Low-temperature Corrosion Research Articles

Reservation of the heat supply system with biofuel heat energy sources

The implementation of the reservation of the heat supply system to a populated area was analyzed. It is noted that increasing the reliability of heat supply to consumers is a fundamental task, especially given the current military situation in Ukraine. The solution to this difficulty is considered when reserving heat supply by constructing a backup source of thermal energy using alternative types of fuel. A combined thermal scheme for connecting a hot water heat generator using biofuel to the existing city heat supply system is proposed to increase its reliability and survivability, which ensures the efficiency and reliability of operation of the heat supply system during the heating and summer periods. It is shown the possibility of operating a solid fuel heat generator with its power significantly less than the total heat load of the city's heating supply in several operating modes: during the heating period - for heating the return coolant at the inlet to the existing gas boiler house; in the summer period - for the needs of hot water supply in full; in the event of an emergency shutdown of a gas boiler house in winter - to maintain the viability of the heat supply system. The results of a calculation analysis of the operation of a 5,0 MW biofuel heat generator under different operating modes are presented. Heating the return coolant by 4...5 °C in winter, in addition to saving natural gas, prevents low-temperature corrosion of existing gas boilers. In the event of an emergency lack of natural gas supply, the combustion of wood chips ensures the production of thermal energy at the level of thermal losses of the heating network while maintaining the coolant temperature at least 3 °C at the calculated external air temperature. Thanks to the accumulation of thermal energy at night in the summer period in the volume of heating network pipes as a buffer tank, compensation for peak hot water consumption is provided with the available power of the solid fuel boiler house equal to the average load of the hot water supply system. When implementing a project to back up the heat supply system in Lutsk using a combined thermal scheme for connecting a solid fuel boiler house, an increase in the reliability and viability of the system and natural gas savings of 40,5% during a year of operation were achieved

Simulation and verification of cylinder head’s temperature field for marine low-speed engine

Abstract The high load capacity of marine low-speed engines poses a challenge to the thermal load of the cylinder head and temperature field simulation can guide its structural design and optimization. The 3-dimensional models of the fluid domain for the cooling water and the solid domain for the cylinder head are established and meshed, the fluid domain model is simulated to research the heat transfer pattern by Fluent software, and the surface temperature and heat transfer coefficient of the cooling water is calculated. The thermal boundary conditions of the gas in the cylinder are simulated through the analysis of the engine working process. The temperature of the cylinder head is calculated based on the fluid-structure interaction model with the thermal boundary conditions by Workbench software. The validation of the model is carried out by comparing the simulated and measured temperature data in the heated inner wall of the cylinder head. The research results show that compared with the measured value, the error of simulation results is small, which shows that the model and method are reasonable. The temperature in the inner wall of the combustion chamber is low, which can provide possibilities to appropriately improve the power density of the low-speed engine and reduce the risk of low-temperature corrosion.

Features of the operation of the lubrication system for lubricating ships' low-speed engines

The article examines the optimization of high-alkaline cylinder oil consumption in lubrication systems of marine low-speed engines (MODs), as well as the peculiarities of operation of the cylinder lubrication system in view of modern trends in the development of low-speed marine engines. An overview of the lubricator lubrication systems of the cylinders of marine main engines was carried out and possible prospects for improving the lubricator lubrication systems of the cylinders of marine diesel engines were considered. Attention is paid to the causes of occurrence and methods of combating a whole range of problems, which represent the possibility of increased wear of cylinder sleeves, pistons, piston rings, this is related to the type and grade of fuel and its sulfur content. Sulfur in the fuel is neutralized with the help of cylinder oils with high alkalinity, problems arise when changing the grade of fuel, this requires changing the brand of cylinder oil (CLO BN). The possibility of using a cylinder oil mixing system (ACOM) to neutralize sulfur in the fuel and improve protection against low-temperature corrosion of MOD cylinder liners is proposed and considered. This system mixes and doses cylinder oil depending on the load of the ship's engine and the type of fuel. ACOM allows the dosing of oil on marine engines operating in dual fuel consumption (SDF) modes, where the ratio between pilot (ignition) and gas fuel is defined, the system mixes two different types of cylinder oil into one with the required BN base number, it can be determined as the oil's ability to neutralize acids produced during use. The results of optimal consumption of cylinder oil, which ensures minimal wear of cylinder liners of a diesel engine, are given. Features of lubrication regimes and control of the technical condition of the cylinder group of marine low-speed diesel engines during their operation at reduced rotation frequency are described

