High Temperature Corrosion Processes Research Articles

Corrosion of 316 SS in chloride molten salt for thermal energy storage: Inhibitory effects of Al powder

Abstract MgCl2-KCl-NaCl is regarded as one of the most prospective high-temperature thermal energy storage mediums and heat transfer fluids (HTF) for 3rd generation concentrated solar power (CSP) systems. However, high corrosion to alloys limits its application. In this paper, corrosion tests were conducted on 316 SS, in MgCl2-KCl-NaCl at 800°C with different content (0 wt.%,1 wt.%, and 10 wt.%) of Al powder addition as a corrosion inhibitor. The impact of Al powder was assessed through electrochemical methods, specifically impedance spectroscopy (EIS) and potentiodynamic polarization (PDP). Following corrosion tests, the morphologies and phase compositions of 316 SS were determined by using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD). The addition of Al powder can significantly reduce the corrosion current density of 316 SS in MgCl2-KCl-NaCl at 800°C, which was 183.29 times higher than that with 10 wt.% without Al addition. Al and the degree increased with increasing content of Al. With the addition of 1 wt.% Al, the thickness of the diffusion layer is significantly reduced, which was 54.6 μm (100 h), 275.1 μm (200 h), 370.4 μm (300 h), and 500 μm (400 h), respectively. When the addition of Al reaches up to 10 wt.%, the inwards diffusion of Al caused the formation of Al enriched layer, which was identified as the FeAl phase, on the surface of 316 SS during the high-temperature corrosion processes. The thickness of the Al enriched layer was associated with the diffusion time of Al, and its depth was 40.4 μm (100 h), 45.3 μm (200 h), 103.5 μm (300 h), and 139.5 μm (400 h).

In-situ observation and directional growth of Ca2Lu8(SiO4)6O2 in Lu2SiO5 attacked by CMAS at 1500 °C

Excellent resistance to CMAS is one of the key parameters for environmental barrier coating (EBC) operated in gas turbine engines. The interaction between CMAS and EBC was usually investigated by the characterization of reaction products after corrosion. The lack of direct observation of the high-temperature CMAS corrosion processes hinders the understanding of CMAS-induced degradation of EBC. In this work, the interaction between CMAS and an EBC candidate (Lu2SiO5) was investigated by an in-situ observation method, where the melting of CMAS and the growth of reaction products were first observed. It was found that the reaction product Ca2Lu8(SiO4)6O2 tends to precipitate vertically to form a textured structure. In addition, Lu2SiO5 tends to decompose into Lu2Si2O7 and Lu2O3 at 1500 °C, which induces an intergranular infiltration of CMAS and leads to the formation of Ca2Lu8(SiO4)6O2 fringes. Compared with the corrosion behavior at 1300 °C, Lu2SiO5 exhibits fast interaction with CMAS and poor CMAS resistance at 1500 °C. The results provide important guidelines on the design of rare earth silicate EBC.

The prospects of applying ionic modification of nitride coatings to high temperature corrosion processes in the oil and gas industry

The use of ion modification methods associated with high-dose irradiation with low-energy ions is one of the promising ways to increase the resistance of materials to external influences, including high-temperature corrosion. The process of ionic modification involves creating a near-surface layer of material with a high dislocation density due to implantation effects. This inhibits destructive processes caused by corrosive oxidation or mechanical influences. In this work, using the ion modification method to improve the resistance to high temperature corrosion, the authors modified nitride coatings of the order of 500 nm thick. The main purpose of using the ion modification method was to increase the strength characteristics of nitride coatings against high-temperature corrosion, as well as to reduce the effects of coating degradation to mechanical softening and wear after corrosion tests. In the course of the studies, it was found that the use of low-energy irradiation with O2+ ions (40 keV) leads to an increase in the stability of strength characteristics during high-temperature corrosion (with prolonged heating in an oxygen-containing environment at a temperature 700 °C), as well as a decrease in wear in comparison with unmodified nitride coatings applied to the surface of 316L steel. The promise of these studies lies in the development of new methods for modifying steel structures, which will improve the efficiency of resistance to corrosion and mechanical damage, as well as increase the wear resistance of the surface under external mechanical influences. The possibility of increasing stability due to deformation inclusions caused by implantation makes it possible to increase corrosion resistance by increasing the resistance of materials to high-temperature oxidation.

