Molten Sodium Sulfate Research Articles

Corrosion fatigue and damage tolerance in the nickel‐based superalloy RR1000 subjected to SO2 environments

AbstractWhen exposed to a high‐temperature corrosive environment, nickel‐based superalloys may experience surface pitting and sulphide diffusion, which will influence concurrent or subsequent fatigue behavior. Sulphur, pre‐existing in the environment or as a bi‐product of burning fossil fuels, reacts with sodium (as an atmospheric pollutant), creating molten sodium sulphate deposits on the metal surface. Combined with sodium chloride, these deposits attack the protective oxide layer allowing sulphides to migrate along grain boundaries. Continued sulphide diffusion promotes a weakened subsurface layer, inducing grain dropout and fatigue crack initiation. The present investigation focussed upon the subsequent effects of exposure to SO2 containing atmospheres on low cycle fatigue performance, together with the impact of an intermediate cleaning process. Damage tolerance data suggest that exposure to a SO2 environment fails to affect fatigue crack growth threshold or Stage II growth behaviors when compared with standard laboratory air.

Comparison of high temperature corrosion resistance of NiCr-based coatings applied by High Velocity Oxygen Fuel and Twin Arc Spray processes

High temperature corrosion in a molten sodium sulfate salt environment is a serious problem of coal-fired boiler components. The promising possibility for surface protection is to apply thermally sprayed coatings. However, the key to success is to choose the optimal material and the proper spraying technology for a given application. Therefore, this study is aimed at a comparison of the high temperature behaviour of two protecting NiCr-based coatings sprayed by different technologies on 1.4903 (W.nr.) chrome-molybdenum steel. NiCrMoWFe was deposited by High Velocity Oxygen Fuel technology (HVOF). Twin wire arc spray technology (TWAS) was used to deposit multi-layered “multicoat” consisting of NiCrMoNbTa and NiCrTi with ceramic overlay Armaguard coated by brush. Chrome-molybdenum steel 1.7715 (W.nr.) commonly used in coal-fired boiler was also exposed to the high temperature corrosion test to evaluate the potential benefit of the coating. Coated as well as the uncoated specimens were exposed to the molten salt environment 18% Na2SO4 and 82% Fe2(SO4)3 at 690 °C under cyclic condition. The thermogravimetric approach was used to determine kinetics of corrosion. The X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyze the corrosion products and determine the corrosion mechanism. The results of high temperature corrosion resistance show that NiCrMoWFe coated by HVOF and multicoat sprayed by TWAS was comparable.

Influence of rheological behavior of molten sodium sulfate on adherent heterogeneous surface on performance of high-temperature thermochemical energy storage

Molecular dynamics simulations were carried out to investigate the influence of the rheological behavior of molten sodium sulfate on the adherent heterogeneous surface on the performance of high-temperature thermochemical energy storage. It was found that the molten salts occurred to migrate on a nanoscale adherent heterogeneous surface due to the energy gradient of the heterogeneous surface. Molten salts underwent three spreading stages during the entire migration process. Poor wettability of the molten salt on the CaO surface came out in the climb spreading stage, and good wettability came out in the fall spreading stage. More importantly, the motion of molten salt caused an additional enhancement effect of potential energy at the interface compared to the pure CaO surface. The enhancement mechanism was the kinetic energy caused a more violent collision of ions between the molten salt and the solid surface, which lead to a local overheat at the interface to promote the ionization of the surface. The structural analysis illustrated that the calcium oxide ions of the heterogeneous surface appear stronger fluidization compared with the calcium oxide ions of the pure calcium oxide surface. The enhancement effect worked on the CaO based adsorbent rather than the inert solid supports.

Wettability of molten sodium sulfate salt on nanoscale calcium oxide surface in high-temperature thermochemical energy storage

Molecular dynamics simulations were carried out to originally investigate wettability of molten sodium sulfate salt on nanoscale calcium oxide surfaces at high temperature and micro-mechanisms on the molten salt promoting the performance of thermochemical energy storage. It was found that in terms of the molten sodium sulfate salt, the nanoscale calcium oxide surface was hydrophilic, and became more and more hydrophilic with increasing temperatures. More importantly, average diffusion coefficients and radial distribution functions of various ions illustrated that the molten sodium sulfate activated the calcium ions and the oxygen ions on the calcium oxide surface, resulting in weak fluidization of the surface at a temperature below the melting temperature. This promoted potential energy of the atom interaction at the contact interface. Additionally, it was observed that the sodium sulfate transited from partial coverage to complete coverage on the spherical nanoscale calcium oxide surface. Higher temperatures induced better coverage performances due to the decreasing surface tension of the molten salt. Therefore, the molten salt formed a screen on the surface to effectively prevent the calcium oxide particles from sintering. This study provides a theoretical foundation to further utilize the molten salt to promote the performance of the thermochemical energy storage.

