Abstract
A modified noise resistance method was developed and applied for corrosion monitoring at several chemical plants. A data-acquisition system capable of measuring the electrochemical noise under the site's conditions also was developed. The noise resistance, Rn, has been regarded as an equivalent factor to the polarization resistance, Rp. However, it was considered that this estimation might not be appropriate because the real dimensions of the anodic and cathodic areas may vary with immersion time as well as with the combination of materials and environments. Therefore, a new factor, G, was introduced instead of the Stern-Geary constant, B, that is used for the conventional polarization-resistance monitoring technique. The factor ‘G’ was named the corrosion factor. In the modified method, the corrosion rate, Cr, was defined as follows: CR = G•I/Rn. The G value was determined experimentally from the relationship between the mass-loss rate and the Rn of electrodes made with equivalent material to the objective of the monitoring effort. A test solution, which simulated the environment of the site, was also used for the experiment. Applying this modified method, the occurrence of unexpected and intense general corrosion at a plant was successfully detected by a combination of plant and laboratory tests. The corrosion loss estimated by the method agreed closely with tube wall thickness loss measured on damaged tubes in the plant.
