Current and potential relations for the cathodic protection of steel in a high resistivity environment

In order to evaluate potential and current criteria for the cathodic protection of bare low-carbon steel in a high-resistivity environment, specimens were exposed in th e laboratory fo r a p eriod of t wo months to a soil having a resistivity of a bout 20,000 ohm-centimeter.Previous work in low-resistivity environments b y the a u thor a nd by other investigators has shown t hat corrosion can be redu ced to a negligible d egree by pola rizing a steel structure to -0.85 volt (protective potential) with reference to a copper-co pper sulfate electrode.In su ch studies by the author, cathodic polarization curves have also been shown to be useful in indicating t he current density required fo r cathodic protection.In t he present study the above criteria were again evaluated.I n add ition to protecting;th e steel at the protective potential (free of IR drop), the eff ect on protection of including IR drop caused by the protective current was also noted.Also, cathod ic polar ization curves were obtained on a record er in conjunction with a bridge ci rcu it to eliminate t h e I R drop.The resul ts show that the best d egree of p rotection was achieved on t he specimen controlled at -0.77 volt (without IR) with refere nce to a saturated calo mel h alf-cell.This is approximately eq uivalent to the protective potential -0.85 volt with reference to t he copper-copper sulfate electrode.Applied current indicated by thc break (change-in-slope) in t he cathodic polarization curve agreed reasonably well with t he actual cu rrent nece sar y to maintain polarization at -0 .77volt (free of IR ).The curre nt rcq uired for protection was abo ut three t im es the m ag nitud e of the co rrosion current; t herefore, the corrosion reaction was either under a nodic control (unli ke prev ious studies) or an equ ivalent type of control which was caliS d by high resistance at anodic areas .