Effect of Rhenium Addition on Hot Corrosion Resistance of Ni-Based Single Crystal Superalloys

Hot corrosion behavior of two Ni-based single crystal superalloys containing 0 Re and 2 wt pct Re in molten sodium sulfate (Na2SO4) at 900 °C in static air has been investigated using the “deposit recoat” method. It was shown that the incubation period of the Re-containing alloy was much longer than that of the Re-free alloy. It was because Re increased the stability of the chromium oxide (Cr2O3) scale, promoted the formation of nickel titanate (NiTiO3), and suppressed the diffusion of Ni. In addition, the overall corrosion kinetics of the Re-containing alloy followed a multistage parabolic law, compared with the Re-free alloy. This could be because Re increased the activity of chromium (Cr) and titanium (Ti), which resulted in the periodically opening and closure of the cracks in the outer oxide layer.

Role of tantalum in the hot corrosion of a Ni-base single crystal superalloy

Hot corrosion behaviour of a Ni-base single crystal superalloy with low Cr, Ti and high Ta contents in molten sodium sulphate (Na2SO4) at 900°C in static air was investigated using the “deposit recoat” method. The corrosion scale was composed of an outer NiO layer, an inner Al2O3-dominant oxide network layer and a (CrSx(1.000<x<1.500), TaS2) sulphide layer. Ta was found to exhibit a beneficial effect by promoting the formation of NaTaO3 and TaS2, and therefore inhibiting the formation of liquid phases Na2MoO4 and Ni-sulphides. The formation mechanism and effect of TaS2 were discussed in detail in the present paper.

Hot Corrosion of SiC/BN/SiC Composites

SiC fiber-reinforced SiC matrix composites with a BN interphase (SiC/BN/SiC) are proposed for hot-section turbine engine applications. NaCl ingested into the engine in marine environments can react with sulfur impurities present in fuel forming molten sodium sulfate deposits resulting in hot corrosion. Hot corrosion of bulk SiC has been investigated, however, the effects of BN on SiC corrosion, as well as an understanding of hot corrosion resistance of SiC fibers and SiC matrix materials produced by different processing routes has not been elucidated. In this study, Na2SO4-induced hot corrosion of model materials (i.e. bulk CVD or Hexoloy SiC with thin BN surface films) and SiC/BN/SiC composites was characterized in furnace studies at temperatures between 900 and 1150°C, for times up to 96 hours, and in O2 with 2.5 or 1000 ppm SO2. Na2SO4 was applied to the surface of coupons prior to furnace exposure. After exposure, the amount of corrosion was determined from weight change measurements. Corrosion products were characterized by Scanning Electron Microscopy, Energy Dispersive Spectroscopy, and dissolution followed by Inductively Coupled Plasma Optical Emission Spectroscopy. Thin film BN appeared to increase the hot corrosion attack of the model materials. The composites exhibited more severe hot corrosion than the model materials, especially in matrix areas with initial porosity. Sylramic fibers underwent more attack than Hi-Nicalon-S fibers. Ceramic matrix composites were also exposed in a Mach 0.3 Burner Rig at 950°C for times of 24 and 45 hours. Simulated seawater (NaCl in deionized H2O) was injected into the burner rig, reacted with sulfur in the fuel to form Na2SO4 (8ppm) to induce hot corrosion. Severe hot corrosion was observed under these conditions. Microstructural and compositional characterization of the corrosion products is underway. Finally, solubility of relevant oxides in sodium sulfate will be determined as a function of Na2O activity at 900°C to elucidate the SO2 and SO3 partial pressure dependence of hot corrosion of silica- and boria- forming materials.

Hot corrosion behaviour and its mechanism of a new alumina-forming austenitic stainless steel in molten sodium sulphate

Abstract Hot corrosion behaviour and related mechanism of an alumina-forming austenitic (AFA) stainless steel at 1173 K in molten sodium sulphate were investigated. Compared with the Ni-based super-alloy K438, K417 and the commercial steel 316L, the current AFA steel exhibited a high hot-corrosion resistance with less internal sulphidation. It was found that a dense, continuous Al2O3 scale created at the early corrosion stage enabled formation of a compact Cr2O3 scale on it in the subsequent corrosion. Such dual layers effectively suppressed the sulphur penetration and protected the matrix from the hot corrosion.

