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 fall-off of oxide scale with poor adhesion inside superheater/reheater tubes in boilers for (ultra) supercritical power unit is the main cause of accidents such as superheater/reheater blockage, tube explosion and solid particle erosion in the steam turbine which cause serious economic losses. However, there is still no method for testing and assessing the adhesion of oxide scale inside the tube. A method for testing the adhesion of corrosion products in tubes by spiral lines is proposed in this paper, and the accuracy of adhesion evaluation is improved by adopting the image recognition method.

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.

A model based on a concept of “fraction of exfoliated area” as a function of oxide scale strain energy was developed to predict the extent of exfoliation of steam-side scale from boiler tube superheater loops. As compared with the Armitt diagram, which can be used to predict when scale damage and exfoliation would be likely to occur, a “fraction of exfoliated area” approach provides an estimation of mass of scale released and the fraction of tube likely to be blocked by the exfoliation. This paper gives results for the extent of blockage expected in a single bend of a superheater loop was predicted as a function of operating time, bend geometry, and outlet steam temperature under realistic service conditions that include outages. The deposits of exfoliated scale were assumed to be distributed horizontally the tubes bends. Three types of bends were considered: regular bends, short bends, and hairpin bends. The progressive increase in steam and tube temperatures along a single loop of superheater tubing and the ensuing variation of oxide scale thickness are considered. Numerical simulation results for a superheater loop made of TP347H austenitic steel indicated that tube blockage fractions larger than 50% are likely to occur within the first two years of boiler operation (with regularly scheduled outages) for outlet tube temperatures of 540-570°C, which is consistent with practical experience. Higher blockage fractions were predicted for tubes with hairpin bends than for tubes with regular bends, of length that are larger than five internal tube diameters. Finally, the blockage model presented can be used with some confidence to devise operating schedules for managing the consequences of oxide scale exfoliation based on projections of time to some critical blockage fraction for specific boiler operating conditions.

‘Chromium-Molybdenum’ ferritic steels are the most popular materials for construction of steam generators. Microstructure of ‘Cr-Mo’ steels is highly susceptible to changes due to heat- treatments. This susceptibility is often exploited for improving creep strength of these alloys, via suitable heat treatments, that are employed in order to incorporate changes in grain size and secondary precipitates. This paper presents a general overview of the role of microstructure in various aspects of gaseous oxidation, and the synergistic role of the alloy microstructure and oxidation in deterioration of mechanical properties. In the light of this overview, the paper also presents oxidation kinetics and oxidation-assisted microstructural degradation in the microstructurally different regions of weldments of a common variety of ‘Cr-Mo’ steel, viz., 2.25Cr-1Mo steel.

The spalling of oxide scales at the steam side of superheater and reheater of ultra-supercritical unit is increasingly serious, which threatens the safe and economic operation of the boiler. However, no effective monitoring method is proposed to provide an on-line real-time detection on the spalling of oxide scales. This paper proposes an on-line magnetic non-destructive testing method for oxide granules. The oxide scale-vapor sample from the main steam pipeline forms liquid-solid two-phase flow after the temperature and pressure reduction, and the oxide granules are separated by a separator and piled in the austenitic pipe. According to the difference of the magnetic features of the oxide scales and the austenitic pipe, the oxide granule accumulation height can be detected through the spatial gradient variations of the magnetic induction. The laboratory test results show that the oxide scale accumulation can be accurately calculated according to the spatial gradient changes around the magnetized oxide granules, with the detection error not exceeding 2%.

During the last decades, new generations of Ni-based superalloys have emerged with judiciously controlled chemistries. These alloys heavily rely on the addition of refractory elements to enhance their mechanical properties at elevated temperatures; however, a clear interpretation of the influence of these minor-element additions on the alloy's high-temperature oxidation behavior is still not well understood, particularly from the standpoint of predicting the transition from internal to external alumina formation. In this context, the present investigation describes a systematic study that addresses the intrinsic effects that minor element additions of Nb, Ta, and Re have on the oxidation behavior of alumina-scale forming γ-Ni alloys. By combining a novel simulation approach with high-temperature oxidation experiments, the present study evidences the generally positive effect associated with 2 at. % addition of Ta and Re as well as the detrimental consequences of Nb additions on the 1100 °C oxidation of (in at. %) Ni-6Al-(0,4,6,8)Cr alloys.

