Relationship between electrochemical characteristics and SCC of X70 pipeline steel in an acidic soil simulated solution

Size: px

Start display at page:

Download "Relationship between electrochemical characteristics and SCC of X70 pipeline steel in an acidic soil simulated solution"

Brenda Mosley
6 years ago
Views:

1 Acta Metall. Sin.(Engl. Lett.)Vol.22 No.1 pp58-64 Feb Relationship between electrochemical characteristics and SCC of X70 pipeline steel in an acidic soil simulated solution Zhiyong LIU 1), Xiaogang LI 1), Yingrui ZHANG 2), Cuiwei DU 1) and Guoli ZHAI 1) 1) Key Lab of Corrosion Erosion and Surface Technique Beijing, Corrosion and Protection Center, University of Science and Technology Beijing, Beijing , China 2) Mechanical Engineering Department, University of Rochester, NY, 14627, USA Manuscript received 3 March 2008; in revised form 31 August Introduction Stress corrosion cracking (SCC) of X70 pipeline steel in simulated solution of the acidic soil in Yingtan in China was investigated using slow strain rate test (SSRT), SEM and potentiodynamic polarization technique. Experiment results indicate that X70 steel is highly susceptible to SCC as applied potential reduces, which is manifested in loss of toughness and brittle fracture. Constant polarization current can detect the occurrence of SCC. The lower the polarization current is the sooner stress corrosion cracking occurs. The SCC mechanisms are different at varying potentials. When potential is higher than open circuit potential, anodic process controls SCC, whereas when potential is far lower than open circuit potential, cathodic process controls SCC, and between these two potential regions, a combined electrochemical process controls the SCC. Stress or strain has a synergistic effect with electrochemical reactions to accelerate the cathodic hydrogen evolution process, which makes the X70 pipeline steel to be more susceptible to SCC. KEY WORDS X70 steel; SCC; Electrochemical process; Acidic soil SCC of pipeline steels in soil has been one of the biggest safety problems in pipeline operation. It has occurred overseas and resulted in great economic loss [1]. Until now, hydrogen has been considered as a key factor for SCC of pipeline steel in low ph condition, including near-neutral ph [2 5] andacidsolutions [6]. Parkins et al. [2] considered that the mechanism of near-neutral ph SCC involved dissolution and the ingress of hydrogen into the steel, however, the evidence in support of hydrogen playing a role in the overall growth process was circumstantial rather than direct. Gonzalez-Rodriguez et al. [7] considered that the mechanism of SCC in API X80 pipeline steel in 0.01 mol/l NaHCO 3 solution was dominated by film rupture and anodic dissolution (AD). Park et al. [8] indicated that transgranular SCC in near-neutral ph bicarbonate solution was developed by a pit-to-stress corrosion crack transition mechanism. Chen et al. [9] considered that hydrogen embrittlement mechanism would become operative following dissolution mechanism for crack initiation and initial growth. Gu et al. [10] indicated that hydrogen could diffuse into Corresponding author. Professor, Tel.: ; Fax: address: lixiaogang99@263.net (Xiaogang LI) DOI: /S (08)60071-X

59 the specimen around the crack tip during SCC of pipeline steels in the near-neutral ph solution, and hydrogen accelerated the dissolution rate of the steel in solution, resulting in increasing SCC

