The Effect of Gas Bubbles and Cavity Dimensions on the Local Electrode Potential within Pits, Crevices and Cracks.
PENNSYLVANIA STATE UNIV UNIVERSITY PARK DEPT OF MATERIALS SCIENCE AND ENGINEERING
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Gas bubbles have been reported to routinely accumulate in cavities during anodic or cathodic polarization in several merals and alloys Fe, Ni, Cu, Al, stainless steel. These in-place gas bubbles have been found to sharply increase the gradients of potential and ionic concentration within the cavities. A limiting electrode potential, E sub LIM, in the cavity has been observed, consistant with theoretical considerations which predict its existence. Mathematical modeling has been reasonably successful, especially in the case of cathodic polarization where the calculation shows that the potential and concentration gradients are strong functions of the sharpness of the crack, the ohmic voltage increasing as the crack opening decreases, in agreement with earlier calculations of current distribution in slots by Wagner. By analogy, a similar result in expected for anodic polarization. The shift of the potential with increasing distance into a cavity is always in the direction of a decrease in anodic or cathodic polarization, is typically very large approaching the E sub LIM value of the system, is largely independent of the cathodic or anodic polarization, is typically very large approaching the E sub LIM value of the system, is largely independent of the cathodic or anodic polarization applied to the external surface and often places that base of the cavity in a potential regime of other reactions, e.g., anodic metal dissolution at the crack tip while the samples outer surface is under effective cathodic protection.
- Physical Chemistry
- Metallurgy and Metallography