Electrochemical study on corrosion of different metals exposed to deaerated geothermal brines containing CO 2

Transcription

1 Electrochemical study on corrosion of different metals exposed to deaerated geothermal brines containing CO 2 N. Mundhenk 1, P. Huttenloch 2, R. Zorn 2, and T. Kohl 1 1 Institute of Applied Geosciences / KIT 2 European Institute for Energy Research (EIFER), Karlsruhe Abstract Herein, we report selected results from corrosion experiments that were conducted with an electrochemical 3-electrode setup. Various metals have been exposed to natural geothermal brines at different environmental conditions to study the reactivity of the metal/brine interface by using different methods. The response on polarization as well as the spontaneous behavior has been investigated. The experiments show that temperature is a crucial parameter for the different materials. For non-alloyed N80, the cathodic partial reaction (hydrogen evolution) is the rate-limiting step and controls the corrosion rate. Thereby, it is strongly influenced by the temperature with a local maximum close to 60 C. The temperature also affects the type of corrosion, as could be shown for duplex alloy 318LN. In a 20 C environment the polarization curves shows no pitting potential and subsequently no pitting corrosion could be observed on the post-polarization coupon. With increasing temperature, however, the tendency to undergo pitting corrosion strongly increases and discrete pitting potentials are discernible in the polarization curves. For alloyed materials the capability to build up a stable passive layer (passivation) becomes crucial. Standard stainless steel 316L fails and undergoes pitting corrosion within short time spans. Higher alloyed materials and non-ferrous alloys show excellent passivation properties and are resistant under the specific test conditions. Introduction Geothermal waters that reside in deep geologic rock formations of the Upper Rhine graben (URG) can be used for power production and heat supply. Most of these natural waters contain considerable amounts of dissolved solids (mainly sodium, calcium, and chloride) and gases (mainly carbon dioxide and nitrogen), lowering the ph to values below 5 [1, 2]. These waters are often referred to as brines with a TDS that often exceeds g l -1. With the kind permission of the Bruchsal (EnBW AG) and Soultz (GEIE) operators we were authorized to sample brine for the laboratory experiments. In geothermal utilization the physico-chemical characteristics of the brine account for a high corrosivity with respect to many construction materials [3]. Often enough material degradation and component failure result in extensive operational downtime periods. In order to enhance the availability of geothermal power plants and to reduce the operation costs, a proper corrosion engineering has to be established. This necessitates an in-depth knowledge of the corrosion system, which includes the material (i.a. microstructure, texture, and composition), the brine (i.a. composition, ph, and redox state), and the environment (i.a. hydrodynamic, temperature). Consequently, corrosion resistance is a system property

2 and not a material property. The outcome of this research can be regarded as a starting point for an improved material qualification. In the experiments we tested commonly used casing and pipe steels (Fe-based mild steels API N80, API P110, P235GH, and P265GH), standard stainless steels (430F, 316L, and 316Ti), and promising candidates for geothermal applications (alloy 904L, duplex alloy 318L, super-duplex alloy 31, Nickel-base alloys 59 and 625, and Titanium gr. 2). The materials have been chosen on the basis of their specification and commercial availability. The experiments were conducted according to the ASTM standards [4, 5]. A three-electrode setup was used to study the electrochemical behavior of metal coupons during exposure in the electrolyte (geothermal water). Cyclic voltammetry is a very useful method to study the passivation-, breakdown-, and repassivation behavior within short-term experiments. Three electrochemical potentials can be determined by cyclic voltammetry, a potentiodynamic polarization technique: the open circuit potential E OC, the pitting potential E P, and the repassivation potential While the E OC and the E P can be determined on the forward polarization sweep, the E R occurs on the reverse sweep and indicates whether or not the metal is capable to repassivate after the onset of pitting corrosion. All electrochemical potentials can be set into relation to determine the individual pitting resistance and the repassivation capability. Since the cyclic voltammetry works with an external stimulation the spontaneous behavior of the materials has been studied in another series of measurements. Here, the E OC is measured over time. Its evolution reveals the tendency to oxidize in the exposure medium and whether or not a stable passive layer forms. Results In a first step the response of mild steel N80 on polarization has been studied (Fig. 1). For this material no passivation is achieved under the test conditions and the current density is a function of the applied potential. Experiments at different temperatures (20, 40, 60, and 80 C) show that the temperature strongly influences the anodic and cathodic reaction and that the cathodic reaction is the rate-controlling step. While the anodic reaction (Fig. 1b) is strongly accelerated with increasing temperature, the cathodic reaction (Fig. 1a), most likely the reduction of hydrogen ions and dissolved carbon species, is highest in the 60 C experiment. The temperature dependence with a local corrosion rate maximum is a well-known feature of CO 2 corrosion and has been described by various authors. It is therefore concluded that CO 2 is a very important, if not the most important corrosion agent in the system. However, whether or not this mechanism is still dominant at temperatures > C has to be confirmed in future experiments. Furthermore, the curves show that with increasing temperature the open circuit potential (E OC ) is shifted towards lower, or more active, values. Fig. 2 shows the polarization curves for the mild steel N80 (solid line) and duplex-alloy 318LN (dashed line) as a function of temperature in a semi-logarithmic diagram. The equilibrium potentials E OC in the 318LN curves are higher potentials than in the N80 curves. This is due to the alloying elements chromium, nickel, and molybdenum. These elements also account for the passivation of this alloy, which is a potential range where the current density is independent of the applied potential. The effect of temperature on the polarization curve of 318LN is very significant. For the 20 C experiment there is no sharp potential above which pitting occurs and the current density increase at high anodic overpotentials is rather smooth. For the 40 and 80 C experiments a more discrete potential is discernible, which is approx. 150 mv higher in the 40 C experiment. The hysteresis loop is larger for the 80 C experiment and the passive current density is also increased.

