Calibration of the electrochemical methods for the corrosion rate measurement of steel in concrete (NT TR 521)

Abstract

  In this study, steel bars were embedded in concrete slabs with different chloride introductions (0%, 1.5%, 3% and 6 % by mass of cement) in order to induce corrosion at different rates. The slabs were cast in one laboratory and distributed to three different laboratories for monitoring of corrosion rate over a period of one year. Four instruments representing two types of electrochemical techniques, that is, linear polarisation technique (GECOR instruments) and galvanostatic pulse technique (GalvaPulse and SP’s instrument), were evaluated in the project through the comparative measurements. Finally, the standard gravimetric method ASTM G1 was used for calibration of the results measured by the non-destructive methods based on the electrochemical techniques. The results show that the three different laboratories measured fairly comparable corrosion rates based on the gravimetric method, indicating that the method used in the project for production of specimens is basically suitable for use in the calibration of any type of non-destructive electrochemical techniques for measuring the corrosion rate of steel in concrete. The corrosion rates measured by GECOR are fairly close to the true mean corrosion rate (mass loss divided by the whole exposed area of steel), while the corrosion rates obtained by the 5 seconds short time galvanostatic pulse measurements from the chloride introduced specimens are close to the true actual corrosion rate (mass loss divided by the corroded area of steel). When multiplying the values measured by GECOR by a factor of 6 (as a pitting factor) for the chloride introduced specimens, the recalculated corrosion rates become very comparable with the true actual corrosion rate. The condition of chloride content in concrete is necessary information for proper judgement of passive or de-passive status of steel when using pulse techniques and proper use of pitting factor when using GECOR instruments.