Effect of Mechanical Stress on the Properties of Steel Surfaces: Scanning Kelvin Probe and Local Electrochemical Impedance Study

Abstract

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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