Papers by Anatolie Carcea
The anodic behaviour of a range of nickel-base materials including pure Ni, Alloy 600, and Ni-21C... more The anodic behaviour of a range of nickel-base materials including pure Ni, Alloy 600, and Ni-21Cr has been examined at 90 o C using strongly and mildly acidic solutions. Lead oxide was injected at potentials above the Pb ++ /Pb equilibrium so that no bulk Pb deposition was possible. Underpotential deposition (UPD) of Pb on Ni was inferred from the strong inhibiting effect of PbO addition in the true active dissolution region of pure Ni in strong acid. In the prepassive region, where a Ni(OH) 2 type of film is thought to be present, competition of Pb and OH for surface sites led to a strong activation of dissolution (delay of true passivation) in the mildly acidic environment. In the passive region, some impairment of the long-term passive current decay was observed. Cr alloying generally suppressed all three effects of lead, but the Cr content of Alloy 600 was insufficient to do this reliably, and evidence of nickel-like behaviour was present for this alloy.
The recent interest in stress corrosion cracking (SCC) of carbon steel by fuel ethanol has prompt... more The recent interest in stress corrosion cracking (SCC) of carbon steel by fuel ethanol has prompted us to reexamine the mechanism of such cracking using methanol, which causes more rapid SCC and presumably does not induce a different mechanism. An absolutely key issue in such a study is the mechanism of transgranular (TG) SCC. Intergranular (IG) SCC could occur by a variety of anodic SCC mechanisms, but we are convinced that TGSCC can only occur by a locally brittle mechanism. Thus our work has focused on conditions that promote TGSCC. Then, by examining the mechanical response of the cracking in a slow strain rate test, one can test for a role of hydrogen embrittlement (at first sight this appears to be ruled out anyway, in view of the necessity of a relatively positive electrode potential, but one can always speculate on the existence of large IR drops down the cracks). If the cracking is due to hydrogen, there is invariably a large plastic strain before cracks initiate, to the po...
A mechanistic study was done on Alloy 600 exposed to a hydrogenated steam environment considered ... more A mechanistic study was done on Alloy 600 exposed to a hydrogenated steam environment considered to simulate primary water conditions of PWR and CANDU reactors. Alloy 600, a Ni-Fe-Cr alloy, is used in the primary circuit of nuclear reactors and has been found susceptible to primary water stress corrosion cracking (PWSCC), even in highly reducing conditions. Initial surface analysis on exposed Alloy 600 specimens indicated the presence of pure nickel nodules on the surface, suggesting the likelihood of internal oxidation of Cr, with Ni being expelled owing to the resulting high internal stress. Diffusion occurs preferentially along grain boundaries due to grain boundary diffusion being dominant at low temperatures, rather than lattice diffusion. A cross-section of the exposed sample was prepared using a focused ion beam which revealed an intergranular micro-crack. Elemental mapping was done on the cross-section which confirmed enrichment of chromium and oxygen and depletion of nickel...
Corrosion Science, 2015
ABSTRACT Electrochemical measurements were made on Ni, Alloy 600, and Alloy 800, immersed in acid... more ABSTRACT Electrochemical measurements were made on Ni, Alloy 600, and Alloy 800, immersed in acid sulfate solutions at 315 °C. For pure Ni, adding sulfate in relatively small concentrations to solutions containing only chloride ions resulted in significantly increased current density in the anodic range and a sharp drop in polarization resistance, suggesting sulfate as the aggressive anion in mixed sulfate and chloride systems. Electrochemical measurements were taken on Alloys 600 and 800 in solutions of varying sulfate concentrations and differences in dissolution kinetics and passivation tendency are reported. Electrochemical reduction of sulfate is likely occurring, accelerating the metal dissolution rate.
ECS Transactions, 2013
ABSTRACT Alloy 600 and Alloy 800 are Ni-Fe-Cr alloys used in nuclear steam generators. Sulfate is... more ABSTRACT Alloy 600 and Alloy 800 are Ni-Fe-Cr alloys used in nuclear steam generators. Sulfate is an impurity found in faulted secondary side water that can accumulate in crevices in the acidic-to-neutral pH range, possibly resulting in acid sulfate stress corrosion cracking. Electrochemical measurements were done on Alloys 600 and 800 exposed to acid sulfate solutions at 315 °C and 120 bar. Results indicate that the addition of sulfate significantly increased the corrosion rate of both alloys - possibly due to reduction of sulfate resulting in impairment of surface passivity by reduced sulfur species. The addition of Fe2+ in solution reduced the corrosion rate due to the formation of a magnetite film. The corrosion rate and anodic current density of the 800 alloy were greater than those of the 600 alloy at high sulfate concentrations but became comparable at lower concentrations, or with the addition of Fe2+. SCC experiments were done and were in broad agreement with electrochemical measurements.
