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

P.E. Barnes 1, 2, S.B. Lyon 2, Tom McLaughlin3, D.L. Engelberg 1,2

1 Advanced Metallic Systems CDT, The University of Manchester, Manchester, UK

2 Corrosion & Protection Centre, The University of Manchester, Manchester, UK 3 Intertek CAPCIS, Manchester, UK

Summary Corrosion fatigue cracks have been observed in a high strength steel wire subjected to a CO2 saturated synthetic seawater environment. These cracks are believed to nucleate at corrosion pits, and a statistical crack and pit population assessment sup-ports this observation. Pit and crack depth morphology data has been analysed. The effect of applied stress and plastic strain during fatigue testing increased the crack and pit population.

1. Introduction Flexible risers are used in the oil and gas industry to transport oil-based products from the seabed to floating production facilities. These risers are exposed to cyclic wave loading and the aggressive seawater/oil/gas environment. The combined cyclic loading and aggressive environment mean that the risers are susceptible to corrosion fatigue. 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 a fluid barrier. The outer layers of the wire, which resist the applied axial load, are made from high strength carbon steel. Condensed seawater and corrosive gas-es, such as carbon dioxide can permeate through the multilayer system into the an-nular space between the different layers of wire, creating an environment in which corrosion fatigue can occur [1],[2]. The objective of the work reported in this paper was to determine the relationship between crack initiation, microstructure susceptibil-ity, exposure environment and the effects of cyclic stress on the corrosion fatigue process. The work forms part of a Ph.D. project to strengthen the understanding of the behaviour of flexible risers and to recommend improvements in material selec-tion.

2. Experimental The material used in this work was a cold drawn, high strength carbon steel wire with dimensions of 12mm x 4mm x 20 mm (W x T x L). The chemical composition and mechanical test data is summarised in Tables 1 and 2.

Table 1: Carbon steel wire chemical composition (wt.%) Chemical Element

Fe C Si Mn P S Cr Mo Ni N

Weight Percent

(%) Bal. 0.61 0.20 0.69 <0.01 0.003 0.01 <0.01 0.01 0.0048

Table 2: High Strength Carbon Steel 12x4mm Wire Mechanical Properties 0.2% Proof

Stress (MPa) Elongation at

Yield (%) Ultimate Tensile Strength (MPa)

Elongation at Failure (%)

1000 0.49 1450 25.3

Corrosion fatigue tests were performed at five nominal stress ranges in CO2 saturat-ed, modified (5%NaCl) ASTM D1141 synthetic seawater without the addition of heavy metal salts. A stress ratio of R=0.1 and a test frequency of 2Hz was used, at a temperature of 20°C. A displacement controlled, four-point bend test was used, with a strain gauge attached to the compression side of the sample. The strain gauges were protected from the environment by rubber sealant and bitumised tape. The synthetic seawater environment was purged with high purity (99.995%) carbon diox-ide, until the oxygen concentration was less than 5ppb. An Orbisphere was used to measure the oxygen concentration, before the test solution was introduced into the corrosion fatigue test chamber. To maintain the oxygen level in the test chamber, continuous CO2 purge of the environment was provided throughout the experiment. A post-fatigue tested sample was selected from the four nominal stress ranges (Ta-ble 3), and the sample sectioned along the maximum stress region, situated between the 4-point bend contact points. A sampling length of 20mm for the pit and crack analysis was used. The secondary fatigue cracks were observed in the normal plane; therefore all samples were then sectioned along the transverse plane, perpendicular to the maximum stress direction. The samples were then mounted in Bakelite and polished to ¼ micron diamond paste finish, followed by an OPS finish. Immersion in 2% Nital (2% Nitric acid, 98% Ethanol) was used to reveal the microstructure, to ac-quire information about the fatigue crack path. Using the optical microscope with ac-quisition software Axiovision, images of the crack depth were obtained, and the verti-cal distance from the edge of the sample to the crack tip measured. Likewise, pit depth was measured to be the vertical distance from the edge of the sample to the base of the pit.  The crack and pit depth data was analysed and a best-fit distribution for the pit and crack depth data log-normal distribution was found to best fit the data [3], and this type of distribution requires the use of the geometric mean which was calculated us-ing the formulae in [4].

