Pitting corrosion is one of the most destructive types of metal loss. The purpose of this study was to investigate the evolution, or in other words, the propagation, of a single pit in carbon steel after the initiation stage. In view of the chemical and electrochemical reactions inside a single pit in carbon steel, a two dimensional model that allows the prediction of pit evolution was developed. Eleven species in aqueous sodium chloride solution and two neutral complexes were considered in the model. Given that the active-passive transition of a metal is a key phenomenon in pitting, the equations used to construct a Pourbaix diagram for iron were incorporated in the model as rules to govern passivation behaviour. By using the finite element package COMSOL Multiphysics as a tool, the Nernst-Planck equations for the mass transport and potential variations were solved. In addition, the multiphysics model was extended with Moving Boundary (ALE) mode to predict shapes of pits. The results of the study were that the model was able to investigate migration of ionic species, account for the active-passive transition of metal and also able to show the effect of solid precipitation. The model was able to show movement of the boundaries of a pit and hence, predict the shapes of pit at a given range of time. The results were discussed in comparison to the Pourbaix diagram of iron and compared with the experimental results and published models reported in literature. The principal conclusion is that modelling corrosion activities with integrated thermodynamic equations based on Pourbaix diagram methods is an outstanding way to model any other corrosion activities.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

L'étude du comportement électrochimique des phases intermétalliques Al2Cu, Al2CuMg, Al7Cu2Fe, (Al,Cu)16Mn4Si3 et MgZn2, caractéristiques des alliages d'aluminium 2214-T6 et 7050-T74 et réalisée à partir d'échantillons massifs synthétisés, a montré que la formation de défauts majeurs dans la couche d'oxyde, obtenue par anodisation des alliages en milieu H2SO4 200 g/l, était directement due à la présence de ces particules au sein des alliages. L'important dégagement gazeux d'oxygène qui se produit à la surface de ces particules lors de l'étape d'anodisation explique la formation de trous et de fractures dans la couche d'oxyde. En milieu marin, ces défauts facilitent la migration des ions chlorures à la surface de l'alliage mise à nu localement et favorisent par conséquent le développement de la corrosion localisée. Les résultats obtenus en milieu NaCl 35g/l ont permis de mieux comprendre le mécanisme de corrosion des phases intermétalliques ainsi que leur rôle sur la propagation des piqûres. L'élimination des particules intermétalliques de la surface des alliages lors de l'étape de décapage (avant anodisation) permet de favoriser une croissance plus régulière de la couche d'oxyde lors de l'anodisation. Par conséquent, la résistance à la corrosion des alliages 2214 et 7050 augmente considérablement, permettant d'atteindre les objectifs fixés lors de ce projet / The electrochemical behaviour of the bulk intermetallic particles such as Al2Cu, Al2CuMg, Al7Cu2Fe, (Al,Cu)16(Mn,Fe)4Si3 and MgZn2 showed that the formation of wide defects in the oxide layer during the 2214-T6 and 7050-T74 alloys anodization was directly due to the presence of these kind of particles in the microstructure which are able to sustain both high dissolution rate and high water oxidation kinetics in acidic electrolyte. In marine environment, these defects facilitate the migration of aggressive ions like chloride on the locally bare alloys and are the onset of the pitting corrosion. The results obtained in 3.5 wt.% sodium chloride electrolyte permit to understand the corrosion mechanism of the intermetallic phases and their influence on the pits propagation. The optimization of the alloys pickling conditions in order to remove this kind of particles (prior to the anodization step) has been investigated in this work. Consequence of removing intermetallic particles, the aluminium enrichment of the surface facilitates the oxide growth and the formation of regular layer. The corrosion tests on treated aluminium alloys have clearly showed the influence of the pickling step on the resistance corrosion of 2214 and 7050 alloys, reaching the objectives fixed in this project

Advanced alloys often present precipitated carbides and nitrides in their microstructure following exposure to elevated temperatures. These secondary phases are usually undesirable, because potentially deleterious for the corrosion and mechanical performances of the material. Carbides and nitrides are enriched in key alloying elements that are subtracted from their surrounding matrix areas, creating alloying element depleted zones, which might become initial sites for corrosion initiation. In this study, the influence of micro- and nano-sized precipitated carbides and nitrides on the corrosion initiation of biomedical CoCrMo alloys and duplex stainless steels has been investigated at microscopic scale, by using a combination of local probing techniques. The microstructures of the alloys were first characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and magnetic force microscopy (MFM). The Volta potential mapping of carbides and nitrides revealed their higher nobility compared to the matrix, and particularly compared to their surrounding areas, suggesting the occurrence of some alloying element depletion in the latter locations, which may lead to a higher susceptibility for corrosion initiation. In-situ electrochemical AFM studies performed at room temperature showed passive behavior for large potential ranges for both alloy families, despite the presence of the precipitated carbides or nitrides. At high anodic applied potential, at which transpassive dissolution occurs, preferential dissolution started from the areas adjacent to the precipitated carbides and nitrides, in accordance with the Volta potential results. Thus, the presence of carbides and nitrides doesn’t largely affect the corrosion resistance of the tested advanced alloys, which maintain passive behavior when exposed to highly concentrated chloride solutions at room temperature with no applied potential. The effect of nitrides on the corrosion initiation of duplex stainless steels was investigated also at temperatures above the critical pitting temperature (CPT). Depending on the type, distribution and size range of the precipitated nitrides different corrosion behaviors were observed. Intragranular (quenched-in) nano-sized nitrides (ca. 50-100 nm) finely dispersed in the ferrite grains have a minor influence on the corrosion resistance of the material at temperatures above the CPT, while larger intergranular (isothermal) nitrides (ca. 80-250 nm) precipitated along the phase boundaries cause a detrimental reduction of the corrosion resistance of the material, in particular of the austenite phase / <p>QC 20130927</p>