Steel corrosion rate studies using sample area as a variable

Corrosion rate data are often considered to be independent of sample size if constant transport conditions can be maintained. The corrosion system of hypoeutectoid sheet steel samples (1020 carbon steel) in aqueous sodium sulfate solution has been studied, and data are presented to show that smaller samples corrode at higher current densities relative to larger samples in the same environment. Corrosion current densities from the varied area samples via weight loss data or electrochemical impedance spectroscopy (EIS) measurements exhibit a 50 mV/decade anodic Tafel slope without the need for ohmic drop corrections to the corrosion potential measurements. For at least the particular case of low C steel and aqueous solution corrosion, the rate controlling step is oxygen diffusion to the cathodic sites and the cathode sites have been suggested previously by others as being at the periphery of sheet- or flag-style samples. In that the periphery to central area ratio increases as the overall sample area decreases, the anodic current density is increased as the sample areas are reduced. The variable sample areas approach with ohmic free corrosion potentials, should have applicability for corrosion studies in low conductivity electrolytes. 14 ref.--AA