution

Abstract The corrosion of stainless steel alloys 316L and 317L in chloridised ammonium sulphate solution at 100°C and varying pH was investigated. Alloy 316L was found to be an adequate material for use in naturally aerated solution within the pH range of 5·4–7·4. At low potentials (<0·23 V) and a high potential (0·75 V), alloy 317L shows better performance. It also exhibits better pitting resistance at high potential, however, in the potential range of 0·23–0·45 V, alloy 316L shows better corrosion resistance. The dissolution of the Mo oxide species is proposed to be the reason for the weak performance of alloy 317L in the potential range of 0·23–0·45 V. On a examiné la corrosion des aciers inoxydables 316L et 317L dans une solution chlorée de sulfate d’ammonium à 100°C et à pH variable. On a trouvé que l’alliage 316L était un matériel adéquat pour utilisation dans une solution aérée naturellement dans la gamme de pH de 5·4 à 7·4. Aux faibles potentiels (<0·23 V) et à potentiel élevé (0·75 V), l’alliage 317L montrait une meilleure performance. Il exhibait également une meilleure résistance à la corrosion par piqûres à potentiel élevé. Cependant, dans la gamme de potentiel de 0·23–0·45 V, l’alliage 316L montrait une meilleure résistance à la corrosion. On propose la dissolution d’une espèce d’oxyde de Mo comme étant la raison pour la faible performance de l’alliage 317L dans la gamme de potentiel de 0·23–0·45 V.

High Temperature Corrosion of Titanium Under Conditions Relevant to Pressure Leaching: Mass Loss and Electrochemistry

The corrosion behavior of titanium was studied using mass loss and electrochemical measurements in sulfuric acid with Cl−, Cu2+, and Fe3+ additions up to 175°C. The corrosion rate of titanium in 30 g L−1 H2SO4 + 12 g L−1 Cl− solution at 150°C measured by mass loss was higher than 62 mm y−1. The corrosion rates of titanium under simulated pressure-leaching conditions determined by both mass loss and polarization resistance measurements were consistent, and also suggest that Cu2+ and Fe3+ ions are excellent corrosion inhibitors for titanium. Results from mass loss and electrochemical measurements all confirmed that the corrosion rate of titanium decreased remarkably with the addition of 1 g L−1 Fe3+ to the leaching solution at 150°C, and that the presence of Fe3+ is the most important factor for the growth of titanium oxide films under pressure-leaching conditions. In addition, iso-corrosion diagrams, with 0.1, 0.5, and 1 mm y−1 lines for titanium in H2SO4 solutions from room temperature to 175°C, were cons...

FIB/SEM Study of Pitting and Intergranular Corrosion in an Al-Cu Alloy

Scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDX) was used to study the formation of stable pits on an aluminum-copper casting alloy in artificial seawater solution at 10°C. The polished specimens were immersed for various times and the development of corrosion around intermetallic particles was monitored. Focused ion beam (FIB)/SEM was used to determine the subsurface microstructure at selected local attack sites within the corroded area. The earliest stages of attack started with pit initiation at the surface of intermetallic particles resulting in dealloying. Cu redistribution occurred during corrosion over the entire surface and its concentration differed from one site to another. FIB imaging revealed that localized corrosion was propagated where continuous particles were buried beneath the surface. Dealloying of the particle and matrix/particle interfacial attack produced a sponge-like remnant and cracks at the interface, respectively. Propagating away from the initia...

On the Development of Thermo-Kinetic Eh-pH Diagrams

It is shown that thermo-kinetic Eh-pH diagrams can be generated through electrochemical measurements. These diagrams offer an accurate method of determining stability regions for leaching without relying on thermodynamic calculations, which may be inaccurate or for which data may be difficult to obtain. The Fe-NH3-CO3-H2O system is studied here. Potentiodynamic polarization experiments were performed on an iron sample in ammoniacal solution in de-aerated condition at different temperature and pH. Polarization plots show that both active anodic dissolution and passive regions are present for pure iron in ammoniacal solution depending on the potential. The electrochemically obtained potential-pH data were used to generate the thermo-kinetic Eh-pH diagrams for the Fe-NH3-CO3-H2O system.