STRUCTURE AND ELECTROCHEMICAL BEHAVIOUR OF Fe-9Cr-W ALLOYS WITH VARIABLE CARBON AND TUNGSTEN CONTENT. Jiří RAPOUCH, Jaroslav BYSTRIANSKÝ

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1 STRUCTURE AND ELECTROCHEMICAL BEHAVIOUR OF Fe-9Cr-W ALLOYS WITH VARIABLE CARBON AND TUNGSTEN CONTENT Jiří RAPOUCH, Jaroslav BYSTRIANSKÝ Department of Metals and Corrosion Engineering, Institute of Chemical Technology, Prague, Technická 5, Prague 6, Czech Republic, Abstract Use of creep-resistant steel P92 in modern power plants enables increase of service temperature up to 650 C. Some alloying elements (Cr, W) can easily increase their oxidation states and form volatile compounds in high steam parameters environments. Very similar degradation mechanism is observed in strongly oxidizing aqueous environments, where these alloys are subject to transpassive dissolution. Model melts containing constant chromium content (9 wt.%) and variable amount of carbon ( wt. %) and tungsten ( wt. %) were used for study of the influence of alloying elements. Structure of these alloys was characterized using optical and scanning electron microscope. Electrochemical behaviour was evaluated by potentiodynamic method. Model alloys showed to have similar structure compared to industrial melts of P92, moreover, shapes of their potentiodynamic curves are also very similar. The curve in transpassive area is accompanied by formation of substantial local peaks whose position differ according to chemical composition and heat treatment of the alloy. The model alloys were potentiostatically etched in local peaks areas and characterized using scanning electron microscope equipped by EDS analyser. Keywords: creep-resistant martensitic steels, tungsten, carbon, electrochemical behaviour, transpassivity 1. INTRODUCTION Heat-resistant steel P92 belongs to the group of (9-12)% Cr modified martensitic steels. These steels are used in components of advanced coal-fired power plants, such as heat-exchange tubes and steam pipelines. The use of P92 enables increase of steam parameters up to 650 C / 30 MPa. Essential service requirements under these conditions are high creep strength and enhanced resistance to steam-side oxidation. From the point of chemical composition chromium is the most important alloying element responsible for sufficient oxidation resistance of the steel. Moreover, alloying with molybdenum and tungsten leads to increase of creep resistance due to solid-solution strengthening [1-4]. Oxidation resistance of steel P92 is dependent on exposure environment. Compared to very thin, highly protective oxide layer formed in dry-air environment, scales formed in steam-containing atmosphere are thick consisting of several sublayers and containing many defects (pores, voids, cracks). The scale is usually composed of outer magnetite layer and inner Fe 3 O 4 / (Fe,Cr) 3 O 4 layer [5]. Another problem of behaviour of these materials in steam environment is selective oxidation of some alloying elements and formation of volatile compounds. Example of this problem is development of volatile CrO 2 (OH) 2 on Fe-Cr alloys in O 2 /H 2 O atmospheres causing decrease of Cr content on the surface. This chromium depletion process leads to formation of less protective oxides [6-9]. Similarly to chromium, other elements such as molybdenum and tungsten can be selective oxidized and evaporate in the form of their compounds [10]. In addition, evaporation of alloying elements can change the overall kinetics of oxidation from parabolic behaviour to linear or even breakaway oxidation. In addition, the same behaviour (that means losing of protective properties of passive layer due to transition of key elements into soluble compounds in higher oxidation states) is observed in aqueous solutions during transpassive dissolution [11, 12]. Conditions for transpassive degradation are achieved in strongly oxidizing

