A Lifetime-Prediction Approach to Understanding Corrosion: The Corrosion-Fatigue and the Corrosion Behavior of a Nickel-Based Superalloy and a Nanocrystalline Alloy

Lifetime-prediction models are useful for simulating a material’s in-service behavior or outcome. Perhaps the greatest advantage of these models is the ability to use the predicted results to help optimize engineering designs and reduce costs. The Hastelloy® C-2000® superalloy is a single-phase material and face-centered cubic in structure at all temperatures. The C-2000® alloy is a commercially designed alloy manufactured to function in both reducing and oxidizing solutions. C-2000® is used as a fabrication material for heat exchangers, piping for chemical refineries, and storage repositories. The corrosion properties of C-2000® are excellent, and the ductility and fatigue properties are good. In this study, C-2000® is used to verify the theoretical basis of an electrochemical-micromechanical crack-initiation corrosion-fatigue model for materials under passive electrochemical conditions. The results from electrochemical and mechanical experiments, along with the findings from the conventional electron microscopy and a laser interferometer will be presented. A nanocrystalline Ni-18 weight percent (wt.%) Fe alloy is examined to investigate its electrochemical behavior in a 3.5 wt.% NaCl solution. Three Ni-18 wt.% Fe samples were annealed at 400ºC for 3, 8, and 24 hours (hrs.) to study the effects of grain sizes on the electrochemical properties of bulk Ni-18 wt.% Fe. The electrochemical results from the annealed samples are compared with those measured for the as-received Ni-18 wt.% Fe material consisting of an average grain size of 23 nanometers (nm). The samples annealed for times longer than 8 hrs. appear to have undergone an abnormal grain growth, where nanometer and micrometer (μm) grain sizes are present. Unlike the electrochemical results for the as-received material, the annealed nanocrystalline materials appear to be susceptible to localized corrosion. Consequently, these larger grains within the nanoncrystalline-grain matrix are preferentially attacked during electrochemical corrosion. Of the four materials studied, the as-received nanocrystalline alloy possesses the best corrosion properties, and the nanocrystalline material coarsened to an average grain size of 23 μm has the poorest electrochemical properties

Investigation on the stress corrosion cracking susceptibility of an alloy 718 prepared by laser powder bed fusion assessed by microcapillary method

openThe study focused on evaluating the stress corrosion cracking (SCC) behaviour of alloy 718 processed by laser powder bed fusion (L-PBF), also called selective laser melting (SLM), with different laser powers, in a solution containing chloride, using the microcapillary method under constant tensile load. An analysis of the surface defects, hardness, and microstructure of the samples under examination was carried out and a correlation was sought between these results and those obtained by electrochemical investigations. The polished surface of the samples was observed under the optical microscope, as was the electro-etched surface. Vickers hardness measurements were carried out. Electrochemical polarization tests were used to evaluate the resistance of the passive layer on the surface of the material. Potentiodynamic and galvanostatic polarization tests were performed. The tests were carried out on as-printed samples and samples under tensile load. The results were compared with conventionally produced counterparts. The microstructure and stress corrosion cracks were observed under the scanning electron microscope (SEM). The 115W laser power produced samples with a lower defect density; therefore, the highest resistance to localized corrosion was found. L-PBF samples under tensile load showed corrosion and SCC resistance superior to that of conventional material. In the L-PBF samples, submicronic cracks were detected adjacent to the boundaries of the subgrain, and the mechanisms that led to their appearance were explained as a synergistic effect of various microstructural factors, specifically, the greater corrosion resistance of the subgrain boundary and the high concentration of dislocations in the adjacent area. The fine microstructure of the L-PBF samples generated much smaller cracks than those observed on the conventional material, which explains the increased resistance to SCC observed in electrochemical tests.The study focused on evaluating the stress corrosion cracking (SCC) behaviour of alloy 718 processed by laser powder bed fusion (L-PBF), also called selective laser melting (SLM), with different laser powers, in a solution containing chloride, using the microcapillary method under constant tensile load. An analysis of the surface defects, hardness, and microstructure of the samples under examination was carried out and a correlation was sought between these results and those obtained by electrochemical investigations. The polished surface of the samples was observed under the optical microscope, as was the electro-etched surface. Vickers hardness measurements were carried out. Electrochemical polarization tests were used to evaluate the resistance of the passive layer on the surface of the material. Potentiodynamic and galvanostatic polarization tests were performed. The tests were carried out on as-printed samples and samples under tensile load. The results were compared with conventionally produced counterparts. The microstructure and stress corrosion cracks were observed under the scanning electron microscope (SEM). The 115W laser power produced samples with a lower defect density; therefore, the highest resistance to localized corrosion was found. L-PBF samples under tensile load showed corrosion and SCC resistance superior to that of conventional material. In the L-PBF samples, submicronic cracks were detected adjacent to the boundaries of the subgrain, and the mechanisms that led to their appearance were explained as a synergistic effect of various microstructural factors, specifically, the greater corrosion resistance of the subgrain boundary and the high concentration of dislocations in the adjacent area. The fine microstructure of the L-PBF samples generated much smaller cracks than those observed on the conventional material, which explains the increased resistance to SCC observed in electrochemical tests