For last half century the development of creep strength enhanced ferritic steels has been continued and presently ASME grades 91, 92 and 122 extremely stronger than conventional low alloy steels have extensively been used worldwide in high efficient power plants. However the use of these creep strength enhanced 9-12%Cr steels is limited to around 630°C or 650°C at maximum in terms of high temperature strength and oxidation resistance. Consequently the appearance of ferritic steels standing up to higher temperature of around 700°C to substitute of high strength austenitic steels is strongly desired. Under the state, the addition of high nitrogen to ferritic steels is attracting considerable attention because of improving high temperature strength and oxidation resistance of them. This work was done to evaluate the oxidation resistance of high nitrogen steels and to investigate the effect nitrogen and microstructure on oxidation resistance using 9-15%Cr steels with about 0.3% nitrogen manufactured by means of Pressurized Electro- Slag Remelting (PESR) method in comparison with ASME grades 91 and 122. As a result, high nitrogen ferritic steels showed excellent oxidation resistance comparing with nitrogen-free steels and ASME grades 91 and 122. The oxidation resistance of 9%Cr ferritic steels depends on the nitrogen content in the each steel. That is, the weight gain decreases with an increase in nitrogen content. Moreover, the oxide scale of high nitrogen steel contained a high concentration of Cr. It is conjectured that, in high temperature oxidation, nitrogen plays a key role in promoting the formation of the oxide scale which has high concentration of Cr, inhibiting oxidation from proceeding. And also it was found that the oxidation resistance of the high nitrogen steels does not depend greatly on Cr content but on their microstructure. The oxidation resistance of high nitrogen ferritic heat-resistant steels increased as the fraction of martensite structure increased. These results indicate for high nitrogen steels Cr diffusion along grain boundaries is further promoted resulting in the formation of protective oxide scale having high Cr concentration. Furthermore as new findings it was confirmed that the Cr diffusion in substrate of steels to form Cr concentrated oxide scale on the metal surface is accelerated by nitrogen while suppressed by carbon in matrix of steel.

Both of high pressure main throttle valves and one governing valves were jammed during the cold start of steam turbine served for 8541 hours at 600 °C in an ultra supercritical power plant. Other potential failure mechanisms were ruled out through a process of elimination, such as low oil pressure of digital electro-hydraulic control system, jam of orifice in the hydraulic servo-motor, and the severe bending of valve stem. The root cause was found to be oxide scales plugged in clearances between the valve disc and its bushing. These oxide scales are about 100~200 μm in thickness while the valve clearances are about 210~460 μm at room temperature. These oxide scales are mainly composed of Fe 3 O 4 and Fe 2 O 3 with other tiny phases. Both of valve disc and its bushing were treated with surface nitriding in order to improve its fatigue resistance, which unexpectedly reduces the steam oxidation resistance. On the other hand, significant fluctuation of valve inner wall temperature during operation accelerated the exfoliation of oxide scales, and the absence of full stroke test induced the gradual accumulation of scales in valve clearances. In light of the steam valve jam mechanism in the present case, treatments in aspects of operation and resistance to steam oxidation are recommended.

CWT (combined water treatment) was introduced in Japan in 1990 and over 50 power generation boilers are now in operation. However, the effect of oxygenated treatment on the steam oxidation of the ferritic-martensitic steels and austenitic stainless steels that are used for superheaters and reheaters is currently far from clear. In this study, laboratory tests were used to examine the effect of the oxygen level of the feed water on the scale growth and the scale exfoliation propensity of T91 ferritic-martensitic steel and 300-series austenitic stainless steels, as represented by TP316H and TP347H (coarse- and fine-grained, respectively). The oxygen level of the feed water had little effect on the steam oxidation rates of all the steels tested. Hematite (Fe 2 O 3 ) formed in the outer layer of the oxide scales on both the ferritic and austenitic steels and is considered to have been encouraged in the simulated CWT atmosphere. The adhesion strength of the oxide scale formed on T91 in the simulated CWT atmosphere, that is, scale in which hematite was present, was lower than that of the oxide scale formed in the simulated AVT (all volatile treatment) atmosphere. The oxidation rate of fine-grained TP347H was confirmed to be slower than that of coarse-grained TP316H. Hematite significantly influenced the scale exfoliation of the austenitic steels and the critical oxide thickness for exfoliation decreased with increasing proportion of hematite in the outer scale.

