Localized Corrosion in Cathodically Polarized A109 Steel in Aqueous Sodium Chloride

Corrosion of ASTM A109 steel (UNS G10230) in aerated aqueous 0.5% sodium chloride (NaCl) solution was studied at the corrosion potential (E_corr) and at different cathodic polarization (CP) potentials. Optical microscopy (OM) was used to characterize localized corrosion, and weight-loss measurements were made to estimate corrosion rates. Uniform corrosion was observed on all samples that were polarized cathodically up to –70 mV from E_corr. Samples kept at E_corr showed uniform corrosion during the initial period of exposure but pitted after longer periods. The number of pits per unit area and the area of individual pits decreased with increasing cathodic potentials. Similarly, the corrosion rate decreased with increasing CP potential and became immeasurably small at potentials ≥ 120 mV cathodic to E_corr. Microscopic studies indicated nonuniform etch-type corrosion in cathodically polarized samples. Etches first appeared at the bottom of the sample and expanded upward with time through a sequence of mantles. At low CP potentials, shallow pits formed at the mantles and spread across the surface between the first and second etched layers. Numerical simulations based on the finite element method (FEM) suggested a nonuniform current density distribution on the sample surface. Localized corrosion in the cathodically polarized coupons was attributed to nonuniform distribution of the cathodic current.