The corrosion behavior of tubing materials carrying steam at high temperature is of great concern to fossil power plant operators. This is due to the fact that the oxide films formed on the steam side can lead to major failures and consequently to reduced plant availability. The wall loss of the pressure boundary caused by oxidation can increase the hoop stresses and cause premature creep failures; second, the increased insulation of the tubes due to the low thermal conductivity of the oxide film can lead to increased metal temperature, thereby exacerbating the fireside corrosion as well as creep problems. The third concern is that thicker oxides may spall more easily when the plant is cooled down. On restart, the spalled material may lodge somewhere in the system with the potential for causing tube blockages, or it may be swept out with the working fluid and enter the steam turbine causing erosion damage to the turbine nozzles and blades. Failures of tubing and turbine components by these mechanisms have been widely reported in the United States. In view of the importance of the steamside oxidation, a major study of the phenomenon is being carried out as part of a major national program sponsored by the U.S. Department of Energy and the Ohio Coal Development Office. As a prelude to the experimental work, a literature survey was performed to document the state of the art. Results of the review are reported here.

Boiler materials for ultra-supercritical coal power plants - steamside oxidation

The focus of this review is the state of knowledge of the oxidation behaviour in steam of alloys with potential for use as pressure parts in steam boilers. The rate of oxide growth on steam-touched surfaces and the characteristics of that oxide are of increasing interest as the quest for improvements in cycle efficiency leads to progressively higher operating temperatures and pressures. The consequences of increased rate of growth of these oxides are of concern because of implications for tube overheating and oxide exfoliation. Mitigation of such problems requires a mechanistic understanding of the influences of alloy composition and microstructure, and especially of the evolution with time of specific scale structures. Similarly, the relative effects of factors such as time, temperature and operating parameters must be understood. The oxidation behaviour of the class of ferritic steels that forms the bulk of the heat transfer surface in steam boilers is of particular importance since allo...

A review of the oxidation behaviour of structural alloys in steam

Correlations of electrochemical noise, acoustic emission and complementary monitoring techniques during intergranular stress-corrosion cracking of austenitic stainless steel

Cycle chemistry and its effect on materials in a supercritical water-cooled reactor: A synthesis of current understanding

By using coupons and compact tension (CT) samples, the corrosion and corrosion fatigue behaviors of modified 9Cr–MoVNb steel, T91, in a deaerated supercritical water (SCW) condition were investigated. Water pressure was 25 MPa and the test temperatures were 370 and 500 °C. Before and after the 200 h exposure tests, the weight of the samples was measured. The weight of the coupons exposed to 500 °C water increased while that in 370 °C water decreased. The samples exposed to both these conditions were analyzed by using SEM-EDX, AES, and XRD. The analyses showed that only an oxidation (oxide formation) on the sample in the SCW condition occurred while an anodic dissolution in the subcritical condition was dominant. Fatigue crack growth rates (FCGR) determined in the water conditions were compared with those in air. The FCGRs of the alloy were higher in the supercritical and subcritical aqueous conditions than those in air. From the standpoint of a corrosion or an oxidation, the corrosion fatigue behavior of the steel in a supercritical environment was discussed.

Corrosion and corrosion fatigue behaviors of 9cr steel in a supercritical water condition

Electrochemical noise behavior of Type 304 austenitic stainless steel (UNS S30400) with various degrees of sensitization was investigated under slow strain rate tensile (SSRT) conditions i...

Current and Potential Fluctuation Characteristics in Intergranular Stress Corrosion Cracking Processes of Stainless Steels

Oxidation kinetics of recently developed ferritic heat-resistant steels, HCM12A, NF616, and HCM2S, were investigated in a superheated steam to evaluate the effects of chemical composition of the steels, testing temperature (560-700°C), steam pressure (1-10 MPa), and degrees of microstructural evolution by aging on oxidation. The contribution of alloyed Cr to oxidation resistance was pronounced above 600°C, while no material dependency was found at 600°C or lower. The apparent activation energy of the oxidation rate clearly changed at around 600°C for NF616 and HCM12A. In contrast, HCM2S showed single activation energy over the range of temperatures. Although temperature and chemical composition were the major factors, steam pressure also showed a clear negative effect on the oxidation rate in the lower temperature range, 570-600°C.

Oxidation Rate of Advanced Heat-Resistant Steels for Ultra-Supercritical Boilers in Pressurized Superheated Steam

Steam oxidation of ferritic heat-resistant steels for ultra supercritical boilers

Transient behavior of short-circuit current of Type 316 stainless steels (UNS S31600) during a slow strain rate tensile (SSRT) test in high-purity oxygenated water at 288 C was investigated and their correlation with stress corrosion cracking (SCC) was discussed Short-circuit current at free corrosion potential between a specimen under slow-rate straining and a counter electrode which was made of the identical alloy to the specimen was continuously monitored using a zero-resistance-ammeter Spike-like current transients were observed after yielding The current pulses had short rise time! typically a few seconds, and slow decay behavior, 100--1,000 seconds; This characteristic was considered to correspond to the slip dissolution processes Although the current transients were observed both on the solution treated material and on the sensitized one, characteristics of the current transients reflected well the difference in SCC susceptibility between the materials Current transients from the sensitized material had higher peak intensity and slower decay rate than those from the solution treated one Capability of the current noise monitoring for detecting SCC initiation in austenitic stainless steels in high-temperature high-pressure water was demonstrated and it was suggested that the current noise can provide real-time evaluation of SCC behavior

Electrochemical noise characteristics of IGSCC in stainless steels in pressurized high-temperature water

In this study, effects of superheated steam on cyclic crack propagation behavior of a heat resistant steel were investigated. Crack propagation experiments were carried out on NF616 (9Cr-0.5Mo-2WVNb) in pressurized superheated steam (600°C/10MPa) under cyclic loading either with or without holding time at constant load. Superheated steam environment has two opposing effects on cyclic crack growth, acceleration and retardation. A modified tarnish rupture (TR) model has been proposed to explain the crack propagation behavior. The crack propagation rate estimated based on the TR-type model well agreed with the experimental data.

Tatsuo Kondo

Papers

Current and Potential Fluctuation Characteristics in Intergranular Stress Corrosion Cracking Processes of Stainless Steels

Oxidation Rate of Advanced Heat-Resistant Steels for Ultra-Supercritical Boilers in Pressurized Superheated Steam

Steam oxidation of ferritic heat-resistant steels for ultra supercritical boilers

Electrochemical noise characteristics of IGSCC in stainless steels in pressurized high-temperature water

Crack growth behavior of ferritic steel for USC boilers in pressurized superheated steam