A novel salt-bath nitrocarburizing process recently developed forms a lithium-iron compound-oxide layer on the surface of steel in concurrence with a nitride layer by adding lithium ions to the molten salt. This process has already been successfully applied to mass-produced products. However, the microstructure and its formation process of the surface layer in alloyed steels during the nitrocarburizing process have not yet been fully understood. In this study, we focus on the effect of Si and Cr, which are included in a common die steel, on the microstructure of an oxide layer of a nitrocarburized alloy. The alloys used in this study are Fe-0.4wt%C, Fe-0.4wt%C-2.0wt%Si, and Fe-0.4wt%C-2.0wt%Cr. These alloys were arc melted into button ingots under an Ar atmosphere. The ingots were annealed at 1123 K for 1.0 h, followed by air cooling and double tempering at 873 K, similar to the heat treatments employed to hot-die steels. Salt-bath nitrocarburizing was carried out at 823 K for 0.1-10 h. The microstructures of the cross-sectional surface layers of the samples were examined using an optical microscope and FE-SEM. Elemental mapping as well as phase identification of the surface layers were done by EDS, XRD, and GD-OES. In the Fe-C binary alloy, a thin continuous oxide layer of α-LiFeθ 2 formed first on the outermost surface, and a thick iron nitride layer developed underneath the oxide layer, with aligned oxide particles along the grain boundaries of the nitrogen compound layer. In the case of Si addition, the outermost oxide layer became thinner and an additional oxide layer consisting of α-LiFeθ 2 and (Li,Fe) 3 Siθ 4 formed between the outer oxide layer and nitrogen compound layer, and the formation of the oxide particles in the nitrogen compound layer was fully suppressed. In the case of Cr addition, internal oxide particles formed in the nitrogen compound layer, similar to those in the binary steel, although an continuous oxide layer of CrfN,O) formed in between those layers. On the basis of these results, the inner oxide layer formed with Si addition contributes to improving the frictional wear characteristics in die steels.

Direct-fired supercritical CO 2 (sCO 2 ) cycles are expected to result in sCO 2 with higher impurity levels compared to indirect-fired cycles. Prior work at ambient pressure showed minimal effects of O 2 and H 2 O additions, however, a new experimental rig has been built to have flowing controlled impurity levels at supercritical pressures at ≤800°C. Based on industry input, the first experiment was conducted at 750°C/300 bar in CO 2 +1%O 2 -0.25%H 2 O using 500-h cycles for up to 5,000 h. Compared to research grade sCO 2 , the results indicate faster reaction rates for Fe-based alloys like 310HN and smaller increases for Ni-based alloys like alloys 617B and 282. It is difficult to quantify the 310HN rate increase because of scale spallation. Characterization of the 5,000 h specimens indicated a thicker reaction product formed, which has not been observed in previous impurity studies at ambient pressure. These results suggest that more studies of impurity effects are needed at supercritical pressures including steels at lower temperatures.

Traditional laboratory steam experiments are conducted at ambient pressure with water of variable chemistry. In order to better understand the effect of steam pressure and water chemistry, a new recirculating, controlled chemistry water loop with a 650°C autoclave was constructed. The initial experiments included two different water chemistries at 550° and 650°C. Two 500-h cycles were performed using oxygenated (OT, pH ~9 and ~100 ppb O 2 ) or all-volatile treated (AVT, pH ~9 and <10 ppb O 2 ) water conditions at each temperature. Coupons exposed included Fe-(9-11)%Cr and conventional and advanced austenitic steels as well as shot peened type 304H stainless steel. Compared to ambient steam exposures, the oxides formed after 1,000 h were similar in thickness for each of the alloy classes but appeared to have a different microstructure, particularly for the outer Fe-rich layer. An initial attempt was made to quantify the scale adhesion in the two environments.

Stress corrosion cracking (SCC) is a potential risk in structural steels used for steam boilers. To investigate the effect of dissolved oxygen (DO) on SCC susceptibility, three steels, T23, T24 and T91 were annealed at 1065°C and then quenched to create a susceptible microstructure and then exposed in a Jones test to stagnant and circulating water at 200°C with varying DO levels. The results indicated that among the tested steels, the SCC susceptibility was highest in T91 but lowest in T23 which did not exhibit crack initiation with 100 ppb DO. T24 showed no cracking with 50 ppb DO but cracked with 100 ppb DO under these conditions. Based on these results, the next planned step is to monitor crack growth in-situ and determine a critical DO content for each material.

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.

Oxide scale, whose ionic conductivity is larger than its electronic one, generate an electro-motive force between a metal/scale and a scale/gas interfaces. When the scale is thin, an electrical potential gradient is large. The large electrical potential gradient may have a possibility to break scales. To confirm the possibility, high temperature oxidation of zirconium on initial stage was observed by an acoustic emission (AE) technique. AE signal was detected before the scale thickness less than 3 μm. And an electrical response on sputtered zirconium oxide thin film was observed. When the applied voltage over 2.00 V, the electrical current was scattered.

The competitive effect of Nb and V additions on the high-temperature oxidation behavior of Ti- 30Al alloys were studied at 800°C in air. Oxidation performance increased with increasing Nb content, however, V additions eliminated the beneficial effect of Nb on oxidation performance, causing higher oxidation mass gains. In-situ high-temperature XRD by means of synchrotron source suggested dissolution of Nb 5+ but lower valence of vanadium ions in the TiO 2 oxide scale during oxidation. Dissolution of Nb and V ions with different valence in TiO 2 during oxidation could cause the beneficial and detrimental effects observed on the performance of high-temperature oxidation of Ti-30Al.

