The effect of fluid flow on growth of single corrosion pits

The influence of fluid flow on pitting corrosion was investigated with use of a flow channel operated under well-characterized hydrodynamic conditions. The system under study was 304 stainless steel in 0.1 M Na 2SO 4 + 0.2 M NaCl at pH 5. By using micro-electrodes, it was found that individual corrosion pits could be created. Measurement was made of single pit current as a function of time for various hydrodynamic conditions. For pits which initiated under stagnant conditions, it was found that the sudden imposition of flow caused pit deactivation provided that the Peclet number (flow rate × pit size/diffusivity) exceeded about ten. Finite element calculations of the fluid flow and transport were carried out for two-dimensional configurations having cross-sections similar to corrosion pits. The calculations were made with the assumption that the dissolving surface was covered with a ferrous sulfate salt film. Numerical results for the average dissolution rate agreed to within a factor of three with experimentally determined rates. Calculations showed the local dissolution rate to be greater at the sides of the cavity than at the bottom, in agreement with the observed shape of shallow pits.