The oxidation behavior of X20 steel in steam environments was studied isothermally between 580-640°C. Initially, the magnetite (Fe 3 O 4 ) phase formed on the surface. With increasing time and temperature, the hematite (Fe 2 O 3 ) phase formed. The oxide scale consisted of an inner layer divided from an outer layer by the original metal surface. A Cr-rich area was observed beneath the original metal surface. Oxide scales formed on a serviced boiler tube at 540°C for 7000h were also analyzed and found to be similar in oxide phase composition to those formed on X20 steel under laboratory conditions. However, differences existed in the microstructure and distribution of the Cr-rich area within the oxide scale. It was concluded that the oxidation mechanism under field conditions differs from that under laboratory conditions.

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.

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.

This paper investigates the combined effect of shot peening and pre-oxidation treatment in air on the subsequent steam oxidation resistance of Modified 9Cr-1Mo steel with different sulfur contents. Cast steel balls (50-180 μm diameter) and pure Cr (50-230 μm diameter) were used for shot peening durations of 5-50 seconds. After shot peening, pre-oxidation was performed in air at 973K for 3.6ks. Then, oxidation testing was conducted in steam at 923K for up to 3.6Ms. Only the combination of Cr shot peening and pre-oxidation treatment facilitated the formation of a protective Cr-rich oxide scale on the specimen surface during pre-oxidation. This Cr-rich oxide scale remained stable during subsequent steam oxidation, resulting in excellent oxidation resistance of the steel.

The steam oxidation behaviour of boiler tubes and steam piping components is a limiting factor for improving the efficiency of the current power plants. Spallation of the oxide scales formed during service can cause serious damage to the turbine blades. Vallourec has implemented an innovative solution based on an aluminum diffusion coating applied on the inner surface of the T/P92 steel. The functionality of this coating is to protect the tubular components against spallation and increase the actual operating temperature of the metallic components. In the present study, the newly developed VALIORTM T/P92 product was tested at the EDF La Maxe power plant (France) under 167b and 545°C (steam temperature). After 3500h operation, the tubes were removed and characterized by Light Optical Metallography (LOM), Scanning Electron Microscopy (SEM), with Energy Dispersive X-ray spectrometry (EDX) and X-Ray Diffraction (XRD). The results highlight the excellent oxidation resistance of VALIORTM T/P92 product by the formation of a protective aluminum oxide scale. In addition, no enhanced oxidation was observed on the areas close to the welds. These results are compared with the results obtained from laboratory steam oxidation testing performed on a 9%Cr T/P92 steel with and without VALIORTM coating exposed in Ar-50%H 2 O at 650°C.

During commissioning of recently built modern, and highly efficient coal-fired power plants, cracks were detected after very short time of operation within the welds of membrane walls made from alloy T24. The root cause analysis revealed transgranular and mostly intergranular cracks adjacent to the heat affected zone beside weld joints. At that time, the degradation mechanism was rather unclear, which led to an extended root cause analysis for clarification of these failures. The environmentally assisted cracking behavior of alloy T24 in oxygenated high-temperature water was determined by an experimental test program. Hereby, the cracking of 2½% chromium steel T24 and 1% chromium steel T12 were determined in high-temperature water depending on the effect of water chemistry parameters such as dissolved oxygen content, pH, and temperature, but also with respect to the mechanical load component by residual stresses and the microstructure. The results clearly show that the cracking of this low-alloy steel in oxygenated high-temperature water is driven by the dissolved oxygen content and the breakdown of the passive corrosion protective oxide scale on the specimens by mechanical degradation of the oxide scale as fracture due to straining. The results give further evidence that a reduction of the residual stresses by a stress relief heat treatment of the boiler in combination with the strict compliance of the limits for dissolved oxygen content in the feed water according to water chemistry standards are effective countermeasures to prevent environmentally assisted cracking of T24 membrane wall butt welds during plastic strain transients.

