Corrosion Behavior of GH4169 Alloy under Alternating Oxidation at 900 °C and Solution Immersion

materials Article Corrosion Behavior of GH4169 Alloy under Alternating Oxidation at 900 ◦C and Solution Immersion Zhongfen Yu 1,2 , Li Liu 2,3, *, Rui Liu 2 , Min Cao 2 , Lei Fan 2 , Ying Li 2 , Shujiang Geng 1 and Fuhui Wang 3 1 2 3 * School of metallurgy, Northeastern University, Shenyang, 110819, China; (Z.Y.); (S.G.) Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China; (R.L.); (M.C.); (L.F.); (Y.L.) Key Laboratory for Anisotropy and Texture of Materials (MoE), School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China; Correspondence: ; Tel.: +86-24-8108-3918; Fax: +86-24-2392-5323 Received: 29 March 2019; Accepted: 6 May 2019; Published: 8 May 2019



 Abstract: In this paper, the corrosion behavior of GH4169 superalloy under alternating oxidation (at 900 ◦ C) and solution immersion (in 3.5% NaCl solution, 30 ± 1 ◦ C) has been studied by SEM, XRD, XPS, and electron probe microanalysis (EPMA). The results show that the alternating environment increases the corrosion rate of GH4169. The reaction of NaCl and Cr2 O3 generates various volatile and soluble corrosion products, such as Na2 Cr2 O7 , CrCl3 , Cl2 , and Na2 CrO4 , at a high temperature. The destruction of the protective Cr2 O3 film leads to the increase of defects in the oxide scale, promoting the formation of oxides, such as NiO and Fe2 O3 , and changes the composition and structure of the oxide film. After repeated iterations, the mixed oxides will result in the spalling of the oxide film because they can reduce the fracture toughness of the corrosion scale. Therefore, the corrosion is comprehensively intensified. Keywords: Ni-based superalloy; high temperature oxidation 1. Introduction Ni-based superalloys are widely used in the turbine blades of gas turbines and aircraft engines due to their excellent mechanical strength and oxidation resistance [1–3]. These materials, however, suffer serious corrosion in filed experiences [4–6]. Many researchers have focused on the high temperature oxidation behavior of Ni-based superalloy in pure O2 or air [7–10]. Previous investigations showed that the GH202 superalloy had excellent oxidation resistance at 800 ◦ C and 900 ◦ C due to the formation of a continuous and dense Cr2 O3 layer, which improved the bonding strength between the oxide scale and the substrate [11]. Some scholars believe that the spalling of NiCr2 O4 spinel could lead to the depletion of Cr, and therefore, metals cannot form a continuous protective Cr2 O3 layer for a long time [11]. The thermal shock resistance of nickel-based alloys was conducted by simulating the oxidation behavior under the operation and parked state of gas turbines. Studies revealed that Superni76, Superni750, and In600 had an excellent resistance to cyclic oxidation since a protective oxide scale consisting of Cr2 O3 could form at 750 to 950 ◦ C [12,13]. However, some researchers believed that the NiCr2 O4 oxide film, resulting from the solid-state reaction between NiO and Cr2 O3 , is easily peeled off from the substrate under the cyclic oxidation, because of the high growth stress at 900 ◦ C [14]. Besides the long time oxidation and cyclic oxidation, Ni-based alloys used in gas turbines also face Materials 2019, 12, 1503; doi:10.3390/ma12091503 www.mdpi.com/journal/materials Materials 2019, 12, 1503 2 of 17 other corrosion, especially salt corrosion [15,16]. When gas turbines are used in marine environments and thermal power conditions, the hot corrosion behavior of the Ni-based superalloys in a high temperature environment containing salt is one important issue. It was found that Na2 SO4 could y− provide SO3 and O2− at high temperatures, which led to the formation of MSx and MOx , promoting cracking and peeling off of the oxide film, and finally accelerated corrosion [17,18]. On the other hand, y− Cl− could react with metals and oxides, and form MOx and Cl2 at high temperatures. Cl2 could penetrate into the inside of the oxide film easily and further react with the oxides, leading to the peeling off of oxide films, and accelerating the corrosion of the alloy [19–21]. As previous studies usually focused on the corrosion behavior of superalloys in a single environment, a more intensive study concerning the environmental details and their relationships with the corrosion of aircraft engine blades is urgently required. In fact, engine blades are used in elevated temperatures and high salt environments in the marine environment during operation and parking, respectively. In this environment, the blade suffers synergistic corrosion between high oxidation and low temperature electrochemical reactions. When working in high temperatures, oxidation happens; on the other hand, when parked in normal temperatures, the electrochemical reaction happens. Therefore, the metal materials used in aircraft engine blades experience a special corrosion: An alternating high temperature oxidation and normal temperature electrochemical reaction. However, limited published papers have focused on these alternating oxidation and electrochemical reactions. In this paper, the alternating corrosion behavior of GH4169 alloy (the blade material) under the alternating high-temperature oxidation (900 ◦ C) and normal temperature corrosion (3.5% NaCl solution, 30 ± 1 ◦ C) were investigated. The corrosion rate was calculated by weighing experiments, and the composition and morphology of the corrosion products were analyzed by XRD, XPS, and SEM. The alternating corrosion mechanism was discussed. 2. Materials and Methods 2.1. Material and Specimens The material used in this study was Ni-based superalloy GH4169, with a chemical composition as listed in Table 1. After cut into a piece with the dimensions of 10 mm × 10 mm × 2 mm, the specimens were ground using SiC papers up to 2000 grit, cleaned with distilled water and alcohol, and dried in cold air before the corrosion tests. Table 1. The compositions of GH4169 alloy. Element C Cr Mo Al Ti Fe Nb + Ta B Ni Mass% 0.045 19.09 3.25 0.88 0.83 18 5.08 0.05 Bal. 2.2. Alternating Oxidation and Normal Temperature Corrosion Test The specimens were suspended by a quartz and inserted inside a tube furnace, in which it was first performed in static air conditions at 900 ◦ C for 10 h. After 10 h of oxidation, these specimens were cooled down in the air to room temperature for 1 h and immersed in a 3.5% NaCl solution at 30 ± 1 ◦ C for another 10 h. Then, the specimens were dried with hot air for 15 min and NaCl was deposited evenly on the surface of the specimens. Next, the specimens were put into the tube furnace to begin the next experimental cycles. Each experimental cycle was 20 h, including 10 h of oxidation and 10 h of normal immersion in 3.5% NaCl solution. The whole test included 10 cycles, as shown in Figure 1. Specimens after high temperature corrosion were recorded as No. x. 5 cycle (x is the number of cy (...truncated)