(Invited) Effect of Mechanical Stress on the Passivity of Steel

Abstract

The influence of the elastic and plastic tensile stress on the electrochemical properties of mild carbon steel and austenitic stainless steel was studied using Scanning Kelvin Probe (SKP) and Localized Electrochemical Impedance Spectroscopy (LEIS) techniques. These two methods give complementary information on the surface of the stressed electrode. SKP characterizes the thermodynamic (surface potential) and LEIS is informative on the conditions of the passive oxide (capacitance of the film). The Volta potential and the admittance were mapped across an area with stress concentrator (notch) “in situ” under load. It was found that the elastic deformation slightly influences both the potential and the capacitance. The plastic strain decreases the potential measured in the lab air for both steels by 150 to 300 mV. LEIS shows an increase of the capacitance measured in borax electrolyte above the yielded surface in the notch relatively to the surrounding surface. Thus, the change inf the potential is reciprocal to the change of the capacitance. After straining, in load free conditions, stainless steel saved low potentials in the area containing residual stresses. It corresponds to LEIS data that shows an increased capacitance. In opposite, unloading of strained mild steel redistributes the surface potential. Mainly the surface increased the potential showing the passivation. However, low potential locations related to residual stress were also found by SKP. Mapping of the capacitance by LEIS also shows partial passivation of the steel. Thus, locations with concentrated stress, which are prone to localized corrosion can be recognized using local electrochemical techniques.After experiments, the strained surfaces were characterized by XPS and AFM. The effect of the plastic tensile strain was explained by the formation and emerging of dislocations and dislocations pile-ups. The dislocations rupture the oxide film making it more defective that weaken the steel passivity. On the other hand, the formation of the stepped surfaces decreased the potential that decreased the activation energy of anodic dissolution of the steel.