Failure Analysis of Fasteners in a Remotely Operated Vehicle (ROV) System
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A failure analysis study was carried out on AISI 204 Cu stainless steel full-threaded fasteners and end-threaded SS 316L fasteners used in a deep water work class ROV system. The AISI 204 Cu stainless steel full-threaded fasteners were used in a Ti–6Al–4V alloy pressure case. The end-threaded SS 316L fasteners were used for attaching the bottom fender in the tether management system (TMS) made of Al 6061-T6 alloy. The mechanical property of both the fasteners was found to be within standard specification. The SEM study of the full-threaded fasteners revealed that the corrosion pit formation in the unfractured interface area is the cause of crack initiation. Subsequently, the crack propagated by stress corrosion cracking and finally, failed by fast overload cleavage fracture. It can be reasoned that a lower wt.% of molybdenum in AISI 204 Cu stainless steel reduced the pitting corrosion resistance of the full-threaded fasteners. The end-threaded fastener failed by stress corrosion cracking, but corrosion pit was not found in the fracture surface, due to the lack of its interface area with the unfractured region. It can be inferred that the chlorine ions of seawater break the oxide film of stainless steel bolts to form corrosion pits. These corrosion pits act as a stress concentration point to initiate a crack, and subsequently the bolts fail by stress corrosion cracking, showing a brittle appearance.
KeywordsPitting corrosion Stress corrosion cracking Remotely operable vehicle (ROV) Tether management system (TMS) Transgranular fracture
The authors wish to acknowledge the support extended by the Marine Biotechnology team for their help during the SEM analysis. The authors also thank Dr. M. A. Atmanand, Director, NIOT, Chennai, for his support and encouragement. This work was funded by the Polymetallic Manganese Nodule (PMN) Programme and the Gas Hydrate Programme of the Ministry of Earth Sciences, Government of India.
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