A polarization behavior of chromium was investigated in H2SO4-Na2SO4 solution of pH 2.0 at 298 K by the potentiostatic oxidation and electrode impedance technique. From the coulometry and the chemical analysis of electrolyte the thickness of the passive films on chromium was estimated to be about 2.1 nm after 3.6 ks potentiostatic oxidation. The frequency response of the electrode impedance were simulated by the equivalent circuit of a cascade connection of two parallel RC in the prepassive (the potential region of the negative loop) and passive region, and of a cascade connection of a parallel RC and a parallel RCL in the active and transpassive region. From the impedance analysis, it was conceivable that the potential drop across the passive film and film/solution interface was constant of about 0.56 V irrespective of the polarization potential in the passive region.
A study was made of corrosion products and their effects on stress corrosion cracking of austenitic stainless steel. Wedging action by solid corrosion products in notches or cracks induces high stresses and eventual failure of specimens by stress corrosion cracking. Data were obtained from stress-relieved and unloaded (externally) specimens so that wedging by corrosion products provided the only source of stress in the specimen.
Pressures were measured in excess of 7000 psi due to wedging action of corrosion products. At the base of a notch these pressures developed stresses of the order of magnitude of the yield strength of the metal.
Wedging action can provide all the energy required for stress corrosion cracking. A mechanism is proposed which involves a discontinuous type of propagation, with fluctuations occurring over one or a few atomic distances. A running or mechanical type of crack propagation for more than a few atomic distances is ruled out on the basis of the mechanics of the system.
3.4.3, 3.5.8, 6.2.5
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