R. D. Davidson

The corrosion of steam generator surfaces under typical secondary coolant conditions : effects of pH excursions on the alloy surface composition

Abstract Specimens of Inconel 600 (I-600), Inconel 690 (I-690), Incoloy 800 (I-800) and Monel 400 (M-400) were exposed to high temperature steam generator water conditions typical of those which might be found when the secondary coolant pH is allowed to fall into the acid regime. The specimens were analysed by surface and electrochemical techniques, either directly following exposure in the acid medium steam generator coolant or after adjusting the coolant pH and chemistry to near-normal steam generator conditions. The initial acid exposure resulted in the growth of a chromium-rich surface corrosion product film on all alloys (except M-400) and the precipitation of nickel-rich sulphates. Following the return to a higher pH, the alloy again had chromium-rich surface oxides but also exhibited sulphide crystallites adhering to the base oxide, particularly for I-600. The tendency to retain these sulphides is attributed to the porosity of the protective oxide through which nickel is transported to the solution. The conversion of sulphate-sulphide is believed to occur as the pH is raised to normal alkaline conditions. In the case of M-400, a chromium oxide layer is not available to restrict the transport of nickel to the solution. As a result, a thick layer of sulphide crystals grows on the surface of the M-400 alloy even when subjected to a mild acid pH excursion. Even trace concentrations of sulphide/sulphate under normal pH control are shown to react with alloys whose oxide films appear permeable to nickel transport.

The use of XPS, FTIR, SEM/EDX, contact angle, and AFM in the characterization of coatings

Coatings are applied to surfaces for a variety of reasons: to enhance their appearance, to protect the substrate, to augment the adhesion to other layers, or to functionalize them for further reactions. To evaluate the efficacy of the coating, it is often necessary to analyze the substrate and the coating to ensure that the needed characteristics are present. To this end, the use of x-ray photoelectron spectroscopy (XPS), contact angle, and atomic force microscopy (AFM) can provide information about the surface composition, its morphology, and its ability to be wetted with various solvents. Scanning electron microscopy with energy dispersive x-ray analysis (SEM/EDX) and Fourier transform infrared spectroscopy (FTIR) can provide a clear picture of the near surface components as well as the continuity of coatings. All of these aspects are valuable in evaluating a coating and essential when problems are encountered. The application of these techniques to the analysis of coatings is discussed.

Quantitative XPS Measurements of Some Oxides, Sulphides and Complex Minerals

A number of binary oxides, sulphide compounds and complex minerals have been analysed by x-ray photoelectron spectroscopy to give the relative intensities of major photoelectron lines. Where possible, compound surfaces were prepared by fracturing in vacuo or in inert gas to avoid contamination. Alternatively, some of these compounds have undergone a mild thermal treatment in vacuo to remove much of the surface contamination which normally affects such intensity measurements. The experimental line intensity ratios were corrected only by their particular photoelectron cross-section and inelastic mean free paths. For most oxides and complex minerals studied, good agreement is obtained with the expected stoichiometry, particularly when using photoelectron lines of higher kinetic energy. This tends to support the use of a simple background approximation when measuring specifically with those oxides involving non-transition elements. The sulphides studied mostly involved transition metals ; agreement between calculated and actual stoichiometry was only modest. Some differences in measured surface stoichiometry occurred as a result of the ambient conditions within which the fracture was made ; fracture in vacuo produced a different S/M ratio than did fracture in an inert gas environment.

Analysis of ZrNb fuel channel surfaces for hydrogen and other elements using Secondary Ion Mass Spectrometry (SIMS)

Abstract Understanding the mechanisms of hydrogen ingress into CANDU pressure tubes fabricated from zirconium-2.5 wt% niobium alloy requires knowledge of the hydrogen concentration in the surface oxide, the oxide/metal interface and in the alloy phase beneath the interface. Secondary Ion Mass Spectrometry (SIMS) has been shown to have attractive capabilities for detecting hydrogen isotopes in such surface films. The depth concentration profiles of deuterium, lithium, fluorine and iron have been measured on specimens taken from several locations on two pressure tubes from Unit 3 of the Pickering Nuclear Generating Station. These tubes were known to have widely differing deuterium concentrations in their bulk phases. SIMS profiles were obtained from both the outside (OD) and inside (ID) tube surfaces. The OD studies show that for regions where bulk deuterium pickup is small, the deuterium concentration within the surface oxide decreases steadily with depth and is 50–100 times lower near the oxide/alloy interface. For regions of high pickup, the profiles showed only a small concentration gradient of deuterium across the oxide thickness. This infers the importance of the surface oxide in suppressing hydrogen ingress. The ID studies also show that regions with high deuterium pickup have anomalous deuterium depth profile structure near the oxide metal interface. This suggests that the integrity of the ID oxide film may also bear some responsibility for enhanced deuterium ingress at these locations. Variations in iron, lithium or fluorine concentrations within the ID or OD oxide scales do not appear to have a significant effect on the deuterium profiles measured.

Deuterium depth distribution investigations in Zr and ZrO2

Abstract We have used the broad resonance at E( 3 He ) ≈ 0.63 MeV in the D + 3 He → p + 4 He nuclear reaction to probe the depth distribution of deuterium implanted at 50 keV energy into ZrO 2 and at 40 keV into ZrNb 2.5% specimens. Nuclear reaction data of two kinds, viz. the excitation functions and the proton energy spectra, have been used to extract the deuterium profiles. These data have been compared with secondary ion mass spectrometry (SIMS) measurements. The nuclear reaction analyses (NRA) reveal the basic features of the distributions - preservation of the primary implant features in the ZrO 2 specimen and redistribution including in-diffusion in the metal specimens. A summary is given of the procedures used to extract D profiles from the nuclear reaction measurements.

Id cracking in uns N06600 nuclear steam generator tubes: evidence for SCC from secondary ion mass spectrometric images

Abstract SIMS images from a cross-section of a partial through-wall ID-initiated intergranular crack have shown clear evidence of separated anodic⧹cathodic processes. At the crack tip, localised deposits of sulphur, chlorine and oxygen from the coolant are detected. Electropositive elements, such as copper and zinc, from the coolant, only migrate to a region about 5 μm back from the tip. This behaviour is attributed to anodic and cathodic regions whose reactions are necessary for crack propagation.

Imaging fibers by atomic force microscopy

Glass textile fibers have been used to demonstrate an atomic force microscope technique presented in this article. This method can be applied for imaging the surface of any micron or larger sized fibers. The fibers are discovered to have a variable corrugated surface texture, rather than the previously supposed uniformly cylindrical form, with individual ridges about 5–10 nm high. Scanning electron microscopy images of equivalent glass fibers are included for comparison.

Corrosion Product Analysis of Aluminum Closure Panels

Painted aluminum panels subjected to several laboratory-based accelerated corrosion tests were examined using surface analytical techniques. This paper presents some of the results from these measurements, which indicated that the nature and the extent of the corrosion attack were greatly influenced by the salt spray conditions. In general, ASTM G 85 acetic acid salt spray produced the greatest amount of corrosion, while exposure to GM 9540P and ASTM B-117 resulted in the least amount of corrosion. Moreover, filiform corrosion was the most common corrosion attack observed from exposures to many of the salt spray tests. Finally, filiform corrosion appeared to propagate by the dissolution of the phosphate conversion coating, thus lifting off the paint layer from the aluminum substrate.