Mitigating low-temperature corrosion in flue-gas heat exchangers for improving thermal storage efficiency in geothermal power plants

Geothermal energy is a renewable and stable energy source that plays a crucial role in providing heat. However, underground sulfur fumes have been restricting the heat exchange and storage rate for years. Accurately predicting the condensation of acidic substances of flue gas heat exchangers is crucial in geothermal heat storage. In this paper, Acid and water vapor have been firstly utilized to forecast the deposition of acidic substances on the heat exchanger’s surface. Utilizing gas–liquid equilibrium data of H2SO4-H2O solution and multi-component transport theory, a numerical model has been developed to analyze the impact of acidic deposition and corrosion characteristics. The results shown a correlation between the distribution of acidic substances deposition within heat exchanger and temperature distribution. When water vapor volume fraction increased from 3 % to 18 %, there was a corresponding increase of acid condensation rate on surface. Specifically, the condensation rate increased by 9.6 %, 16.1 %, and 21.8 % respectively under the flow rates of 5 m/s, 8 m/s, and 10 m/s, while the acid mass fraction decreased by 22.4 %, 20.7 %, and 20.1 % respectively. Acidic substances predominantly deposit in specific areas of the heat exchanger. Moreover, Water vapor content increasing can lead to reduction deposition of acidic substances, this may be a potential method to reduce loss of heat exchanger and increase heat storage. These findings offer a significant step forward heat exchanger technology and geothermal energy application.

Numerical investigation of stable combustion at ultra-low load for a 350 MW wall tangentially fired pulverized-coal boiler: Effect of burner adjustments and methane co-firing

In the context of deep peak regulation, utility boilers are often required to operate at ultra-low loads, significantly deviating from their designed conditions, which can lead to unstable combustion or even furnace flameout. However, their operating characteristics at 20 % or even lower loads remain unclear. This study focuses on a 350 MW wall tangentially fired pulverized-coal boiler, exploring its combustion characteristics and pollutant emission patterns at 20 % ultra-low load. Through numerical simulation analysis of 14 different combustion scenarios, this research investigates the impacts of burner arrangements, burner horizontal swing angles (ranging from 5° to 20°), and methane (CH4) co-firing ratios (ranging from 0 % to 20 %, calculated by heat value) on flue gas temperature and main flue gas components (O2, CO, NOx, etc.). The results indicate that as the boiler load decreases to 20 %, the average flue gas temperature inside the furnace drops by about 200 K, accompanied by widespread flame impingement on the water-cooled walls, leading to decreased combustion stability of the pulverized coal. Operating only the lower two layers of burners can achieve a higher average flue gas temperature (1540 K) and lower NO emissions (294 mg/m3). Furthermore, setting the horizontal swing angle of the burners to 15° can mitigate the impact of flames on the water-cooled walls and elevate the flame center, with the average flue gas temperature reaching 1559 K, making it the recommended burner swing angle setting. As the CH4 co-firing ratio increases from 0 % to 20 %, the average flue gas temperature rises by 50 K, and the emissions of NO and CO2 decrease by 15.3 % and 6.2 %, respectively, while H2O concentration increases by 21.3 %, potentially leading to low-temperature corrosion issues. The recommended CH4 co-firing ratio is 15 %.

