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Cordoba-Torres P.,Spanish University for Distance Education (UNED) | Mesquita T.J.,Joseph Fourier University | Mesquita T.J.,Ugitech Research Center | Nogueira R.P.,Joseph Fourier University
Electrochimica Acta | Year: 2013

Complex nonlinear-least-squares fitting of impedance data to an equivalent circuit is probably the most intuitive method used to represent the whole electrode impedance from experimental electrochemical impedance spectroscopy data. However, among other questions it has the primary problem of identifying a physically significant model representing the system under examination. In that context it then becomes very suitable the study of other analysis procedures to complement this approach. With regard to constant-phase element (CPE) behavior characterization, direct impedance analysis seems to be a good choice since CPE behavior is explicitly revealed in the high frequency decay of the imaginary component of the impedance. For that reason it is becoming popular among electrochemists who can get the CPE exponent from a simple fit of the imaginary part of the impedance to a frequency power law. There are, however, important limitations to the frequency range employed in this analysis that are commonly ignored. In the case of disk electrodes, geometrical constraints related to the insulating-metal boundary induce current and potential distributions that screen the real underlying CPE behavior in the high frequency domain. On the other hand, for lower frequencies, CPE behavior can also be masked by the effects of diffusion. In this paper we present some methodological considerations taking these effects into account in order to get a more reliable characterization of the CPE behavior from direct impedance analysis. We illustrate these issues with different experimental conditions and show that in many cases deviations can be remedied thanks to a convenient theoretical treatment that allows correct values of CPE parameters to be retrieved. © 2013 Elsevier Ltd.

Mesquita T.J.,Ugitech Research Center | Chauveau E.,Ugitech Research Center | Mantel M.,Ugitech Research Center | Kinsman N.,IMOA International Molybdenum Association | And 7 more authors.
7th European Stainless Steel Conference: Science and Market, Proceedings | Year: 2011

The use of stainless steels (SS) as concrete reinforcement is becoming increasingly popular in coastal and marine constructions in order to prevent corrosion induced by chloride ions penetrating into the concrete. However, the influence of Mo addition on pitting corrosion resistance of these steels is not clearly understood in alkaline chloride conditions even being well stablished the benefictial effect of this element in acidic and neutral environments. Therefore, the understanding of the Mo role on the corrosion resistance of SS in alkaline media is hence of major importance for the setting of optimized alloy composition. The present work aims at studying the effect of Mo addition on pitting corrosion properties of three different stainless steel families (austenitic, ferritic and duplex SS) in alkaline environments (solution simulating a chloride-contaminated-concrete at two alkaline pH values: 10, to simulate carbonated concrete, and 12, to simulate sound concrete). For comparison, an acid (pH=4) and neutral (pH=7) solution were also included in this study and a real cement medium). In order to establish the real role of Mo addition on pitting and passivation properties of the three studied families of SS, the alloy samples were prepared in laboratory under highly controlled conditions. The obtained results were discussed with respect to the influence of Mo addition on pitting potential for industrial alloys (AISI 304, ISI 316, AISI 430, AISI 434, SAF 2304 and SAF 2205) .

Mesquita T.J.,Total S.A. | Chauveau E.,Ugitech Research Center | Monnot M.,Ugitech Research Center | Bouvier N.,Ugitech Research Center
NACE - International Corrosion Conference Series | Year: 2015

The good mechanical properties as well as the large range of corrosion resistance justify the use of super martensitics in oil and gas production fields. Nevertheless with UNS 17400, which is largely used, some in-service failures have occurred. In this context, the super martensitic stainless steel 15Cr5Ni0.8Mo, which has good mechanical and corrosion properties, could be a good candidate to replace it. In this work, different alloying elements are studied with laboratory heats to optimize its chemical composition for oil & gas application. The control of microstructure eliminates residual ferrite and retained austenite as parameters in corrosion behavior. The positive influence of buffer test solution is described and the reinforcement of passive film in non sour medium is explained. Then the influence of H2S on passive film and corrosion mechanism is highlighted, in particular when the gas destabilizes a passive film allowing generalized corrosion. Influence of alloying elements is related to the corrosion mechanism: a positive effect of chromium, molybdenum and copper is observed on corrosion resistance in sulfur medium. The effect of molybdenum on SSC resistance is described. © 2015 by Nace International.

Mesquita T.J.,Ugitech Research Center | Chauveau E.,Ugitech Research Center | Mantel M.,Ugitech Research Center | Nogueira R.P.,Joseph Fourier University
Applied Surface Science | Year: 2013

The objective of this work is to study the effect of Mo additions on film passive properties of three different stainless steels (SS) types (austenitic, ferritic and duplex alloys). A comparison between Mo containing (3 wt% Mo) and free Mo (0 wt% Mo) grades of highly controlled laboratory heats was done considering their passive film formed in different aggressive conditions, from neutral to alkaline pH. The presence of oxidized Mo on the passive layer was confirmed by X-ray photoelectron Spectroscopy (XPS). The presence of Mo within the passive film improved the passivity breakdown potential for the duplex and ferritic SS, but seemed to have no effect for austenitic SS.

Mesquita T.J.,Ugitech Research Center | Chauveau E.,Ugitech Research Center | Mantel M.,Ugitech Research Center | Mantel M.,Grenoble Institute of Technology | And 2 more authors.
Corrosion Science | Year: 2014

The corrosion properties of two supermartensitic stainless steels were studied in chloride and H2S environments. A comparison between 1.4542 and 1.4418 SS grades was made considering the effects of microstructure, different steps of the heat treatment on the pitting and sulfide stress cracking SSC properties. The SSC limits of 1.4418 were determined in terms of different pH and PH2S conditions. In H2S media, the 1.4418 presented a higher stability and passivation compared to 1.4542. The Electron back-scattering and X-ray diffraction were used to characterize the amount of' retained austenite in the 1.4418 and then its influence on pitting and SSC resistance was investigated. © 2014 Elsevier Ltd.

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