Corrosion behavior of Al–O coating in oxygen-saturated lead-bismuth eutectic at 550 °C

The amorphous Al–O coating with a thickness of approximately 6 μm was prepared on 316 L stainless steel using multi-arc ion plating technology to investigate the corrosion resistance of the coating in static lead–bismuth eutectic at 550 °C. The corrosion time ranged from 100 to 1000 h. The samples were characterized using different instruments, and the results showed that the Al–O coating had poor wettability and could effectively protect the substrate material from lead–bismuth corrosion in a static lead–bismuth eutectic. However, during the long-term high-temperature corrosion process, the coating changes from an amorphous phase to a polycrystalline doped structure, resulting in pores on the coating. Further, elemental interdiffusion was identified between the substrate and coating, indicating risk of coating failure.

High temperature corrosion of particulate matter during MSW combustion: Effects of particle size, inorganic minerals and additives on corrosion rates and mechanisms

The high temperature corrosion behavior and corrosion mitigation of PM with different sizes generated by MSW combustion were studied. The experiment result showed that the corrosion rate of 304 stainless steel increases significantly as the PM size decreases. The corrosive ability of PM1 on 304 stainless steel was about 3–5 times higher than that of PM10+ at 600 °C. Kaolin can greatly restrain the corrosiveness of PM. After 5% of kaolin was added, the corrosion rate of the PM generated by MSW combustion on 304 stainless steel was reduced by 12%. Calculating the composition of the particles after the formation of a stable slagging layer and ash deposition layer on the heating surface, the corrosion control efficiency improved to 71%. High-temperature corrosion process of PM was dominated by NaCl, KCl, CaCl2, Na2SO4 and K2SO4, the corrosion of alkali and alkaline earth chloride was generally stronger than that of sulfate at 600 °C. This study confirms that the addition of kaolin can effectively mitigate high-temperature corrosion in the superheater tubes of waste incineration furnaces.

Long-term corrosion of HTGR graphite with SiC coating

When creating a Kazakhstani high-temperature gas-cooled reactor (HTGR), a type of fuel with improved graphite and its gradient protective coating based on silicon carbide (SiC) will be used. So, the purpose of this work is determination the rate and features of the long-term high-temperature corrosion of reactor graphite and its SiC coating at a temperature of 1200K and a pressure of 200-250 kPa in helium with an admixture of water and oxygen in concentrations close to the normal operation of an HTGR; development a numerical model of a corrosion experiment with the convection method of supplying the working mixture. As a result, four successive test stages lasting 30 days each (the total duration of corrosion was 120 days), the weight loss of pure graphite samples (without SiC coating) range from 1.8% to 10.4%, depending on the manufacturer. In general, the average corrosion rate of unprotected graphite corresponds to the literature data. The use of a SiC coating over graphites reduces the corrosion rate of samples by a factor of 100-500. The developed model made it possible to describe both the high-temperature corrosion processes of HTGR graphite and the dynamics of the mass transfer of all gas components throughout the entire volume of the installation during the experiment.

Hot corrosion behavior of hybrid double-layered LZ/LZC + YSZ thermal barrier coatings made using powder and solution precursor feedstock

Lanthanum zirconate (La2Zr2O7; LZ) and cerium doped lanthanum zirconate (La2(Zr0.7Ce0.3)2O7; LZC) has gained immense attention as thermal barrier coating (TBC) materials because of their superior thermophysical properties, high-temperature phase stability and chemical inertness than yttria-stabilized zirconia (YSZ). In the present study, LZ and LZC double-layered TBCs are prepared by solution precursor plasma spray process (SPPS) as a top layer over the atmospheric plasma sprayed YSZ coatings, and their hot corrosion behavior is studied. The as-deposited SPPS coatings exhibited single phases, and the microstructural morphologies reveal a hierarchically structured surface with vertically cracked microstructure. Exposure to the high temperature corrosion process results in damage of the morphology and microstructure within the SPPS layer by forming lanthanum vanadate (LaVO4) and lanthanum cerium vanadate ((LaCe)VO4) in LZ and LZC coatings, respectively, while leaching out zirconia (ZrO2). In the case of conventional YSZ, the corrosive salts infiltrate entirely through the coating and result in complete densification and associated phase changes. While combining the above aspects, the hybrid double-layered architecture of LZ/YSZ and LZC/YSZ coatings can be beneficial to protect the underlying YSZ layer against the detrimental infiltration of molten corrosive salt.

