Analysis and TEM examination of corrosion scales grown on Alloy 690 exposed to pressurized water at 325 °C

Abstract

Corrosion scales grown on Alloy 690 during exposure to pressurized water were examined and analysed by SIMS, scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM), XPS and x-ray diffraction at grazing incidence. All the corrosion tests were performed in pure water on spark-machined and 1 µm polished specimens at a temperature of 325• C and a pressure of 155 bar. This surface finish results in the formation of a highly defective and thin perturbed zone similar to the standard surface finish of tubes used in pressurized water reactors. Two different testing devices were used: a static autoclave made of stainless steel and a recirculating autoclave made of titanium alloy. Scale thicknesses deduced from XTEM, XPS and SIMS are in good agreement; the average value is ∼50 nm for an exposure of 2160 h. The microstructure of these scales cannot be considered as duplex, as is usually described in the open literature. The scale is formed of a rather continuous and uniform internal Cr-rich thin layer and an external layer, the composition, morphology and microstructure of which depend on the test conditions and test duration. For example, scales grown in the static autoclave contain higher concentrations of Fe and Ni than scales grown in the recirculating titanium alloy autoclave.

Influence of later-dissolved oxygen on microstructural changes in oxide films formed on Alloy 690TT in hydrogenated primary water

“…The Cr-rich oxides are also found to contain a mixed iron and nickel chromite (Ni 1Àx Fe x Cr 2 O 4 ), and Cr 2 O 3 nodules are found at the interface between this spinel and alloy [10]. Thus, the microstructure and chemical composition of the oxide film formed in hydrogenated primary water used in this study agree well with the results reported elsewhere [6][7][8][9][10]. The growth mechanism of different parts in the oxide film has been discussed extensively in previous reports, and this paper addresses the effects of later-dissolved oxygen on the microstructural changes in the oxide film formed under the DH condition.…”

“…To form such a protective oxide film and to decrease the corrosion rate, a certain hydrogen partial pressure is maintained in primary water to maintain a reducing environment during the nuclear reactor operation. The microstructure and chemical composition of surface oxide films formed on Ni-based alloys in simulated normal hydrogenated primary water containing B and Li have been extensively studied [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. It is widely agreed that an oxide film grown on Alloy 690TT consists of an outer layer containing mixed spinel oxide particles of Ni, Cr, and Fe, and an inner layer containing continuous and compact Cr-rich oxides.…”

Photoelectrochemical study of nickel base alloys oxide films formed at high temperature and high pressure water

“…During the corrosion process, a passive layer is formed, whose structure and properties play a key role on degradation phenomena such as nickel release in the water, which has for consequence an increase of the contamination, or such as stress corrosion cracking initiation and growth. Different papers describe the oxide film growing on nickel base alloys in PWR simulated primary water as a duplex structure [1][2][3][4][5][6][7][8][9][10][11][12]. The internal layer corresponds to an oxide rich in chromium whereas the outer one, which is assumed to be formed by precipitation process [3,6], is rich in iron or nickel.…”

“…Different papers describe the oxide film growing on nickel base alloys in PWR simulated primary water as a duplex structure [1][2][3][4][5][6][7][8][9][10][11][12]. The internal layer corresponds to an oxide rich in chromium whereas the outer one, which is assumed to be formed by precipitation process [3,6], is rich in iron or nickel. Recently, a characterisation by transmission electron microscopy of the oxide layer formed on alloys 690 and Ni-30Cr exposed to PWR primary simulated medium has identified two different * Corresponding author.…”

Influence of dissolved oxygen concentration on the oxide film formed on Alloy 690 in high temperature water

“…The oxidation behavior of nickel-base alloys has been one of the hot topics because the oxide films formed are important to their performance. Up to now, in order to elucidate the oxidation process, some studies have been done to analyze the formed oxide films [2][3][4][5][6][7][8][9]. And the effects of different water chemistry parameters such as dissolved hydrogen (DH) [10][11][12][13][14], pH [15,16], temperature [15,17,18], Li concentration [19] and Zn injection [20,21] on the oxidation behaviors of Ni-base alloys have also been investigated.…”