2 59 the specimen around the crack tip during SCC of pipeline steels in the near-neutral ph solution, and hydrogen accelerated the dissolution rate of the steel in solution, resulting in increasing SCC susceptibility. Beavers et al. [7] pointed out that SCC initiation and propagation conform to different mechanisms, in which SCC occurs due to AD in the beginning and then propagates in a mechanism of hydrogen induced cracking (HIC). Wang et al. [11] reported that SCC was controlled by hydrogen embrittlement (HE) below 800 mv SCE and by AD within mv SCE. By far, more and more evidences in soil environments have demonstrated the effect of hydrogen on SCC of pipeline steel [12 14]. However, only few literatures clearly demonstrated what factor leads to hydrogen absorption in SCC crack-tip zone and how hydrogen works during the SCC process. Meanwhile, solutions with high ph and near neutral ph had been widely used in laboratory tests to simulate the conditions in soils overseas [15,16], but little research on SCC in Chinese soil has been carried out, and a simulation solution has not been developed. Sour soil in southeast China is with low oxygen, high water concentration, and is very likely to induce SCC; therefore it is important to study the SCC of X70 pipeline steel and its weld area in this sour soil. The objective of this work is to investigate interactions between stress or strain rate and cathodic process, using electrochemical polarization technique, SSRT and SEM to find out the effect of cathodic electrochemical process on SCC of an API X70 pipeline steel in natural acidic soil in Yingtan, China. 2 Experimental The X70 pipeline steel used in this work is cut from a hot-rolling plate supplied by Bao steel Co. Ltd. of China, with the following chemical composition (in wt pct): C 0.045, Si 0.26, Mn 1.48, Nb 0.033, Ni 0.16, Cu 0.21, Mo 0.23, Al 0.035, S 0.001, P 0.017, and Fe balance. The microstructure of X70 steel matrix is shown in Fig.1. To obtain a suitable solution to simulate the acidic soil environment in southeast China, sour soil in Yingtan is chosen because it is one of representative soil environment in the north center of acid soil area in southeast Fig.1 Microstructure of X70 pipeline steel (fine polygonal ferrite with small marten-austenitic particles at grain boundaries, which result in a small grain size and high strength). China, with a ph range from 4.0 to 5.5, and the soil is taken from underground of 1.5-meter depth in Yingtan, China. The simulating solution is to simulate the liquid mixture in soil, prepared according to the analysis data of soil leach extracted from mixed pure water and dry soil in 1:1 mass ratio, the main chemical composition (g/l) of which is CaCl , NaCl , Na 2 SO , MgSO 4 7H 2 O , KNO , NaHCO , and the ph is 4.0 controlled by HAc. Pure nitrogen gas is bubbled into the solution during all experiments to eliminate oxygen. SSRTs were performed at 650 mv SCE, 850 mv SCE, 1200 mv SCE, as the open circuit potential is about 700 mv SCE, at a strain rate of s 1 at room temperature

3 60 to detect the SCC susceptibility of X70 pipeline steel in the simulating solution. Tensile test samples were made according to GB/T All samples were polished parallel to the tension direction to 800 grits using emery paper and soaked in solution for 24 h before tension tests. Furthermore, electrochemical techniques including dynamic polarization technique and constant potential polarization were used to investigate electrochemical characters and their impact on the mechanism of SCC for X70 pipeline steel. Dynamic polarization curves were measured under two cases, the ones with strain or stress, and the others without stress. Three-electrode system is used in all tests, with X70 steel as the working electrode, platinum plate as auxiliary electrode, and saturated calomel electrode (SCE) as the reference electrode. Specimens were polished to 800 grits with emery paper before tests. 3 Results and Discussion 3.1 SCC susceptibility of X70 pipeline steel obtained by slow strain rate test The SSRT [17] was conducted at a constant strain rate of s 1 using a computerassisted slow strain tensile test machine made in Xi an, China. The stress-strain curves of the X70 steel exhibited reductions both of the strain to rupture and ultimate tensile strengths (UTS) obtained in the Yingtan soil simulated solution at varying potentials compared to that in air, as shown in Fig.2. Toughness loss is calculated to describe the SCC susceptibility here, results of which are shown in Fig.3. The area-contraction loss and loss of elongation-percentage are defined as I ψ and I δ, respectively. I ψ =(1 ψ E /ψ 0 ) 100%, and I δ =(1 δ E /δ 0 ) 100%, where ψ E represents the area-contraction at applied polarization potentials, and ψ 0 is area-contraction in air. Similarly, δ E is the elongation percentage at polarization potentials, and δ 0 is the elongation percentage in air. As shown in Fig.3, I ψ is similar to I δ at every applied potential. Both I ψ and I δ increase as polarization potential reduces, which indicates that X70 pipeline steel has substantial SCC susceptibility at various potentials and the SCC susceptibility increases when potential gets lowered. As already known, below the zero current potentials, which are approximat- Fig.2 SSRT curves of X70 pipeline steel at different potentials and in air. Fig.3 SCC susceptibilities at various potentials.

61 ely 700 mv SCE for X70 steel in simulated solution in this work, hydrogen blister will occur and will become more intensive as the potential is further lowered, such as at 850 mv SCE and 1200 mv

