CURRENT DEMAND FOR CATHODIC PROTECTION OF COATED STEEL - 5 YEARS DATA

摘要

The current demand for cathodic protection of coated steel samples have been measured for 5 years. Organic coatings of various thickness and type have been tested, representing different classes of coatings according to DNV RP 401 and NORSOK M501. The samples were exposed in natural seawater and polarised with zinc anodes. For samples with only a primer coating (50 mu m thick) the current demand started to increase immediately after immersion. However, after a few months the current demand stabilised and it has been stabile since. The current demand after five years was 10 - 20 percent of the value for bare steel. For samples with 150 mu m thick coatings the current demand increased during the first two - three years of exposure, but was stabile since. The current demand for these coatings after five years was 3 - 6 percent of the value for bare steel. For samples with 450 mu m thick coatings or more the current demand has been low during the five years of exposure. For one of the coatings the current demand started to increase during the last year, but the current demand was still below 1 percent of that for bare steel. Cathodic disbonding around mechanical damage in the coatings does not seem to affect the current demand. Blistering due to cathodic reduction of oxygen under the coating seems to be the most important factor for the current demand. A conceptual model for predicting the current demand as function of coating thickness and time is suggested.