Effect of Flow Rates on Localized Corrosion Behavior of 304 Stainless Steel in Ozonated 0.5 N NaCl Available to Purchase

The effect of flow rates on the corrosion behavior of 304 stainless steel in a solution of 0.5 N NaCl containing 0, 0.02, and 1.2 mg/L dissolved ozone was studied at room temperature. Flow rates ranging from 0.3 to 2.0 m/s were simulated by the use of a rotating cylinder electrode (RCE). Corrosion potentials and passive current densities were measured, and values of breakdown and repassivation potentials were derived from cyclic polarization curves. It was found that the transition from laminar to turbulent flow for this system (supported by a Reynold’s number calculation) occurs between 0.2 and 0.4 m/s. In this velocity regime, an inverse relationship is observed between the breakdown and repassivation potentials. Under laminar flow conditions, increasing velocity shifted the repassivation potential in the active direction due to the increased driving force for pitting, which resulted from the stabilization of the passive film. This stabilization was also reflected by an increase in the breakdown potential in the noble direction. Values of the passive current density decreased with increasing velocity in this regime due to the diffusion boundary layer decreasing at the alloy surface. Under turbulent flow conditions, increasing velocity caused the passive current density to stabilize in deaerated and low ozone concentration solutions. However, solutions containing high concentrations of ozone experienced an increase in the anodic current density with the onset of solution flow, and pitting and crevice corrosion were observed. These results indicate that ozone does stabilize the passive film on stainless steel and that increasing solution velocities, at least in the laminar regime, further stabilize the passive film. However, the combination of high concentrations of ozone and significant flow velocities may increase the pitting and crevice corrosion susceptibility of stainless steels in chloride solutions.