Thermodynamic Analysis and Performance Improvement in Biomass Power Plant: A Comprehensive Review

Biomass power plants play a crucial role in the sustainable energy sector by converting organic materials such as agricultural residues, forest biomass, and dedicated energy crops into electricity and heat. Biomass power plants represent a viable and renewable energy solution that can contribute to transitioning to a low-carbon economy. Their ability to convert organic materials into electricity and heat, coupled with proper management of feedstocks and emissions, can provide a sustainable alternative to fossil fuel-based power generation. Biomass has appeared as one of the most encouraging renewable energy sources for the replacement of fossil fuels. An extensive study about the prospective of biomass to produce renewable energy in the world has been exhibited in this article. The biomass-driven combined heat and power plant demonstrates 67% and 12% efficiency improvement compared to the stand-alone biomass power plant. BFP-CCS performs best at the H<sub>2</sub>O/Mn<sub>2</sub>O<sub>3</sub> mass ratio of 1.6, the H<sub>2</sub>O/O<sub>2</sub> molar ratio of 2.8, the O<sub>2</sub>/biomass mass ratio of 0.22, and the fuel utilization factor of 0.65. The exergo-economic and exergo-environmental factors obtained are 51.5% and 0.0288% respectively at the favorable operating conditions. The round-trip efficiency of the process using R1233zd was 8.77%, which was slightly lower than that of the process using R245fa (8.84%). The net power output of the final CLC integrated configuration and conventional power plants are 492.19 kW and 273.12 kW respectively. A lot of energy can be recovered without low-temperature corrosion problems, and 7% of the total input energy can be saved. The maximum obtainable exergy efficiency was 42.03%, which was related to MSW (Municipal Solid Waste). The primary aim of this review is to furnish a thorough understanding of the thermodynamic complexities and potential improvements within biomass power plants. Through a critical analysis of current research and emerging technologies, this review aims to establish the groundwork for more effective and sustainable energy production from biomass, playing a crucial role in shaping a cleaner and more environmentally friendly future.

A novel methodology for monitoring low-temperature corrosion caused by hygroscopic salts using linear polarization resistance

A new methodology for monitoring low-temperature corrosion caused by hygroscopic salts was developed and tested with ZnCl2. First, deliquescence and efflorescence points of ZnCl2 were determined by electrochemical impedance spectroscopy and chronoamperometry measurements in the range of 10–35 vol % H2O. Then, low-temperature corrosion was monitored using a probe that combines linear polarization resistance and mass loss measurements. Suitable potential and scan rate for the linear polarization resistance measurements were experimentally determined. The feasibility of the probe was tested by monitoring the corrosiveness of ZnCl2 on P235GH steel under different temperatures (100–180 °C) and water vapor concentrations (15–35 vol %). With the developed corrosion probe, corrosion caused by hygroscopic salts during changes in testing conditions can be monitored and evaluated.

Low-Temperature Corrosion Performance of Laser-Cladded Co-WB Coatings in Simulated Seawater

Low-Temperature Corrosion Performance of Laser-Cladded Co-WB Coatings in Simulated Seawater

Kinetics in low-temperature-isothermal oxidation of tungsten carbide under different humidity conditions

The isothermal oxidation experiments of tungsten carbides (WC) were carried out at levels of inequality in relative humidity (RH) levels and below 200 °C. Evaluated resistivity fluctuations and structural transformations during the low-temperature corrosion process were detected. In this paper, we present a detailed investigation into the resistivity behavior of WC under varying humidity conditions. Our innovative approach enabled us to witness a rapid decrease in resistivity under conditions of 85 % RH and 85 °C, from 0.00374 Ω-cm to 0.00177 Ω-cm, followed by a stable linear increase after 0.7 h at a constant rate of 0.000924 Ω-cm/h. Notably, a clear correlation between humidity levels and resistance change rates was found, where higher humidity led to faster resistivity changes. In sharp contrast, a resistivity change rate of less than 5 % under the conditions of water and oxygen independently was observed. The low-temperature oxidation mechanism of WC was described by XRD and XPS spectra. Moreover, the Nernst-Einstein equation and Wagner's model are used to establish the relationship between resistivity and oxidation degree for analyzing and predicting the reaction kinetics and oxidation behavior, indicating that the oxidation degree of WC increased logarithmically with oxidation time.