Dynamic simulation on high-temperature corrosion behaviour of tube surface with fouling in utility boiler fired by high-chlorine coal

A dynamic model on high-temperature corrosion with fouling-type ash deposition was established to predict the thickness distribution of corrosion layer and the outer/inner layer of ash deposit. It provides an observation on variation of heat transfer characteristics of the alloy test probe over time. This model involves the combustion characteristics of high-chlorine coal, the process of formation, migration, and deposition of fly ash particles, and the corrosion of alloy test probe. In this study, the dynamic model has been integrated into the numerical simulation of high-chlorine coal combustion in a 50 kW long-period corrosion test furnace. The prediction results show that the amount of fly ash deposition on the windward surface is the most while the least amount appears on the side surface. There is a great correlation between the corrosion layer and the inner layer of ash deposit. Because the inner layer of ash deposit and the alloy substrate are spatially adjacent, the high temperature corrosion process is greatly affected by the growth and property of fly ash deposit layer. The simulation results show that the heat flux of probe surface beneath ash deposit is 2.5 kW/m2 which is 50% of the clean surface (5 kW/m2). The changing pattern of thermal resistance versus time at different positions of the alloy surface is consistent with the variation of thickness of the corrosion layer and ash deposit layer. The windward face has the highest thermal resistance and the least heat flux due to the highest thickness of the corrosion layer and ash deposit layer. With the increase of probe wall temperature, the ratio of thermal resistance of corrosion layer to total heat transfer resistance increases, and the ratio at the side face is larger than that at other locations. And the proportion of the corrosion layer in thermal resistance for alloy TP347H reaches 8% at the probe wall temperature of 973 K.

Comparison of H2S high-temperature corrosion characteristics of three kinds of low-alloy and heat-resistant-steel tubes

ABSTRACT This study investigates the H2S high-temperature corrosion characteristics of three common heat – exchanging tubes for supercritical boilers, namely SA-213T12, 15CrMoG and 12Cr1MoVG. The high-temperature corrosion rates and wall thickness reduction rates of these three materials were investigated. The results indicate that the spalling of corrosion fragments produced by high-temperature H2S corrosion is the main cause for the reduction in the tube wall thickness. Moreover, as a pressure component, the thinning of the water wall tube thickness is an important index to judge its service life. The bulk aggregation of corrosion products can easily cause the corrosion layer to fall off and make the corrosion reaction go inside. By comparison, the abilities of the three kinds of tubes for anti-H2S high-temperature corrosion are ranked from weak to strong as follows: SA-213T12、15CrMoG and 12Cr1MoVG. Besides, the high-temperature H2S corrosion process is the most sensitive to temperature.

Determination of High Temperature Corrosion Rates of Steam Boiler Evaporators Using Continuous Measurements of Flue Gas Composition and Neural Networks

The use of low-emission combustion techniques in pulverized coal-fired (PC) boilers are usually associated with the formation of a reduced-gas atmosphere near evaporator walls. This increases the risk of high temperature (low oxygen) corrosion processes in coal-fired boilers. The identification of the dynamics and the locations of these processes, and minimizing negative consequences are essential for power plant operation. This paper presents the diagnostic system for determining corrosion risks, based on continuous measurements of flue gas composition in the boundary layer of the combustion chamber, and artificial intelligence techniques. Experience from the implementation of these measurements on the OP-230 hard coal-fired boiler, to identify the corrosion hazard of one of the evaporator walls, has been thoroughly described. The results obtained indicate that the continuous controlling of the concentrations of O2 and CO near the water wall, in combination with the use of neural networks, allows for the forecasting of the corrosion rate of the evaporator. The correlation between flue gas composition and corrosion rate has been demonstrated. At the same time, the analysis of the possibilities of significantly simplifying the measurement system by using neural networks was carried out.

Oxidation studies on nickel-base superalloy 617 OCC

Nickel-base superalloy 617 OCC has been indigenously developed for potential use in Advanced Ultra-Super Critical (USC) Power Plants. Hot corrosion is one of the serious issues associated with superheater and reheater tubes in the boiler part of the USC power plant. To understand the high temperature corrosion processes coming into play, it is useful to have a good understanding of the oxidation behavior of the alloy at the same temperature in ambient air environment. The present study reports, how the Alloy 617 OCC behaves in laboratory air environment at 700 °C. Studies were carried out at intervals of 500 h for a cumulative exposure time of 5000 h. Thermogravimetry, XRD of the corroded surface, SEM/EDAX analysis of corroded surface and cross-sectional surface were carried out after each of the 10 exposure times. Thermogravimetry revealed that there was essentially no weight change of the test specimen over the entire exposure time range. XRD revealed that chromium oxide is essentially the only corrosion product. SEM/EDAX and XRD of the corroded surface showed the progressive increase in the extent of chromium oxide formation on the surface. A well-developed chromium-depleted region was seen below the chromium oxide layer in the X-ray mapping studies. The present research showed that considerable levels of nickel and cobalt occur above the chromium oxide layer at the surface. There is evidence that metallic agglomerates form above the chromium oxide layer. Titanium present in the superalloy gets preferentially oxidized and gets into the chromium oxide layer at the surface. Preferential surface oxidation of aluminum was also noticed. There was evidence for internal oxidation of aluminium at long exposure times. The above findings will be presented and discussed, bringing out the oxidation mechanisms coming into play for 617 OCC at 700 °C.

