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

doi:10.1016/j.msea.2006.05.035

Materials Science and Engineering: A

Volume 429, Issues 1–2, 15 August 2006, Pages 161-168

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Abstract

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.

Introduction

It is generally expected that the demand for energy will dramatically grow in the near future. To satisfy future energy needs, energy supply systems that can provide a higher efficiency and reduce the environmental impacts are required. Recently, an international forum, called the Generation IV International Forum (GIF), started to develop future-generation nuclear energy systems, known as Generation IV systems. Among the six systems that were selected in the program, the supercritical water-cooled reactor (SCWR) is anticipated to operate above the thermodynamic critical point of water (22.1 MPa, 374 °C) to achieve a thermal efficiency approaching 44% [1]. Owing to the SCWR's higher operating temperature than conventional reactors and the unique properties of SCW, the behavior of materials in the SCW condition is one of the main issues to be investigated [2]. It is well known that, in conventional light water reactors (LWRs), stress corrosion cracking and corrosion fatigue of the structural components represent a possible life limiting factor for their operation [3]. This experience from the LWRs requires that the material properties of candidate alloys related to such degradation mechanisms should be substantially evaluated and reviewed during the stages of a plant design and material selection. For this reason, corrosion and corrosion-related cracking of various alloys for SCWRs have been investigated. Among the materials that have been studied here, modified 9Cr–MoVNb steel, designated as T91, is a candidate for reactor core components, nuclear fuel cladding, etc. The steel has already been used, especially in the thick sections in the supercrtitical water cooled fossil power plants [4]. The steel is also a candidate material for the steam generators in liquid metal cooled fast breeder reactors (LMFBRs) due to its adequate creep and corrosion resistance properties [5], [6]. In this study, the corrosion and corrosion fatigue behaviors of a ferritic/martensitic steel, T91, are investigated.

Section snippets

Materials and test samples

A ferritic/martensitic steel, T91, which is one of the candidate materials for the core structures in SCWRs was used in this study and its chemical composition is shown in Table 1. Compact tension (CT) samples with a width of 30 mm and a thickness of 6 mm were used in the fatigue tests, with an orientation of L–T. Fig. 1 shows the dimensions of the CT samples used in this study. For the corrosion measurements, coupons with dimensions of 10 mm × 10 mm × 2 mm were prepared. The coupons were polished with

Corrosion in water

Fig. 2 shows the SEM micrographs and EDX analyses of the samples exposed to (a) 370 °C water (Corr-370) and (b) 500 °C supercritical water (Corr-500) for 200 h. In the SEM micrographs, the darker color on the fringes of the samples separates the surface oxide from the base alloy. The SEM and EDX analysis results showed that a much thicker oxide was formed on the sample exposed to the supercritical water at 500 °C. It was also shown that, on sample Corr-500, two layers were formed, which is known to

Conclusions

Using ferritic/martensitic steel, T91, corrosion and corrosion fatigue tests were performed in high temperature water conditions including supercritical water at 500 °C. The following conclusions can be drawn.

1.
In the 370 °C water and 500 °C SCW conditions, two layers of surface oxide were formed on the T91 samples. The outer and inner layers consist mainly of magnetite and the Fe₃O₄-FeCr₂O₄ binary, respectively.
2.
The formation of the oxide was the main corrosion mechanism of the steel in the 500 °C

Acknowledgement

The research was sponsored by the Ministry of Science and Technology, Korea under contract M2-0211-00-0011.

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

Abstract

Introduction

Section snippets

Materials and test samples

Corrosion in water

Conclusions

Acknowledgement

Int. J. Fatigue

Mater. Sci. Eng.

Corros. Sci.

J. Supercrit. Fluids

J. Supercrit. Fluids

Corros. Sci.

J. Supercrit. Fluids

J. Nucl. Mater.

Corrois. Sci.

Int. J. Fatigue