Enhancing the Fuel Efficiency of Cogeneration Plants by Fuel Oil Afterburning in Exhaust Gas before Boilers

Cogeneration or combined heat and power (CHP) has found wide application in various industries because it very effectively meets the growing demand for electricity, steam, hot water, and also has a number of operational, environmental, economic advantages over traditional electrical and thermal systems. Experimental and theoretical investigations of the afterburning of fuel oil in the combustion engine exhaust gas at the boiler inlet were carried out in order to enhance the efficiency of cogeneration power plants; this was achieved by increasing the boiler steam capacity, resulting in reduced production of waste heat and exhaust emissions. The afterburning of fuel oil in the exhaust gas of diesel engines is possible due to a high the excess air ratio (three to four). Based on the experimental data of the low-temperature corrosion of the gas boiler condensing heat exchange surfaces, the admissible values of corrosion rate and the lowest exhaust gas temperature which provide deep exhaust gas heat utilization and high efficiency of the exhaust gas boiler were obtained. The use of WFE and afterburning fuel oil provides an increase in efficiency and power of the CPPs based on diesel engines of up to 5% due to a decrease in the exhaust gas temperature at the outlet of the EGB from 150 °C to 90 °C and waste heat, accordingly. The application of efficient environmentally friendly exhaust gas boilers with low-temperature condensing surfaces can be considered a new and prosperous trend in diesel engine exhaust gas heat utilization through the afterburning of fuel oil and in CPPs as a whole.

Numerical research on ammonia-coal co-firing in a 1050 MW coal-fired utility boiler under ultra-low load: Effects of ammonia ratio and air staging condition

Co-firing ammonia (NH3) with coal for thermal power generation is a promising strategy for achieving global carbon neutrality. However, the drawbacks of NH3 combustion (e.g., narrow flammability limits, low theoretical flame temperature, high NOX emissions, etc.) can negatively impact the performance of coal-fired boilers, particularly for large-scale ones operating under ultra-low loads. As such, this study was conducted to investigate the ultra-low-load combustion characteristics of a 1050 MW coal-fired boiler when employing NH3-coal co-firing technology. Three-dimensional numerical simulations of ten combustion scenarios were performed to examine the effects of the NH3 co-firing ratio (ranging from 0% to 80%, by calorie) and air staging condition (Separated Over Fire Air (SOFA) ratio ranging from 4% to 24%) on temperature, gas species distributions, and emissions. Results show that as the NH3 co-firing ratio increases to 80%, the in-furnace average gas temperature decreases by 100 ∼ 200 K. The oxidation of NH3 takes precedence over the reduction of NO with NH3, leading to a surge in NO emissions· NH3 slip can be prevented at ultra-low loads due to the rapid consumption of NH3 near coal burners. Meanwhile, CO2 emissions reduce to 20% of that observed in pure coal firing. The increased H2O content in flue gas promotes coal burnout through the water–gas reaction, while the high H2O emissions may potentially cause low-temperature corrosion. At a SOFA ratio of 16%, a relatively large high-temperature region is achieved in the furnace, indicating more stable combustion under ultra-low loads. Excessively small or large SOFA ratios can reduce combustion intensity in the primary combustion zone while contributing to lower NO emissions.

Features of low-temperature corrosion protection of cylinder sleeves of ship low-speed engines

The article examines the specifics of solving one of the problems of low-speed marine engines, during which low-temperature corrosion of cylinder liners occurs during operation at minimum and partial load modes. An overview of the cooling systems of ship's main engines was conducted and possible prospects for improving the cooling systems of ship's diesel engines were considered. Attention is paid to the causes and methods of combating low-temperature corrosion of ship's main engines. The methods of regulating the cooling system of marine diesel engines were considered, and it was determined that the modern concept of regulation in the cooling system should include both automatic regulation of operational parameters due to the installation of frequency-regulated cooling system pumps, and regulation of the water-chemical parameters of the cooling system. Methods of regulating the operating parameters of the pumps in the cooling system of the ship's main engine were also considered, which use the principle of frequency regulation of centrifugal pumps to ensure the modes of operation with the highest efficiency. Thus, frequency regulation is an energy-efficient way of regulating centrifugal pumps with variable coolant flows in ship engine cooling systems and one of the ways to maintain an optimal temperature state. The possibility of modernizing the cooling system of cylinders of low-speed engines according to the principle of the LDCL system to reduce the impact and occurrence of low-temperature corrosion was considered