Spatial Distribution of Heavy Metal Ions in Glass Forming Melts: Spectroscopic and Physical-Chemical Study

Spatial distribution of heavy metal ions (HMI) in inorganic glass forming melts was studied by measuring HMI distribution coefficient between two unmixable melts: sodium-borate glass forming melt and non-glass forming molten sodium sulfate. Combining the data on glass host composition dependence of HMI distribution coefficient with the data of Rayleigh and Mandel’shtam-Brillouin scattering (RMBS) spectroscopy of glasses doped with HMI and undoped ones made it possible to evidence the segregation of HMI into alkali-enriched inhomogeneities of glass hosts and offer the approach to choosing compositions of glass host in which HMI segregation will be minimized.

Effect of Temperature on the Synthesis of ZrO&lt;sub&gt;2&lt;/sub&gt;-Mullite Powders in Molten Sodium Sulfate

ZrO2-mullite compound powders were synthesized with zircon and aluminum sulfate in molten sodium sulfate. Na2ZrSiO5 was prepared as intermediate with zircon and sodium carbonate at 900°C for 3h. Then the mixture of resultant Na2ZrSiO5 and aluminum sulfate was heated at different temperatures in molten sodium sulfate salt. The ZrO2-Al2O3-SiO2 compound powders were obtained after washing away the solvable salts. The results showed that the compound powders synthesized at 800°C were tetragonal ZrO2, SiO2 and Al2O3; the powders synthesized at 1000°C were monoclinal ZrO2 and mullite. The mullite in compound powders became to decompose when the reaction temperature increased over 1100°C.

Preparation of Mullite Powder by Aluminum Sulfate and Silica in Molten Sodium Sulfate

Mullite powder was prepared through reaction of aluminum sulfate and silica in molten sodium sulfate media. The phase composition and microstructure of mullite powder were investigated. The results showed that high purity mullite is prepared by this method. The mullite powder exhibits needle-like whisker crystal with 50~100 nm in diameter and 3~8 #m in length. It is found that by this method mullite starts to form at 900oC, the quartz phase is disappeared about 1000oC, and mullite begin to decompose when the temperature is over 1100oC. Thus, the reasonable synthesizing temperature is about 1000oC. This synthetic method is characterized by process simplicity, the temperature of mullite formation is lower than 200-400oC compared with other conventional methods.

Molten salt dynamics in glass melts using millimeter-wave emissivity measurements

A novel application of millimeter-wave radiometry has been made for the first time to non-contact detection and monitoring of molten salt layer formation on a nuclear waste glass melt (without the nuclear waste) in a joule-heated melter, which could eventually be implemented for on-line monitoring in nuclear waste vitrification facilities. The experiments were carried out at a frequency of 137 GHz in the EV-16 melter at Clemson Environmental Technology Laboratory (CETL) with 245 lbs. (111 kG) of glass and a total of 4.2 lbs. (1.9 kG) of added salt. The dynamics of salt layer build up were observed from the initial formation of small drops of about 5 mm diameter or less to larger pools >28 mm cross-section that were coincident with the increase in millimeter-wave surface level fluctuations causing the salt to flow back and forth until a continuous layer was formed. The millimeter-wave emissivity at 137 GHz of DWPF black frit glass melt and molten sodium sulfate salt at 950 °C was determined to be 0.64 ± 0.05 and 0.44 ± 0.05, respectively.

Reaction-bonded silicon carbide refractories

Silicon nitride-bonded silicon carbide (SNBSC) and aluminosilicate-bonded silicon carbide (ASBSC) refractories have been prepared by reaction bonding. The percentage of silicon nitride or aluminosilicate in the refractory was varied from 15 to 25 wt.%. The modulus of rupture of the refractories increases with molybdenum disilicide (MoSi 2) infiltration. These are resistant to molten zinc, lead, and aluminium, but are attacked by molten sodium sulphate. The thermal expansion coefficient of silicon carbide refractories varies from 3.1×10 −6 to 5.3×10 −6 ° C −1 for the temperature from room temperature to 1000 °C.

