Performance of NiB inhibitor for PbSCC of alloy 600 in Pb-contaminated water and caustic solutions

Nickel-based iron-chromium alloys are used as steam generator (SG) tubing materials in nuclear power plants (NPPs) but experience intergranular stress corrosion cracking (IGSCC) and lead stress corrosion cracking (PbSCC) due to the harsh operating conditions of NPPs. In order to improve the integrity of SGs, many studies have sought suitable inhibitors for IGSCC, while those for PbSCC have been sought to a limited extent. In this study, the performance of nickel boride (NiB) as an inhibitor for PbSCC was evaluated. Nickel boride has been shown to be a good inhibitor for IGSCC of Alloy (or Inconel) 600 in a caustic aqueous solution containing no lead (Pb). No significant SCC was observed to occur in Alloy 600 tested in pure water under the slow strain rate test (SSRT) condition in this study. A mixed mode of IGSCC and TGSCC, however, occurred relatively easily in Alloy 600 tested in pure water containing lead oxide (PbO) at 315 °C. The susceptibility of Alloy 600 to SCC decreased with the addition of NiB, implying that NiB can be used as an inhibitor for PbSCC in Pb-contaminated water.     

Electrochemical noise analysis of PbSCC of Alloy 600 SG tube in caustic environments at high temperature

An electrochemical noise (EN) monitoring technique was applied to analyse an initiation and a propagation of Pb-assisted stress corrosion cracking (PbSCC) of Alloy 600 material in a simulated environment of steam generator (SG) sludge piles at high temperature. A typical electrochemical potential decrease concurrent with a current increase was observed during the EN measurements, indicating the occurrence of localized corrosion events. The EN data were analysed for the processes of a uniform corrosion, an initiation and a propagation of PbSCC using a stochastic theory and microscopy.     

Effect of hydrogen in Inconel Alloy 600 on corrosion in high temperature oxygenated water

Corrosion test on hydrogen charged and uncharged coupons of Inconel Alloy 600 in high temperature oxygenated water showed more weight loss of charged coupon. Observation of the oxide film by transmission electron microscopy (TEM) showed a defective, thicker oxide layer on charged coupon. Analyses of the oxide film by TEM-energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy indicated enrichment of Ni but depletion of Cr in the oxide film on charged coupon. The changes in corrosion behavior and microstructure of the oxide film were most likely due to the hydrogen enhanced preferential dissolution of Cr cations in the water.     

Internal oxidation of Alloy 600 exposed to hydrogenated steam and the beneficial effects of thermal treatment

Alloy 600 in the solution annealed (SA) and thermally treated (TT) conditions was exposed to a hydrogenated steam environment considered to simulate primary water in pressurized water reactors. The likely susceptibility to primary water stress corrosion cracking (PWSCC) was evaluated using the internal oxidation model. A FIB was used to extract cross-sections from Alloy 600 samples and elemental maps were generated; internal oxidation was observed intragranularly in all cases, resulting in expulsion of metallic nickel to the surface. Intergranular oxidation and embrittlement was observed in SA samples, while a thick and dense Cr-rich oxide was formed intergranularly on TT samples.     

Effect of dissolved hydrogen on the electrochemical behaviour of Alloy 600 in simulated PWR primary water at 290 °C

With an increase in dissolved hydrogen (DH) content from 0 to 5 cm³ STP H₂/kg H₂O the electrochemical behaviour of Alloy 600 in deaerated PWR primary water at 290 °C was investigated, using corrosion potential (E_corr) monitoring, potentiodynamic polarization, cyclic voltammetry and electrochemical impedance spectroscopy (EIS). DH content controls the E_corr of Alloy 600. Raising DH content directly promotes the cathodic process and reduces the passivity of Alloy 600 significantly. EIS results show that increasing DH content results in a thinner inner-layer oxide film and ions diffusion becomes easier. The mechanism of these DH effects is discussed.     

