Comportement vis-à-vis de la corrosion électrochimique d'un revêtement duplex, utilisé pour protéger les canalisations enterrées en fonte (élaboration d'un modèle physique)

La recherche de tests permettant de qualifier des revêtements protecteurs de canalisations enterrées a été menée sur deux types de dépôt métallique recouverts d'une couche de finition organique : zinc+peinture bitumineuse, zinc-aluminium (85-15 % en masse)+peinture époxy. Au cours de ce travail, la grande complexité de structure et de répartition des phases, au sein des matériaux, a été mise en évidence. Celle-ci résultant du procédé de fabrication (projection à l'arc électrique) engendre aussi une complexité dans l'interprétation des résultats électrochimiques obtenus. L'environnement sol a été réduit à une solution simple mais agressive (NaCl 6 g.L 1). Quant aux incidents pouvant se produire lors de la mise en place des canalisations, ils ont été simulés par une blessure mettant à nue la fonte constituant les tuyaux et leurs raccords. Un modèle physico-chimique, exprimant les réactions électrochimiques du système duplex, a pu être établi à l'aide de la spectroscopie d'impédance électrochimique, en s'appuyant sur la théorie de l'électrode poreuse. L'effet cicatrisant des produits de corrosion du dépôt métallique a été évalué. Il s'avère que les mécanismes de corrosion sont plus rapides pour les revêtements à base de Zn+peinture bitumineuse que pour les revêtements Zn-Al+peinture époxy ; celui-ci est, à long terme, plus efficace dans le milieu choisi.The research of tests permitting to qualify protective coatings applied on buried pipelines was performed on two kinds of metallic deposit covered by a finishing organic coating : zinc + bituminous paint, zinc-aluminium (85-15% in weight) + epoxy paint. In this work, an important structure complexity of phase distributions was revealed inside of the investigated materials. This one results from the industrial process (projection by electric bow) and creates also a complexity of the interpretation of the electrochemical results. The soil environment was represented by a simple but aggresive solution (NaCl 6 g.L-1). The incident defects, resulting from the set up of the pipelines were simulated by a notch by which means the cast iron substrate was achieved. A physico-chemical model, expressing the electrochemical reactions of the duplex system, was established in terms of the porous electrode theory and with help of electrochemical impedance spectroscopy. A healing effect of the corrosion products of the metallic deposit was evaluated. It proves that the corrosion mechanisms are faster for the Zn+bituminous paint coating than the Zn+Al+epoxy paint coating. Consequently, Zn-Al+epoxy paint coating is a more efficient corrosion protection for longer immersion time in the chosen environment.CHATENAY MALABRY-Ecole centrale (920192301) / SudocSudocFranceF

Effets combinés du frottement et de la corrosion, dans le cas d’un alliage fer-nickel en milieu sulfurique

En étudiant le comportement de l’alliage Fe-30 % Ni en milieu sulfurique sous frottement, nous avons montré que plusieurs processus interviennent simultanément (synergie) pour modifier la cinétique de l’usure corrosive, parfois de manière antagoniste :- la destruction locale des couches superficielles qui se forment au contact du milieu corrosif (adsorbats, couches de produits de corrosion, ou film passif qui protège l’alliage de la corrosion) et qui tend à accélérer la dissolution de l’alliage ;- un écrouissage s’accompagnant d’une transformation martensitique, qui tend à diminuer la réactivité électrochimique de la surface dans le domaine actif.Parallèlement, la formation de certaines couches superficielles par les phénomènes électrochimiques modifie les conditions de frottement et réduit l’importance des endommagements de la surface et par conséquent la vitesse d’usure

Kinetics of austenite to martensite transformations in iron-nickel-carbone alloys during their cooling or tensile testing

Vol. 5, C8 (1995): International Conference on Martensitic Transformations : ICOMAT 95, Lausanne, Switzerland, August 20-25, 1995Two austenitic Fe-Ni-C alloys, one having 23.9 wt.% Ni, 0.39 wt.% C, and a martensit start point of 228 K, the other with 21.8 wt.% Ni, 0.48 wt.% C, and an M(S) of 233 K, were tested to determine the kinetics of their austenite to martensite transformation both during cooling, at a rate of 5 K.min(-1), and during tensile testing at strain rates from 8.3x10(-5) to 8.3x10(-3) s(-1) and at temperatures from 233 to 323 K. Both alloys displayed variations in the critical plastic strain epsilon(c) for the onset of serrated yielding. Metallography confirmed that serrated yielding can occur in austenite before any strain-induced martensite is formed. Although strain-induced martensite formed at all of the examined strain rates and temperatures, inflections on the true stress versus true strain, or sigma-epsilon diagrams, were observed only at low test temperatures. The effect of transformation-induced plasticity, or TRIP, assessed by the maxima of uniform plastic strain epsilon(u), (epsilon(u) = 0.8), was detected only in tests run at 253, 273 anti 293 K. Kinetic diagrams for the formation of strain-induced martensite, in dependence on epsilon, were compiled for all the test temperatures and strain rates, and were compared with kinetic diagrams of heat release gained by differential scanning calorimetry for the austenite to martensite transformation. Diagrams of the amounts of heat released, Delta Q(A-->M), in the martensite formation process, against the plastic strain magnitude in the presence of similar proportions of martensite, were found to yield linear relationship

Martensitic transformation induced by quenching or by plastic deformation

Nowadays the possibility to use the TRIP effect (Transformation Induced Plasticity) is stated not only for the high alloy steels but also for the low alloy steels. This effect is connected with the martensitic transformation, but not necessarily the strain induced martensitic transformation induced by deformation provokes the TRIP effect: decisive factors are the chemical composition, the stability of the austenite, its grain size, the particles geometry and their distribution, and also the temperature, amplitude and rate of deformation during the mechanical loading. The change of energy is compared between the creation of the quench and strain induced martensite with and without the TRIP effect. The molar enthalpy released during the creation of quench induced martensite in the continually cooled austenite of the alloy Fe-23.87wt %Ni-0.39wt %C, was calculated from DSC (differential scanning calorimeter) measurements. For this same material, the consumed work during tensile tests was determined at different temperatures. The temperatures interval was between Ms and Md. At each température, a volume traction of strain induced martensite was created and thé TRIP effect could occur, that is both extraordinary great elongation at high ultimate tensile strength and nearly no local deformation (no creation of a neck). This paper conclues with a discussion on the preferences and lacks in the use of TRIP effect in low alloy steels

Nouvelles possibilités d'évaluation de la stabilité stucturale des aciers inoxydables austénitiques

CHATENAY MALABRY-Ecole centrale (920192301) / SudocSudocFranceF

Physical metallurgy of plastic deformation of as quenched martensite

Translated from Czech. (Kovove Mater. 1988 v. 26(3) p. 336-344)Available from British Library Document Supply Centre- DSC:9023.19(VR-Trans--3938)T / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo