Search Results for corrosive attacks

Solar salts are used as an energy storage media and heat transfer fluid in power plants. The salts can cause significant corrosion to various steels that are in contact with the salt. Static corrosion tests performed with different steels show, that the corrosive attack by industrial grade salt melts is more severe than by defined grade salt melts and the sample corrosion is faster (i.e. the weight gain is larger) for higher temperatures. Slow strain rate (SSR) tests in salt are difficult to conduct due to the corrosive attack of the salt also on the test setup. The SSRT setup in salt could be realized and tests could be conducted successfully. No clear evidence for an accelerated failure of samples tested in salt compared to samples tested in air could be found on Alloy 347 Nb. Comparative low cycle fatigue (LCF) tests at air and in molten salt atmosphere were successfully performed and showed similar results on tubes out of Sanicro 25. No evidence of accelerated crack growth in molten salt could be found.

Coatings are an essential part of the materials system to protect the turbine blades from oxidation and corrosive attack during service. Inter-diffusion of alloying elements between a turbine blade substrate and their coatings is a potential concern for coated turbine blades at ever increasing operating temperatures because this can cause the formation of undesirable Secondary Reaction Zones (SRZs), which may degrade the mechanical properties of coated Ni-based superalloys. Understanding the effects of each element on the SRZ formation is essential in order to understand both the mechanism and inter-diffusion behaviour between coatings and substrates. In this research, a number of simpler aluminized ternary Ni-Al-X (where X is Co, Cr, Re, Ru or Ta) alloys were investigated in order to elucidate the separate effects of each element on the microstructural evolution, especially at the coating/substrate interface. The aluminized ternary alloys developed distinctive diffusion zones, depending on the third alloy element, ‘X’. Specifically, it has been found that both Ni-Al-Re and Ni-Al-Ta alloys developed a continuous SRZ-like diffusion layer. This diffusion zone persisted in the Ni-Al-Re alloys after high temperature exposure, indicating that Re has a stronger effect on SRZ formation than Ta.

A combined pilot-scale combustion test and long-term laboratory study investigated the impact of oxy-firing on corrosion in coal-fired boilers. Four coals were burned under both air and oxy-firing conditions with identical heat input, with oxy-firing using flue gas recirculation unlike air-firing. Despite higher SO 2 and HCl concentrations in oxy-firing, laboratory tests showed no increase in corrosion rates compared to air-firing. This is attributed to several factors: (1) Reduced diffusion: High CO 2 in oxy-firing densified the gas phase, leading to slower diffusion of corrosive species within the deposit. (2) Lower initial sulfate: Oxy-fired deposits initially contained less sulfate, a key hot corrosion culprit, due to the presence of carbonate. (3) Reduced basicity: CO 2 and HCl reduced the basicity of sulfate melts, leading to decreased dissolution of metal oxides and mitigating hot corrosion. (4) Limited carbonate/chloride formation: The formation of less corrosive carbonate and chloride solutes was restricted by low O 2 and SO 3 near the metal surface. These findings suggest that oxy-firing may not pose a greater corrosion risk than air-firing for boiler materials.

Inconel Filler Metal 72 (FM 72) and Incoclad 671/800H co-extruded tubing have been successfully used for over 20 years to protect boiler tubing from high-temperature degradation. A newer alloy, FM 72M, offers superior weldability and the lowest corrosion rate in simulated low NOx environments. Both FM 72 and 72M show promise in addressing challenges like circumferential cracking and corrosion fatigue in waterwall tubing overlays. Additionally, 72M’s superior wear resistance makes it ideal for replacing erosion shields in superheater and reheater tubing. Beyond improved protection, these alloys exhibit increased hardness and thermal conductivity over time, leading to reduced temperature difference across the tube wall and consequently, enhanced boiler efficiency and lower maintenance costs. This paper discusses the historical selection of optimal alloys for waterwall and upper boiler tubing overlays, analyzes past failure mechanisms, and highlights the key properties of successful choices like FM 72 and 72M.

