Full Length ArticleDegradation of the oxide film formed on Alloy 690TT in a high-temperature chloride solution
Graphical abstract
Introduction
Nickel-base alloys are now in widespread use in pressurized water reactors (PWRs) like steam generator (SG) tubing and control rod driving mechanism nozzles due to their higher resistance to corrosion and stress corrosion cracking (SCC) than stainless steels. Was et al. [1] have reviewed SCC initiation in Alloy 690 in high-temperature water and concluded that Alloy 690 is resistant to crack nucleation both in service and in laboratory studies which is closely related to its higher chromium content and lower carbon content [2]. However, some SCC cracking occurs in components operating under abnormal PWR water chemistry because of the release of detrimental anionic impurities, like SO42− and Cl−, from degraded insulations, sealing materials and leaked chillers [2], [3], [4], [5], [6].
According to the report [7] that the reduction of nuclear power plants capacity factor caused by corrosion is about 5% a year, with more than 80% of SG failure due to SCC since 1990s. It is generally recognized that there is an interrelation between SCC and the properties of the surface film [8], [9], [10], [11], [12], [13], [14], [15], [16], [17] and that the water chemistry exert an important effect on the properties and the oxidation kinetics of the oxide films formed on metal surfaces and/or grain boundary (GB) planes [18], [19], [20], [21], [22], [23], [24], [25], [26]. Investigating the influences of SO42− and Cl− on the water chemistry in crevices and SCC of Alloy 600 in high-temperature water indicates that only in extremely acidic conditions (pH25°C ≤ 1.5) is the SCC of Alloy 600 observed [6]. SCC may occur in salt environments at pH25°C 1 if the content of NaCl is higher than 1 M [5] and some significant secondary cracks are visible on the gauge section of specimens tested in 5 M NaCl solutions at pH25°C < 3 [2]. Investigation of the impedance characteristics in the pitting evolutionary process of Alloy 690 [20] suggests that Cl− ions can enter into the pits during pitting, thus to inducing the competition between the formation and destruction of the oxidation film. Because of the strong activity of Cl−, the dense oxidation film protecting the metal will be destroyed gradually. Results from investigations [27], [28], [29] suggest that a localized highly aggressive environment can produce in the crevices between the pipes and the tube support plates of PWR since the concentration of Cl− within the crevices may be several orders of magnitude higher than that in the secondary circuit tubing. Moreover, acidification within the crack tip due to the auto-catalytic nature of occluded cells, that Cl− is concentrated in the crack and is charge balanced by H+ [4], may suppress the formation of the protective oxide film [30] and promote the initiation of SCC [6].
Recently, considerable efforts have been made in an attempt to obtain a general understanding of the corrosion mechanism of Alloy 690 under various water chemistry conditions at high temperature [16], [31], [32], [33]. Experimental evidences [31] imply that a duplex oxide film can be formed on Alloy 690 in oxygenated high-temperature water and the inner layer is composed of primarily NiO which is poor protective. The oxide film on Alloy 690TT formed in hydrogenated water also shows a duplex structure, that the outer layer consists of big Ni- and Fe-rich oxide particles and underlying compact small Cr-rich oxide particles and the inner layer is composed of continuous Cr-rich oxides [32]. In the high-temperature alkaline environment, a duplex structure passive film on Alloy 690 contains an inner layer of fine-grained Cr-rich oxides or spinels and an outer layer of Ni-Fe spinels and Ni-rich hydroxides [33]. At the presence of Pb, both Cr-rich and Ni-rich oxides are alternatively distributed within the outer layer, whereas the inner layer composed of Ni-rich oxides and amorphous structures is porous and poorly protected [16]. However, the influence of only Cl− ions on the properties of the oxide film on Alloy 690 is as yet poorly understood, and no comprehensive theory has been reported to describe this phenomenon.
So the study of corrosion behavior of Alloy 690TT in a high-temperature chloride solution and characteristics of the oxide film is helpful to provide the designers and operators with useful information to improve the safety of the nuclear power plant. Based on our previous work [16], comparative conditions are utilized to investigate the influence of Cl− ions on corrosion behavior of Alloy 690TT in an aqueous solution at 320 °C and 10 MPa, in the present work. Many researchers found that impurities in water, such as chloride and sulfate, even at μg/kg (10−9) levels, can accelerate the crack growth rates [4]. The impurities in the SG feed-water are concentrated within the crevices and the local concentrations of impurities can be increased up to six orders of magnitude, resulting in a localized highly aggressive environment within the crevices, where SCC is likely to occur [27]. The emphasis is put on the composition and structure of the oxide film on Alloy 690TT in high-temperature water with 500 ppm Cl− ions and the related oxidization mechanism is also discussed.
Section snippets
Materials and experimental procedures
Commercially nuclear grade Alloy 690 tubing material was obtained from Baosteel in the mill-annealed condition, with a thickness of 1.09 mm and 19.05 mm in an outer diameter. The chemical composition (wt%) is 30.47 Cr, 9.97 Fe, 0.21 Si, 0.14 Mn, 0.02 C, 0.007 P, 0.027 N, 0.001 S and bal. Ni. As Alloy 690 is thermally treated in practical application, before experiment the as-received materials were thermally treated that consisted of solution annealing at 1100 °C for 5 min, followed by water
Results
Comparing this paper with our previous work [16], which depict the oxidization of Alloy 690TT in pure water, clearly reveals the effects of Cl− ions on corrosion behavior of Alloy 690TT in high-temperature water. The metallographic morphology presents austenitic phase in the thermal treated specimen with carbides on GBs, Fig. 2a. GB characteristics of Alloy 690TT measured by EBSD reveal that GBs consist of primarily RHGBs and CSLs (mainly Σ3 twins) with little amount of LABs, as presented Fig. 2
Discussion
Based on the results in this work, a continuous monolayer oxide film, which primarily contains NiCr2O4 spinel oxides and Cr(OH)3 gel-like hydroxides with trace amount of NiO on the top-surface, is formed on Alloy 690TT after exposure for 4400 h in a high-temperature chloride solution. After immersion in pure water at 320 °C [16], a duplex oxide film is formed with an outer layer mainly composed of NiFe2O4 spinel oxides with certain Ni-rich oxides and/or hydroxides, and a compact inner layer of
Conclusions
Based on the comparative analysis of corrosion behavior of Alloy 690TT in high-temperature water with and without Cl− ions, the following conclusions can be drawn about the properties of the oxide film and the mechanism of Cl−-induced corrosion.
- 1.
Unlike the duplex layer oxide film formed in high-temperature water, a thinner and monolayer, primarily composed of NiCr2O4 spinel-type oxides and Cr(OH)3 gel-like hydroxides with trace amount of NiO on the top-surface, is formed on Alloy 690TT after
Acknowledgements
This work is supported by the Chinese National Science Foundation (Nos. U1260201 and 51771028).
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