Effect of transport and resistivity characteristics of the environment on the corrosion of steel

Conditions of low transport and high resistivity can cause localized corrosion, as observed in other environmental systems, resulting in higher penetration rates than would develop under conditions of uniform corrosion. In this study, transport and resistivity are controlled over several orders of magnitude, and their effect on the rate and form of corrosion are observed. Small steel coupons are immersed in a matrix of environmental conditions in which transport and resistivity are controlled, and the corrosion rate of each coupon is measured periodicially. Measurements of corrosion rate are carried out using polarization resistance and a.c. impedance techniques. The technique of polarization resistance allows a rapid determination of corrosion rate, however, correction for iR error in the potential measurements, in high resistivity media, must be performed. During the corrosion measurements, specimens are polarized from -10 to 10 mV, versus open circuit potential, and the current applied to obtain this change is measured. Corrosion rates are calculated from the data that results. More information is obtained using a.c. impedance, but the measurement requires more sophisticated evaluation compared to the polarization resistance technique. In addition to corrosion rate data, without the problem of iR compensation, a.c. impedance provide information on the mechanism of corrosion and resistivity of the media-oxide. Gravimetric weight loss measurements are made for verification and total weight loss. The extent of pitting and uniform corrosion are also evaluated at the end of the exposure period