The Advanced Ultrasupercritical (A-USC) power plants are aimed to operate at steam inlet temperatures greater than 700°C; consequently, a complete materials overhaul is needed for the next-generation power plants. HAYNES 282, a gamma-prime strengthened alloy, is among the leading candidates because of its unique combination of properties, superior creep and LCF strength, fabricability and thermal stability. It is currently being evaluated in wrought and cast forms for A-USC turbine rotors, casings, boiler tubings, header, and valves. The candidate materials for A-USC applications not only require oxidation resistance for steam cycles but fireside corrosion resistance to coal ash is also of an extreme importance. In order to study the effect of both environments on the performance of 282 alloy, the alloy was exposed for extended periods in various oxidizing environments, such as air, air plus water vapor (10%), and 17bar steam up to 900°C. The fireside corrosion resistance of 282 alloy was evaluated at 700°C in synthetic coal ash and at 843°C in alkali salt deposits in a controlled gaseous environment.

This presentation compares the corrosion resistance of uncoated Haynes 230 and SS316HS substrates to the same substrates coated with a Fe-based amorphous alloy. The substrates were exposed to highly corrosive media, FLiNaK, for 120 hours at 700 °C. The findings indicate that the thermal spray amorphous alloy coating provided superior corrosion resistance within the coatings while protecting the substrates against the aggressive environment. As a result, the new amorphous metal coating improved the substrate's lifespan by providing better protection against high-temperature corrosion, paving the way for a more efficient and cost-effective future in various industrial applications.

This paper outlines a comprehensive UK-based research project (2007-2010) focused on developing fireside corrosion models for heat exchangers in ultra-supercritical plants. The study evaluates both conventional materials like T22 and advanced materials such as Super 304H, examining their behavior under various test environments with metal skin temperatures ranging from 425°C to 680°C. The research aims to generate high-quality data on corrosion behavior for materials used in both furnace and convection sections, ultimately producing reliable corrosion prediction models for boiler tube materials operating under demanding conditions. The project addresses some limitations of existing models for these new service conditions and provides a brief review of the fuels and test environments used in the program. Although modeling is still limited, preliminary results have been presented, focusing on predicting fireside corrosion rates for furnace walls, superheaters, and reheaters under various service environments. These environments include those created by oxyfuel operation, coal-biomass co-firing, and more traditional coal firing.

Research has demonstrated that creep damage in power plant steels is directly linked to grain boundary precipitates, which serve as nucleation sites for cavities and micro-cracks. The formation of M 23 C 6 carbides along grain boundaries creates chromium-depleted zones vulnerable to corrosion and significantly reduces creep life due to rapid coarsening. Through combined Monte Carlo grain boundary precipitation kinetics and continuum creep damage modeling, researchers have predicted that increasing the proportion of MX-type particles could enhance creep performance. This hypothesis was tested using hafnium-containing steel, which showed improved creep and corrosion properties in 9% Cr steels. Ion implantation of Hafnium into thin foils of 9 wt% Cr ferritic steel resulted in two new types of precipitates: hafnium carbide (MX-type) and a Cr-V rich nitride (M 2 N). The hafnium carbide particles, identified through convergent beam diffraction and microanalysis, appeared in significantly higher volume fractions compared to VN in conventional ferritic steels. Additionally, Hafnium was found to eliminate M 23 C 6 grain boundary precipitates, resulting in increased matrix chromium concentration, reduced grain boundary chromium depletion, and enhanced resistance to intergranular corrosion cracking.