Effect of NaCl and Na2SO4 on Low Temperature Corrosion of Vapour- and Pack-Aluminide Coated Single Crystal Turbine Blade Alloys CMSX-4 and RR3010

The current work presents a systematic study of two alloy compositions (RR3010 and CMSX-4) and two types of coatings: inward grown (pack) and outward grown (vapour) deposited aluminides, exposed to 98Na2SO4–2NaCl mixture. Grit blasting was used on some of the samples, prior to coating, to mimic in-service procedures and remove oxides from the surface prior to coating. Two-point bend tests were then performed on the coated samples, with and without applied salt at 550 °C for 100 hours. Samples were pre-strained at 0.6 pct strain to deliberately pre-crack the coating and then strained at 0.3 pct for the heat treatment. Exposure to 98Na2SO4–2NaCl under applied stress of vapour-aluminide coated samples of both alloys, revealed significant coating damage in the form of secondary cracks in the intermetallic-rich inter-diffusion zone, although only CMSX-4 exhibited cracks propagating further into the bulk alloy while RR3010 proved more resistant. The pack-aluminide coating proved more protective for both alloys, with cracks propagating only into the coating and never into the underlying alloy. In addition, grit blasting proved beneficial in reducing spallation and cracking for both types of coating. The findings were used to propose a mechanism based on thermodynamic reactions, to explain the crack width changes through the formation of volatile AlCl3 in the cracks.

Cold-end corrosion in biomass combustion – Role of calcium chloride in the deposit

Using sustainable fuels in power production may lead to an increased risk of corrosion of various boiler parts. In biomass and waste combustion, hygroscopic and deliquescent deposits can cause corrosion of, e.g., preheaters and flue gas cleaning equipment. In the colder part of the boiler, calcium chloride (CaCl2) may form when utilizing fuels containing chlorine. Calcium chloride is a deliquescent salt which can absorb moisture from the flue gas leading to the dissolution of the salt itself. When calcium chloride deliquesces, it forms a corrosive electrolyte that can cause severe corrosion. In this work, full-scale corrosion probe measurements in a biomass boiler and laboratory experiments were carried out to study whether calcium compounds and especially CaCl2 might cause corrosion at different temperatures and humidity conditions. The impact of CaCl2 on the corrosion of carbon and stainless steel was studied in more details at various temperatures (80, 100 and 120 °C) and flue gas moisture levels (15–25 vol% H2O) in a laboratory furnace. Different exposure times were used (24, 72 and 168 h), and the effect of mixtures of CaCl2 and calcium carbonate (CaCO3) was also studied. The corrosion rate was gravimetrically measured, and the sample cross-sections were analysed with SEM-EDX to verify the impact of different elements on the corrosion. Full-scale measurements showed chlorine-induced low-temperature corrosion. The main species in the deposit were calcium and chlorine, and corrosion was observed at conditions at which CaCl2 deliquesces. The laboratory work showed that although the deposit contains mainly calcium carbonate, which is not deliquescent, the highest corrosion rate (>1 mm/year) was found with 5 wt% CaCl2 in the salt deposit mixture. The tests showed that the corrosion rate is linear with time, and severe corrosion of carbon steel occurs when CaCl2 deliquesces. Stainless steel did not show any measurable corrosion at the conditions tested.

Root cause analysis for 410 S stainless steel structured packing failure in vacuum distillation column (case study)

A root cause analysis was conducted on the premature failure of 410 S stainless steel structured packing material in a vacuum distillation column. The multidisciplinary team identified four potential causal factors that could contribute to the failure, including: low temperature corrosion, high temperature attack, design aspects and operational parameters. For each causal factor, sub-factors were determined and classified as primary contributor, secondary contributor and noncontributor. The analysis revealed that low temperature chloride condensation in the tower top bed led to severe hydrochloric acid corrosion while high temperature coking in the lower beds led to intergranular cracking of the stainless steel. Also, the 0.1 mm packing material thickness used was not sufficient to obtain a 10–15 years design life. Recommendations include changes to the operating parameters to prevent condensation and coking as well as mechanical design modifications to get acceptable lifetimes.

