Experimental Observations and Analysis of Hydrodynamic Effects on Growth of Small Pits

Authors:
  • TBT. R. Beck
    T. R. Beck
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Abstract
Potentiostated pitting experiments were conducted with zone refined iron and commercial Type 304 stainless steel in sodium chloride solutions. Growth of single small pits initiated at small holes in a lacquer surface layer was observed under a microscope at 60X under stagnant and controlled hydrodynamic flow conditions. Three modes limiting pit growth rate were determined; hemispherical diffusion, hydrodynamic boundary layer diffusion, and solution ohmic resistance. Experiments showed that for a given size pit, there is a threshold velocity below which hydrodynamic flow is not important and that there is a critical velocity above which current density is limited by ohmic drop in solution. Stainless steel repassivates at the ohmic limited current density and pitting stops.

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