Supercritical electrochemical corrosion

High temperature and high pressure processing of materials often involves the use of supercritical fluids. Corrosion studies are quite essential for evaluation of the equipment in supercritical fluid operations. Previous electrochemical measurements for alloys in supercritical fluids are rare (1-1). The reported measurements (3) show that passivation of iron alloys is different at supercritical conditions compared to ambient conditions. The study of the electrochemistry of iron alloys can lead to control of corrosion of equipment utilizing the alloys. Thermodynamic analysis provides the information about stable species, i.e. corrosion products under given temperatures and pressures. 

L.B. Kriksunov and A.A. Me Donald, Corrosion in Supercritical Water Oxidation Systems a Phenomenological Analysis, J. Electrochemical Society, 142, (1995), 4069. 

Kriksunov LB, Macdonald DD. Corrosion in supercritical water oxidation systems—a phenomenological analysis. J Electrochemical Sci 1995 142(12) 4069, 4073. 

Electrochemical Measurements of Corrosion of Iron Alloys in Supercritical Water... 

Electrochemical potentlostat measurements have been performed for the corrosion of iron, carbon steel, and stainless steel alloys in supercritical water. The open circuit potential, the exchange or corrosion current density, and the transfer coefficients were determined for pressures and temperatures from ambient to supercritical water conditions. Corrosion current densities increased exponentially with temperature up to the critical point and then decreased with temperature above the critical point. A semi-empirical model is proposed for describing this phenomenon. Although the current density of iron exceeded that of 304 stainless steel by a factor of three at ambient conditions, the two were comparable at supercritical water conditions. The transfer coefficients did not vary with temperature and pressure while the open circuit potential relative to a silver-silver chloride electrode exhibited complicated behavior. 

There are few published electrochemical studies of supercritical water (temperatures and pressures above 374 and 218 atm). Some of the data pertain to the power generation industry for example, a 1975 report summarizes some industrial experience with corrosion of steam generator tubing in pressurized water reactors (4). Most of the other literature references concern... 

Most of the data available in the literature are for subcritical conditions. Corrosion studies of iron alloys in supercritical water have not been reported. For supercritical fluid extraction and corrosion studies, a supercritical fluid reactor system for temperatures up to 530 C and pressures up to 300 atm was constructed. This system was used to determine the electrochemical behavior of type 304 stainless steel (304 S.S.), 316 S.S., 1080 carbon steel (1080 C.S.), and pure iron in supercritical water. 

Construction of Apparatus. The schematic of the apparatus for supercritical corrosion studies is shown in Figure 1. The important components include a type 396-89 Simplex Minipump which can accurately meter (between 46 and 460 ml/hr) a wide variety of solvents at pressures up to 6000 psi (about 400 atm) an EG G Model 362 Scanning Potentiostat the electrochemical cell an IBM PC computer with interface hardware for electrochemical potential and current, temperature, and pressure measurement and control and a 316 stainless steel reactor, which holds the supercritical fluid for the measurements. The alloy was selected for excellent corrosion resistance properties and relatively low cost when compared with other exotic alloys such as Hastelloy C. 

This article details the thus far developed experimental techniques to carry out potentiometric, pH, electrokinetic, electrochemical kinetics, corrosion, and conductivity measurements in high-temperature (>300 °C) subcritical and supercritical aqueous environments. The author of this chapter recently reviewed the electrochemical processes in high-temperature aqueous solutions [2], an experience that has had a significant impact on the content of this chapter. N ote that the treatment and interpretation of the obtained high-temperature electrochemical data are out of the scope of this review, but there are a number of excellent papers [3-6], which the author recommends to a reader who is interested in the treatment of electrochemical data. Also, two of these papers [4, 5] are useful to anyone interested... 

In another study [35], the electrochemical emission spectroscopy (electrochemical noise) was implemented at temperatures up to 390 °C. It is well known that the electrochemical systems demonstrate apparently random fluctuations in current and potential around their open-circuit values, and these current and potential noise signals contain valuable electrochemical kinetics information. The value of this technique lies in its simplicity and, therefore, it can be considered for high-temperature implementation. The approach requires no reference electrode but instead employs two identical electrodes of the metal or alloy under study. Also, in the same study electrochemical noise sensors have been shown in Ref. 35 to measure electrochemical kinetics and corrosion rates in subcritical and supercritical hydrothermal systems. Moreover, the instrument shown in Fig. 5 has been tested in flowing aqueous solutions at temperatures ranging from 150 to 390 °C and pressure of 25 M Pa. It turns out that the rate of the electrochemical reaction, in principle, can be estimated in hydrothermal systems by simultaneously measuring the coupled electrochemical noise potential and current. Although the electrochemical noise analysis has yet to be rendered quantitative, in the sense that a determination relationship between the experimentally measured noise and the rate of the electrochemical reaction has not been finally established, the results obtained thus far [35] demonstrate that this method is an effective tool for... 

