Effect of Hydrodynamics on Pitting^★

Studies were made with an artificial pit in the side of a flow channel to determine quantitative hydrodynamic effects on pitting rate. Experiments were conducted under potentiostatic conditions with iron, titanium, and aluminum in chloride, bromide, and iodide solutions. Two modes of pitting were observed: (1) a high current density limited only by ohmic resistance in solution, and (2) a lower current density limited by mass transfer. Only the ohmic mode was observed for aluminum and for titanium in chloride solution. Corrosion in these cases was accompanied by a large evolution of hydrogen which caused local agitation of the diffusion layer even at a zero solution flow. The mass transfer limited mode was observed as well as the ohmic mode for iron in chloride solution and for titanium in bromide and iodide. The mass transfer limited current density increased with flow rate, but was not always well defined because solid corrosion products in the pit disturbed the solution flow pattern. Transport of FeCl₂ away from the corroding surface limited the corrosion rate of the iron and the measured current density was, for the most part, within ±60% of theoretical based on saturation solubility of FeCl₂ in NaCl solution. For titanium, the calculated saturation solubility of the mass transfer limited species is a function of the flow conditions. This species is interpreted to be an oxy-salt with composition and solubility dependent on local acidity developed in the pit.