Titanium Dissolution-Passivation in Highly Chloridic and Oxygenated Aqueous Solutions

Abstract

The principal objective of this paper is to show that we developed an experimental setup and a dissolution-passivation monitoring technique for use in super critical water oxidation ͑SCWO͒ conditions (T Ͼ 374.15°C, P Ͼ 22 MPa͒. The principal barrier to this technology development is the corrosion of the reactor materials. High corrosion rates were experienced in industrial plants. A major goal is to develop effective tools to determine and control the electrochemical and corrosion parameters. Until now, electrochemical data concerning metallic materials submitted to aqueous solutions at pressures and temperatures beyond the critical point are scarce. This research, in the context of organic wastes SCWO, first proposes, for both sub and supercritical conditions, technical solutions for an electrochemical cell able to give access to quantitative electrochemical information in aggressive solutions (pH у 1.5 and chloride ions content of 1 g/L͒. Its second purpose is to obtain information on a titanium alloy ͑T60 grade͒ behavior in such conditions. The present study leads to a model establishing a reaction mechanism, which operates under normal conditions of pressure and temperature and can be extended to supercritical ones. The limits of use of this material as a plant reactor material for supercritical water oxidation ͑SCWO͒ are evaluated.Supercritical water oxidation ͑SCWO͒ is a high-efficiency, thermal oxidation process capable of treating a wide variety of hazardous and nonhazardous wastes. 1-12 The SCWO reaction takes place at elevated temperatures and pressures above the critical point of water (Pc ϭ 22.1 MPa, Tc ϭ 374°C). Supercritical water ͑SCW͒ under these conditions undergoes a drastic alteration of its density, dielectric constant, hydrogen bonding, and other physical properties. It behaves as a moderately polar liquid under ambient conditions 13 and due to its low dielectric constant, presents very low solubilizing properties for inorganic salts. SCWO processing setups are fully enclosed and do not produce hazardous air pollutants ͑HAPS͒. In a traditional SCWO waste treatment system, dilute aqueous organic waste ͑generally with less than 10% organic wastes͒ is combined with an oxidizer at elevated pressure and temperature ( P Ͼ 22.1 MPa, T Ͼ 400°C) in a reactor for short residence times ͑10-15 s͒. At supercritical conditions, the reactants form a homogeneous single-phase mixture facilitating complete reaction. For most wastes, these conditions are sufficient to achieve 99.99% destruction. However, the high-temperature environment within SCWO reactors and processing systems present significant reliability and performance problems. Experiences have shown that corrosion rates can be rapid when treating wastes containing halogens, such as chlorine. [14][15][16][17][18] The most common corrosion-resistant alloys 19 such as ironbased 10,11 or nickel-based ones 9,10,12,19-22 do not provide adequate protection against chloride attack under the oxidizing conditions performed in SCWO systems. Recently, SCWO reacto...

The influence mechanism of Fe3+ on corrosion behavior of Ti6Al4V in sulfuric acid solutions

“…Studies [5][6][7][8][9][10][11][12] on behavior of corrosion and passivation in acidic environment showed that titanium performed poor corrosion resistance in reductive acids such as sulfuric acid and hydrochloric acid. Blackwood et al and Su et al [5,8] have studied the passivation behavior of titanium at different temperatures in sulfuric acid, and found that the surface of the titanium formed a layer of titanium hydride when the active dissolving corrosion of titanium occurred in sulfuric acid [8,9]. Yu et al [10,11] studied the passivation behavior of titanium in sulfuric acid and found that sulfuric acid made surface oxide film of titanium dissolved and instable.…”

Anodic Behavior Modeling of the C-276 Alloy in Chlorinated Media Simulating the Hydrothermal Conditions of Organic Waste Treatment

“…1,6,[16][17][18][19] The resulting interpretations of the electrochemical behavior are also illustrated by observation of scanning electron microscopy ͑SEM͒ images of the C276 alloy surfaces. They are compared to those obtained for titanium in a previous article.…”

“…It is performed for C276 alloy at a scan rate of 0.1 mV/s, two times smaller than those used in the same conditions of pressure and temperature in previous works. 1,6,9 Therefore, with this scan rate, experiments were carried out in quasistationary state conditions, and localized attacks, which could be formed in chlorinated solutions, cannot significantly occur. In a similar way, there is a minor effect of the diffusion transport of oxygen either for titanium or C276 alloy as already known.…”

“…The main goal of the paper is to conceive a model for the reaction mechanisms as for titanium alloys in the same experimental conditions. 1,6 It is also related to previous works that were carried out on other nickel-base alloys ͑I800͒ 7,8 and pure nickel 9 in environments simulating the conditions of primary and secondary media in PWR nuclear plants ͑for high pHs and minor quantities of chloride ions͒. The similitude of the current-potential curves led us to use the same type of model already efficient for titanium metal 6 and pure nickel by taking into account some reference works on this material generally performed in sulfuric acid media and at low temperature.…”

Effect of the Bromide Ions on the Titanium Electrode Dissolution Polarized by Alternating Current in Aqueous Solutions

“…Titanium dissolution in hydrochloric acid with fluoride ions and in phosphoric acid solution in the presence of industrial alternating current has been shown in scientific works [10][11][12][13][14][15]. However, the works do not specify the electrochemical properties of titanium electrode in the bromide aqueous media [16][17][18][19][20]. In this regard, the present work considered the electrochemical dissolution behavior of titanium electrode in bromide aqueous media.…”