The current work presented a study of isothermal-oxidation behavior of the additive manufactured (AM) Alloy718 in air at 800°C. The oxidation behavior of Alloy718 specimens produced by selective laser melting (SLM) and electron beam melting (EBM) process were comparatively examined. No significant differences were observed in oxidation kinetics while different microstructures of the oxide scale were found. Coarse and columnar chromia grains developed on SLM specimens, whereas the chromia scale of EBM specimens consisted of extremely fine grains. Glow Discharge Optical Emission Spectrometry (GD-OES) analysis revealed that SLM specimens contain a higher content of Ti in chromia compared with EBM specimens. Process-induced supersaturation in SLM specimens might lead to a relatively high concentration of Ti in the chromia, which may affect the grain morphology of oxide scale in the SLM specimen.

The oxide exfoliation is one of the main problems that cause the explosion of superheater or reheater, which threaten the safety of power plant units, but there is no direct test method of the particle concentration of the scales in high temperature steam. Based on the study of ferromagnetic and optical characteristics of scales, the technology and equipment were developed for on-line measurement based on magnetic sensitivity and granularity behavior. Through numerical simulation and dynamic simulation experiments of scale movement under high temperature and high pressure steam, calculating method of the particle concertation of scales in the main steam or reheated steam pipeline was retrieved by local sampling concentration.

To stay competitive in today’s dynamic energy market, traditional thermal power plants must enhance efficiency, operate flexibly, and reduce greenhouse gas emissions. This creates challenges for material industries to provide solutions for harsh operating conditions and fluctuating loads. Higher efficiency demands steels with excellent steam oxidation resistance, favoring ferritic steels for cycling operation due to their limited thermal expansion. This paper presents a study modeling a combined cycle power plant using GE 9HA0.2 GT technology. The analysis compares different maximum live steam temperatures (585°C, 605°C, 620°C) and four alloys (grades 91 and 92, stainless S304H, and Thor 115) for heat exchangers exposed to steam oxidation. Results indicate that Thor 115, a creep strength enhanced ferritic (CSEF) steel, is a viable alternative to stainless steel for live steam temperatures above 600°C, offering improved oxidation resistance with minimal weight increase. Modern CSEF steels outperform stainless steel in power plants with lower capacity factors, reducing thermal fatigue during load changes. Increasing the live steam temperature boosts plant efficiency, leading to significant CO 2 savings for the same power output.

The effect of gas impurities on corrosion behavior of candidate Fe- and Ni-base alloys (SS 316LN, Alloy 800HT, Alloy 600) in high temperature CO 2 environment was investigated in consideration of actual S-CO 2 cycle applications. Preliminary testing in research and industrial grade S-CO 2 at 600 °C (20 MPa) for 1000 h showed that oxidation rates were significantly reduced in industrial-grade S-CO 2 environment. Meanwhile, controlled tests with individual impurity additions such as CH 4 , CO, and O 2 in research-grade CO 2 were performed. The results indicated that CH 4 and CO additions did not seem to significantly affect oxidation rates. On the other hand, O 2 addition resulted in lower weight gains for all alloys, suggesting that O 2 may be primarily affecting corrosion behavior.

Model alloys of Fe-20Cr and Ni-20Cr (all compositions in weight %) and variants containing small amounts of Si or Mn were exposed to Ar-20CO 2 and Ar-20CO 2 -H 2 O (volume %) at 650 or 700°C. Protective Cr 2 O 3 scale was more readily formed on Fe-20Cr than Ni-20Cr, as a result of the different alloy diffusion coefficients. Silicon additions slowed chromia scale growth, promoting passivation of both alloy types. Water vapour accelerated chromia scaling, but slowed NiO growth.

The combustion of coal and biomass fuels in power plants generates deposits on the surfaces of superheater / reheater tubes that can lead to fireside corrosion. This type of materials degradation can limit the lives of such tubes in the long term, and better methods are needed to produce predictive models for such damage. This paper reports on four different approaches that are being investigated to tackle the challenge of modelling fireside corrosion damage on superheaters / reheaters: (a) CFD models to predict deposition onto tube surfaces; (b) generation of a database of available fireside corrosion data; (c) development of mechanistic and statistically based models of fireside corrosion from laboratory exposures and dimensional metrology; (d) statistical analysis of plant derived fireside corrosion datasets using multi-variable statistical techniques, such as Partial Least Squares Regression (PLSR). An improved understanding of the factors that influence fireside corrosion is resulting from the use of a combination of these different approaches to develop a suite of models for fireside corrosion damage.

Structural alloy corrosion is a major concern for the design and operation of supercritical carbon dioxide (sCO 2 ) power cycles. Looking towards the future of sCO 2 system development, the ability to measure real-time alloy corrosion would be invaluable to informing operation and maintenance of these systems. Sandia has recently explored methods available for in-situ alloy corrosion monitoring. Electrical resistance (ER) was chosen for initial tests due the operational simplicity and commercial availability. A series of long duration (>1000 hours) experiments have recently been completed at a range of temperatures (400-700°C) using ER probes made from four important structural alloys (C1010 Carbon Steel, 410ss, 304L, 316L) being considered for sCO 2 systems. Results from these tests are presented, including correlations between the probe measured corrosion rate to that for witness coupons of the same alloys.

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.