The aim of the present work was to study the influence of the stress on the electrode potential of the austenitic stainless steel301LN using Scanning Kelvin Probe (SKP). It was found that elastic deformation reversibly ennobles the potential whereas plasticdeformation decreases the potential in both tensile and compressive deformation mode and this decrease is retained even 24 h afterremoval of the load. To interpret the stress effects, different surface preparations were used and the composition and thickness ofthe passive film were determined by GDOES. Slip steps formed due to plastic deformation were observed using AFM. The effect ofplastic strain on the potential is explained by the formation of dislocations, which creates more a defective passive film.

QC 20160516

The influence of mechanical stress on the electrochemical properties of ferritic steel SAE 1008 and austenitic stainless steel 301LN was studied using Scanning Kelvin Probe and Localized Electrochemical Impedance Spectroscopy (LEIS) techniques. The probeworking electrode Volta potential difference was mapped in situ under load. It was found that the influence of elastic deformation on the potential was small. Plastic deformation decreased the potential of steel by 150-300 mV, whereas the relaxation of the load from the plastic domain increased the Volta potential. However, some locations, which can contain residual stress, remained at low potential. The pre-strained surfaces were characterized by X-ray Photo Electronic Spectroscopy and by Atomic Force Microscopy. Distribution of the capacitance across strained and strain-free surfaces was studied by LEIS in boric/borate electrolyte. The plastic stress increases the capacitance and decreases the ability of the steels to passivate the surface indicating that emerging of pile-ups of dislocations create defective oxide films. Stress corrosion cracking (SCC) is a well-known complex corrosion process caused by the combination of stress and corrosion, which can lead to wear, cracking or fatigue failures. In many cases, the residual stress from plastic deformation or wear accelerates the corrosion rate.1 In the classic film rupture model, tensile stress breaks the passive film creating anodic locations at the bottom of the crack, 2,3 which propagates through an activation/passivation process. This model was developed to the "slip dissolution-film rupture model" pointing out the importance of formation of dislocations and metal dissolution through dislocation slip lines.3,4 The slip dissolution-film rupture model of crack advance was discussed in details previously. 5 This model can predict the crack growth rate for the stainless steels, nickel alloys, and low-alloy steels in high temperature water. 6 The effect of the yielding on the rate of dissolution of many metals was found to be much pronounced in comparison with the influence of elastic deformation. 7 The dissolution rate showed a marked rise at the beginning of the plastic region that is asymptotic with increasing strain. Similar effects of plastic deformation on the anodic current during dissolution were also found for stainless steels.8 To explain this mechanical-electrochemical effect, both the increase of dissolution rate at slip edges and dislocations, and the increase of surface roughness from plastic deformation were pointed out.8 On the other hand, the selective slip dissolution can be ascribed to the local excess of Gibbs potential, 7,8 thus contradicting earlier works. Hoar 9 has shown that both enthalpy and entropy of activation were not significantly altered by cold work, which meant that the free energy of activation for anodic reaction should remain almost constant. A calorimetric study 10 showed that residual energy from cold work was less than 7 calories per gram without any significant impact on ...

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The interplay between mechanical stresses and electrochemical reactions may lead to stress corrosion cracking or hydrogen embrittlement for many materials. In this work, the effect of the tensile stress on the electrochemical properties of AISI 304 stainless steel was studied using scanning Kelvin probe (SKP) in air and scanning electrochemical microscopy (SECM) in an aqueous 0.5 M Na 2 SO 4 electrolyte. The measurements were performed under load-and load-free conditions. No influence of the elastic stress on the electrochemical potential of the steel was found. In contrast, the plastic strain induces dislocations and dislocation pileups , which emerge to the surface. The formation of new active surfaces is accompanied by an increase in the roughness and a 150-200 mV decrease in the steel potential. After activation, the potential increased due to passivation of the emerging surfaces by a newly grown oxide film, which took place under both the load and load-free conditions and followed a time dependence of = A log t + B. Formation and then passivation of the new surfaces increased and then decreased the reduction current of the mediator in the SECM measurements. The effect of residual stress stored in the steel due to the development of dislocations on the reactivity of the re-passivated surface was investigated.

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Low cycle fatigue tests are performed on a high strength tempered martensitic steel at different plastic strain amplitudes at room temperature. Internal and effective components of the flow stress are analyzed using Handfield and Dickson's method. The internal stress is affected by the plastic strain amplitude. Conversely, the evolution of the athermal component of the effective stress with the number of cycles is independent of the plastic strain amplitude. The thermal part of the effective stress increases with the plastic strain amplitude, but remains constant with plastic strain accumulation. Microstructural changes in the cyclically deformed material are investigated by means of transmission electronic mycroscopy and X-Ray characterizations. Internal and effective stress evolutions are discussed based on these observations.

The aim of this study is to evaluate fatigue performance of joined assemblies (spot weld and/or adhesive bonding) in corrosive environment. Various assemblies have been tested in alternated and simultaneous fatigue‐corrosion modes. Adhesive joints are strongly affected by simultaneous fatigue‐corrosion with a large drop of the fatigue life compared to results in air. By alternating fatigue and corrosion, the reduction of fatigue life is important. For spot welding, fatigue life is decreased at higher load amplitudes and increased at lower amplitudes. These results are strongly linked to the opening of the gap near the spot weld at high load amplitudes. At low amplitudes, corrosion might limit the local stress at the notch root of the weld.