Martín A. Rodríguez

Properties of the Passive Film on Alloy 22 in Chloride Solutions Obtained by Electrochemical Impedance

Impedance measurements were performed for alloy 22 in the passive and transpassive range, in 1 M NaCl at 90°C. A R Ω -(R//CPE) circuit model was applied in the full passive range, where R was the film resistance. This model also applied for a wide range of chloride concentrations and pH values, at the open circuit potential. Two time constants were observed at the beginning and at the end of the passive range. In these cases, the resistances for the ion transfer might be located at the film interfaces, and not in the film itself. The protective properties of the film improved with polarization time due to the thickening and ageing of the film. The film resistance and the space charge layer thickness increased with the potential. The oxidation of Cr to Cr 6+ occurred in the film at high potentials, followed by the transpassive dissolution. In the pre-transpassive range of potentials, the film showed a p-type electronic character, while the ionic properties were that of a passive film. The passive film of alloy 22 was an n-type semiconductor, which changed to a p-type for the high passive potentials. N D = 2.7 x 10 20 cm ―3 and E FB = ―0.551 V SCE were determined.

Passivation and Depassivation of Alloy 22 in Acidic Chloride Solutions

Alloy 22 (N06022) is a Ni―Cr―Mo alloy that offers an outstanding corrosion resistance in a wide variety of highly corrosive environments. In the present work, the general corrosion of alloy 22 in the passive and active states and the transition from passive and active states were studied in acidic chloride solutions at 90°C. Electrochemical studies, including electrochemical impedance spectroscopy and polarization tests, were performed using mill-annealed and thermally aged (10 h at 760°C) alloy 22 specimens. The depassivation pH (pH D ) was 1.7 in deaerated conditions and pH D = 0.3 in aerated conditions. The transition from passive to active states was characterized by a 3 orders of magnitude increase in the corrosion rate (CR) and a significant increase in the interfacial capacity. The CRs obtained via electrochemical tests for mill-annealed (MA) and thermally aged alloy 22 were comparable in all the tested conditions used in the present work. Intergranular attack was observed in thermally aged alloy 22 corroding in the active state due to the presence of precipitates and adjacent depleted zones of the protective alloying elements.

Pharmacological evidence for the existance of different subtypes of muscarinic acetylcholine receptors for phosphoinositide hydrolysis in neonatal versus adult rat atria

The negative inotropic effect of carbachol, as well as phosphoinositide hydrolysis, was measured in atria from neonatal and adult rats. Carbachol increased phosphoinositide hydrolysis and decreased dF/dt of both neonatal and adult atria; however, the neonatal atria showed hypereactivity to carbachol as compared with adult atria. Inhibition of phospholipase C reduced the supersensitivity to carbachol upon contractility in neonatal atria producing values similar to those of the adult atria, suggesting that muscarinic acetylcholine receptor (mAchR) stimulation is secondary to receptor-mediated hydrolysis of phosphoinositides. Pharmacological analysis with mAchR antagonists tends to support the idea that m1 and m2 subtypes are the most important mediators of the response to carbachol in neonatal atria. In adult atria, the effect of carbachol is coupled only to mAchR m2 subtypes.

Oxide Film Aging on Alloy 22 in Halide Containing Solutions

Passive and corrosion behaviors of Alloy 22 in chloride and fluoride containing solutions, changing the heat treatment of the alloy, the halide concentration and the pH of the solutions at 90 deg. C, was investigated. The study was implemented using electrochemical techniques, which included open circuit potential monitoring over time, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements carried out at open circuit and at passivity potentials. Corrosion rates obtained by EIS measurements after 24 h immersion in naturally aerated solutions were below 0.5 {mu}m/year. The corrosion rates were practically independent of solution pH, alloy heat treatment and halide ion nature and concentration. EIS low frequency resistance values increased with applied potential in the passive domain and with polarization time in pH 6 - 1 M NaCl at 90 deg. C. This effect was attributed to an increase in the oxide film thickness and oxide film aging. High frequency capacitance measurements indicated that passive oxide on Alloy 22 presented a double n-type/p-type semiconductor behavior in the passive potential range. (authors)

Cholinoceptor activation subserving the effects of interferon gamma on the contractility of rat ileum.