Steam oxidation of a novel austenitic steel, of which composition is Fe-20Cr-30Ni-2Nb (at.%), has been conducted at 973 K to evaluate steam oxidation resistance based on detail analyses of scale morphology and scale growth. Two types of scale morphologies were observed in the solution treated sample, depending on the grain of the steel. Although thin duplex scale with the Cr-rich layer was formed in the early stage, most of the surface was covered with thick duplex scale which consists of magnetite as the outer scale and the mixture of Fe-Cr spinel and metallic Ni as the inner scale. On the other hand, surface morphology of the oxide scale was independent of grain of the steel and thick duplex scale as seen on the solution treated sample was formed on the pre-aged sample. Steam oxidation resistance of the steel is almost the same as that of commercial austenitic steels and it can be improved by the surface treatment such as shot peening. Based on the results, this steel has both enough creep rupture strength and good steam oxidation resistance for A-USC power plants.

Oxide scale formation in the inner bore of steam tubing has been identified as a key metric for determining operational parameters and life expectancy of modern boiler systems. Grade 91 tubing is commonly used for the construction of key components within boiler systems designed for power generation operating in the temperature range of 500 to 650 °C. Standard laboratory test procedures involve grinding the surface of test coupons to homogenise their surface structure and improve experimental consistency, however, data presented here shows a discrepancy between laboratory and industrial practices that has long term implications on scale growth kinetics and morphological development. Microstructural analysis of both virgin and ex-service tubing reveals the presence of a pre-existing oxide structure that is incorporated into the inwardly growing scale and is implicated in the formation of multiple laminar void networks. These void networks influence thermal diffusivity across the scale and may function as regions of spallation initiation and propagation.

This paper describes the steam oxidation behavior of two 18Cr-8Ni austenitic fine-grained stainless steels, TP347HFG and SUPER304H, which have been developed for ultra-supercritical (USC) boilers. A field exposure test was conducted by installing these tubes, along with comparative materials, in the tertiary superheater of a utility power boiler. After periodic service, the fine-grained tubes were removed to examine their steam oxidation behavior. Examination of the steam oxidation scale on the inner surface of the tubes indicated an extremely low scale growth rate for the fine-grained steels, even after 10 years of service. The oxidation structure is discussed and compared with conventional materials, TP321H and TP347H. Accelerated steam oxidation tests were conducted using an oxidation test with saturated dissolved oxygen concentration. The combination of fine-grained steel and a shot-peening layer exhibits high steam oxidation resistance at 700°C or higher temperatures.

For high-strength steels developed at the National Institute for Materials Science (NIMS) in Japan, a dispersion of nano-sized MX nitride particles along boundaries and in the matrix is achieved by reducing carbon concentration below 0.02%. This structure results in excellent creep strength at 923K, approximately two orders of magnitude longer rupture time than P92. Additionally, adding a large amount of boron exceeding 0.01% combined with minimized nitrogen effectively improves creep rupture strength by stabilizing the martensitic microstructure during creep. Efforts have been made to enhance the steam oxidation resistance of these 9Cr steels strengthened by boron and fine MX nitrides. A combination of 0.7% Si, 40-60 ppm S, and pre-oxidation treatment was applied. Steam oxidation tests were conducted at 923K for up to 4000h. Pre-oxidation treatment in argon gas at 973K for 50h significantly improved oxidation resistance in steam at 923K by forming a protective Cr-rich oxide layer. The pre-oxidized steels exhibited much lower mass gain in steam at 923K than Mod.9Cr-1Mo steel at 873K, and lower than T91 at 873K after 1000h. After 4000h, their mass gain was about zero, much lower than P91 at 873K and 923K. SEM/EDS analysis and low mass gain suggest a protective Cr-rich oxide scale formed on the pre-oxidized steel surface, exhibiting excellent oxidation resistance in steam at 923K.