Abstract
With offshore structures being increasingly installed in frigid regions and deeper waters, it is important to clarify the effects of temperature and dissolved oxygen (DO) content of seawater on fatigue strength. The effect of applied potential for high tensile strength steel should also be clarified, as hydrogen can be introduced under cathodic protection (CP).
In this study, fatigue tests were conducted on welded Tee joints of a 600 MPa-strength steel, by controlling temperature and DO concentration in synthetic seawater with and without CP. The temperature was kept at 30 or 4°C, and the DO concentration was 0 ppm, 5 ppm or of air-saturated state. In the tests with CP, the potential of -0.8, -1.0 or -1.2 V (vs. SCE) was applied on the toes of welded joints.
While the effect of temperature was not remarkable, lowering DO content increased the fatigue strength for the steel under free corrosion. In longer fatigue endurance region, CP of -0.8 or -1.0 V (vs. SCE) improved the fatigue strength in seawater to exceed that in the air. However, cathodic protection at -1.2 V decreased the fatigue strength regardless of DO content.