Table 3: Sample data showing a summary of conducted tests

Sample ID

σmean (MPa)

σcalculated range (MPa)

Log Cycles to Failure

No. of Secondary

Cracks

No. of Corrosion

Pits

R1C0 428 700 5.09 104 518

R2C0 367 585 5.30 95 325

R3C3 275 424 5.37 16 236

R4C2 214 337 5.87 19 174

3. Results and Discussion Table 3 summarises the number of secondary cracks and corrosion pits observed in all four test samples. Secondary cracks were observed to initiate at the free surface of the wire in the normal plane. The crack paths run parallel to the path of the ap-plied bending stress, and as the applied stress increases, the crack population and the number of pits increases.

Two types of crack path are observed in the corrosion fatigued samples, transgranu-lar and intergranular, See Figure 1. The cracks tend to initiate as transgranular, from localised corrosion pits, before bifurcating into transgranular and intergranular cracks. Intergranular cracking along the grain boundary is caused by a variety of situations, such as the precipitation of brittle phases on the grain boundary, in this case cement-ite. Fatigue crack paths tend to display regular deflections, large branching’s and bifurcations [5]. Figure 2 shows that the secondary cracks generated vary greatly in depth between 23 - 2428µm, with a geometric mean of ~140µm. It is widely observed in the litera-ture that fatigue cracks initiate from stress raisers [6][7]. These act to breakdown the protective surface film, in this case Calcite (CaCO3) found using Glancing Angle X-ray Diffraction (GAXRD) techniques, thus exposing bare metal to the corrosive envi-ronment and initiating a localised corrosion pit. Figure 2 also shows that the local-ised corrosion pits vary greatly in depth between 4 - 40µm, with a geometric mean of 10µm. The results in Figure 3 show that by increasing the applied stress the number of lo-calised corrosion pits is also increased, meaning that the fatigue process is causing more plastic deformation at the sample surface. Similarly, the number of secondary cracks is increasing with applied stress due to the increase in initiation sites.

(a)

(b)

Figure 1: Images of (a) Transgranular Crack and (b) Intergranular Crack from a Corrosion Pit

4. Conclusions Overall, as the applied stress increases, the crack population increases, however, the geometric mean crack depth decreases. The crack depth measurement statistics satisfy a lognormal distribution over the whole range of applied stress. Localised cor-rosion pitting is observed on the surface of the fatigue-tested samples, and pit depth measurement statistics indicated a log-normal distribution over the whole range of applied stress. It has been observed that as the applied stress increases the corro-sion pit population increases. This is thought to be due to the increase in plastic de-formation at the surface of the material from the fatigue process.

(a)

(b)

Figure 2: Probability Plots (a) Crack Depth and (b) Pit Depth

(a)

(b)

Figure 3: Applied Stress against Population (a) Secondary Cracks and (b) Localised Corrosion Pits

5. References [1] A. Rubin and J. Gudme, “Qualification of Steel Wire for Flexible Pipes,” in

NACE International, 2006, no. 06149, pp. 1–19.

[2] H. Morand, A. Felix-Henry, and J. Remery, “Design and Qualification of High Pressure Flexible Pipes,” PetroMin Pipeliner, pp. 26–32, 2008.

[3] M. Kowaka, H. Tsuge, M. Akashi, K. Masamura, and H. Ishimoto, Introduction to Life Prediction of Industrial Plant Materials: Application of the Extreme Value Statistical Method for Corrosion Analysis. 1994, p. 300.

[4] Z. Szatrowski, “Calculating the Geometric Mean from a Large Amount of Data,” Journal of the American Statistical Association, vol. 41, no. 234, pp. 218–220, 1946.

[5] J. Toribio, B. González, and J. C. Matos, “Fatigue and fracture paths in cold drawn pearlitic steel,” Engineering Fracture Mechanics, vol. 77, no. 11, pp. 2024–2032, Jul. 2010.

[6] F. Church and T. S. Srivatsan, “Fatigue Processes in Metals - Role of Aqueous Environments,” Engineering Fracture Mechanics, vol. 36, no. 6, pp. 827–852, 1990.

[7] Y. Wang and R. Akid, “Role of Nonmetallic Inclusions in Fatigue, Pitting, and Corrosion Fatigue,” Corrosion Science, no. February, pp. 92–102, 1996.