軟鋼の腐食疲労と塗料の効果

Abstract

Rotating bending and reversed flexure tests on mild steel coated with paints were carried out. Paints used were vinyl and oily ship bottom paints and zinc-rich paint (ZRP). The effectiveness of the impressed current cathodic protection on the painted surface was also examined. In the rotating bending, the fatigue limit was obtained for the specimens coated with vinyl or oily paint, but in the reversed flexure tests, the S-N curves of the painted specimens were similar to those of the bare metal in sea water and no definite fatigue limits were obtained. The difference in the S-N diagrams of the two test methods was probably due to the large deformation at the side edges of the specimen in the reversed flexure test. The S-N diagram of the ZRP coated specimens at relatively high stresses nearly coincided with those of the bare metal in the air, but at the stresses below the fatigue limit in the air, specimens fractured at a nearly equal number of reversals of stress. This might be due to the consumption of the protective ability of ZRP, judging from the change of the electrode potential of the specimens. The fracture of the specimen was protected by repeated painting. The application of the impressed current cathodic protection togather with the oily paint was very effective and the current densities necessary to protect corrosion were reduced to about one-twentith or-thirtith of those of the bare specimens cathodically protected.