Comparison of Corrosion Rate-Measuring Devices for Determining Corrosion Rate of Steel-in-Concrete Systems

The principles of the various corrosion rate-measuring instruments (3LP, Nippon Steel Company, and GECOR devices) are presented. The corrosion rate determined by these devices is compared with that obtained by standard electrochemical and nonelectrochemical corrosion rate-measuring methods. For corroding steel coupons in acid, very good agreement was found between the electrochemical and nonelectrochemical rate-determining systems, but for passive specimens, the agreement was not so good. A linear relationship was observed between the results obtained by the Nippon device and impedance spectroscopy for small mortar specimens irrespective of the test solution, exposure to solution, or test cycle. The significance of this relationship is discussed. Testing of large concrete specimens shows that for passive steel-in-concrete systems, all devices were incapable of confining the signal distribution from the counter electrode to the rebar area directly below the counter electrode when the size of the working electrode far exceeded that of the counter electrode. This may be due to either the inherent difficulty in confining signal distribution in a passive steel-in-concrete system or the small size of the counter electrode used. However, for an active system, the confinement achieved resulted in calculated R_p values that are much closer to the actual average rate.