Microscopic distributions of elastic and plastic strains have been studied in Alloy 600 during ac... more Microscopic distributions of elastic and plastic strains have been studied in Alloy 600 during accelerated corrosion. Laue diffraction is employed with a sub-micron beam of highly coherent polychromatic (white) radiation. The diffraction patterns are analysed to detect elastic and plastic deformations associated with the crack initiation and propagation processes. Stressed C ring and unstressed mill annealed samples of Alloy 600 were corroded under hydrothermal controlled electrochemical conditions. In the C ring, the filamentous surface cracks produced had compressive strain fields along each crack. In the mill annealed sample, changes in strain fields in the same area were measured as oxidation progressed. Cracking at the metal grain boundaries appeared to be induced by expansive growth of surface oxides. For the mill annealed sample, accumulation of elastic strains in the grain boundaries appeared in advance of crack propagation.
Faraday discussions, Jan 21, 2015
The pitting of nuclear steam generator tubing alloys 600, 690 and 800 was studied at 60 °C using ... more The pitting of nuclear steam generator tubing alloys 600, 690 and 800 was studied at 60 °C using dilute thiosulfate solutions containing excess sulfate or (for Alloy 600) chloride. A potentiostatic scratch method was used. In sulfate solutions, all alloys pitted at low potentials, reflecting their lack of protective Mo. The alloys demonstrated the most severe pitting at a sulfate : thiosulfate concentration ratio of ∼40. Alloy 600 pitted worst at a chloride : thiosulfate ratio of ∼2000. The results are interpreted through the mutual electromigration of differently charged anions into a pit nucleus, and differences in the major alloy component.
Laue diffraction measurements made using polychromatic synchrotron x-radiation have revealed sign... more Laue diffraction measurements made using polychromatic synchrotron x-radiation have revealed significant changes to the microscopic strain distributions in an Alloy 600 (UNS N06600) tube following the application of a circumferential (hoop) stress and then as a result of stress corrosion cracking (SCC). The changes in the elastic and plastic strain distributions were studied for the same area near the outer diameter of the tube prepared as a C-ring. Following the application of the circumferential stress, there was a notable loss of crystal order within the outermost 3 µm to 5 µm of the tube surface as a result of plastic processes; further into the sample the elastic strains in grain boundaries become oriented toward the direction of the impressed stress. The average intensity of oriented tensile strain is 5×10 -3 within the outermost 50 µm of the C-ring. After corrosion, the elastic strain distribution in the same region becomes slightly compressive. The strains associated with a network of fine incipient cracks on the outside diameter of the C-ring were explored; elastic compressive strains were found within few micrometers from the crack center presumably as a result of crack opening, while plastic deformation was limited to the immediate crack region.
Corrosion, 2015
Microscopic distributions of elastic and plastic strains have been studied at three intervals dur... more Microscopic distributions of elastic and plastic strains have been studied at three intervals during an electrochemically-accelerated corrosion test of a flat polished surface of a millannealed and unstrained sample of Alloy 600 in 10% NaOH at 315 °C. Laue microdiffraction was employed using a micron size polychromatic (white) X-ray beam to map strain changes in the sample as a function of corrosion time. After two 12 hour corrosion periods, tensile elastic strains were detected along a grain boundary. After a third 24 hour corrosion period, this particular boundary was found to have cracked to a depth of several microns; other adjacent boundaries exhibited tensile elastic strain, but no cracking. No significant changes to plastic strain distributions in the same area could be detected as a function of corrosion exposure.
Busby/Environmental Degradation, 2012
Micron, 2014
Alloy 800 (Fe-21Cr-33Ni) has been found susceptible to cracking in acid sulfate environments, but... more Alloy 800 (Fe-21Cr-33Ni) has been found susceptible to cracking in acid sulfate environments, but the mechanism is not well understood. Alloy 800 C-ring samples were exposed to an acid sulfate environment at 315°C and cracks were found with depths in excess of 300μm after 60h. Preparation of a TEM sample containing crack tips is challenging, but the ability to perform high-resolution microscopy at the crack tip would lend insight to the mechanism of acid sulfate stress corrosion cracking (AcSCC). The lift-out technique combined with a focused ion beam sample preparation was used to extract a crack tip along the cross-section of an acid sulfate crack in an Alloy 800 C-ring. TEM elemental analysis was done using EDS and EELS which identified a duplex oxide within the crack; an inner oxide consisting of a thin 3-4nm Cr-rich oxide and an outer oxide enriched in Fe and Cr. Preliminary conclusions and hypotheses resulted with respect to the mechanism of AcSCC in Alloy 800.
Journal of The Electrochemical Society, 2011
ABSTRACT Artificial pit electrodes have been used to measure the pitting dissolution kinetics in ... more ABSTRACT Artificial pit electrodes have been used to measure the pitting dissolution kinetics in the anodic limiting current region for a range of Ni-22Cr-xMo (wt %) alloys with x = 0, 3, 6, 9 and 13. Most of the data were obtained at 25–90 °C using 1 M NaCl solution. Type 316 stainless steel and pure nickel were used for comparison. At high temperatures the anodic limiting current density for the NiCr(Mo) alloys (x = 0, 3, 6, 9) is nearly the same as that of stainless steel, other things being equal; at intermediate temperatures the NiCr(Mo) alloys give higher i lim values. Changing the NaCl concentration between 0.1 and 1 M causes a 20% change in i lim at a given pit depth, and the reasons for this are discussed. Comparison of pure Ni and Ni-22Cr shows a 35–40% higher i lim for the latter at a given pit depth, and the reasons for this are discussed. Literature values of solubility and diffusivity, and a 1-D finite difference model, are used to rationalize – in part – the limiting current densities and associated average diffusivities in terms of concentration-dependent diffusivities.