Corrosion performance and possible mechanism of aluminium nitride-molybdenum gradient material in a simulated liquid metal battery environment

After being fabricated successfully by spark plasma sintering technology, corrosion performance of aluminum nitride-molybdenum gradient material was investigated using X-ray diffraction, scanning electron microscope, XPS techniques, etc. in a simulated liquid metal battery environment with LiF–LiBr–LiCl eutectic salt as electrolyte. The research results indicated that low temperature corrosion at 550 °C occurs mainly in pure aluminum nitride region with the corrosion products grown on the grain boundaries including alumina, nitrogen-lithium, aluminum-lithium intermediate phase or their complex compounds. Moreover, the formed corrosion product morphology is influenced significantly by aluminum nitride distribution. As the gradient compositions developed towards the pure molybdenum zone, the product morphologies tended to evolve in a sequence of sphere, flake and octahedron. At temperatures higher than 600 °C, alumina tends to be dissolved in molten salt vapor and molybdenum starts to be corroded simultaneously resulting in a substitute of the formed lithium aluminum molybdenum oxide phases for alumina in the final corrosion products. Mass change study on the samples suggested both the holding time and temperature have significant influences on the corrosion behavior of aluminum nitride-molybdenum gradient material. At temperatures higher than 700 °C, sudden change in sample mass accompanied by interface cracking would happen within the holding time of 100 h. Therefore, under long-term isothermal conditions, the serviced temperature of aluminum nitride-molybdenum functional gradient materials is suggested to be lower than 600 °C.

Effect of ammonia co-firing on heat transfer, safety, and economy of coal-fired boilers

Ammonia (NH3) co-firing provides a promising route to reducing CO2 emissions of coal-fired boilers. The thermodynamic calculation model of a 600 MW supercritical coal-fired boiler was established to investigate the effects of NH3 co-firing on the heat transfer, safety, and economy of the boiler. Co-firing 50 % of NH3 (on the lower heat value basis) reduced the emissivity and temperature of the flame, and decreased the heat load of the furnace by 6.2 %-7.1 %. Nevertheless, the heat absorption of both the main steam and reheat steam can still be satisfied by adjusting the ratio of flue gas in two vertical flues without modifications of the heating surfaces. With the NH3 ratio rising from 0 % to 50 %, the temperature of flue gas at the furnace outlet grew by only 7.0 °C. Besides, the tube wall temperatures of the platen superheater and the high temperature superheater increased by < 3.0 °C, ensuring the safety of the heating surfaces. At boiler rated load, low temperature corrosion of air pre-heaters was hardly aggravated by co-firing NH3 of 50 %, but it became more severe at lower boiler loads. Compared with single coal combustion, the temperature of exhaust flue gas rose by 6.2–13.9 °C when NH3 ratio was 50 %, and the boiler efficiency decreased by 0.7 %-1.2 %. The high price of NH3 greatly increased the running cost of the boiler, but a higher CO2 tax can highlight the economic benefits of NH3 co-firing.

Research of Exhaust Gas Boiler Heat Exchange Surfaces with Reduced Corrosion When Water-Fuel Emulsion Combustion

