Effect of Fe ion concentration on fatigue life of carbon steel in aqueous CO₂environment

Abstract

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 [...

Crack growth behaviour and crack tip chemistry of X70 pipeline steel in near-neutral pH environment

“…Secondly, a higher frequency results in a higher crack tip strain rate, which promotes the formation of a sharp crack tip and accelerates the CGR.

8 Comparison of the CGR in this work with the values reported in the literature 20,23

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“…20,24 Figure 8 compares the CGR in this work with the values reported in the literature in which the experiments are conducted under the similar R ratio and Δ K . 20,23 For ease of comparison studies, all the CGRs are expressed in terms of d a /d t with a unit of mm/s. It shows that the crack growth rate increases with the increase of the loading frequency.…”

“…The important role of cyclic loading in the crack growth has been demonstrated in laboratory investigations. [18][19][20][21][22][23] Parkins pointed that the cyclic loading could raise the crack tip strain rate and reactivated the dormant cracks. The cracks would keep dormancy under a constant load and could re-start propagating when cyclic loads were applied.…”

An electrochemical and X-ray computed tomography investigation of the effect of temperature on CO2 corrosion of 1Cr carbon steel

“…Corrosion of mild carbon steel due to the presence CO 2 is a common problem in the oil and gas industry. The CO 2 gas dissolves in the water phase and hydrates to carbonic acid, promoting electrochemical reaction at the metal-water interface leading to iron dissolution [1][2][3][4] [5].…”

Corrosion of carbon steel under CO2 conditions: Effect of CaCO3 precipitation on the stability of the FeCO3 protective layer

“…This is due to its relatively low price, its availability in the volume required and the possibility of easily satisfying mechanical and fabrication requirements [1]. When CO2 dissolves in water and hydrates to carbonic acid (H2CO3), it promotes an electrochemical reaction between the steel and the aqueous phase, leading to iron dissolution [2][3][4] [5][6] [7]. This process has been under investigation for more than four decades, but the corrosion mechanism occurring at the metal-electrolyte interface is still a subject of research [8] [9].…”