Effect of Experimental Procedures on the Cyclic, Hot-Corrosion Behavior of NiCoCrAlY-Type Bondcoat Alloys

A simplified test procedure was established to assess the hot-corrosion behavior of MCrAlY-type nickel-base alloys under the influence of molten sodium sulfate as well as sodium sulfate/potassium sulfate salt blends. Salt-coated specimens were exposed to 1-hr thermal cycles at 950°C in flowing oxygen for up to 500 cycles. Mass-change data of the specimens revealed a significant dependence of the corrosion attack not only on the average contaminant flux rate, as expected, but also on the initial amount of salt deposited during each recoating cycle. Furthermore, deposit removal before salt recoating markedly influenced the corrosion attack of the alloys. This was apparently related to changes in salt chemistry by the dissolution of elements such as Cr from the alloy, which can shift the basicity of the salt and thus affect the extent of attack. Substituting Na for K in sodium sulfate/potassium sulfate salt blends generally resulted in decreased attack. For K-containing salt deposits, increasingthe gross amount of alkali compared to sulfur resulted in increased sample weight losses due to scale spallation. In contrast, decreasing the amount of sulfur in such deposits which contained exclusively Na as the alkali resulted in a significantly reduced corrosion attack compared to stoichiometric sodium sulfate.

Cathodic corrosion of nickel in molten sodium sulphate and sodium carbonate at 900°C

The corrosion behaviour of nickel under cathodic polarization in both molten Na 2SO 4 and Na 2CO 3 at 900°C has been investigated. Corrosion losses of nickel under cathodic polarization in both molten salts increased with an increasing quantity of electricity for the cathodic reaction. Large amounts of powdered corrosion products, rich in nickel, were detected in both salts after cathodic polarization. It is thought that such cathodic corrosion of nickel proceeded by a chemical reaction of nickel with the cathodic products.

Evaluation of the environmentally assisted cracking of aluminide intermetallic compounds

A series of mechanical and electrochemical experiments were performed on samples of titanium aluminides, Ti 3Al( α 2) and iron aluminide (D0 3-Fe 3Al). Electrochemical studies of these alloys in NaCl solution showed that Ti 3Al is less susceptible to pitting corrosion than Fe 3Al. Both materials were susceptible to stress corrosion cracking whenever the applied potential during stressing was greater than the breakdown potential. The mode of cracking was intergranular and most often multiple cracks were observed on the surface of tested samples. At applied potentials less than the breakdown potential susceptibility to localized corrosion decreased sharply and at the open circuit potential these alloys did not show any susceptibility. Hydrogen embrittlement (HE) was studied by cathodically charging samples with hydrogen. Ti 3Al was very susceptible to HE and its tensile properties after hydrogen charging dropped sharply, while Fe 3Al was less susceptible and no major changes in mechanical properties were observed. Oxidation rate of both materials were low up to 800 °C, but hot corrosion resistance in molten salt was extremely low. Severe localized corrosion was observed when samples were exposed to a molten sodium sulfate salt.

The Determination of Electronic Transport Properties in Molten Sodium Sulfate at 1173 Κ

Article The Determination of Electronic Transport Properties in Molten Sodium Sulfate at 1173 Κ was published on January 1, 1992 in the journal High Temperature Materials and Processes (volume 10, issue 2).

On the electrical conductivity and other transport properties of molten sodium sulphate

Kim and Simkovich have recently published a study of transport properties in molten sodium sulphate at 1173 K. Surprisingly enough, their electrical conductivity is one order of magnitude lower than according to all other studies of the pure salt. Their result is also in conflict with the general systematics found for high-conducting melts. The possible sources of error in conductivity measurements are discussed, and so are also the concept of transport number for pure molten salts and the validity of the Nernst-Einstein relation. Their transport number experiment is reinterpreted, which has some consequences for the calculated sodium ion mobility and self diffusion coefficient.

ChemInform Abstract: Synergistic Dissolution of Oxides in Molten Sodium Sulfate.

AbstractThe dissolution kinetics of pure Cr2O3 and pure Fe2O3 powders in molten Na2SO4 are compared with those of the dissolution of a mixture of these oxides.

Synergistic Dissolution of Oxides in Molten Sodium Sulfate

The dissolution kinetics of pure and pure powders, respectively, into , were each measured and compared with those for the same oxides during simultaneous dissolution of the mixed oxide powders in the same environment. For dissolution of mixed oxides, accelerated kinetics for each oxide resulted from an acid‐base coupling reaction. Measurements of the changes in the and oxygen activities in the melt during oxide dissolution suggest that the dissolution reaction for the single oxides is controlled by the diffusion of (as ). Therefore, the enhanced dissolution kinetics for mixed oxides is interpreted in terms of a synergistic reaction to promote this diffusion step.