Characterisation of non-stoichiometric cementite (Fe3C)

In order to measure the composition range of cementite, a sample has been prepared by carburisation at 700 °C at a high carbon activity aC = 100. Such strongly carburising conditions normally result in a quick onset of the high-temperature corrosion process metal dusting, which leads to cementite decomposition at the surface. The effect of sulphur to retard this corrosion process was used by adding a small amount of 1 ppm H2S to the gas mixture.The measurements of the carbon contents of the cementite particles formed at the sample surface were performed by using electron-probe microanalysis (EPMA). It was found that cementite (Fe3C) which was formed at a very high carbon activity aC » aC (Fe3C/α-Fe) reveals a measurable carbon gradient. The homogeneity range of cementite at 700 °C in the investigated sample was determined to be of at least 0.6 at.% carbon. Notethis study has been performed at Max-Planck-Institut für Eisenforschung already two decades ago, but it was not yet published internationally. However, the respective experimental results were already described in [1].

Cellular automata simulation on the corrosion behavior of Ni-base alloy in chloride molten salt

Chloride molten salt is required to study its corrosion behavior since it is recommended to be used as heat transfer and heat storage material in high-temperature thermal power generation system. In this paper, a simplify model was established by the reaction of diffusive corrosive gas and metal substrate in a chloride molten salt. A cellular automaton model based on the stochastic approach is adopted to simulate the uniform corrosion and the migration of metallic element. The corrosion behavior is also simulated considering the practical working condition, such as oxygen, moisture and so on. By comparing the simulation results with experimental data, it proves that the cellular automata simulation is an effective alternative method to describe the high-temperature corrosion process of molten salts. Simulation results of 25,000 time steps is taken as a comparison of the 21-day corrosion which reflects the Cr distribution, and the growth of corrosion layer with the migration of Cr element was reproduced. The change of corrosion layer thickness is similar to the mass loss. The simulation method provides insight on the distribution of corrosion products, corrosion kinetics and morphologies for the long-term application.

Corrosion assessment of 9Cr-1Mo steel in molten LiCl-KCl eutectic salt by electrochemical methods

Ferritic chromoly steels are being considered for the process crucibles for the electrorefining of the pyrochemical reprocessing of spent metallic fuels in India. Pyrochemical reprocessing uses molten LiCl-KCl salt at 500–600 °C under an inert argon atmosphere for the electrorefining. The corrosion assessment of the 9Cr-1Mo steel in LiCl-KCl molten salt at 500 °C under inert argon atmosphere has been attempted using electrochemical measurements such as open circuit potential (OCP), linear polarization resistance and electrochemical impedance spectroscopy (EIS). The OCP of the sample was shifted towards noble direction during the total duration of 98 h. The linear polarization resistance, measured at regular intervals of exposure of 9Cr-1Mo steel to molten salt, increased with the increase of the exposure duration. The electrochemical impedance technique has been used to evaluate and monitor the molten salt-corrosion processes of the base metal. The EIS results showed the formation of a large semi-circle, with the characteristics of double loops capacitance attributed to the nature of intermittent oxide film over the surface. The surface oxide films are mostly composed of iron and chromium oxides confirmed by XRD and SEM-EDS analysis. It is also evident from the EDS analysis that depletion and enrichment of Cr and Mo across the surface of the 9Cr-1Mo steel. This work indicated that the electrochemical techniques show considerable promise for the monitoring of high-temperature molten salt corrosion processes.

Kinetics of volatilization of high temperature corrosion products and its application to chlorine corrosion

Operating atmospheres of many industrial high temperature processes contain a certain amount of halogens, in most cases chlorine. Halogens have the tendency of forming volatile metal chlorides of the general formula MxCly, which are well known to play a critical role in high temperature corrosion processes, when present at a significant amount. Thermodynamic calculations give a valuable hint, whether a reaction product can occur and how volatile such products can potentially be, when the partial pressures are calculated. However, such calculations reflect thermodynamically stable conditions, while in open systems the local kinetics control the process of surface reactions. For each specific condition, the reaction rates and thus the corrosion rates have to be determined. In previous work, we have shown that corrosion kinetics which lead to the formation of volatile products follows different steps. In this work, a mathematical model is developed in which the different steps, which are required for the formation of metal chlorides and their removal from the surface are described. It is assumed that the slowest of such formation or transport steps determines the kinetics of high temperature corrosion. The application of this model on the literature data shows that by this model it becomes possible to identify the slowest and therefore rate determining step.

