Corrosion Fatigue Cracking of High Strength Carbon Steel Wires in CO2-Saturated Seawater Environment

Flexible risers are used to transport oil-based products from the seabed to floating production facilities. These risers are exposed to cyclic wave loading and the seawater/oil/gas environments. Their structure consists of a stainless steel carcass, which is helically wound with multiple layers of carbon steel wire separated by extruded thermoplastics, which act as fluid barrier. The outer layers of the wire are made from high strength carbon steel. Condensed seawater and corrosive gases, such as carbon dioxide can permeate through the multilayer system into the annular space between the different layers of wire, creating an environment in which corrosion fatigue can occur. The work in this paper addresses the interaction between exposure environment, stress range, and frequency with microstructure to identify a strategy to improve the resistance to corrosion fatigue.Corrosion fatigue experiments in aerated and CO2 saturated synthetic seawater environment have been carried out. The number of fatigue cracks observed after testing in the CO2 saturated environment was an order of magnitude higher than in the aerated solution. This was attributed to a significantly increased number of pits that may have acted as stress raisers for crack nucleation. Also, for the CO2 saturated environment, the number of fatigue cracks increases with increasing applied stress, whereas for the aerated environment it is independent of applied stress. Statistical analysis of secondary cracks in the CO2 saturated environment indicates a lognormal distribution of crack depth. The corrosion fatigue cracks in a CO2 saturated environment seem to propagate in a transgranular manner when the stress intensity factor (K) is high, followed by bifurcation along grain boundaries as K decreases. Corrosion fatigue cracks developed in an aerated environment exhibit only transgranular crack paths.