Stress corrosion cracking of alloy 600 in an aqueous solution containing lead oxide

The stress corrosion cracking (SCC) behavior of Alloy 600 was studied in aqueous solutions containing lead. Electrochemical polarization and current transient experiments were performed at 315 °C in a 40% sodium hydroxide solution to assess the effect of lead on a passive film formed on Alloy 600. The influences of the alloy microstructure and the addition of an inhibitor to the environments on lead-induced SCC were investigated using C-ring and slow strain-rate tensile (SSRT) tests in demineralized high-purity water and caustic solutions containing PbO at 315 °C. The surface films on Alloy 600 were examined using Auger electron spectroscopy (AES) and energy dispersive x-ray spectroscopy (EDXS). The PbO markedly accelerated SCC of Alloy 600 in the caustic solution and the high-purity water at 315 °C. The addition of nickel boride (NiB) or cerium boride (CeB₆) to the test solutions decreased the susceptibility of Alloy 600 to SCC. Thermally treated Alloy 600 (Alloy 600 TT) tended to crack in a transgranular mode, while the solution-annealed Alloy 600 (Alloy 600 SA) tended to crack in an intergranular mode in water containing PbO.     

Effects of the solution temperature and the pH on the electrochemical properties of the surface oxide films formed on Alloy 600

The surface oxide films on Alloy 600 have been investigated as a function of the solution temperature and the pH by using a cyclic voltammetry, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and a depth profiling by Auger electron spectroscopy (AES). H₃BO₃, Na₂SO₄ and NaOH aqueous solutions with temperatures in the range of 30–300 °C were used as the test solutions. As the solution temperature of the 0.5 M H₃BO₃ increased, the thickness of the passive film increased but the resistance of the passive film was diminished, which is coincident with a solution temperature dependency of the passive current in the potentiodynamic curve. The inner oxide film on Alloy 600 was distinguishable from the Cr-rich outer oxide film above 100 °C. From the Mott–Schottky relation, the oxide formed in 0.5 M H₃BO₃ at 300 °C showed a p-type semiconductor property, accompanied by a Cr-rich oxide film throughout the whole oxide film unlike the n-type oxide films up to 250 °C. The oxide resistance of the passive film decreased in the order of 0.5 M H₃BO₃, 0.1 M NaOH and 0.5 M Na₂SO₄, which is consistent with the pH dependency of the passive current. Ni-rich oxide films of a p-type were formed in the 0.5 M Na₂SO₄ or 0.1 M NaOH.     

Electrochemical properties and growth mechanism of passive films on Alloy 690 in high-temperature alkaline environments

Passive films formed on Alloy 690 in high-temperature alkaline environments were investigated by potentiodynamic polarization, X-ray photoelectron spectroscopy, transmission electron microscopy and Mott-Schottky approach. Passive current density and donor density of the passive films increase with increasing temperature, due to increased diffusion rates of metallic ions and dehydration of hydroxide phases. The passive films show a duplex structure including an inner layer of fine-grained Cr oxide or spinel oxide and an outer layer of Ni-Fe spinel oxide and Ni hydroxide. A growth model of the passive films on Alloy 690 in high-temperature alkaline environments is proposed and discussed.     

The effect of prior deformation on stress corrosion cracking growth rates of Alloy 600 materials in a simulated pressurized water reactor primary water

The effect of prior deformation on stress corrosion cracking (SCC) growth rates of Alloy 600 materials in a simulated pressurized water reactor primary water environment is studied. The prior deformation was introduced by welding procedure or by cold working. Values of Vickers hardness in the Alloy 600 weld heat-affected zone (HAZ) and in the cold worked (CW) Alloy 600 materials are higher than that in the base metal. The significantly hardened area in the HAZ is within a distance of about 2-3 mm away from the fusion line. Electron backscatter diffraction (EPSD) results show significant amounts of plastic strain in the Alloy 600 HAZ and in the cold worked Alloy 600 materials. Stress corrosion cracking growth rate tests were performed in a simulated pressurized water reactor primary water environment. Extensive intergranular stress corrosion cracking (IGSCC) was found in the Alloy 600 HAZ, 8% and 20% CW Alloy 600 specimens. The crack growth rate in the Alloy 600 HAZ is close to that in the 8% CW base metal, which is significantly lower than that in the 20% CW base metal, but much higher than that in the as-received base metal. Mixed intergranular and transgranular SCC was found in the 40% CW Alloy 600 specimen. The crack growth rate in the 40% CW Alloy 600 was lower than that in the 20% CW Alloy 600. The effect of hardening on crack growth rate can be related to the crack tip mechanics, the sub-microstructure (or subdivision of grain) after cross-rolling, and their interactions with the oxidation kinetics.     