3 50 40 API N b API N80 I j I / (µa cm -2 ) a 20 C 40 C 60 C 80 C I j I / (µa cm -2 ) C 40 C 60 C 80 C Fig. 1: Polarization curves for N80 in deaerated and CO 2 -purged Soultz brine with a ph of 4.8; (a) Cathodic response; (b) Anodic response. j / (µa cm -2 ) C 40 C 80 C 318LN: dashed lines N80: solid lines E vs. Ag/AgCl RE / mv Fig. 2 Comparison of polarization curves for mild steel N80 and duplex-alloy 318LN in deaerated and CO 2 -purged Soultz brine with a ph of 4.8. Temperature: 20, 40, and 80 C. Fig. 3a shows the E OC evolution from two measurements for 316L in CO 2 -purged 80 C Soultz brine. The spontaneous passivation process is interrupted by fluctuations. One curve exhibits discrete sets of E OC oscillation with increasing amplitudes, which can be explained by short-term activation and passivation changes. Discrete pits surrounded by tempering colors could be detected on this post-exposure coupon, which proves insufficient stability. Another measurement shows a very smooth spontaneous passivation, depassivation/activation and repassivation development (Fig. 3a. red curve), where it took more than 5 hours to achieve full E OC recovery. It is therefore assumed that a more substantial surface alteration occurred. In both cases, the interruptions start after exceeding a potential of -200 mv. Incipient E OC ennoblement during exposure could be observed in virtually all measurements in both. Bruchsal and Soultz brine. This indicates a high tendency for spontaneous passivation and proves the oxidative capacity of the environment. If localized corrosion does not occur, the E OC will shift harmonically to noble direction until it achieves a stable value. In the case of 318LN, alloy 31, 625, and the Ti alloy (Fig. 3b), the potential shift can approach several hundred millivolts, approx. 450 mv for the Ti alloy. This behavior indicates very good passivation properties. Fig. 3c give the potentiodynamic polarization curves for all tested alloys in 80 C Soultz brine. It can be seen that the alloys 31, 59, 625, and the Titanium gr. 2 exhibit outstanding performances in the test environment. The stable state of passivity for these materials is also corroborated by the harmonic E OC curves in Fig. 3b. The stainless steels are under constant threat to suffer a form of localized corrosion (pitting) as the E OC for these materials strongly approach the upper stability limit of passivity.

4 E vs. (Ag/AgCl)/mV (a) a Start of OCP fluctuation at ca mv? Depassivation/ repassivation event OCP oscillation set (d) b Alloy 31 Alloy 625 Ti gr Time / h t / hours c Fig. 3 (a) E OC measurements of 316L; (b) E OC measurements of alloy 31, alloy 625, and Titanium grade 2; (c) Potentiodynamic polarization curves for all tested alloys in 80 C Soultz brine. Conclusions The experimental approach in this work enables us to assess the corrosion resistance of various conventional and candidate materials and to identify parameters that control to the corrosion process in geothermal brines. The corrosion behavior of the different materials is very individual and comprises several forms of corrosion, including uniform and localized corrosion. Subsequently, suitable materials for geothermal applications could be identified. Acknowledgements We would like to thank GEIE (EDF, EnBW AG, ES, Bestec). Furthermore, Outokumpu VDM GmbH is gratefully acknowledged for providing test materials.

5 References [1] B. Sanjuan, R. Millot, Ch. Dezayes, M. Brach, Main characteristics of the deep geothermal brine (5 km) at Soultz-sous-Forêts (France) determined using geochemical and tracer test data, C.R. Geosci. 342 (2010) [2] H. Pauwels, C. Fouillac, A.M. Fouillac, Chemistry and isotopes of deep geothermal saline fluids in the Upper Rhine Graben: Origin of compounds and water-rock interactions, Geochimica et Cosmochimica Acta 51 (1993) [3] N. Mundhenk, P. Huttenloch, B. Sanjuan, H. Steger, T. Kohl, R. Zorn, Corrosion and scaling as interrelated phenomena in an operating geothermal power plant, Corrosion Science 70 (2013) pp [4] ASTM G5-82, Standard practice for standard reference method for making potentiostatic and potentiodynamic anodic polarization measurements, in: Haynes, G.S., Baboian R. (Eds.), Laboratory Corrosion Tests and Standards. ASTM Committee G-1 on corrosion of metals, ASTM special technical publication 866. [5]. ASTM G 1-03, 1985, Standard practice for preparing. cleaning. and evaluating corrosion test specimens. in: G.S. Haynes. R. Baboian (Eds.) Laboratory Corrosion Tests and Standards. ASTM Committee G-1 on Corrosion of Metals. ASTM Special Technical Publication, vol. 866, 1985.