Steel Corrosion Rate Studies Using Sample Area as a Variable

Abstract

Corrosion rate data are often considered to be independent of sample size if constant transport conditions can be maintained. The corrosion system of hypoeuteetoid sheet steel samples 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 withouf the need for ohmic drop corrections to the corrosion potential measurements.For at least the particular case of low carbon steel and aqueous solution corrosion, the rate controlling step is 02 diffusion to the cathodic sites and the cathodic 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.The application of the Stern-Geary equation (i) for converting polarization resistance data to corrosion current densities requires some knowledge or assumptions of the anodie and cathodic Tafel slopes for the reaction being studied. Because the polarization resistance measurement is normally conducted over a small polarization range, typically less than • 50 mV and normally within • 20 mV from the corrosion potential, supplementary potentiodynamic scans are necessary to obtain Tafel slope data. This is especially important where Tafel slope values for the system in consideration are not available in the literature. The authors do acknowledge there are other methods for obtaining corrosion rates from polarization resistance values which do not require the use of Tafel slopes, but the interest here is in an alternative method for obtaining anodic Tafel slopes for a corroding metal.In low conductivity electrolytes such as dilute (less than 0.01M) solutions, distilled water or soils, the generation of proper polarization resistance and Tafel slope values is complicated because of the voltage drop correction made necessary by high electrolyte resistance values in most research cell designs.Several investigators (2, 3) have discussed the problems in polarization curves in low conductivity electrolytes which are mainly caused by insufficient IR correction being applied to the voltage measurements. Recently a novel approach involving a "chemical potentiostat" and a rotating disk electrode was reported which eliminates the need for IR voltage drop correction and was shown to be effective for evaluating the polarization behavior of 1020 carbon steel in artificial seawater diluted tenfold (4). In that study, the 02 concentration at the surface of a conventional rotating disk electrode (RDE) was adjusted by the use ...