In an earlier paper, preliminary data for HAYNES 282 alloy was presented for potential advanced steam power plant applications. Since then, 282 alloy has continued to be evaluated for a variety of A-USC applications: superheater boiler tubing, large header piping, rotors, casings, etc. Per current practice the alloy achieves its strengthening by a two-step age hardening heat treatment. Given the difficulty of such a procedure, particularly for larger components in the power plant, interest has focused on the development of a single step age hardening treatment. While considerable work on 282 alloy is still going on by a number of investigators, during the preceding years a large amount of data was generated in characterizing the alloy at Haynes International. This paper will briefly review the behavior of 282 alloy in air and water vapor oxidation (10% H 2 O) at 760°C (1400°F), low cycle fatigue properties at 649°C to 871°C (1200°F to 1600°F) and long-term thermal stability at 649°C to 871°C (1200°F to 1600°F). Special focus of the paper will be mechanical behavior: tensile and creep; microstructural analysis, and weldability of 282 alloy as a result of single step age hardening heat treatment: 800°C (1475°F)/8hr/AC.

The development of oxy-fuel combustion technology for coal-based power generation may produce combustion products different from those typically found in traditional boilers. In particular, the enrichment of CO 2 and perhaps SO3 could alter the chemical equilibrium to favor the formation of certain carbonates and sulfates in the deposit. Higher concentrations of these gases would also increase the potential for condensation of carbonic and sulfuric acids in lower-temperature areas of the boiler. To address these concerns, B&W has instituted a comprehensive research program to better understand the effect of oxy-coal combustion on fireside corrosion. The scope of this program includes gas and deposit analyses of actual combustion products sampled from B&W's Clean Environment Development Facility (CEDF) during the oxy-coal combustion of three commercial coals. The sampling locations consist of regions representing the lower furnace, superheater bank, and pulverizer outlet. Following the gas and deposit analyses, a series of laboratory corrosion tests will be performed to expose candidate alloys and coatings to conditions simulating the oxy-coal combustion environments. The technical approaches and results of the fireside corrosion program obtained to date are discussed.

Using oxygen, rather than air, in coal-fired boilers has been studied for several years as a strategy to reduce NOx and concentrate CO 2 for capture. In combination with flue gas recirculation, higher levels of CO 2 are expected but increased H 2 O and SO 2 levels also may occur. In order to understand the role of substrate composition on corrosion, a combination of commercial and model alloys were investigated with synthetic coal ash and gas compositions simulating air- and oxyfiring environments. Exposure temperatures ranged from 600°-800°C to cover current operating temperatures up to advanced ultrasupercritical conditions. Using 500h exposures, no consistent negative effect was found for switching to the oxy-firing environment with the same synthetic ash. For model Fe-Cr alloys, 30%Cr was needed to form a thin protective reaction product across this temperature range. Among the commercial stainless steels, 310-type stainless steel showed low reaction rates with the maximum attack at 650°C. At higher temperatures, the depth of attack on Fe-base type 310 stainless steel was less than for Ni-base alloy 740. Initially, this difference was attributed to the Al and Ti additions in alloy 740. However, cast and hot rolled model Ni-18Cr and -22Cr alloys with various Al and Ti additions showed decreased metal loss with increasing Al and Ti additions in the oxy-firing environment at 700° and 800°C. As expected, metal loss was very sensitive to Cr content. A second set of model alloys also examined the effect of Co and Mo.

Nitridation is a high-temperature material degradation issue that can occur in air and in environments containing nitrogen, ammonia, etc., and in a variety of industrial processes. The nitridation behavior of several commercial nickel- and cobalt-based alloys is reviewed in this paper. The alloys include Haynes 230, Haynes 188, Haynes 625, Haynes 617, Haynes 214, Hastelloy X, and Haynes 233. The environments discussed are high-purity nitrogen gas between 871°C and 1250°C, 100% ammonia gas at 982°C and 1092°C, and a simulated combustion atmosphere at 982°C. The results showed that nitridation occurred in all the environments containing nitrogen. The nitridation attack was strongly influenced by the alloy compositions and the type of oxide formed (i.e., chromia or alumina), as some degree of oxidation was expected in the environments in which residual oxygen was present. Thermal cycling is briefly discussed because the integrity of protective oxides is also an important factor in resisting high-temperature oxidation and nitridation attack.