Corrosion, 2014
Laue diffraction measurements made using polychromatic synchrotron x-radiation have revealed sign... more Laue diffraction measurements made using polychromatic synchrotron x-radiation have revealed significant changes to the microscopic strain distributions in an Alloy 600 (UNS N06600) tube following the application of a circumferential (hoop) stress and then as a result of stress corrosion cracking (SCC). The changes in the elastic and plastic strain distributions were studied for the same area near the outer diameter of the tube prepared as a C-ring. Following the application of the circumferential stress, there was a notable loss of crystal order within the outermost 3 µm to 5 µm of the tube surface as a result of plastic processes; further into the sample the elastic strains in grain boundaries become oriented toward the direction of the impressed stress. The average intensity of oriented tensile strain is 5×10 -3 within the outermost 50 µm of the C-ring. After corrosion, the elastic strain distribution in the same region becomes slightly compressive. The strains associated with a network of fine incipient cracks on the outside diameter of the C-ring were explored; elastic compressive strains were found within few micrometers from the crack center presumably as a result of crack opening, while plastic deformation was limited to the immediate crack region.
Corrosion Science, 2005
The deleterious eVect of dissolved Pb ++ on passivity of Ni and Ni-21Cr has been explored at 90°C... more The deleterious eVect of dissolved Pb ++ on passivity of Ni and Ni-21Cr has been explored at 90°C using two solutions: 0.1 M perchloric acid, and a pH 3.5 acetic acid/sodium acetate buVer. Pb ++ strongly inhibits true (low-potential) active dissolution of pure Ni; this is consistent with underpotential deposition (UPD) of elemental Pb, apparently a new observation. The range of potentials where this occurs is greater for the more acidic solution. By inoculating Pb ++ at diVerent potentials, the maximum underpotential can be shown to be at least 300 mV. In the less-aggressive acetic acid environment, a 'pre-passive' state of Ni is more noticeable than in the perchloric acid, and in contrast to the true active region, this region is subject to a strong activating eVect of Pb. Dissolution of Ni proceeds rapidly on a surface that is trying to form its pre-passive Wlm but is partly covered by Pb. The time-dependence of the passive current in the acetic solution is altered subtly by incorporation of Pb into the metal-Wlm interface, such that the normal decay with time is truncated, leading to a higher long-term dissolution or Wlm thickening rate. The presence of 21% Cr suppresses active dissolution, greatly ennobles the open-circuit potential, and eliminates (for these particular conditions) both the inhibiting and the activating eVects of Pb. The relevance of these results to corrosion and SCC caused by lead in nuclear power systems is brieXy discussed.
Corrosion Science, 2007
De-alloying of a 70Cu-30Pt alloy has been studied with a view to testing the thermodynamic predic... more De-alloying of a 70Cu-30Pt alloy has been studied with a view to testing the thermodynamic prediction that a novel kind of underpotential or sub-potential electrodeposition may be possible in such highly curved nanoporous solids. After de-alloying in H 2 SO 4 solution containing CuSO 4 , successive cathodic processes of PtO monolayer reduction, UPD of Cu, and a Cu plating process were identi-Wed, all occurring at potentials above the Cu equilibrium potential. The maximum amount of charge in the third plating process was determined to be about three monolayers, averaged over the whole porous surface. This seems to rule out the possibility that this process is some kind of second UPD layer or other non-bulk form of Cu plating. A probable explanation is that small amounts of bulk Cu are plating into regions of high negative curvature within the de-alloyed material. However X-ray diVraction did not show any evidence of bulk Cu.
Acta Materialia, 2012
The microscopic strain distributions were studied for stress corrosion cracks produced electroche... more The microscopic strain distributions were studied for stress corrosion cracks produced electrochemically in C-rings of Alloy 600 (0.65 Ni, 0.16 Cr, 0.08 Fe). The strain data were obtained using polychromatic X-ray microdiffraction (PXM) and (in part) by electron backscatter diffraction (EBSD). PXM was used to measure plastic and elastic strain distributions around the tip of a short crack, along with the changes to the direction and shape of the diffraction spots (ellipticity). For a sample with a short (30 lm) crack, the misorientation map showed a well-defined region of plastic deformation along the grain boundary in advance of the crack tip, extending to the next triple point. For the large crack sample, plastic and elastic stains as well as crystalline order could be measured in high detail with respect to the crack path. However, no correlation between these could be obtained, except for a notable degradation of crystalline order near the crack mouth. A comparable EBSD misorientation map shows strong correlation between misorientation and the crack edges; this may in part reflect the role of sharp edges in the more surface-sensitive approach.
Acta Materialia, 2012
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Papers by Anatolie Carcea