Microstructural characteristics of the oxide films formed on Alloy 690 TT in pure and primary water at 325 °C
Introduction
Nickel-based Alloy 690 (Ni–30Cr–10Fe) is a technologically important material which is widely used as structural material for steam generator tubes in pressurized water reactors (PWRs). The higher chromium content than Alloy 600 (Ni–16Cr–9Fe) endows an improved corrosion resistance [1], [2], and failures of Alloy 690 tubes have rarely been reported. However, its long-term corrosion behavior under service conditions is still a highly concerned issue. For example, corrosion leads to nickel release in the water, which can be activated into 58Co in the nuclear core under neutronic flux, and as such increases the global radioactivity of the primary circuit of PWR [3]. Corrosion is also the primary cause for the initiation and growth of stress corrosion cracking [4].
Characteristics of the oxide films formed on an alloy, including chemical composition, microstructure and thickness, etc., determine the protect ability of the oxide film and thus plays a crucial role in the corrosion behavior [5], [6], [7], [8]. A couple of material and environment relevant factors influence the oxide film characteristic. In addition to the chemical composition and microstructure of the alloy, water chemistry and surface finishing condition are two key factors that can vary the characteristic of the oxide film. According to McIntyre et al. [9], [10], the boiler chemistry has a strong effect on the surface oxide composition of chromium-containing alloys such as alloys 600, 690 and 800. It is well-known that PWR primary water operates with H3BO3 as a chemical shim for excess thermal neutron absorption to control the chain reaction, while LiOH is used to adjust the pH value [11]. Compared with pure water in the secondary circuit, addition of H3BO3 and LiOH increases the pH value and solution conductivity. Besides, oxygen could be introduced into both primary and secondary circuits occasionally by aerated feed water [12], or by adding of oxygen or H2O2 during the shutdown process of PWRs to stabilize the oxide and decrease the radiation activity of the coolant [13]. Consequently, an off-water chemistry is formed temporarily. It is therefore worthy to investigate the influence of changes in water chemistry on microstructure and composition of the formed oxide film.
Surface finish is also an important factor influencing corrosion behavior of an alloy. According to Perez et al. [14], surface finish produces a subsurface deformation layer with high dislocation density, phase transformation and recrystallization, which would impact corrosion mechanisms. In general, studies in laboratories use ground or polished specimens revealed the effects of surface finish on corrosion resistance [15], [16], [17]. However, under normal service conditions, the surface of components is not polished, and it is of great importance to study the corrosion on such as-machined surfaces.
Our previous work [18] has reported that surface finish (ground, mechanical polished and electropolished) has obvious effect on the microstructure of oxide films formed on Alloy 690 TT in oxygenated primary water. Corrosion on as-machined surface of such a component as heat-exchange tube, however, has not yet been studied. As such, the objective of this work is to obtain a better understanding of the microstructural characteristics of the oxide films grown on the as-received commercial Alloy 690 TT tube specimens after the exposure in pure and primary water both containing 2 ppm O2 at 325 °C. The related oxidation mechanism, and effects of water chemistry and surface finish on oxidation are discussed.
Section snippets
Experimental
The tubing material used in this work was Alloy 690 TT with an outer diameter of 19.07 mm and a wall thickness of 1.07 mm. Chemical composition of the alloy is listed in Table 1. The tubes were half sectioned along the longitudinal axis (tube direction). Specimens for exposure tests with dimensions of 10 × 10 × 1.07 mm3 were then cut from the sectioned tube, as shown in Fig. 1. Surface roughness (Ra, which is the arithmetical mean deviation of the profile), and morphology of the specimen with
Surface morphology of the oxide film
Figs. 3a–d show the surface morphology of the oxide film formed on Alloy 690 TT tube specimens after the exposure experiments. As shown in Fig. 3a, following the exposure in pure water at DO = 2 ppm, the specimen was covered by irregularly shaped oxide particles, which appear to be randomly packed on the surface. The size of the oxide particles is inhomogeneous. Large particles such as particle A in Fig. 3a disperse on the surface, while small, needle-like and granular particles develop from the
Oxide film structure and thermodynamic considerations
The present results indicate an obvious effect of water chemistry on the oxide film structure of Alloy 690 TT tube specimens, as schematically shown in Figs. 9a and b. After immersion in oxygenated pure water (pH325°C = 5.8, conductivity = 0.05 μS/cm), a duplex oxide layer (Fig. 9a) is formed with an outer layer of scattered Fe-rich spinel and NiO particles, and a porous inner layer mainly composed of nickel oxides. In comparison, as shown in Fig. 9b, the oxide scale formed in oxygenated primary
Conclusions
The morphology, chemical composition and microstructure of the oxide films formed on as-received Alloy 690 TT tube specimens after the exposure in oxygenated pure and primary water were investigated, and the related oxidation mechanism was discussed. The following conclusions can be drawn from the present investigation.
- (1)
A duplex oxide film is formed in pure water, including an outer layer of granular Fe-rich spinel and NiO, and a porous inner layer mainly composed of nickel oxides. The oxide
Acknowledgements
The authors gratefully acknowledge Electric Power Research Institute (EPRI) for providing commercial Alloy 690 TT tube. We also appreciate Shanghai Synchrotron Radiation Facility BL14B1 beamline for the GIXRD measurement. This work is supported by the Special Funds for the Major State Basic Research Projects (G2011CB610502) and the National Science Foundation of China (No. 51025104).
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