Long-term performance of high temperature alloys is critically linked to the oxidation behavior in power generation applications in wet air and steam. As power generation systems move towards higher efficiency operation, nextgeneration fossil, nuclear and concentrating solar power plants are considering supercritical CO 2 cycle above 700°C. Wrought solid solution strengthened and precipitations strengthened alloys are leading candidates for both steam and Supercritical CO 2 power cycles. This study evaluates the cyclic oxidation behavior of HAYNES 230, 282, and 625 alloys in wet air, flowing laboratory air, steam and in 1 and 300 bar Supercritical CO 2 at ~750°C for duration of 1000 -10,000h. Test samples were thermally cycled for various times at temperature followed by cooling to room temperature. Alloy performances were assessed by analyzing the weight change behavior and extent of attack. The results clearly demonstrated the effects of alloy composition and environment on the long-term cyclic oxidation resistance. The extents of attack varied from alloy to alloy but none of the alloys underwent catastrophic corrosion and no significant internal carburization was observed in supercritical CO 2 . The performance of these alloys indicates that these materials are compatible not only in oxidizing environments, but also in Supercritical CO 2 environments for extended service operation.

The EU NextGenPower-project aims at demonstrating Ni-alloys and coatings for application in high-efficiency power plants. Fireside corrosion lab and plants trials show that A263 and A617 perform similar while A740H outperforms them. Lab tests showed promising results for NiCr, Diamalloy3006 and SHS9172 coatings. Probe trials in six plants are ongoing. A617, A740H and A263 performed equally in steamside oxidation lab test ≤750°C while A617 and A740H outperformed A263 at 800°C; high pressure tests are planned. Slow strain rate testing confirmed relaxation cracking of A263. A creep-fatigue interaction test program for A263 includes LCF tests. Negative creep of A263 is researched with gleeble tests. A263 Ø80 - 500mm trial rotors are forged with optimized composition. Studies for designing and optimizing the forging process were done. Segregation free Ø300 and 1,000mm rotors have been forged. A263 – A263 and A293 – COST F rotor welding show promising results (A263 in precipitation hardened condition). Cast step blocks of A282, A263 and A740H showed volumetric cracking after heat treatment. New ‘as cast’ blocks of optimized composition are without cracks. A 750°C steam cycle has been designed with integrated CO 2 capture at 45% efficiency (LHV). Superheater life at ≤750°C and co-firing is modeled.

Alloy 33 is a weld overlay material that has generated a lot of interest in the fossil boiler industry. The high chromium content of Alloy 33 has been shown to provide excellent corrosion protection in both waterwall and superheater/reheater tube applications. For waterwall applications, the corrosion resistance has been demonstrated in both laboratory and field tests conducted over the last 5 years. In addition to corrosion resistance, the Alloy 33 has also shown that it is also resistant to cracking (although no material is 100% immune). In the superheater/reheater, the use of spiral clad weld overlay tubes is able to provide resistance to excellent coal ash corrosion. Laboratory and field tests have shown Alloy 33 to have among the best corrosion resistance of all materials studied. The application of Alloy 33 is also easier than other more highly alloyed materials (such as FM-72) and is less expensive. As a result of these favorable experiences, Alloy 33 is now being used commercially to weld overlay both waterwall and superheater/reheater tubes on fossil boilers.

The INCONEL filler metals 72 and 72M have been utilized significantly for weld overlay protection of superheaters and reheaters, offering enhanced corrosion and erosion resistance in this service. Laboratory data conducted under simulated low-NOx combustion conditions, field exposure experience, and laboratory analysis (microstructure, chemical composition, overlay thickness measurements, micro-hardness) of field-exposed samples indicate that these overlay materials are also attractive options as protective overlays for water wall tubes in low-NOx boilers. Data and field observations will be compared for INCONEL filler metals 72, 72M, 625 and 622.

