Effects Of Aluminum Spray Coating On Marine Corrosion Fatigue Crack Initiation Of X80Steel

The drilling risers are subjected to both alternating stress and corrosion medium duringthe exploitation of offshore oil and natural-gas. Their main failure modes are corrosion andcorrosion fatigue. Previous research indicates that aluminum spray coating can significantlyimprove the corrosion fatigue properties of the offshore structures such as drilling risers. Butvery few studies are focused on the influence and effect mechanisms of the aluminum spraycoating on corrosion fatigue crack initiation. The corrosion fatigue crack initiation period hasimportant influence on corrosion fatigue life. In this paper, slow strain tension electrode testand corrosion fatigue test were carried out to study the influence and mechanisms of thecoating‘s cathodic protection on the corrosion fatigue crack initiation of X80steel combinedwith surface morphology observation, fractography and numerical simulation.Strain electrode tests under various strain rates were carried out to quantitatively evaluatethe influence of aluminum spray coating’s cathodic protection on X80steel’s crack initiation.The results indicated that X80steel’s crack initiation life under different strain rates wasincreased when coated steel potential was applied on X80steel. With the rising of strain rate,the coated steel potential’s improving influence on crack initiation life and total life wasweakened. The proportion of crack initiation life in total life was reduced.Results of corrosion fatigue test under different stresses indicated that aluminum spraycoating’s improving influence on corrosion fatigue life of X80steel was enhanced with thedecreasing of stress. Surface morphology observation and cross-section morphology of pittingcrack source of bare and coated X80steel showed that a large number of conic corrosion pitscould be observed on the surface of both bare and coated X80steel. Compared with the barecorrosion fatigue specimen, the dimensions of coated specimen’s pits with crack were smaller; the pits were sharper and the stress concentration was more serious.Based on the simulated result of the stress field around the pit based on ANSYS, KICforcrack initiation at the bottom of corrosion pit was calculated. With the existence of coatingKICwas significantly increased. Thus the critical stress components needed by the crackinitiation at the bottom of corrosion pit should be higher. In this case, only the pit was sharperand the stress concentration was more serious could the crack initiate at the bottom of thecorrosion pit. By the way, the effect of cathodic protection of aluminium coating prolongedthe initiation of the corrosion pit significantly. Because the process of corrosion pit initiationand crack initiation at the bottom of the pit were both inhibited. With the existence of thealuminum spray coating’s cathodic protection, the initiation of the corrosion fatigue crack wasinhibited effectively; and the corrosion fatigue life was prolonged.