Comportement d’aciers inoxydables en eaux naturelles

International audienceBehaviour of stainless steels immersed in natural waters: Electrochemistry and bacterial adhesion. The free corrosion potential of a stainless steel immersed in natural seawater rises quickly until it reaches values ranging between +100 and +350 mV/SCE, which increases the risk of initiation of pitting corrosion. According to litterature this phenomenon also occurs in fresh waters. The aim of this study is to confirm or to invalidate this trend; the electrochemical behaviour of samples of stainless steels immersed in river water and the influence of the biofilm formed on the surface of the samples are studied. The free corrosion potentials of three different stainless steels (S30403 or AISI 304L, S31603 or AISI 316L, S31254 or 254SMO) have been measured continuously during their immersion in the Seine river. SEM observations of the samples surface show the presence of a biofilm on the three kinds of stainless steel. The free corrosion potentials increase and end up between +100 and +300 mV/SCE. This increase is not immediate, the latency time being around 20 days. This could be related to an effect of the low temperature of the water during the immersion (8-10 °C) and/or to an effect of the Total Organic Carbon (TOC), which would limit the growth rate of the biofilm, hence its influence on the evolution of the free corrosion potential.Un acier inoxydable immergé dans de l'eau de mer naturelle voit son potentiel de corrosion libre Ecorr augmenter rapidement jusqu'à atteindre des valeurs comprises entre +100 et +350 mV/ECS, ce qui augmente le risque d'initiation de la corrosion par piqûres. D'après la littérature, il semblerait que le même phénomène ait lieu en eaux douces naturelles. Le but de cette étude est de confirmer ou infirmer cette tendance en étudiant le comportement électrochimique d'échantillons d'aciers inoxydables immergés dans une eau de rivière et en examinant l'influence du biofilm formé sur la surface de l'acier inoxydable. Pour cela, un suivi du potentiel de corrosion libre d'échantillons d'aciers inoxydables S30403 (AISI 304L), S31603 (AISI 316L) et S31254 (254SMO) a été effectué durant leur immersion dans la Seine. Des observations de la surface des échantillons ont été pratiquées au MEB et ont montré la présence d'un biofilm sur les trois nuances d'acier inoxydable. L'augmentation du potentiel de corrosion libre a bien eu lieu et a atteint les mêmes valeurs que celles de la littérature (entre +100 et +300 mV/ECS). Par contre, cette augmentation n'est apparue qu'après un temps de latence d'environ 20 jours. L'explication de ce retard peut provenir de la faible température de l'eau (8-10 °C) et de sa faible teneur en carbone organique total (COT) qui limiteraient la vitesse de développement du biofilm et donc son impact sur l'évolution de Ecorr

Intergranular corrosion evaluated by metallographic and atomic force microscope

International audienc

Behaviour of hydrogen in Fe-Ni-C alloys

A particular cathodic charging technique was used to evaluate the behaviour of hydrogen on Fe-25.66Ni-0.31C alloy. This technique is based on the electrolysis of water injected in a molten salts bath. The electrolytic charging conditions were chosen as −2.05V/Ag and 300°C. The quantities of hydrogen extracted from specimens of different diameters after electrolysis over different durations were used to calculate both the substantial surface concentration (C-0=3cm(3) H-2/cm(3) metal) and the hydrogen concentration profiles in austenite. After quenching at −65°C, a linear relationship between the hydrogen concentration and the amount of retained austenite was determined. A critical concentration C-k=0.06 cm(3) H-2/cm(3) metal, initiated microcracks in martensite. The behaviour of hydrogen on austenite is discussed in terms of grain boundary and dislocation trapping

Mechanical study of instability of austenitic Fe-Ni-C alloys — Effect of hydrogen

The mechanical properties of different Fe-Ni-C alloys, in which Ni and C contents are correlated in order to ensure roughly equal Ms temperatures, are investigated considering three austenitic states: water-cooled (gamma), cathodically hydrogen charged at 300°C (gamma + 300°C/H-2), and heat-treated at 300°C (gamma + 300°C) for comparison. The true stress σ versus true strain are approximated by σ = K-1 + K-2 epsilon(1/2). Except for 0.006 wt %C, the fitting displays two or three domains of strain characterized by higher values of the slope K2 at high deformations. For carbon content beyond ≈ 0.2 wt% C this slope increase is due to strain induced martensite. As a consequence transformation induced plasticity (TRIP) effect, confined to medium carbon contents, is observed. At the same time the stress-strain diagrams exhibit instabilities in the form of serrated yieldings. The critical stress and strain of their onset is correlated to the number of Frank-Read sources (FRS) activated by the plastic flow. In the case of higher carbon alloys and higher strains, the increased slope K2 is thought to be due to another strengthening mechanism involving carbon atoms in the solid solution, associated with Portevin-Le Chatelier (PLC) effect. At low carbon content the effect of hydrogen in prior austenite is negligible, but at high contents the embrittlement and cracking of the strain induced martensite is immediate

Statistical normalisation of M23C6 carbides precipitation kinetics, changes in chromium concentration in their vicinity and electrochemical charateristic of EPR tests

International audienc