4 61 ely 700 mv SCE for X70 steel in simulated solution in this work, hydrogen blister will occur and will become more intensive as the potential is further lowered, such as at 850 mv SCE and 1200 mv SCE. Hydrogen diffusing into the metal reduces the ductility and enhances the susceptibility to SCC. Therefore I ψ and I δ increase as potential drops from anodic range to cathodic range. SEM micrographs of fracture surface at different potentials are shown in Fig.4, which provide good evidences to the rupture mechanism. It can be seen that specimens exhibit a ductile-type failure of dimple in air and in solution at 650 mv SCE, but a cleavage fracture morphology at 850 mv SCE and 1200 mv SCE, which reveals that the decline of cathodic potential leads to an increase of SCC susceptibility because of the intensification of hydrogen evolution [11]. Fig.4 SEM fractographs in air (a), in solution at 650 mv (b), at 850 mv (c) and at 1200 mv (d). 3.2 Relationship of constant potential polarization and the initiation of SCC The current recorded during constant potential polarization test fluctuated as tensile specimen is elongated, and a sudden up or down in the current recorded indicates a sharp change on the surface of sample, which means SCC has occurred. Fig.5 shows that all current peaks appeared within plastic deformation region during elongation, which means that SCC takes Fig.5 Constant potential curves at varying polarizing potential showing sudden waves in plastic deformation range.

62 place in plastic status and decline of applied polarizing potential will accelerate the occurrence of cracks. This is also confirmed by the side surface morphology of tensile specimen in Fig.6. At 850 mv SCE, there are no micro-cracks found on a sample sidesurface when elongation percentage is small and no sudden polarization current peaks were observed, as shown in Fig.

5 62 place in plastic status and decline of applied polarizing potential will accelerate the occurrence of cracks. This is also confirmed by the side surface morphology of tensile specimen in Fig.6. At 850 mv SCE, there are no micro-cracks found on a sample sidesurface when elongation percentage is small and no sudden polarization current peaks were observed, as shown in Fig.6a. However, when the elongation of sample is large enough for a sudden current peak to appear, micro stress-corrosion-cracks appeared on its surface as shown in Fig.6b. Fig.6 Surface morphologies of specimen at 850 mv before (a) and after (b) constant polarization current sudden alteration according to Fig Dynamic polarization curves and their implications on SCC susceptibility Potential dynamic polarization curves measured by slow and fast sweep rate tests on samples of X70 pipeline steel in the simulation solution are shown in Fig.6, which can be used to forecast the different electrochemical processes between non crack-tip area and the crack-tip [2]. In this work, slow sweep rate is 0.5 mv/s, and fast sweep rate is 50 mv/s. Fast sweep curve is used to estimate the electrochemical reactions at the crack-tip and slow sweep curve is used to estimate those at non crack-tip area. As shown in Fig.7, there are great differences between the fast and slow sweep polarization curves. According to these two curves, there are three polarization current regions in logarithm exhibiting different electrochemical reactions within varying potential range. When potential is higher than 690 mv SCE, in which 650 mv SCE is located, the current is positive for both fast and slow sweep curves, indicating that the electrochemical reactions are anodic both at crack-tip and in non-cracktip area. For the case of potential within range of 1030 mv SCE to 690 mv SCE, where 850 mv SEC is located, the current of slow sweep curve is minus whereas the current fast-sweep-rate curve is positive, Fig.7 Fast and slow sweep curves of the X70 pipeline steel.

6 63 meaning that the electrochemical process is opposite between in and out of crack-tip area, and crack-tip is anode whereas non-crack-tip area is cathode. However, when potential is further lower, such as at 1200 mv SCE, the current of both these curves is minus, manifesting that there is little difference between crack-tip and non-crack-tip zone when both are controlled by cathodic electrochemical process [18]. Above results reveal that mechanism of SCC of X70 steel in Yingtan simulation solution is anodic dissolution at 650 mv SCE,a combination of anodic dissolution and hydrogen embrittlement at 850 mv SCE, and hydrogen embrittlement at 1200 mv SCE. According to fracture morphologies shown in Fig.3 and Fig.7, hydrogen evolution will increase the SCC susceptibility of X70 steel, whereas anodic dissolution leads to a low SCC susceptibility in acidic soil simulating solution. However, according to the results of constant polarization tests (Fig.5), both at 850 mv SCE and 1200 mv SCE, polarization currents fluctuate up suddenly when SCC occurs, which indicating that not only at 850 mv SCE but also at 1200 mv SCE crack-tip is anode when SCCs take place actually. These results support the data in Fig.7. This is mainly due to the fact that HCO 3,H 2CO 3 and H + are important factors to SCC in tested solution according to Ref.[19]. H 2 CO 3 comes from HCO 3 in acid environment. The H + H process speed up the permeation of hydrogen into metal matrix and microcracks-induced hydrogen embrittlement will increase the susceptibility of SCC [10]. Meanwhile, stress status will interact with the electrochemical process [20], which is one of the characteristics of SCC. As shown in Fig.8a, stress mainly impacts cathodic reaction whereas has little effect on the anodic process. Dynamic electrochemical cathodic curves with stress in elastic deformation status move toward the direction of the increase in current, comparing to those without stress, and the movement is related to the strain rate. The movement shows a maximum within a certain strain rate range such as s 1, however at the status of higher strain rate ( s 1 ) or without strain (strain rate is 0), less movement takes place. This reveals that stress or strain status will accelerate the hydrogen evolution process to expedite the hydrogen embrittlement of X70 pipeline steel. What is more, the most intensive increase of current in the strain rate range of s 1 is a good evidence of synergistic effect of stress and electrochemical process leading to SCC. As shown in Fig.8b, this synergistic effect becomes more intensive at plastic deformation status both for the cathodic and anodic process. Fig.8 Effect of stress state on electrochemical process without plastic deformation (a) and with plastic deformation (b).