2 environments that could occur in overheated steam or supercritical water [13]. Transpassive behaviour of materials could be observed using electrochemical polarization in area of very positive potentials (around 1-2 V/ACLE for Fe-Cr alloys). In addition, electrochemical polarization technique could be used for detection of phases enriched with alloying elements that are easily transpassive soluble in suitable electrolytes. This method was successfully used for steels P91 (9Cr-1Mo) [14] and P92 [15] after creep exposures. In this paper model Fe-9Cr-W steels with various carbon and tungsten content are studied from the point of electrochemical behaviour in the transpassive area and its relation to microstructure. 2. EXPERIMENTAL Four different laboratory melts (2 kg) of Fe-9Cr steels with various carbon and tungsten content and commercial P92 were used for experiments. Chemical composition of all samples is given in Tab. 1. Heat treatment of specimens consisted of quenching (1050 C, 20 min, air) followed by tempering at 780 C for 10 min. Tab. 1 Chemical composition of samples steel chemical composition [wt. %] C Mn Si S Cr Ni Mo W other elements P V: 0.23, Nb: 0.06, N: 0.37 C1W C15W C53W C63W All samples were grinded, polished and etched in nital and Villela-Bain solution mixture before study of the structure. Scanning electron microscope Tescan Vega 3 equipped with EDS analyser was used for microstructure observation. A common three-electrode cell with a Pt wire as counter-electrode and a silver/silver chloride electrode (ACLE) as the referent electrode (197 mv compared to the standard hydrogen electrode) was used for electrochemical measurements. All potentials reported in this paper are related to the ACLE. Electrochemical measurements were conducted in aerated Na 2 SO 4 solution adjusted with sulphuric acid to ph 3,4. Potentiodynamic measurements were carried out by the PC3 system and Gamry electrochemical software. Corrosion potential was stabilized for 15 min before beginning of polarization measurement. The used rate of polarization (scan rate) was 1 mv/s. 3. RESULTS 3.1 Microstructure Microstructures of steel P92 and two model alloys C15W2.3 and C63W4.0 are shown on Fig. 1a-1c. The structure of alloy P92 is well known from many publications [1-4] and consists of tempered martensite with carbide and carbide-nitride particles. Model Fe-9Cr-W alloys (Fig. 1b-1c) have very similar structure composed of tempered martensite and precipitated particles. Significantly more particles are observed in sample C63W4.0 which is probably given by higher carbon content in this alloy. Because of substantially

3 lower nitrogen content (compared to P92), no nitride particles are expected in structure of model alloys. Precipitated phases are anticipated to be carbides of iron, chromium and/or tungsten. 3.2 Electrochemical behaviour Fig. 1. Structures of steels a) P92, b) C15W2.3, c) C63W4.0 Anodic potentiodynamic curves of all used steels in Na 2 SO 4 solution (ph = 3.4) measured from corrosion potential (E corr ) are plotted on Fig. 2a. The curve for commercial steel P92 (black dashed line) shows an ordinary electrochemical behaviour of metal with active-to-passive transition. The value of passivation potential is -260 mv and the critical passivation current density reaches 5.1 A/m 2. Very sharp increase of current density from the passive area probably due to transpassive dissolution takes place at potential around 500 mv. The area of our interest is the potential region mv, where two local significant peaks at potentials 970 mv (current density 0.68 A/m 2 ) and 1220 mv (0.40 A/m 2 ) were observed on polarization curve of P92 steel. Formation of these peaks is assumed to be connected with dissolution of phases enriched with alloying elements (Cr, W), which can easily form soluble compounds in higher oxidation states (Cr VI, W VI ). Samples C1W1.5 and C15W2.3 show very similar behaviour compared to P92, increased amount of carbon and tungsten in alloy C15W2.3 lead to higher value of critical current density and current density of observed peaks. Very broad area of active dissolution was measured in the case of samples C53W1.9 and C63W4.0. The reason of such behaviour is probably high carbon content in these steels, which leads to formation of carbide particles surrounded by Cr-depleted areas. Decrease of current density followed by its increase due to transpassive dissolution was observed after this broad area. Positions of detected local peaks correspond to those of lower carbon steels. Fig. 2. Anodic polarization curves of alloys, a) measured from E corr, b) in transpassive area ( mv)

4 Polarization started at potential 600 mv (exception of passive surface) was conducted to suppress the influence of active dissolution. Potentiodynamic curves measured in potential area mv for all used alloys are plotted on Fig. 2b. Formation of two peaks at potentials 970 and 1220 mv was noticed on the curve of steel P92 (black dashed line). Compared to Fig. 2a, these peaks are less significant and current densities in maximum are lower (0.14 and 0.25 A/m 2 ). Therefore, polarization from passive area is obvious to lead to less intensive electrochemical processes (transpassive dissolution of phases). This behaviour could be explained by dissolution of matrix in active region, which increases the area of more resistant phases. Increased area for following transpassive dissolution could explain higher current densities of local peaks on Fig. 2a. Comparison of individual alloys shows that increasing carbon and tungsten content leads to emphasizing of peaks in area mv. Most significant examples of this trend are samples C53W1.9 and C63W4.0, where increase of current density by 1-2 orders of magnitude compared to P92 was observed. This behaviour is explained by higher amount of precipitates (enriched with alloying elements) in the structure and more intensive transpassive dissolution of these precipitated phases. 3.3 Observation after potentiodynamic measurement Sample C63W4.0 with most significant peaks in transpassive area (the highest current density) was studied more in detail on scanning electron microscope. Structure of this sample before potentiodynamic measurement is shown on Fig. 3a. Large bright particles with size up to several μm were present in the structure. Chemical analysis performed by EDS analyser detected around 41 % Fe, 36 % Cr and 23 % W (all in wt. %) in these particles. Determination of accurate carbon content is not possible using this method. However, relative amount of carbon is higher in these particles compared to surrounding matrix. Therefore, these phases are supposed to be carbide particles, probably of type M 23 C 6. Structure of this sample after polarization ( mv) is shown on Fig. 3b. Carbide particles are evident to be etched out of the matrix. This observation confirms our assumption that formation of local peaks in area mv is connected with dissolution of phases enriched with alloying elements (Cr, W). Fig. 3. Structure of sample C63W4.0 a) before polarization, b) after potentiodynamic measurement The possibility of detection of phases enriched with alloying elements could be used for evaluation of microstructural status of long-term heated creep-resistant Fe-9Cr-W steels and for prediction of service life of operated equipment.

