Key Takeaways & Executive Findings
- •• Increasing impurity content significantly reduces the hot corrosion resistance of DD98M alloy at 950 °C in Na2SO4+NaCl salt. • Nitrogen impurities increase alloy porosity, facilitating rapid diffusion of molten salt and oxygen into the alloy. • Bilateral diffusion of oxygen and sulfur leads to accumulation at the oxide-matrix interface, promoting interfacial crack formation and propagation. • A growth model for hot corrosion products was developed, accounting for varying impurity levels.
Abstract
The influence of varying levels of impurity elements on the hot corrosion resistance of the DD98M alloy in Na2SO4+NaCl salt at 950 °C was investigated. The results indicate that the corrosion resistance of the DD98M alloy significantly decreases with an increase in impurity content, and the presence of nitrogen leads to an increase in alloy porosity. These porosities promote the rapid diffusion of molten salt and oxygen into the alloy, resulting in a bilateral diffusion of oxygen and sulfur, which leads to an accumulation of these elements at the oxide−matrix interface. This process contributes to the formation and propagation of interfacial cracks. A growth model was developed for hot corrosion products in alloys with varying impurity elements.
1. Introduction
Single-crystal (SX) superalloys are widely used in the manufacturing of aeroengines and gas turbine blade materials due to their exceptional mechanical properties at high temperatures, resistance to oxidation and hot corrosion, as well as good stability under high temperatures [1−4]. However, alkali metal impurities in the fuel and sulphates (mainly sodium sulphate) formed by the oxidation and combustion of sulphur can adhere to the leaves, leading to hot corrosion at high temperatures and causing unpredictable degradation of materials. This poses significant challenges in predicting the lifespan and maintenance schedules of these components, which can ultimately lead to catastrophic failure of components [5,6]. Consequently, the investigation into the hot corrosion behavior of superalloys remains a significant and pressing area of research, particularly in terms of enhancing the durability and reliability of modern turbine blade materials.
The hot corrosion of superalloys can be classified into two types. Type-I occurs at temperatures ranging from 900 to 1000 °C, whereas type-II occurs within the temperature range of 600−750 °C [7,8]. Type-I hot corrosion occurs through a reaction between the protective oxide layer on the alloy surface and molten sodium sulfate (Na2SO4), which melts at 884 °C [7]. In type-II hot corrosion, sodium sulfate (Na2SO4) reacts with nickel sulfate (NiSO4) and cobalt sulfate (CoSO4) to form a eutectic phase (with a melting point of 550 °C), which accelerates the surface degradation [8,9]. In contrast to type-I, the initiation of type-II hot corrosion requires a high partial pressure of sulfur trioxide (SO3) [10]. Researchers primarily focus on studying the hot corrosion behavior at about 900 °C and the oxidation behavior at approximately 1000 °C [11]. As the inlet temperature of industrial gas turbines (IGTs) continues to rise, the blade surface temperature exceeds 1000 °C, necessitating a more in-depth investigation into the hot corrosion behavior of superalloys at temperatures above 950 °C.
Currently, the enhancement of hot corrosion resistance in superalloys primarily depends on the strategic regulation of alloying elements and the innovation in surface protective coatings. Chromium (Cr) is recognized as the most important element in preventing hot corrosion. Chromium, vital for its ability to form a stable chromium oxide (Cr2O3) layer on the alloy surface, acts as a barrier that prevents the interaction between the molten salt and the underlying matrix [12,13]. In addition, Cr can effectively capture sulfur and inhibit the formation of other harmful liquid sulfides [1...
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Geng-yi DONG, Yijiala YILITI, Run-ze YU, Jie MENG, Wen-jun HAN, Kai CHANG, Qi-fei ZHANG, Xiao-gang YOU, Yi-nong WANG (2025). Effect of trace impurity elements on high-temperature corrosion resistance of DD98M alloy. SinoTechIntel Verified Research. https://doi.org/10.1016/S1003-6326(25)66979-2
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Frequently Asked Questions
What is the effect of impurity elements on DD98M alloy corrosion resistance?
The presence of impurity elements, especially nitrogen, significantly decreases the hot corrosion resistance of DD98M alloy at 950 °C in Na2SO4+NaCl salt. Nitrogen increases alloy porosity, which promotes rapid diffusion of molten salt and oxygen, leading to interfacial crack formation and propagation.
How does nitrogen affect the microstructure of DD98M alloy during hot corrosion?
Nitrogen impurities increase the porosity of the alloy, creating pathways for rapid diffusion of molten salt and oxygen into the alloy. This results in bilateral diffusion of oxygen and sulfur, which accumulate at the oxide-matrix interface and contribute to crack formation.
What is the significance of studying hot corrosion at 950 °C?
As industrial gas turbines operate at higher inlet temperatures, blade surface temperatures exceed 1000 °C. Studying hot corrosion at 950 °C provides insights into the degradation mechanisms relevant to modern turbine materials, helping to improve their durability and reliability.
What are the two types of hot corrosion in superalloys?
Type-I hot corrosion occurs at 900-1000 °C through reaction with molten sodium sulfate, while type-II occurs at 600-750 °C involving a eutectic phase formed by sodium sulfate with nickel and cobalt sulfates. Type-II requires high partial pressure of sulfur trioxide.
What is the role of chromium in hot corrosion resistance?
Chromium forms a stable Cr2O3 layer on the alloy surface, acting as a barrier against molten salt and matrix interaction. It also captures sulfur and inhibits the formation of harmful liquid sulfides, thereby enhancing hot corrosion resistance.
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