ENHANCING OXIDATION RESISTANCE OF Ni-BASED ALLOYS FOR HIGH TEMPERATURE APPLICATIONS

The demand for energy significantly increases with increasing of world population and industrial activity in developing countries. Nuclear energy can be considered to make an important contribution to future demands for electricity and thermal energy. In order to continue providing inexpensive and safe energy, the operation temperatures of the next generation nuclear reactors should be remarkably increased. Consequently, the alloys used in the high temperature environment will face oxidation degradation problems. Therefore, it is critical to find ways to improve the oxidation resistance of the alloys to be used for high temperature applications. In this thesis, two potential solutions are proposed to enhance the oxidation resistance of three Ni-based alloys. The effect of crystallographic orientation on the oxidation behavior of Hastelloy 230 was investigated to evaluate if texture would affect the oxidation behavior of Ni-based alloys. The obtained results clearly demonstrate that the oxidation rate highly depends on the grain orientation. The oxidation resistance of grains are found to increase in the order of (100) < (110) < (111). The substrates with different grain sizes and similar textures were prepared to inspect the influence of grain sizes on the oxidation behavior of Hastelloy 230 and Hastelloy N. It is demonstrated that the grain size only has a small influence on the oxidation behavior of the high-Cr Hastelloy 230 while it significantly changes the oxidation behavior of the low-Cr Hastelloy N. The oxidation mechanisms of high and low-Cr alloys with coarse and fine grain sizes are discussed. Increasing of the grain boundary density of the low-Cr alloy is believed to promote the diffusivities of Cr ions which results in the formation of a uniform Cr-rich protective oxide layer. The grain refinement process can be an efficient way to enhance the oxidation resistance of investigated low-Cr alloy. The effect of reactive element coating on oxidation behavior of chromium oxide forming alloy was examined. The parameters of electro-deposition process like the current density, deposition time and temperature were optimized to synthesize ceria coating. The coatings were applied to Hastelloy 230 and 625 to investigate the influence of this reactive element on the oxidation behavior. For both alloys, the coating significantly enhanced the oxidation resistance and adherence of the oxide to the substrate. The segregation of reactive element onto grain boundaries is linked to change of the oxidation mechanism from outward diffusion of cations to inward diffusion of anions. Specifically for Hastelloy 230, the addition of ceria coating changed the chromium oxide morphology from columnar to equaxied structure and that is believed to enhance the spallation resistance. In Hastelloy 625, a porous and cracked oxide layer is observed on the uncoated sample, while a layer that is protective against oxidation is formed on the coated sample. The prepared coating suppressed the formation of iron oxide. The oxidation mechanism is suggested and discussed

Wire Arc Additive Manufacturing of Ni-based Hastelloy C276 Alloy and Ameliorated Processes

Hastelloy C276 alloy, a typical Ni-based solid solution strengthened trademark superalloy, possesses excellent practical performance under severe environmental conditions and elevated temperatures. Hence, Hastelloy C276 has been investigated as a candidate for structural material for nuclear reactors, chemical processing and aerospace, particularly as aero-engine components. Wire arc additive manufacturing (WAAM), with its relatively high deposition efficiencies and cost-effectiveness, is revolutionising the 3D fabrication of advanced alloy components. However, the effectiveness of WAAM, for fabricating Ni-based alloys, is not fully understood. This thesis explores Hastelloy C276 structures deposited by the WAAM process, aiming to provide an insightful understanding of this process in the fabrication of Hastelloy C276 alloy and to assess and optimise different processing approaches, permitting a systematical investigation of process-microstructure-property interrelationships in WAAM fabricated Hastelloy C276

Corrosion behavior of metallic alloys in molten chloride salts for thermal energy storage in concentrated solar power plants - A review

