In this work, the corrosion fatigue behaviour of steel armours used in the flexible pipes, in aqueous solutions initially containing different concentrations of Fe 2þ , was investigated by four-point bending testing under saturated 1 bar CO 2 condition. Corrosion fatigue results were supported with ex situ measurements of Fe 2þ and pH. Characterisation of the corrosion scales and crack formations was performed using microscopic and diffraction techniques. Fatigue results showed two times better fatigue life, at the stress ranges of 250 MPa, for samples tested in solutions containing the concentration of Fe 2þ marginally above the solubility limit of FeCO 3 compared to the samples tested in highly supersaturated solution of Fe 2þ. Results revealed that the impact of the alternating stresses on the corrosion behaviour of samples reduces with lowering the applied stresses. At the stress range of 100 MPa, fatigue samples experienced the same corrosion rate as samples that were not subjected to dynamic loading.Keywords: Carbon steel, Carbon dioxide, Corrosion fatigue, Supersaturation, Iron carbonate
IntroductionFlexible pipes, which are used in the oil and gas industry, are an attractive alternative to rigid pipes due to their shorter installation times and longer durability. The typical structure of flexible pipes is composed of polymeric and steel layers, which are not bonded together, to ensure the flexibility of the pipe. 1 The fatigue life of a flexible pipe is determined by the fatigue life of the steel armours, which is placed in the annular space between the inner liner and outer sheath. It was reported 2 that the fatigue life of tensile armour depends on the applied stresses, which are generated due to waves and water current, and the corrosive operating environment. The annulus region might be water filled either due to diffusion and condensation of water from the bore or as a result of a breakage of the outer sheath allowing the entry of sea water. In addition, the severity of the corrosion environment may be increased due to diffusion of small molecules such as methane (CH 4 ), carbon dioxide (CO 2 ) and hydrogen sulphide (H 2 S) from the pipe bore through the inner liner into the annulus.1-3 However, due to a low ratio of free water volume (V) to steel surface area (S), typically v0.1 mL cm 22 , 3 a rapid, high supersaturation of Fe 2þ in the liquid is possible. As a consequence, low corrosion rates, usually v10 mm year 21 , are achieved.
3,4The corrosion behaviour of steel in aqueous CO 2 solution is influenced by the formation of protective films, which might decrease their corrosion rate. 5,6 Typical corrosion scales, formed in such an environment, consist of iron carbonate (siderite, FeCO 3 ) and undissolved cementite (Fe 3 C).7,8 A large number of environmental variables, such as pH, 9-11 temperature, 9,12,13 partial pressure 11 and flowrate, 14-17 influence the corrosion behaviour of steel. Further, material characteristics such as microstructure, chemical composition and heat treatment condition [...