Monitoring methodologies of the acid cleaning of steam generators

Abstract

AbstractSome general considerations about the validity and usefulness of corrosion rate monitoring using the polarization resistance method are developed and the theory of Stern and Geary is shortly discussed. The basic aspects of the acid cleaning of steam generators, the role of corrosion inhibitors and the approach of the author with respect to the problem of monitoring this operation are also reviewed. The main aim of this applied research was to verify the reliability of a corrosion meter in predicting the correct order of magnitude of the corrosion rate of a plain carbon steel and two low alloy steels. Measurements under dynamic conditions, with the solution flow rates of 0.5, 1.0 and 1.5 ms−1, were performed in 5% by weight HCl solutions at 75°C containing a specific commercial inhibitor at two concentrations. Results indicate that the manufacturer calibration of the corrosion meter is not able to provide a realistic evaluation of the corrosion rate of the probe electrodes. Furthermore, it was experimentally demonstrated that probe and tubular specimens of the same material exhibit a different electrochemical behaviour. Therefore, a reliable evaluation of the true corrosion rate of tubular specimens should be made using appropriate calibration charts for the given environment and geometry of the electrochemical probe. At last, some alternating current measurements performed at 10 Hz showed that corrosion rate monitoring based on low‐frequency signals could prove very useful for controlling the aggressiveness of the acid‐cleaning solution.