Roland Oltra

SVET, AFM and AES study of pitting corrosion initiated on MnS inclusions by microinjection

As pitting is a random phenomenon, it is difficult to predict where a pit will appear on the surface and consequently the use of local probes is rendered difficult. In this work, a new method to study pitting corrosion on a MnS inclusion on 316L stainless steel is proposed. It consists in modifying locally the chemistry in its vicinity by injecting with a microcapillary an aggressive solution of NaCl, H2SO4 or HCl. Once a pit appears, scanning vibrating electrode technique (SVET) is used to follow the current fluctuations over and around the pit when the metal is polarized at a passive potential. In another series of experiments the effect of local activation of MnS inclusion was studied ex-situ using Auger electron spectroscopy (AES) and atomic force microscopy. It is observed that a single pit can be initiated only when hydrochloric acid is injected, whereas sulphuric acid only partially dissolved the inclusion. On another hand, the surface morphology is not affected when a sodium chloride solution is injected. A significant enrichment in sulphur is detected around the inclusion by AES, and micropits are observed in the metal at the edge of the inclusion after HCl activation. Anodic zones are detected by SVET around the inclusion, whereas a cathodic current flows from the inclusion. The anodic current is clearly ascribed to the breakdown of passivity induced by adsorbed sulphur coming from the MnS dissolution, whereas various assumptions can be proposed for the origin of the cathodic current.

Influence of the Chemical Composition and Electronic Structure of Passive Films Grown on 316L SS on Their Transient Electrochemical Behavior

Modifications of the passive film formed on 316L stainless steel (SS) in acidic media were performed under dc or ac polarization. The effects of these ageing treatments on the chemical composition and the electronic structure of the passive film were studied. Chemical analysis with angle resolved X-ray photoelectron spectroscopy revealed that ageing induces a net enrichment in chromium oxide in the inner part of the passive film and an increase of the thickness of the inner oxide. Semiconducting behavior of the passive films was revealed by a Mott-Schottky analysis. The passive film was considered as a dielectric with different trapping levels lying in the bandgap. The observed n- or p-type semiconductivity can be assumed to be the result of the ionization of these trap levels under polarization. The most important modification of the electronic structure observed after ageing was a net decrease of the density of acceptor levels, which is associated with a less disordered inner chromium oxide. In the light of these results, the effect of ageing on the transient electrical coupling of the passive film with an active surface was investigated in free potential conditions. The capacitive response revealed by the potential and current transients was related to the localized states in the bandgap of the passive film. The current transient during the potential drop was related to the repassivation of the active surface. At the same time, electrons from the active site are transiently stored in the capacity formed by trap levels in the inner oxide of the passive film before being consumed by the cathodic reaction. The kinetics of the overall process should be limited by the heights of the potential barriers at the different interfaces and by the energy level of the acceptor-type traps.

Analogy between the effects of a mechanical and chemical perturbation on the conductivity of passive films

Abstract In order to study the passivity breakdown of a type 316L stainless steel, the electrochemical impedance and surface stresses were measured under straining conditions. The Mott–Schottky analysis, performed at high frequency, appears as very useful method to study, below the apparent yield strength, the effects of a mechanical stress on the capacitance values. The results obtained in sodium chloride media indicate that the semiconducting properties of passive films formed in a tensile stress field reflect a higher vacancies concentration than those formed in a stress-free state, suggesting that the passive film conductivity is increased. It was also shown that the Mott–Schottky plots drop in the presence of tensile surface stresses, indicating that the passive film composition may be different from that formed in the presence of compressive surface stresses. A comparison between the effects of mechanical stresses and those of chloride is proposed.

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.

Quantitative Analysis of Iron Dissolution during Repassivation of Freshly Generated Metallic Surfaces

A channel flow double electrode was coupled with the pulsed laser depassivation technique to investigate the repassivation kinetics on iron in sulfuric acid (pH 1 to 3). The amount of ferrous ion dissolved from the activated iron electrode during repassivation was measured. The total charge for repassivation is separated into partial charges for dissolution of ions, Q{sub d}, and formation of adsorbed species, Q{sub f}. The charge Q{sub d} decreases with the electrode potential. The charge Q{sub f} does not depend on the electrode potential. Its value corresponds to the formation of a monolayer of trivalent oxide film. A reaction model for repassivation of iron involving two adsorbed species was simulated numerically in good agreement with the experimental results. The actual rate of repassivation was characterized by the fractional coverage of passivating species.

