Fatigue strength of coated steel plate in seawater

Corrosion fatigue tests were carried out in synthetic seawater in order to clarify the temperature effect on the corrosion fatigue strength of coated steel and the effectiveness of coating films of welded joints for the corrosion fatigue strength, which ship classification societies' rules stipulate. Specimens, made of Mt class KA32 TMCP steel plate in 10mm thickness had the notches of stress concentration factor 2.0 at both sides to simulate the stress concentration due to welding at the welded joints. They were exposed to air for 20 days, sprinkled water twice a day to simulate the shipbuilding practice, made surface preparation and finally coated with tar epoxy resin paint. As the results of the tests, the fatigue strengths of the coated specimens in seawater were almost equivalent to those of non-coated ones in lower cycle region, while they were improved in higher cycle region. However, the fatigue strengths of coated specimens of 300 mu m in film thickness were still inferior to those of non-coated specimens in air. The temperature effects were explained in terms of corrosion factor Kc, i e., reduction ratio of corrosion fatigue strength against the one at 25 degrees C. K(c) indicates 1.03 similar to 1.17 at 40 similar to 60 degrees C. Cracks started from small pits of 30 similar to 40 mu m in depth under coating films, which had initiated during water sprinkling, and the films were fractured later. Specimen with film thickness of 200 mu m and over could maintain the same level of fatigue strength as the one of exposed specimens tested in air.