Volta Potential on Stainless Steel Under Mechanical Stress Conditions

It is well documented fact that mechanical work or surface treatment creates stresses in the metal. In corrosive environment the stress can lead to stress corrosion cracking and failure of the metal construction. It is often supposed that stress influences the properties of the surface oxide film by creating defects like metal vacancies and thus activating the anodic dissolution. On the other hand, the kinetic of the partial corrosion reactions is determined by the local electrochemical potential. Thus it is important to know how the stress influences on the surface potential.

The aim of the present study was to measure the influence of stress on the electrochemical potential of stainless steel 301 grade in situ under loading using Scanning Kelvin Probe. The measurements were carried out in the laboratory air at 50%RH and the measured Volta potential was related to the potential of the steel in the passive state. Previously this kind of measurements was carried out elsewhere [A. Nazarov, D. Thierry "Application of Volta potential mapping to determine metal surface defects",Electrochimica Acta 52 (2007) 7689-7696.].

In this work a constant load cell was applied on the sample with notches that concentrates the stress in a particular location. The load was measured and it was in the elastic or plastic deformation range. It was found that elastic deformation resulted in rather low potential changes whereas plastic deformation decreased the surface potential.

Figure 1: A: Potential profile in the notched area, B: Impact of the compressive stress produced by indentation.

Figure 1A shows the potential profile for the steel surface in the notched area under tensile stress conditions. Figure B shows the impact of the compressive stress produced by indentation.

It is possible to conclude that stress decreased the surface potential of the steel by about 200-300 mV. Thus the stress decreased the level of metal passivity that can facilitate the local anodic dissolution. After unloading under residual stress conditions the surface very slowly shifted the potential to the initial level. The effect of the plastic deformation on the steel potential is explained on the basis of the formation of dislocations and defective oxide film. The surface analytical studies are in progress.