Noncontact determination of the elastic moduli of β-Sn up and through the melting point

The aim of this study is to determine the elastic moduli of metals versus temperature, especially around the melting point. Using a technique called laser-ultrasonics which allows one to measure elastic waves time of flight in the metal, longitudinal and shear velocities are calculated. This “noncontact method” is very appropriate for ultrasonic characterization at high temperature. In this paper, the presentation will be focused on the study of tin. Phenomena occurring during the phase transition are discussed. Original calculation of the Lindemann constant is presented. Results concerning the shear velocity VSS and modulus G are presented, from room temperature to the melting point. Furthermore, these results are accurately described by a modeling of G(T) versus temperature.

A mechanical-electrochemical approach for the determination of precursor sites for pitting corrosion at the microscale

The influence of metallurgical defects and residual surface stresses generated by polishing on the pitting susceptibility of duplex stainless steels was studied by combining macro- and microelectrochemical measurements with thermal-mechanical simulation and metallography tests. It has been shown that pits initiate in both phases at metallurgical point defects (such as oxide inclusions in the ferrite and dislocation lines in the austenite). By contrast, the surface stress state was the driving force for pit initiation along the austenite/ferrite interface. Experiments at the macroscale revealed that this process represents about 40% of the total number of pits observed. It has been demonstrated that the local stress gradient was the key-parameter in pit initiation and that the local average stress was the parameter governing the transition from metastable to stable pitting.

Influence of Elastic Deformation on Initiation of Pits on Duplex Stainless Steels

Pitting corrosion of a duplex stainless steel is studied under straining conditions. An original method based on successive X-ray diffraction measurements was first used both to quantify the surface stresses and to determine the elastoplastic behavior in the close vicinity of the surface of metallic phases. The time to pit was then measured under various levels of applied stress below the apparent yield strength by classical electrochemical tests under potentiostatic control. The evolution of the time to pit was then analyzed with respect to the individual elastoplastic behavior of phases. The results obtained indicate that the electrochemical behavior of duplex steels can be correlated to the surface stress state measured in the austenite phase.

Preliminary study on the laser cleaning of stainless steels after high temperature oxidation

Abstract The objective of the present work was to estimate the influence of pulsed laser irradiation on the removal of the oxide layer, which is developed on the surface of stainless steels during their exposure to high temperature oxidation. In general, this layer is a protective one, mainly against corrosion. However, in many manufacturing applications or maintenance work, the removal of the oxides is necessary; for example, the metallic surfaces should be cleaned before welding, otherwise the presence of oxides increases the tendency to brittle behaviour of the joint. In this study, a pulsed Nd:YAG laser (λ=1.064 μm, 10 ns) was used for the surface cleaning of three stainless steels with different chemical compositions and/or surface treatments. The influence of the laser irradiation on the material, as well as on the mechanisms involved, was investigated by in-situ mass measurements and post-situ microscopic observations (Scanning Electron Microscopy/SEM, and Energy Dispersion Spectroscopy/EDS). For energy density applied (1.0–2.0 J cm−2), irrespective of the composition and the thickness of the surface layer, the laser irradiation resulted in the expulsion of the oxide layer, while no material removal of the underlying metal occurred.

Laser plasma plume structure and dynamics in the ambient air: The early stage of expansion

Laser ablation plasma plume expanding into the ambient atmosphere may be an efficient way to produce nanoparticles. From that reason it would be interesting to study the properties of these laser induced plasmas formed under conditions that are known to be favorable for nanoparticles production. In general, plume behavior can be described as a two-stage process: a “violent” plume expansion due to the absorption of the laser beam energy (during the laser pulse) followed by a fast adiabatic expansion in the ambient gas (after the end of the laser pulse). Plasma plume may last a few microseconds and may have densities 10−6 times lower than the solid densities at temperatures close to the ambient temperature. Expansion of the plasma plume induced by the impact of a nanosecond laser beam (λ = 1064 nm) on the surface of metallic samples in the open air has been investigated by means of fast photography. Spatio-temporal evolution of the plume at the early stage of its expansion (first 330 ns) has been recorded. ...