Corrosion resistance of HR3C to a carbonate molten salt for energy storage applications in CSP plants

This paper studies the behaviour of austenitic steel HR3C in a eutectic ternary Li2CO3-Na2CO3-K2CO3 molten salt mixture through an isothermal immersion test at 700°C for 2000h. To this aim various techniques were used in order to examine the effects of long-term, high-temperature corrosion processes on the selected substrate. It was found that the corrosion products were arranged in a multilayer structure, with LiFeO2, LiCrO2, NiO and FeCr2O4 being the main compounds in the different layers. The composition of the remaining salt after the corrosion tests was also studied, the chromium increase being one of the main points to highlight. The results suggested that longer immersion tests should be carried out to clearly confirm the suitability of HR3C for solar thermal applications.

Chloride corrosion in biomass-fired boilers – Fe-O-Cl system thermodynamic analysis

The most common and easiest alternative technologies for conventional fossil fuel combustion are biomass combustion and co-combustion. However, high-chlorine fuels (Clar >0,2%) like: biomass, waste and high chlorine coals generate the risk of intensified corrosion process and a limited steel mechanical strength is observed.The paper presents a thermodynamic analysis of chloride-induced corrosion in the Fe-O-Cl system. The ranges of the metallic, oxide and chloride phase stability are determined within the temperature range T = 750-1000 K. Based on the parametric equations the equilibrium concentration of gaseous phase determined by Deacon reaction are presented. The effect of H2 O concentration in the gaseous phase on high-temperature corrosion process and gaseous NaCl influence on NaFeO2 formation in the passive oxide scale layer (FeO/Fe3 O4 /Fe2 O3 ) are discussed as well. The results are correlated with available in the literature laboratory experimental data and industrial corrosion process observations. Presented thermodynamic analysis is compared with assumptions of “active oxidation” model. The results may be used for experimental research prediction and a corrosion prevention in the industry.

The Effect of Different SO2/SO3 Catalytic Media on High-Temperature Corrosion Processes (Hot Corrosion, Fireside Corrosion, Sulfidation–Oxidation)

In many laboratory hot corrosion (HC) experiments and other experiments involving sulfur commercial catalytic converters are used to catalyze the $${\text{SO}}_{ 2} + { 1}/ 2 {\text{O}}_{ 2} \to {\text{SO}}_{ 3}$$ reaction. The conversion rate of SO2 to SO3 highly depends on the catalyst used and thus the HC results, especially in low-temperature HC conditions, may vary significantly depending on the catalyst selected. Different elements present in the catalyst affect the SO2 to SO3 conversion and with it the degree of HC induced attack. Modern automotive catalysts have low activity for the oxidation of SO2 to SO3. The conversion of SO2 to SO3 can be enhanced by modifications of the catalyst, e.g. by using a Pt-gauze or dip-coating the catalyst in a Fe2O3 solution.

Molten nitrate salts at 600 and 680 °C: Thermophysical property changes and corrosion of high-temperature nickel alloys

This paper examines the effects of long-term, high-temperature corrosion processes on materials used in concentrated solar power tower systems. More specifically, the heat transfer fluid and the containment metal are examined at operating temperatures of 600 and 680°C. A 60/40% NaNO3/KNO3 mixture of molten salt and nickel-based alloys HA230 and In625 were tested for up to 4000h at the given temperatures. HA230 and In625 alloys showed metal losses of 688 and 594μm/year, respectively, after 1000h at 680°C. The solar salt used in this study showed significant decreases (as much as 60°C) in melting point. Further experiments showed that the primary reason for the decrease in melting point is due to high nitrite concentrations in the molten salt. However, other thermophysical properties of aged solar salt appear to change little during high-temperature corrosion tests.

Thermogravimetry and insitu mass spectrometry at high temperatures compared to thermochemical modelling – The weight loss during selective decarburisation at 800 °C

The insitu mass spectrometry detection of volatile species during high temperature corrosion processes, exemplified by decarburisation of plain carbon steel, will be presented. Without influencing the thermobalance signal, changes of the gas composition can be measured by introducing a small quartz capillary into the reaction chamber in close proximity to the sample surface.Experiments with the eutectoid composition Fe, 0.8wt% C have been chosen intentionally as this alloy represents one of the most intensely studied compositions in steel research. This allows good verification with commonly accepted knowledge. Furthermore, comparison between experimental observations and theoretical simulations will be given and critically discussed.