The application of water-fuel emulsion (WFE) in internal combustion engines enables to reduce the consumption of sulfurous fuel oils, thereby protecting the environment from emissions of sulfur and nitrogen oxides, as well revealing a great potential for the heat utilization of exhaust gases. The efficiency of utilization of exhaust gas heat in exhaust boilers (EGB) depends on their temperature at the outlet of EGB, id est. the depth of heat utilization. Exhaust gas temperature is limited by the rate of low-temperature corrosion (LTC), which reaches a level of 1.2 mm/year at the wall temperature of about 110 °C for the condensing heat exchange surfaces (HES) and reduces the reliability of the HES operation. Therefore, decreasing the corrosion rate of condensing HES at wall temperature below 110 °C to an acceptable level (about 0.2 mm/year) when undergoing WFE combustion will make it possible to reduce the exhaust gas temperature and, consequently, increase the efficiency of EGB and fuel saving during the operation of the ship power plant. The aim of the research is to assess improvements to the reliability, durability and efficient operation of condensing HES in marine EGB undergoing WFE combustion in a diesel engine based on experimental studies of the LTC process. A special experimental setup was developed for investigation. The use of WFE with a decreased wall temperature of HES below 80 to 70 °C would improve the reliability of the EGB along the accepted service life, increase the lifetime of the HES metal by almost six times as well as the overhaul period, and reduce the cost of repairing condensing HES. Furthermore, due to the reducing corrosion rate under WFE combustion, the application of low-temperature condensing HES makes it possible to enhance the efficiency of deeper exhaust gas heat utilization and provide sustainable efficient operation of a diesel engine plant on the whole at a safe thermal and environmentally friendly level.

Low-temperature corrosion behavior of laser cladding metal-based alloy coatings on EH40 high-strength steel for icebreaker

Abstract In this paper, four kinds of coatings Ni + 30% WC, Ni + 15% WC, Ni, and Co are prepared on EH40 steel by laser cladding technology. Electrochemical experiment tests at a simulating low-temperature (–40°C) environment in 0.5 mol·L−1 hydrochloric acid (HCl) solution and 3.5 wt% NaCl solution were conducted, and the polarization curve and electrochemical impedance spectroscopy (EIS) impedance spectrum were obtained. The results show that the Ni + 15% WC coating has the smallest corrosion current and 0.5 mol·L−1 HCl solution and 3.5 wt% NaCl solution have the maximum polarization resistance and impedance arc. Coatings will be treated in a 0.5 mol·L−1 HCl solution and 3.5 wt% NaCl solution at –40°C immersion for 30 days; finally, X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM) were used for test passivation films of Ni + 15% WC samples and sample surface morphology after corrosion. XPS results show that Fe, Mo, and Ni elements can form metal oxides in the 0.5 mol·L−1 HCl solution, and Cr, Fe, Mo, Ni, and W elements can form metal oxides in the 3.5 wt% NaCl solution. SEM results display that relatively severe corrosion appeared on the substrate near tungsten carbide in a 0.5 mol·L−1 HCl solution, and coating will be corroded form inside of the coating owing to the corrosive solution will penetrate the substrate. In 3.5 wt% NaCl solution, severe corrosion of the substrate has not been observed; however, the binding part has been obviously corroded.

Effect of Fouling Layer (Acid–Ash Reaction) on Low-Temperature Corrosion Covering Heating Surface in Coal-Fired Flue Gas

Improving the efficiency of coal-fired boiler is beneficial for greenhouse gas control, mainly for carbon dioxide (CO2). The low-temperature corrosion covering heating surfaces is a frequent threat for coal-fired thermal equipment. The corrosion is induced by a fouling layer, where the ash deposition and condensed acid in coal-fired flue gas react mutually. The corrosion experiments were designed to investigate the reactions of representative acid solution between basic oxides, non-basic oxides, and fly ash particles covering metal surfaces. Scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS), X-ray fluorescence (XRF) and X-ray diffraction (XRD) were used to analyze the reaction particles and metal samples collected from experiments. The corrosion rates of 316L steel, 20# steel, Corten steel and ND (09CrCuSb) steel by the sulfuric solution of different concentrations with and without particles were obtained. The results showed that corrosion rate could be reduced by reacted particles, followed as: basic oxides particles &gt; fly ash particles &gt; non-basic oxides particles. Meanwhile, the deposited ash particles with smaller sizes contribute to a deeper acid–ash reactions due to more alkaline oxides accumulated. Thus, the metal surfaces will be covered by denser attachments, playing a function of corrosion resistance. The effect of fouling layer on low-temperature corrosion was obtained, guiding a safe and efficient operation of heat equipment in coal-fired flue gas.