Chemical composition and electronic structure of passive films formed on Alloy 600 in acidic solution

The chemical composition and the semiconducting properties of passive films formed on nickel based alloy (Alloy 600) in acidic sulphate solution, pH 2.0 at room temperature were studied using Auger analysis, voltammetric techniques and the Mott-Schottky approach. The results obtained revealed that the presence of both chromium and mixed nickel-iron oxides in the films leads to the development of a p-n heterojunction, which controls their electronic structure, similarly manner to the case of stainless steels and Alloy 600 in borate buffer solution. This behavior has been interpreted as representing of an oxide system, which has a duplex character, with an inner p-type semiconducting region, mainly formed by chromium oxide and an outer n-type semiconducting region, containing iron oxide. It could also be observed that the nickel oxide present in the films acts as a barrier layer conferring improved protection.     

Corrosion behavior of Hastelloy C-276 in supercritical water

The corrosion behavior of a nickel-based alloy Hastelloy C-276 exposed in supercritical water at 500–600 °C/25 MPa was investigated by means of gravimetry, scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. An oxide scale with dual-layer structure, mainly consisting of an outer NiO layer and an inner Cr₂O₃/NiCr₂O₄-mixed layer, developed on C-276 after 1000 h exposure. Higher temperature promoted oxidation, resulting in thicker oxide scale, larger weight gain and stronger tendency of oxide spallation. The oxide growth mechanism in SCW seems to be similar to that in high temperature water vapor, namely solid-state growth mechanism.     

Influence of Zn on oxide films on Alloy 690 in borated and lithiated high temperature water

To clarify the corrosion control effect of Zn injection into hydrothermal environments, the oxide films on Alloy 690 in the deaerated borated and lithiated water have been investigated using potentiodynamic polarization curves, electrochemical impedance spectra at 300 °C and ex-situ X-ray photoelectron spectroscopy. With Zn injection in the solution, ZnCr₂O₄ and ZnFe₂O₄ were formed in the inner and outer layers of the oxide films on Alloy 690, respectively, through exchange reactions between Zn²⁺ and Fe²⁺/Ni²⁺. A simple model for oxide film structure change and the mechanism of corrosion inhibition by Zn injection is proposed and discussed.     

Characterization of microstructure and corrosion properties of cold worked Alloy 800

X-ray diffraction studies indicated that cold worked (∼50%) Alloy 800 was austenitic and transmission electron microscopy revealed the presence of a small volume fraction of hexagonal ε-martensite along with deformation bands, high dislocation density and primary TiN particle with a few dislocations within it. The passivity of cold worked alloy was very stable in H₂SO₄ solution but unstable in HCl solution at room temperature. The exposure of cold worked alloy in 673 K steam (initial pH of water was 10.1) for a period of 264 h showed almost nil corrosion rate. Scanning electron microscopy revealed a number of small oxide particles on the surface exposed in steam indicating initiation of oxide formation. Energy dispersive X-ray analyses of the surface containing small oxide particles indicated that the surface composition was similar to bulk composition of the alloy. X-ray photoelectron spectroscopy revealed that the alloy surface exposed in steam contained mixed oxides of iron and chromium as well as elemental form of iron, nickel and chromium.     

Effects of hydrogen on the anodic behavior of Alloy 690 at 60 °C

Polarization and electrochemical impedance spectroscopy (EIS) measurements, Mott-Schottky (M-S) analysis and X-ray photoelectron spectroscopy (XPS) were used to investigate the effects of hydrogen on the anodic behavior of a one-dimensionally (1D) 25% cold worked (CW) Alloy 690 thermal treated (TT) in a boric acid and sodium sulphate solution at 60 °C. The pre-hydrogen-charged specimen exhibited a higher anodic current than that of the uncharged specimen below the transpassive potential. The charged hydrogen can be trapped in the metal. Electrochemical impedance spectroscopy (EIS) showed that the resistance capacitance loop of the hydrogen-charged specimen was significantly smaller than that of the uncharged specimen. Mott-Schottky analyses indicated that the passive film formed on Alloy 690 at −0.2 V_SCE was an n-type semiconductor, with a p-n hetero-junction at 0.2 V_SCE. Charged hydrogen increased the carrier density and the thickness of the passive film both at −0.2 V_SCE and 0.2 V_SCE. The Ni/Cr ratio in the surface film decreased after hydrogen charging, indicating that charged hydrogen could enhance the oxide film growth by increasing the OH⁻ (O²⁻) concentrations through its reaction with vacancies.     

