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Fatigue crack growth characteristics of offshore structural steel in marine environment

Memorial University of Newfoundland
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Corrosion fatigue of a metal is a more severe form of fatigue phenomena in the presence of a corrosive environment like seawater or sour gas. The enhanced fatigue crack growth rate has been attributed in part to the anodic dissolution (oxidation of metal) at the crack tip and to hydrogen embrittlement. Numerous studies have produced a wide variety of results regarding the effect of anodic dissolution and cathodic protection on the corrosion fatigue rate of high strength steel. Anodic dissolution can either cause an enhanced crack growth rate or a crack tip blunting causing a much delayed crack initiation or even a crack arrest. There is, however, some consensus about an optimum range of cathodic protection potential (-830mV SCE), around which the offshore steels achieve the maximum corrosion fatigue life. A more positive potential fails to stop the anodic dissolution. A more negative potential causes a higher crack growth rate, (most possibly by generation of hydrogen and hydrogen embrittlement). Micro-organisms, especially Sulfate Reducing Bacteria have been known to have enhanced metal corrosion fatigue. There are mainly two mechanisms responsible for such enhancement. During the respiration process hydrogen is consumed from the system which causes a cathodic depolarisation. Hydrogen sulphide is produced at the end of the respiration cycle, which is known for its contribution to hydrogen embrittlement. -- It is important to understand more about the corrosion fatique phenomena of offshore structural steel in a marine environment. In the present study corrosion fatigue behavior of CSA G 40.21 M 350 WT steel is investigated. Tests were conducted in air, seawater and seawater with cathodic protection. Compact Tension specimens and ASTM substitute seawater had been used in the present study. Crack growth data was acquired using the Alternating Current Potential Drop technique. The data were then analysed according to the ASTM recommendation E647, 1981. A potential of -830 mV (SCE) was applied during the cathodic protection tests. The loading of frequency was 3Hz in air and 0.167Hz in seawater. The air tests were conducted at room temperature and the seawater tests were conducted at 5゚C. The stress ratio for all the tests was 0.05. -- Multiple tests conducted in similar environments produced highly agreeable results. Free corrosion crack growth rate for the intermediate range of ΔK was about 1.5 to 2.0 higher than that in air. However, applying a cathodic protection of -830mV (SCE) reduced the crack growth rate to the level of crack growth rate in air. For all the corrosion fatigue tests (both cathodically protected and freely corroding) ΔKth value was found to be higher than that in air.

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