Recombinant rat interferon γ stimulated the contractility of isolated rat ileum at doses of 4–12 units/ml. Muscarinic cholinoceptors were involved, as treatment of the tissue with atropine prevented the contractile response of the ileum. Furthermore, interferon γ increased the affinity of carbachol for the cholinoceptors and did not change its maximum effect. Neurogenic pathways were also involved since pretreatment of ileum with hexamethonium, hemicholinium or tetrodotoxin impaired the contractile effect of interferon γ. In contrast to the action of exogenous carbachol, the effects of interferon γ are indirect. They appear to involve a G protein regulating phosphoinositide turnover and cytoskeletal structures since they could not be induced in ileum strips that were pretreated with pertussis toxin, phospholipase C inhibitors (2-nitro-carboxyphenyl, NN-diphenyl carbamate and neomycin), cytochalasine B or colchicine.

Determination of Crevice Corrosion Susceptibility of Alloy 22 Using Different Electrochemical Techniques

Alloy 22 belongs to the Ni-Cr-Mo family and it is highly resistant to general and localized corrosion. It may suffer crevice corrosion in aggressive environmental conditions. This alloy has been considered as a corrosion-resistant barrier for high-level nuclear waste containers. It is assumed that localized corrosion may occurs when the corrosion potential (ECORR) is equal or higher than the crevice corrosion repassivation potential (ER,CREV). The latter is measured by means of different electrochemical techniques using artificially creviced specimens. These techniques include cyclic potentiodynamic polarization (CPP) curves, Tsujikawa-Hisamatsu electrochemical (THE) method or other non-standard methods, such as the PD-GS-PD technique. The aim of the present work was to determine reliable critical or protection potentials for crevice corrosion of Alloy 22 in pure chloride solutions at 90°C. Conservative methodologies (which include extended potentiostatic steps) were applied for determining protection potentials below which crevice corrosion cannot initiate and propagate. Results from PD-GS-PD technique were compared with those from these methodologies in order to assess their reliability. Results from the CPP and the THE methods were also considered for comparison. The repassivation potential resulting from the PD-GS-PD technique was conservative and reproducible, and it did not depend on the amount of previous crevice corrosion propagation.

Effect of Weak Acid Additions on the General and Localized Corrosion Susceptibility of Alloy 22 in Chloride Solutions

Electrochemical studies such as cyclic potentiodynamic polarization (CPP) and electrochemical impedance spectroscopy (EIS) were performed to determine the corrosion behavior of Alloy 22 (N06022) in 1M NaCl solutions at various pH values from acidic to neutral at 90 C. All the tested material was wrought Mill Annealed (MA). Tests were also performed in NaCl solutions containing weak organic acids such as oxalic, acetic, citric and picric. Results show that the corrosion rate of Alloy 22 was significantly higher in solutions containing oxalic acid than in solutions of pure NaCl at the same pH. Citric and picric acids showed a slightly higher corrosion rate, and acetic acid maintained the corrosion rate of pure chloride solutions at the same pH. Organic acids revealed to be weak inhibitors for crevice corrosion. Higher concentration ratios, compared to nitrate ions, were needed to completely inhibit crevice corrosion in chloride solutions. Results are discussed considering acid dissociation constants, buffer capacity and complex formation constants of the different weak acids.

Optimization of the Double Loop Electrochemical Potentiokinetic Reactivation Method for Detecting Sensitization of Nickel Alloy 690

Nickel alloy 690 (UNS N06690) is one of the current choices for nuclear power plant steam generator tubing. The severity of certain stress corrosion cracking submodes in Ni-Cr-Fe alloys, such as Al...

Crevice corrosion of nickel-based alloys considered as engineering barriers of geological repositories

Nickel-based alloys are considered among other candidate materials as engineering barriers of geological repositories due to their excellent corrosion resistance. These alloys possess unique advantages: they may be used in saturated and unsaturated repositories, hosted by practically any rock type, while also compatible with any (or no) backfill, and have minimal impact in other barriers. Alloy-22 (UNS N06022) has been the most studied of this class of alloys for its potential application in the proposed repositories (namely Yucca Mountain, USA). Crevice corrosion is however an important and often unintended degradation process that may limit the waste container lifetime if a nickel-based alloy is selected. Alloy susceptibility to crevice corrosion is influenced by environmental and metallurgical variables. This review gives an account of the current knowledge regarding crevice corrosion of nickel-based alloys as candidate materials for the corrosion-resistant layer of high-level nuclear waste containers. Although there is a significant amount of research supporting the use of nickel-based alloys for this application, the effect of the different variables on crevice corrosion resistance is described. Special focus is given to the current criterion for crevice corrosion occurrence in repository environments, recent works and criticisms. The presently established criterion appears robust for ruling out crevice corrosion in saturated repositories; however, the development of a less conservative criterion for crevice corrosion occurrence is necessary to use these alloys in unsaturated repositories.