Effects of cold working path on strain concentration, grain boundary microstructure and stress corrosion cracking in Alloy 600

Grain boundary microstructure, strain distribution and stress corrosion cracking (SCC) in one dimensional (1D), two dimensional (2D) and three dimensional (3D) cold worked Alloy 600 were investigated. The cold working decreased the annealing twins and increased low angle boundaries. 2D cold working caused lower strain concentration at grain boundaries than 1D and 3D cold working. The intergranular SCC (IGSCC) susceptibility was the highest in 1D cold worked alloy while lowest in 2D cold worked alloy. The IGSCC susceptibility displayed a strong correlation with the grain boundary strain concentration and the grain boundary microstructure.     

800、600和690合金的铅致应力腐蚀破裂

在很多核电站的蒸汽发生器中都发现了金属态铅或氧化态铅,因而蒸汽发生器传热管的铅诱导（铅致）应力腐蚀破裂（PbSCC)已引起广泛的注意,已证实在压水堆的蒸汽／水系统,特别对管材,存在着铅的许多可能的来源,本文介绍了PbSCC的现场事例,讨论在压水堆（PWR）二回路条件（碱性,酸性和近中性）下铅对镍合金SCC的影响及其作用机理研究的进展。     

Transgranular SCC mechanism of thermally treated alloy 600 in alkaline water containing lead

This work aims to understand a SCC failure mode of thermally treated steam generator tubing materials in high temperature water containing lead. The effect of lead contents on the anodic polarization curves of alloy 600 (UNS NO6600) and alloy 690 (UNS NO6900) has been studied in a solution of pH 10 at 200 °C and 315 °C. Lead increased the active peaks of alloy 600 and alloy 690 in mild alkaline water at high temperatures. A reduction of PbO to a metallic lead in alloy 690 is easier than that of alloy 600. When lead was added into the solution, a relative ratio of Cr from among the main metallic elements (Cr, Fe, and Ni) of alloy 600 and alloy 690 decreased in the outer corrosion film. Alloy 690 TT showed a transgranular stress corrosion cracking (TGSCC) in a 10 M NaOH solution with 5000 ppm of lead. Intergranular stress corrosion racking (IGSCC) was observed in the 100 ppm lead condition, and some TGSCC was detected on the fracture surface of the alloy 600 MA cracked in the 10,000 ppm lead solution. IGSCC seemed to be retarded by a crack blunting around the grain boundaries, and the TG cracking mode of the thermally treated alloy 600 and 690 seemed to be related to a crack blunting at the grain boundary carbide and a film dissolution by lead in an alkaline solution.     

Electrochemical behaviour of Alloy 600 tubing in thiosulphate solution

Electrochemical and surface properties of passive films formed on Alloy 600 in a thiosulphate solution were studied. Oxide films formed at various passive potentials contained a bilayer oxide, whose composition changed as a function of the applied potential resulting in a change in the impedance behaviour. Destabilisation of the oxide film at potentials within the passive region was observed, which was due to the breakdown of the oxide film and coincided with the loss of Cr within the passive film. In contrast, a higher degree of corrosion protection was obtained when the Cr content within the oxide film was elevated.     

690TT合金在特殊环境下的腐蚀问题

在广泛的电厂运行及实验室研究基础上,690TT合金被证明是目前最佳的蒸汽发生器（SGs）管材之一,690TT合金的使用有效地提高了PWR蒸汽发生器的可靠性,因而成为在役第二代核电站中最常用的传热管管材,并将大量应用于第三代商用核电厂。然而在水质恶化以及随服役时间的增加,690TT合金不可避免的也会遭遇腐蚀。本文对690TT合金使用安全性造成潜在威胁的脱合金成分腐蚀（Cr贫化）、铅致应力腐蚀破裂（PbSCC）、低价硫应力腐蚀开裂（Sy-SCC）的情况加以较详细的介绍。     

Influence of dissolved oxygen on oxide films of Alloy 690TT with different surface status in simulated primary water

Oxidation of Alloy 690 with different surface conditions were performed in oxygenated primary water containing B and Li at 325 °C and 15.6 MPa for 2 months. Oxide scales were analyzed using various methods. Results showed triple-layer oxide films were formed: the outmost layer with dispersed big oxide particles; the intermediated layer with loose sheet-like or needle-like oxides and the inner layer with incompact cellular oxides, which were lack of protectiveness. Oxide morphologies were affected by the surface status of samples. Dissolved oxygen increased the intergranular attack (IGA) sensitivity of Alloy 690TT. Growth mechanism of the oxide film was discussed.