Jean-Marc Olive

Hydrogen embrittlement of 316L type stainless steel

Hydrogen embrittlement tests on type 316L stainless steel are performed including cathodic charging during slow strain rate tests. Brittle multiple cracking is observed and relationships between crack growth rate and diffusion are analysed. The influence of hydrogen on the morphology of ductile fracture is found after fractographic examination. Two aspects of ductile failure are observed in accordance with the hydrogen content of the sample; a reduced density of microvoids for higher hydrogen contents and brittle secondary cracking in addition to ductile fracture surfaces for lower hydrogen contents.

Influence of the Chemical Dissolution of MnS Inclusions on the Electrochemical Behavior of Stainless Steels

Immersion of stainless steel containing a well-controlled density of MnS inclusions in 1 M NaCI, pH 3 leads to the chemical dissolution of these heterogeneities. This process was studied using in situ atomic force microscopy and the dissolution rate of MnS inclusions was estimated between 0.01 and 0.19 μm 3 /min. The effects of MnS dissolution on the chemical composition and the local electrochemical behavior of the specimen surface were investigated using secondary ion mass spectroscopy, X-ray photoemission spectroscopy, and the electrochemical microcell technique. It was shown that stable CrS and unstable FeSO 4 were formed. The size of the areas around MnS inclusions affected by the presence of sulfur-containing species depends on the immersion time and the composition of the native film. The local electrochemical measurements reveal that the chemical dissolution of MnS inclusions promotes pitting corrosion in the surrounding grains at moderate applied potentials for short immersion time and general corrosion of the whole specimen surface for long immersion time.

Effect of grain boundary trapping kinetics on diffusion in polycrystalline materials: hydrogen transport in Ni

Due to experimental limitations, the solute distribution in polycrystalline materials is difficult to obtain directly, especially in the vicinity of grain boundaries. Using a newly developed computational method which mixes continuum diffusion equations and atomic scale jump rates, we study the interstitial diffusion in solids containing interfaces taking into account trapping kinetics. The model is applied to hydrogen diffusion in Ni in elementary configurations: fast intergranular diffusion with no segregation (in agreement with Fishers model), slow intergranular diffusion with trapping, diffusion through a triple junction and solute redistribution due to stress gradients across the interface. It is shown that the classical diffusion modes can be captured and a new diffusion regime with the effect of grain boundary trapping is revealed.

Modification of Plastic Strain Localization Induced by Hydrogen Absorption

Modification of Plastic Strain Localization Induced by Hydrogen Absorption In order to highlight hydrogen effects on the plasticity, the slip morphology after straining (under tension up to 4% of plastic strain in ambient air) of hydrogenated (at 135 wt.ppm) and non-hydrogenated 316L stainless steel polycrystals was compared. A statistical analysis of both slip band spacings (SBS) and slip band heights (SBH) was performed using atomic force microscopy. Tensile tests were performed at low strain rate, specimens being previously charged at controlled hydrogen concentration. The plastic strain field heterogeneity in polycrystals was taken into account thanks to numerical simulation of crystalline plasticity. On each grain, the calculated plastic shear was correlated with the distribution of SBS and the average number of emerging dislocations per slip band. In comparison with uncharged specimen and for an equivalent cumulated plastic strain, the hydrogenated specimen shows an increase of the slip band spacing (SBS) and of emerging dislocations. This result confirms a plastic localization induced by absorbed hydrogen.

Hydrogen degradation of pre-oxidized zirconium alloys.

Abstract The presence of the oxide layers on Zr alloys may retard or enhance the hydrogen entry and material degradation, depending on the layer features. This research has been aimed to determine the effects of pre-oxidation of the Zircaloy-2 alloy at a different temperature on hydrogen degradation. The specimens were oxidised in laboratory air at 350°C, 700°C, and 900°C. After, some samples were tensed at 10-5 strain rate and simultaneously charged with hydrogen under constant direct voltage in 1 N sulfuric acid at room temperature. Other specimens were charged without any tension, then annealed at 400°C for 4 h and finally tensed at above strain rate. The SEM examinations were performed on the cross-sections and fracture faces of specimens. The obtained results demonstrate the effects of the oxide layer on the cathodic current and hydrogen entry, mechanical properties and the appearance of hydrides and fracture behaviour.