5 3. CONCLUSIONS Structures of alloys Fe-9Cr-W with various carbon and tungsten content are similar to commercial steel P92 and consist of tempered martensite and precipitated carbide particles. Potentiodynamic curves of these alloys show common active-to-passive behaviour and significant local peaks in transpassive area. Polarization from passive area leads to decrease of these peaks parameters. Rising amount of carbon and tungsten in the alloy causes increase of current density in highest points of formed peaks. This trend is assumed to be connected with increasing amount of carbides in the structure, which can easily transpassive transit into compounds in higher oxidation states. Observation on scanning electron microscope showed preferential etching of phases determined as Cr- and W-rich carbides during potentiodynamic measurement. ACKNOWLEDGEMENT Financial support from MPO FR-TI1/086 and specific university research (MSMT No 20/2013). REFERENCES [1] VAILLANT, J. C., VANDENBERGHE, B., HAHN, B., et al. T/P23, 24, 911 and 92: New grades for advanced coalfired power plants - Properties and experience. International Journal of Pressure Vessels and Piping, 2008, vol. 85, no. 1-2, p [2] MAYER, K.-H., et al. Creep resistant steels, Woodhead Publishing: Cambridge, England, [3] VISWANATHAN, R., BAKKER, W. Materials for Ultrasupercritical Coal Power Plants Boiler Materials: Part 1. Journal of Materials Engineering and Performance, 2001, vol. 10, no. 1, p [4] KAYBYSHEV, R., SKOROBOGATYKH, V., SHCHENKOVA, I. New martensitic steels for fossil power plant: Creep resistance. The Physics of Metals and Metallography, 2010, vol. 109, no. 2, p [5] ENNIS, P. J., QUADAKKERS, W. J. Mechanisms of steam oxidation in high strength martensitic steels. International Journal of Pressure Vessels and Piping, 2007, vol. 84, p [6] LIU, F., TANG, J. E., JONSSON, T. et al. Microstructural investigation of protective and non-protective oxides on 11% chromium steels. Oxidation of Metals, 2006, vol. 66, no. 5/6, p [7] HOLCOMB, G. H. Steam Oxidation and Chromia Evaporation in Ultra-Supercritical Steam Boilers and Turbines. Journal of the Electrochemical Society, 2009, vol. 156, no. 9, p [8] OSGERBY, S., FRY, A. The role of Alloy Composition on the Steam Oxidation Resistance of 9-12%Cr Steels. Materials Science Forum, 2006, vol , p [9] MEIER, G. H., JUNG, K., MU, N., et al. Effect of Alloy Composition and Exposure Conditions on the Selective Oxidation Behavior of Ferritic Fe Cr and Fe Cr X Alloys. Oxidation of Metals, 2010, vol. 74, p [10] YUN, D. W., SEO, H. S., JUN, J. H., et al. Molybdenum effect on oxidation resistance and electric conduction of ferritic stainless steel for SOFC interconnect, International Journal of Hydrogen Energy, 2012, vol. 37, p [11] BETOVA, I., BOJINOV, M., LAITINEN, T., et al. The transpassive dissolution mechanism of highly alloyed stainless steels I. Experimental results and modelling procedure. Corrosion Science, 2002, vol. 44, p [12] FATTAH-ALHOSSEINI, A., SAATCHI, A., GOLOZAR, M. A., et al. The transpassive dissolution mechanism of 316L stainless steel. Electrochimica Acta, 2009, vol. 54, p [13] KRITZER, P., BOUKIS, N., DINJUS, E. Review of the corrosion of nickel-based alloys and stainless steels in strongly oxidizing pressurized high-temperature solutions at subcritical and supercritical temperatures. Corrosion, 2000, vol. 56, no. 11, p [14] HYUN, Y., LEE, J., KIM, I. The Evaluation of Material Degradation in Modified 9Cr-1Mo Steel by the Electrochemical Technique. Key Engineering Materials, 2004, vol , p [15] RAPOUCH, J., BYSTRIANSKÝ, J., SVOBODOVÁ, M. Detection of Structural Changes in Heat-Resistant Steels after Heat and Creep Strain Using Electrochemical Methods, In Conference proceedings Metal Ostrava: TANGER, 2012, p