Nickel-base alloys were exposed to flowing supercritical CO 2 (P = 20MPa) at temperatures of 700 to 1000°C for up to 1000 h. For comparison, 316L stainless steel was similarly exposed at 650°C. To simulate likely service conditions, tubular samples of each alloy were internally pressurised by flowing CO 2 , inducing hoop stresses up to 35 MPa in the tube walls. Materials tested were Haynes alloys 188, 230 and 282, plus HR120 and HR160. These alloys developed chromia scales and, to different extents, an internal oxidation zone. In addition, chromium-rich carbides precipitated within the alloys. Air aging experiments enabled a distinction between carburisation reactions and carbide precipitation as a result of alloy equilibration. The stainless steel was much less resistant to CO 2 attack, rapidly entering breakaway corrosion, developing an external iron-rich oxide scale and internal carburisation. Results are discussed with reference to alloy chromium diffusion and carbon permeation of oxide scales.

A research program has been initiated to develop the first predictive methodology for corrosion fatigue life in steam turbine blades, addressing a critical gap in current understanding despite extensive research into corrosion pitting and fatigue failure. The study focuses initially on dual-certified 403/410 12% Cr stainless steel, utilizing a newly developed test facility capable of conducting high-cycle fatigue tests in simulated steam environments at 90°C with controlled corrosive conditions. This testing platform enables the investigation of various steady and cyclic stress conditions, establishing a foundation for future testing of other blade steels and the development of comprehensive blade life estimation techniques.

Recently, boiler waterwall tube damage such as fireside corrosion and circumferential cracking in low NOx environments has become a serious issue in Japan, despite the typical use of relatively lower sulfur content coal is typically being used than in US. Thermal spray coating has been the most popular method for tube protection in Japan, and thermal spray coated tubes have been used for this purpose. However, extensive damage to thermal spray coating tubes from cracking and exfoliation has been recently experienced. It has been reported that the thermal fluctuations occurring due to operational changes create alternating stress, leading to cracking and exfoliation of the thermal sprayed thin coating. Corrosion-resistant weld overlays, such as Type 309 stainless steel (in sub-critical boilers) and Alloy 622 (in sub-critical and super-critical boilers), are commonly used to protect boiler tubes from corrosion in low NOx coal fired boilers in U.S. In order to develop a fundamental understanding of the high temperature corrosive behavior of Alloy 622 weld overlay, gaseous corrosion testing and certain mechanical tests for consideration of long-term aging were undertaken. After four years of service in the low NOx combustion environment of a coal fired supercritical boiler, field tests on Alloy 622 weld overlay panels are in continuation. This paper describes the field test behavior of Alloy 622 weld overlay panels installed in a Japanese supercritical boiler, the laboratory results of weight loss corrosion testing, and the results of cyclic bend tests with overlay welded tubes related to aging.

There is a critical lack of data on the mechanical behavior of candidate structural materials for advanced nuclear reactors under molten halide salt environments. Limited legacy data from the molten salt reactor experiment (MSRE) program showed a significant reduction in creep rupture strength of a Ni-base alloy in molten fluoride salt. With ongoing efforts to commercialize different molten salt reactor concepts, the industry can considerably benefit from quantitative information on the impact of molten halide salts on the engineering properties such as creep and fatigue strength of materials of interest. The present work aims to assess the role of molten salt corrosion on the creep behavior of three alloys 316H, 617 and 282 at 650-816 °C. Creep tests were conducted in fluoride (FLiNaK) and chloride (NaCl-MgCl 2 ) salts. Initial results from the ongoing testing will be presented which suggest that the molten salt environment caused a 25-50% reduction in creep rupture lifetime compared to air exposures. Physics-based corrosion and creep models were employed to gain some insights into the potential degradation mechanisms.