Oxidation Behaviour of IN-738 LC Superalloys in the Presence of Ionic Salts at 1173 K

Portugaliae Electrochimica Acta 21 (2003) 389-398 PORTUGALIAE ELECTROCHIMICA ACTA Oxidation Behaviour of IN-738 LC Superalloys in the Presence of Ionic Salts at 1173 K M. Misbahul Amin * Department of Chemistry, Faculty of Science & Technology, University College of Science & Technology Malaysia, Mengabang Telipot, 21030 Kuala Terengganu, Malasya. Received 14 July 2003; accepted in revised form 11 November 2003 Abstract Oxidation of the alloy having nominal composition Ni-16Cr-8.5Co(wt.%) was studied in the presence of KCl and K2CO3 at 1173 K in air. Chemical composition of oxide scales and scale morphologies were determined by means of X-ray diffraction analysis and scanning electron microscopic studies. The oxide scales of KCl- and K2CO3- coated alloys showed complex microstructures and compositions. This behaviour was ascribed to the release of volatile metallic chlorides, evolution of CO/CO2 gas as well as formation of fluxing products. The high temperature oxidation resistance is not only related to the nature of the passive film but is also strongly dependent on the salt environments and the structure of alloy. Keywords: Oxidation, IN-738 LC Alloy, KCl, K2CO3. Introduction Nickel-based alloys containing chromium as a major constituent together with cobalt, titanium and aluminium additions are the most widely used materials in turbine blades of jet engines, nuclear power reactors and land-based turbines. Among a great variety of protective systems to increase the scaling resistance of super-alloys the overlay M-Cr coatings (where M= Ni, Co) offer an advantage of appreciable compositional flexibility. The aluminium addition provides good oxidation resistance where relatively high chromium and cobalt levels ensure hot corrosion resistance. Compositional modification of the overlay coatings, particularly of the Cr:Co ratio, makes them suitable for a variety of service * Corresponding author. E-mail: ; . M.M. Amin / Portugaliae Electrochimica Acta 21 (2003) 389-398 conditions. Oxidation and hot corrosion behaviour of M-Cr materials alone or deposited on superalloys substrates have been a subject of many investigations [1-8]. The role of alkali earth metal salts inducing high temperature oxidation studies has been reported [9-12]. The passivity of nickel in Na2SO4 / NaCl melts, in air, is destroyed by SO3, SO2, and O2 [13, 14]. Blistering and cracking produced by NaCl exposure have been attributed to the formation of volatile metallic chlorides, like NiCl2, CrCl3 in locally reducing environments [15-17]. Limited attention has been paid to the morphology of attack on nickel-base alloys in the presence of potassium in KCl and K2CO3, although chlorides transform to oxides releasing Cl2 gas under oxidizing atmospheres [18-20]. The purpose of this work was to get information on high temperature corrosion of Ni-16Cr8.5Co alloy in the presence of KCl and K2CO3, oxidized at 1173 K in air, including oxidation kinetics as well as structure and morphologies of oxide scales. Experimental The alloy having nominal compositions Ni-16Cr-8.5Co (wt. %) was obtained and homogenized for 4 h in an evacuated (10–2 Pa) quartz tube at 1173 K. Chemical compositions of the experimental alloy are given in Table 1. The ingots were spark-machined to get flat discs, diameter of 15 mm and thickness of 1 mm, which were ground on SiC papers up to 800 grit no., then finally degreased in water with detergent and ultrasonically cleaned in acetone. The polished specimens were uniformly coated with thin films of KCl or K2CO3. The specimens were heated to about 573 K and were sprayed with aqueous solutions of the salt till a nearly uniform coating (about 6 µm thick coatings) of the salt was obtained. The salt coated specimens were dried in an oven for about 1 h, cooled to room temperature and weighed. The salt coated specimens were then used for high temperature oxidation studies. 390 M.M. Amin / Portugaliae Electrochimica Acta 21 (2003) 389-398 Table 1. Chemical composition of IN-738 LC superalloy (wt.%). Co Cr Al Si C P Fe B 8.59 16.08 3.43 0.18 0.11 0.01 0.50 0.05 S W Ta Nb Ti Mn Cu Ni 0.01 2.67 1.75 0.90 3.38 0.03 0.03 Bal Kinetic studies on the oxidation of coated IN-738 LC superalloy were carried out at 1173 K in air for periods up to 48 h. The coated specimens were transferred into silica buckets and suspended by helices in a helical thermal balance using a vertical tubular furnace. Variation in mass change was recorded at intervals of every 4 h. Table 2. Constituent identified in the scales by X-ray diffraction analysis, IN-738 LC superalloy, KCl- and K2CO3- coated specimen, oxidized at 1173 K for 48 h in air. Salt environment KCl Constituent identified in the scales Cr2O3, NiO, K2CrO4, CrCl3, NiCl2, K2NiO2, Al2O3, Co3O4 K2CO3 Cr2O3, NiO, K2CrO4, Al2O3, Co3O4, K2NiO2 Oxide scales of coated IN-738 LC superalloy were examined by taking crosssections of the specimens, conventionally mounted and polished to a 1 µm diamond finish, in a scanning electron microscope, Jeol JSM-35. After oxidation studies, the representative specimens were examined by X-ray diffraction analysis. The various constituents were identified using a Phillip PW-1730 X-ray diffractometer fitted Fe-, Co, or Cu-Kα targets. Table 2 summarizes the results of the XRD analysis. 391 M.M. Amin / Portugaliae Electrochimica Acta 21 (2003) 389-398 Results The oxidation kinetic data are shown in Fig. 1 for IN-738 LC superalloy with KCl and K2CO3, oxidized at 1173 K in air for 48 h. The oxidation rate of KClinduced specimens result mass gain with increasing time up to 32 h followed by a decrease in mass at longer time. K2CO3-coated specimens exhibit invariable mass loss during the first 28 h after which there was mass gain with further increase in time. This could be attributed to the evolution of CO/CO2 gas which facilitates oxidation attack [21, 22]. ∆M/A in g/cm2 × 10-3 2.00 1.50 Œ KCl o K2CO3 1.00 0.50 0.00 0 10 20 30 40 50 -0.50 -1.00 -1.50 -2.00 Time in hour Figure 1. The change in mass with time (oxidation kinetics) for IN-738 LC superalloy coated with KCl and K2CO3, oxidized at 1173 K in air for 48 h. The scales are separated from substrate at some regions, possibly due to polishing artifacts. The chlorides of chromium and nickel form the outer layers of scales incorporated with Cr2O3 and NiO (Fig. 2). Cr2O3 has not been found in the inner scales. This might be due to the formation of intermediate volatile species CrO2Cl2, some of which evaporates and condenses on the walls of the reaction tube and some of it decomposes and accumulates at the alloy/salt interface in the 392 M.M. Amin / Portugaliae Electrochimica Acta 21 (2003) 389-398 form of Cr2O3 [23]. It appears that during cooling of fluxed mass, stresses are developed resulting in the production of cracks and voids [24]. Figure 2. Scale morphology of IN-738 LC superalloy coated with KCl, oxidized at 1173 K for 4 (...truncated)