Type-I Hot Corrosion of Ni-Base Superalloy CM247LC in Presence of Molten Na 2 SO 4 Film

Type-I Hot Corrosion of Ni-Base Superalloy CM247LC in Presence of Molten Na2SO4 Film MAHESH K. KUMAWAT, CHANDRAKANT PARLIKAR, MD. ZAFIR ALAM, and DIPAK K. DAS Type-I hot corrosion behavior of CM247LC superalloy is evaluated in the air at 950 C against low (3 to 4), intermediate (7 to 9), and high (12 to 14 mg cm2) Na2SO4 deposits. Duration of thermal exposure is varied from a very short duration of 5 minutes to long duration of 1000 hours. The alloy shows poor corrosion resistance and undergoes complete disintegration after 500 hours of thermal exposure. Degradation of alloy increases with the increase in the duration of exposure as well as the initially deposited amount of the salt. Based on the systematic analysis of the corrosion scale, degradation mechanisms supported by the microstructural evidence are proposed. Fluxing, sulfidation-oxidation, and sulfide-undercutting are reported as the primary degradation mechanisms for CM247LC alloy in the presence of Na2SO4. Self-sustaining degradation of alloy leading to complete disintegration of specimen is caused by the changeover of fluxing mechanism from basic fluxing to alloy-induced fluxing. https://doi.org/10.1007/s11661-020-06068-6  The Minerals, Metals & Materials Society and ASM International 2020 I. INTRODUCTION THE Ni-base superalloy components such as blades and vanes operating in the turbine engines undergo hot corrosion damage. Material degradation by hot corrosion occurs due to the reaction of a superficial film of salt with the metallic components at high temperatures. The corrosive salts such as sulfates, chlorides, and vanadates are formed in situ during the operation of the engine.[1–4] It is well established that hot corrosion causes greater damage to the material than pure high-temperature oxidation.[5–12] Fluxing and sulfidation-oxidation are two well-accepted mechanisms for hot corrosion of Ni-base superalloys.[7,8,13–20] The corrosion mechanism is affected by the service conditions as well as the elements contained in the superalloy.[1,8,10,15,16] For example, the degradation of superalloys at temperatures above the melting point of salt occurs by basic fluxing.[1,19,20] On the other hand, acidic fluxing occurs at temperatures lower than the melting point of salt.[1,19] The oxides of the refractory elements such as W, Mo, and V present in the alloy are known to promote acidic fluxing even at temperatures above the melting point of salt.[1,19,20] Considering the presence of multifarious alloying elements in the advanced MAHESH K. KUMAWAT, CHANDRAKANT PARLIKAR, MD. ZAFIR ALAM, and DIPAK K. DAS are with the Defence Metallurgical Research Laboratory, Hyderabad 500 058, India. Contact e-mail: Manuscript submitted July 5, 2020; accepted October 11, 2020. METALLURGICAL AND MATERIALS TRANSACTIONS A superalloys, their hot corrosion behaviors can be expected to be complex and cannot be stereotyped. The present study examines the Type-I hot corrosion behavior of the directionally solidified (DS) Ni-base superalloy CM247LC. This alloy is optimized for casting of turbine blades and vanes operating in the hot sections of advanced gas turbine engines and has a maximum temperature capability of 1000 C.[21–25] Reports on the hot corrosion behavior of the above alloy are limited in the open literature.[26–28] Gurrappa examined the comparative corrosion behavior of CM247LC immersed in pure Na2SO4 and Na2SO4-NaCl mixture over a temperature range from 700 C to 1000 C.[26] The study proposed that the formation of superficial Al2O3 layer prevented severe hot corrosion during the initial 25 hours of immersion in pure Na2SO4 molten salt. The presence of NaCl in molten Na2SO4, however, induced cracking and spallation of the protective Al2O3 scale leading to accelerated degradation of the alloy by acidic fluxing. Sumner et al. examined the hot corrosion behavior of CM247 alloy under dynamic flow conditions during exposure to combustion flame in a burner rig test and reported a short incubation stage preceding rapid corrosion.[27] Tsao et al. compared the hot corrosion behavior of this alloy with that of high entropy superalloys in the presence of Na2SO4-25 pct NaCl deposit using immersion and salt film techniques.[28] It was reported that degradation of the alloy ceased during the 20 hours of exposure in the salt film test due to the depletion of salt with time and only oxidation occurred in the latter part of the 20 hours exposure. The degradation continued only after washing and re-coating of the specimen was done after 20 hours of exposure. However, aggravated degradation occurred in the immersed samples due to the abundance of molten salt. The objective of the present study is to evaluate the progressive corrosion damage in the directionally solidified (DS) CM247LC superalloy caused by Na2SO4 salt during isothermal exposure at 950 C in air for various durations ranging between 5 minutes and 1000 hours. The gradual microstructural evolution of the scale during corrosion is characterized and the operating mechanisms for corrosion ascertained. The study reports synergistic effects of fluxing and sulfidation-oxidation mechanisms. It proposes sulfide-undercutting at the corrosion front and examines its role in the degradation process. II. EXPERIMENTAL DETAILS DS CM247LC Ni-base superalloy, having a nominal composition of (in wt pct) Ni 62.7-Co 9.2-W 8.5-Cr 8.1-Al 5.6-Ta 3.2-Hf 1.3-Ti 0.7-Mo 0.5-C 0.05-Zr 0.015-B 0.015, was obtained in the form of 12 mm diameter and 100 mm long rods. The [001] growth direction of the grains was along the length of the rods. Cylindrical specimens having 4 mm diameter and 25 mm length were cut from the rods using wire electro-discharge machining (EDM). The above specimen dimensions conformed to the BS ISO21608:2012 standard recommended for isothermal hot corrosion tests.[29] The samples were grit blasted to remove the oxide and recast layer formed on the surface during the EDM process and ultrasonically cleaned in the acetone bath. Subsequently, each sample was heated to 180 C in an oven and a film of Na2SO4 was deposited on the sample by spraying a saturated aqueous solution of Na2SO4 salt. The specimens were weighed before and after the deposition of the salt layer to determine the weight of the deposited salt. The specific weight change, i.e., weight of the deposited salt divided by the initial surface area of the sample, was measured. Samples with three different salt deposits, namely, 3 to 4, 7 to 9, and 12 to 14 mg cm-2, were prepared for assessing the effect of initially available salt concentration on the hot corrosion behavior of the alloy. These samples are referred to as 3MC, 7MC, and 12MC, respectively, in the subsequent text. The melting temperature of the Na2SO4 salt used in this study was determined as 883.5 C using a Setaram Labsys Evo Differential Scanning Calorimeter (DSC). The salt-coated specimens were subjected to isothermal exposure at 950 C in air using a muffle furnace for various (...truncated)