If one wants to obtain a comprehensive understanding of the interaction between a metal (or metal alloy) and a hydrothermal solution, then electrochemical kinetics and/or corrosion studies must be carried out. In particular, an electrochemical system capable of reliably operating at temperatures above 300 °C should be developed. It is a matter of fact that there are almost no data on the exchange current densities and the anodic and cathodic transfer coefficients for even the most fundamental electrochemical reaction in high-temperature subcritical and supercritical aqueous systems. Even the primary HERs and OERs have been poorly studied at temperatures above 100 °C. Therefore, the creation of a well-established method for measuring electrochemical kinetics and corrosion processes over a wide range... 

Delville, M., BoteUa, P., Jaszay, T., et al. (2002). Electrochemical study of corrosion in aqueous high pressure, high temperature media and measurements of materials corrosion rates applications to the hydrothermal treatments of organic wastes by SCWO, J. Supercrit. Fluid, 26, pp. 169-179. 

X. Guan, D.D. Macdonald, Determination of corrosion mechanisms and estimation of electrochemical kinetics of metal corrosion in high subcritical and supercritical aqueous systems. Corrosion 65 (6) (2009) 376—387. 

Advances in the Study of Electrochemical and Corrosion Phenomena in High Subcritical and in Supercritical Aqueous Solutions... 

It is clear from the above discussion that the phase behavior of snpercritical aqneous solutions in a complicated matter, bronght on primarily by the decrease in the dielectric constant of water. Un-fortnnately, as is evident from an examination of the literature, the phase behavior of supercritical aqueous solutions is poorly understood, but an appreciation of that behavior is vital for interpreting corrosion and electrochemical phenomena in supercritical aqueous media. 

In summary, the study by Zhou et al. " ° confirmed the findings of the prior work by Liu et al., ° that electrochemical noise analysis is an effective method for monitoring the corrosion rate of metals and alloys in high subcritical and supercritical aqueous solutions. The method is readily calibrated and, when used to estimate the noise resistance, and yields a quantity (the polarization resistance) that is directly related to the corrosion current density and hence the corrosion rate through the Stem-Geaiy relationship. This... 

Due to the unique properties of high subcritical and supercritical aqueous systems (SCAS), two corrosion mechanisms, i.e., electrochemical oxidation (EO) and chemical oxidation CO), have been postulated to describe the corrosion of metals and alloys in high temperature media,as outlined above. EO usually involves two or more coupled partial redox reactions at different sites on the corroding metal surfaces in relatively high-density SCAS. On the other hand, CO is postulated to occur through direct reaction of aggressive species with the metal in one act (but possibly in several elemental steps) on one site in low-density supercritical aqueous... 

Figure 72 displays the electrochemical current noise of Type 304 SS in deaerated 0.01 M HCI as a function of pressure at a supercritical temperature of 450°C. Similar to the pressure dependence of corrosion processes at subcritical temperatures (Fig. 50), the corrosion rate increases with increasing pressure yielding an apparently negative value for the activation volume. The current noise rises from 8.7 pA/cm at 197 bar to 114.1 pA/cm at 255 bar... 

No crack growth rate data under well controlled fracture mechanics and electrochemical conditions are currently available for any alloy in supercritical aqueous solutions. Indeed, it may well be that under conditions where the Chemical Oxidation (CO) mechanism dominates (low density) the electrochemistry may not be important, but that deeds to be demonstrated. At high density (p > 0.06 g/cm ) general corrosion occurs via an Electrochemical Oxidation (EO) mechanism and it is likely that SCC also will be an electrochemical phenomenoa... 

The selection of materials for use in many industrial environments, including supercritical thermal power plants (SCTPPs) and Supercritical Water Oxidation (SCWO) systems requires a broad study of the forms and the rates of corrosion of various metals and alloys in supercritical (T > 374°C) aqueous environments. Nickel is an important component of many corrosion resistant alloys and is a classic model for corrosion studies. Extensive electrochemical polarization studies of this metal in... 

Finally, we have demonstrated that it is now possible to perform electrochemical polarization studies in high subcritical and supercritical aqueous systems, which suggests that the whole range of powerful electrochemical techniques, ineluding transient methods and electrochemical impedance speetroseopy (EIS) can be brought to bear on the behavior of metals at the interface between wet and dry corrosive enviromnents. 

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