Influence of Alloying with Cobalt and Hafnium on the Corrosion and Hydrogen Resistances of Refractory Nickel Alloy
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INFLUENCE OF ALLOYING WITH COBALT AND HAFNIUM ON THE CORROSION AND HYDROGEN RESISTANCES OF REFRACTORY NICKEL ALLOY L. М. Іvas’kevych
UDC 620.178.4: 669.14.018
We study the influence of alloying with cobalt (13.0 and 18.5 wt.%) and hafnium (0.3 and 0.7 wt.%) on the hydrogen embrittlement and salt corrosion of cast nickel alloy containing (wt.%) 0.08 C; 21.3 Cr; 2.4 Al; 2.8 Ti; 0.5 Nb; 0.015 B, and 0.005 Zr. At room temperature and under a hydrogen pressure of 35 MPa, we determined its characteristics of short-term strength and plasticity and the number of cycles to fracture. The resistance to high-temperature corrosion was estimated after holding in a 0.25 NaCl + 0.75 Na 2SO 4 salt mixture at 1073°K. It was discovered that hafnium additives significantly increase the corrosion and hydrogen resistance of the alloy. We also determined the optimal combination of strength, plasticity, low-cycle fatigue strength, and hydrogen and corrosion resistances for the alloy with 18.5% cobalt and 0.7% hafnium. Keywords: heat-resistant nickel-cobalt alloy, rate of high-temperature salt corrosion, hydrogen embrittlement.
Introduction Cast alloys based of nickel serve as main materials used for the production of components of the hot circuits of gas-turbine plants (GTP). In this case, as criteria for their choice, parallel with high heat resistance, it is necessary to use the ability to withstand the embrittling influence of working gases and products of their combustion [1–11]. As the cause of catastrophic failures of stationary and transport gas-turbine engines, we can mention intense high-temperature corrosion (HTC) caused by the interaction with oxygen and sulfur, as components of the fuel, and with maritime air. In view of the prospects of development of hydrogen power engineering, the problem of hydrogen resistance of the alloys becomes topical. Under the action of hydrogen, the relative elongation δ , relative narrowing ψ , and low-cycle fatigue strength N , as well the characteristics of strength and crack resistance of the materials are noticeably deteriorated [12–16]. The methods used to increase the serviceability of GTP are based on the optimization of the chemical composition of material in order to guarantee both its high heat resistance (alloying with refractory elements and elements which form intermetallic compounds [1–10]) and high corrosion resistance (formation of stable protective oxide films, such as Al 2O 3 and Cr2O 3 , as a result of alloying with chromium, aluminum, tantalum, rhenium, and hafnium [1–10]). The degree of hydrogen embrittlement is determined by the type and form of the hardening phases and the microstructure of grain boundaries, which also depend on the chemical composition of the alloy [12–16]. Note that the role of cobalt is studied insufficiently. This element changes the relationship between lattices of the γ - and γ ′ -phases and, thus, increases the heat resistance [4, 6, 9, 10]. However, according to various available data, it either does not affect the HTC resistance or insignificantly
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