Recently, more and more attention is paid on applications of molten chlorides in concentrated solar power (CSP) plants as high-temperature thermal energy storage (TES) and heat transfer fluid (HTF) materials due to their high thermal stability limits and low prices, compared to the commercial TES/HTF materials in CSP - nitrate salt mixtures. A higher TES/HTF operating temperature leads to higher efficiency of thermal to electrical energy conversion of the power block in CSP, however causes additional challenges, particularly increased corrosiveness of metallic alloys used as containers and structural materials. Thus, it is essential to study corrosion behaviors and mechanisms of metallic alloys in molten chlorides at operating temperatures (500-800°C) for realizing the commercial application of molten chlorides in CSP. The results of studies on hot corrosion of metallic alloys in molten chlorides are reviewed to understand their corrosion behaviors and mechanisms under various conditions (e.g., temperature, atmosphere). Emphasis has also been given on salt purification to reduce corrosive impurities in molten chlorides and development of electrochemical techniques to in-situ monitor corrosive impurities in molten chlorides, in order to efficiently control corrosion rates of metallic alloys in molten chlorides to meet the requirements of industrial applications

Coatings for alloys used in molten salt nuclear reactor

Rising CO2 levels due to the production of energy from fossil fuels are major contributors to environmental pollution. Nuclear energy, on the other hand, is clean energy and does not contribute to environmental pollution. Conventional water-cooled reactors are safe; however, they still suffer the consequences of the Fukushima accident. A molten salt-cooled reactor, on the other hand, is intrinsically safe and more efficient. But the problem of corrosion of structural alloys in a molten salt environment is an obstacle to the success of Molten Salt Reactors (MSR). The purpose of this research is to analyze the corrosion behaviour of different alloys in a molten salts environment and develop coatings for inhibiting this corrosion. The alloys Hastelloy® N, Haynes® X-750, AISI 304, and AISI 316 were tested for corrosion in molten FLiNaK for 100 h at 700 °C under an argon gas cover. The Cr depletion was found to be the major cause of corrosion in the FLiNaK environment. The highest corrosion observed in the Haynes® X-750, is related to the presence of Al and Ti. However, Hastelloy® N is well protected against corrosion in the FLiNaK environment. Furthermore, AISI 316 was coated with Ni of varying thicknesses to see its corrosion resistance behaviour in FLiNaK at 700 °C. The results indicate thicker Ni coatings of 75 µm are sufficient to provide corrosion resistance to the stainless-steel samples under the testing conditions. But the chromium was still observed to be diffusing toward the Ni coating from the steel substrate and could eventually lead to corrosion during longer exposures to FLiNaK salt. The Ni coating was also modified with the addition of Mo to evaluate the corrosion resistance in the FLiNaK environment. The AISI 304 samples coated with Ni-Mo did not protect well against corrosion. The presence of Mo in the Ni-Mo coating was associated with the formation of carbides at the surface of the Ni-Mo coated sample, which led to accelerated corrosion. Finally, a SiC diffusion barrier was deposited between the Ni plating and the steel surface. The results obtained showed that this coating improved the corrosion resistance of the AISI 304 samples by five times

Hydrogen Embrittlement

Hydrogen embrittlement (HE) is a process resulting in a decrease in the fracture toughness or ductility of a metal due to the presence of atomic hydrogen. In addition to pure hydrogen gas as a direct source for the absorption of atomic hydrogen, the damaging effect can manifest itself from other hydrogen-containing gas species such as hydrogen sulfide (H2S), hydrogen chloride (HCl), and hydrogen bromide (HBr) environments. It has been known that H2S environment may result in a much more severe condition of embrittlement than pure hydrogen gas (H2) for certain types of alloys at similar conditions of stress and gas pressure. The reduction of fracture loads can occur at levels well below the yield strength of the material. Hydrogen embrittlement is usually manifest in terms of singular sharp cracks, in contrast to the extensive branching observed for stress corrosion cracking. The initial crack openings and the local deformation associated with crack propagation may be so small that they are difficult to detect except in special nondestructive examinations. Cracks due to HE can grow rapidly with little macroscopic evidence of mechanical deformation in materials that are normally quite ductile. This Technical Memorandum presents a comprehensive review of experimental data for the effects of gaseous Hydrogen Environment Embrittlement (HEE) for several types of metallic materials. Common material screening methods are used to rate the hydrogen degradation of mechanical properties that occur while the material is under an applied stress and exposed to gaseous hydrogen as compared to air or helium, under slow strain rates (SSR) testing. Due to the simplicity and accelerated nature of these tests, the results expressed in terms of HEE index are not intended to necessarily represent true hydrogen service environment for long-term exposure, but rather to provide a practical approach for material screening, which is a useful concept to qualitatively evaluate the severity of hydrogen embrittlement. The effects of hydrogen gas on mechanical properties such as tensile strength, ductility, fracture, low and high cycle fatigue, crack growth rate, and creep rupture are analyzed with respect to the general trends established from the HEE index values. It is observed that the severity of HE effects is also influenced by environmental factors such as pressure, temperature, and hydrogen gas purity. The severity of HE effects is also influenced by material factors such as surface finish, heat treatment, and product forms, compositions, grain direction, and crystal orientations

