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

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.