7 64 4 Conclusions (1) It is clear that X70 pipeline steel is highly susceptible to SCC in simulation solution of Yingtan soil, which is characterized by loss of toughness and brittle fracture. (2) Constant polarization current can detect the occurrence of SCC, and the lower the polarization is the sooner stress corrosion cracking comes into being within the potential range of mv SCE. (3) The SCC mechanisms are different. Within the potential region of upper open circuit potential, anodic process controls SCC; within the region of far lower than open circuit potential, cathodic process controls SCC; between these two regions, a combined electrochemical process controls the SCC. (4) Stress or strain will interact with electrochemical reactions to accelerate the cathodic hydrogen evolution process, which makes the X70 pipeline steel susceptible to SCC in the simulated acidic soil solution. Acknowledgements This work was supported by Chinese National Science and Technology Infrastructure Platforms Construction Project (No.2005DKA10400), and Major Foundation in the Tenth Five-Year Development Plan of China (No ). REFERENCES [1] C. Manfredi and J.L. Otegui, Eng Failure Anal 9 (2002) 495. [2] R.N. Parkins, W.K. Blanchards Jr and B.S. Delanty, Corrosion 55 (1999) 312. [3] L.J. Qiao, J.L. Luo and X. Mao, Corrosion 54 (1998) 115. [4] T. Kushidu, K. Nose, H. Asabi, M. Kimura, Y. Yamane, S. Endo and H. Kawano, Corrosion/2001/NACE Int (Houston, TX, 2001) p.213. [5] M.P.H. Brongers, J.A. Beavers, C.E. Jaske and B.S. Delanty, Corrosion 56 (2000) [6] Z.Y. Liu, G.L. Zhai, C.W. Du and X.G. Li, Acta Metall Sin 44 (2008) 163. [7] J.G. Gonzalez-Rodriguez, M. Casales, V.M. Salinas-Bravo, J.L. Albarran and L. Martinez, Corrosion 58 (2002) 584. [8] J.J. Park, S.I. Pyun, K.H. Na, S.M. Lee and Y.T. Kho, Corrosion 58 (2002) 329. [9] W. Chen, F. King and E. Vokes, Corrosion 58 (2002) 267. [10] B. Gu, J.L. Luo and X. Mao, Corrosion 55 (1999) 96. [11] Z.F. Wang and A. Atrens, Metall Mater Trans 27A (1996) [12] B.Y. Fang, A. Atrens, J.Q. Wang, E.H. Han, Z.Y. Zhu and W. Ke, Mater Sci 38 (2003) 127. [13] B.W. Pan, X. Peng, W.Y. Chu, Y.J. Su and L.J. Qiao, Mater Sci Eng A 434 (2006) 76. [14] Y.F. Cheng, Int J Hydro Ener 32 (2007) [15] R. Chu, W. Chen, S.H. Wang, F. King, T.R. Jack and R.R. Fessler, Corrosion 60(3) (2004) 275. [16] J.Q. Wang and A. Atrens, Corros Sci 45 (2003) [17] W. Chen, F. King, T.R. Jack and M.J. Wilmott, Metall Mater Trans A 33A (2002) [18] B. Gu, W.Z. Yu, J.L. Luo and X. Mao, Corrosion 55 (1999) 312. [19] S. Nesic, J. Postlethwaite and S. Olsen, Corrosion 52(4) (1996) 280. [20] S.A. Serebrinsky and J.R. Galvele, Corros Sci 46 (2004) 591.

Synergistic Effect of AC and Cl - on Stress Corrosion Cracking Behavior of X80 Pipeline Steel in Alkaline Environment

Int. J. Electrochem. Sci., 13 (2018) 10527 10538, doi: 10.20964/2018.11.48 International Journal of ELECTROCHEMICAL SCIENCE www.electrochemsci.org Synergistic Effect of AC and Cl - on Stress Corrosion