Effect of Laser Powder Bed Fusion Parameters and Hot Isostatic Pressing on the Microstructure and Mechanical Properties of C22 Alloy

A nickel-based Hastelloy C22 powder with a nominal composition of Ni-21.30Cr-13.36Mo-1.92Co-2.94W-3.85Fe-0.23Mn-0.3V(wt%) and particle size distribution of D₁₀=23.8±0.3 μm, D₅₀=37.5±0.2μm, and D₉₀=59.3±1.1 μm was gas atomized to be used as a feedstock in Laser Powder Bed Fusion (LPBF). In this study, laser power and scanning speed were optimized to manufacture specimens with the highest density. Samples manufactured with power of 150 W and scanning speed of 200 mm/s showed relative density of 99.6%. Additionally, for higher production rate, samples were manufactured with power of 225 W and scanning speed of 1200 mm/s. Additively manufactured parts were post processed by conducting solution heat treatment, hot isostatic pressing and hot isostatic pressing+ solution heat treatments. Total 8 conditions were evaluated for phases and microstructure using optical microscope, X-ray diffraction, scanning electron microscopy, and electron backscatter diffraction. Furthermore, microhardness and tensile test were performed on all optimized and post processed samples. Typical cellular and columnar dendritic structure were founded in the as-printed samples. Mo-rich nanoparticles were observed in both as-printed samples and HIPPed samples and were responsible for higher strength. HIP effectively eliminated most of defects in the as-printed samples, but fine precipitates were observed in HIPPed samples. Solution heat treatment led to a full recrystallization with annealing and complete dissolution of the Mo-rich second phases into the Ni matrix. LPBF followed by HIP seems necessary to achieve full density parts with the desired mechanical properties, whereas role of solution treatment on enhancing mechanical properties was not as significant as HIP

Corrosion and other properties comparison of AISI 316L stainless steel surface alloyed with Ru/Ni mixtures with the parent metal and with Hastelloy© C-276

A dissertation submitted to the University of the Witwatersrand in fulfilment of the requirements for the degree of Master of Science in Engineering (Metallurgy & Materials) 2016The surfaces of AISI 316L stainless steel plate were laser alloyed with ruthenium powder as well as a mixture of ruthenium and nickel powders using a Nd:YAG laser set at fixed operating parameters. The microstructure, elemental composition, and corrosion characteristics of the alloyed zone were analysed using optical and scanning electron microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and corrosion potential measurements. EDS analysis of the alloyed specimen showed that through the laser surface alloying, 2 mm surface layers with 12.5wt % Ru and 5.2wt% Ru were produced on an AISI 316L stainless steel. Similar microstructures which were dendritic and columnar grains, typical of weld beads under non-equilibrium cooling conditions were observed for all samples. Hardness profile measurements showed a significant increase from 160 HV for the substrate to a maximum of 247 HV for the alloyed layer. Using an Autolab potentiostat, the corrosion behaviour and resistance of the laser alloyed layers, substrate AISI 316L, and Hastelloy© C-276 were evaluated and compared in sulphuric acid solution of different concentration and temperatures. The Hastelloy© C-276, followed by the 12.5wt% Ru presented the most noble corrosion potential (Ecorr) and the lowest corrosion current density (icorr). However, in 60wt% H2SO4 and 40oC, the 5.22 wt% Ru alloys exhibited slightly better anticorrosive properties than 12.5wt% Ru. The observed corrosion potential, Ecorr, for untreated AISI 316L stainless steel sample in 40wt% sulphuric acid solution at 40oC was -277 mV. The 5.22 wt% Ru and 12.5wt% Ru alloyed stainless steel samples presented -240 mV, and 61 mV respectively in the same solution. Besides showing comparable performance to 5.2wt%Ru sample within specific short potential ranges, Hastelloy© C-276 was generally superior in all solutions. In addition it was found that the stability of the passive layer was improved with additions of Ru. Based on the developed costing equation the cost of 5 mm AISI 316L stainless steel plate with surface area (A = 1 m2) surface alloyed with 5.2wt% Ru to a depth of 2 mm using Nd: YAG laser is estimated at R15 989, and it is less than the cost of a Hastelloy© C-276 plate of similar size which is estimated at R19 900. As the material thickness increases, the cost benefit of laser surface treatment increases and vice versa. Reduction of the Ru additions to levels below 5.2wt% would improve cost competition without detracting from performance.MT201