Development mechanism of internal local corrosion of X80 pipeline steel

doi:10.1016/j.jmst.2019.10.023

Abstract

Abstract:

The occurrence and development mechanism of internal local corrosion has always been a controversial topic, and especially under flow conditions. In this paper, an improved high shear force loop was experimentally used, and local flow field is induced by simulating corrosion defects on the surface of X80 pipeline steel specimens. The characteristics of corrosion products deposited on the surface of specimens in CO₂-saturated NACE solution were investigated by means of electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectrometry (EDS). The 3D micromorphology of the corrosion test surface after remove the corrosion scale used to measure the size of localized corrosion pit. Under the influence of local defects, the wall shear stress (WSS) and turbulent kinetic energy of local flow fields enhanced significantly, and pressure fluctuations in local flow field were induced. The results showed that the characteristics of surface corrosion products varied with flow velocity. The corrosion scales formed in various regions of specimens with defects exhibited different surface micro-morphologies and chemical compositions. Overall, these data offer new perspectives for better understanding the mechanisms behind local corrosion.

Key words: X80 pipeline steel, CO₂corrosion, Flow conditions, Surface defects, Wall shear stress, Cavitation

Zhuowei Tan, Liuyang Yang, Dalei Zhang, Zhenbo Wang, Frank Cheng, Mingyang Zhang, Youhai Jin. Development mechanism of internal local corrosion of X80 pipeline steel[J]. J. Mater. Sci. Technol., 2020, 49: 186-201.

Figures/Tables 22

Table 1 Chemical composition of X80 pipeline steel (wt.%).

X80 pipeline steel (balance Fe)
Mn	Si	C	Cr	S	P	Ni	Ti	Nb	Mo	V
1.83	0.28	0.063	0.03	0.0006	0.011	0.03	0.016	0.061	0.22	0.059

Fig. 1. Specification and flow orientation of SAD: left: side view, right: top view, unit: mm.

Fig. 2. Schematic diagram of the experimental system: 1) gas-pressure meter, 2) gas master valve, 3) gas storage tank, 4) gas flowmeter, 5) gas flow regulating valve, 6) cooling pipe outlet, 7) cooling pipe inlet, 8) solution tank, 9) corrosion resisting centrifugal pump,10) flow master valve, 11) electrochemical workstation, 12) data acquisition computer, 13) fluid buffer box, 14) test channel, 15) testing electrode, 16) fluid distribution box, 17) vertical rotor flow meter, 18) flow control valve, and 19) return valve.

Fig. 3. Details of test channel.

Fig. 4. Configuration of the electrodes: 1) epoxy resin, 2) counter electrode, 3) reference electrode, and 4) working electrode (units are in mm).

Fig. 5. Distinct geometrical regions of the corrosion surface used for SEM analysis (units are in mm, side view).

Fig. 6. EIS spectra: (a) PS at v = 3 m/s, (b) SAD at v = 3 m/s, (c) SAD at v = 5 m/s, (d) SAD at v = 7 m/s. Left: Nyquist plots; Right: Bode and |Z| plots.

Fig. 7. Electrochemical equivalent circuit obtained by fitting the experimental impedance data.

Table 2 Equivalent circuit fitting of EIS data of the PS, v = 3 m/s.

Time (h)	R_s (Ω cm²)	Q_dl (Ω^-1 cm^-2 s^-n)	n_dl	R_ct (Ω cm²)	Q_pf (Ω^-1 cm^-2 s^-n)	n_pf	R_pf (Ω cm²)	Ave err (%)
1	11.06	2.6E-4	0.8223	284.1				4.82
3	11.04	2.9E-4	0.8003	275.5				5.16
6	11.24	4.1E-4	0.7684	269.3				4.04
7	11.36	6.9E-5	1	39.89	4.8E-4	0.8614	214.2	8.31
10	11.31	6.9E-5	1	38.95	5.3E-4	0.8570	208.7	8.92
13	11.05	7.4E-5	1	36.29	6.5E-4	0.8529	188.7	9.14
16	11.37	7.6E-5	1	35.87	7.3E-4	0.8515	183.9	9.01

Fig. 8. SEM views of surface morphologies of the PS after corrosion at v = 3 m/s.

Fig. 9. XRD patterns of the corrosion scales: (a) PS after corrosion at v = 3 m/s, (b) SAD after corrosion at v = 5 m/s.

Table 3 Equivalent circuit fitting of EIS data of the SAD at v = 3 m/s.

Time(h)	R_s (Ω cm²)	Q_dl (Ω^-1 cm^-2 s^-n)	n_dl	R_ct (Ω cm²)	L (H cm^-2)	R_L (Ω cm²)	Ave err (%)
1	11.57	3.7E-4	0.9292	59.97	119.0	214.3	6.32
4	11.73	5.4E-4	0.9775	31.25	87.51	80.83	7.98
7	11.74	8.3E-4	1	21.98	63.51	67.07	4.67
10	11.59	1.1E-3	1	19.53	71.10	66.66	7.17
13	11.07	2.4E-3	1	16.29	91.98	97.67	6.52
16	11.42	4.5E-3	1	11.92	22.26	44.17	6.04

Fig. 10. SEM surface morphologies of the SAD after corrosion at v = 3 m/s.

Table 4 Equivalent circuit fitting of EIS data of SAD at v = 5 m/s from 1-9 h of corrosion.

Time(h)	R_s (Ω cm²)	Q_dl (Ω^-1 cm^-2 s^-n)	n_dl	R_ct (Ω cm²)	L (H cm^-2)	R_L (Ω cm²)	Ave err (%)
1	11.53	9.7E-4	1	14.87	19.63	34.87	5.26
5	11.36	2.1E-3	1	18.97	496.6	91.76	6.31
9	11.58	2.4E-3	1	21.39	1655	168.3	4.44

Table 5 Equivalent circuit fitting of EIS data of SAD at v = 5 m/s from 10-16 h of corrosion.

Time (h)	R_s (Ω cm²)	Q_dl (Ω^-1 cm^-2 s^-n)	n_dl	R_ct (Ω cm²)	Q_df (Ω^-1 cm^-2 s^-n)	n_df	R_df (Ω cm²)	Ave err (%)
10	11.24	2.1E-3	1	15.81	9.4E-3	0.2711	24.23	8.74
13	11.31	2.0E-3	1	19.24	8.2E-3	0.4292	26.22	10.53
16	11.27	1.9E-3	1	21.74	9.9E-3	0.4664	25.45	7.32

Fig. 11. SEM surface morphologies of the SAD after corrosion at v = 5 m/s.

Table 6 Equivalent circuit fitting of EIS data of the SAD at v = 7 m/s.

Time (h)	R_s (Ω cm²)	Q_dl (Ω^-1 cm^-2 s^-n)	n_dl	R_ct (Ω cm²)	L (H cm^-2)	R_L (Ω cm²)	Ave err (%)
1	11.80	3.1E-3	1	15.12	49.85	64.03	8.53
4	11.73	2.9E-3	1	16.02	80.79	76.3	5.24
7	11.74	2.6E-3	1	17.38	108.6	101.73	4.69
10	11.51	2.5E-3	1	18.72	129.5	107.3	6.41
13	11.04	2.2E-3	1	19.33	167.8	133	7.04
16	11.81	2.0E-3	1	20.68	215.4	151.6	5.92

Fig. 12. SEM surface morphologies of the SAD after corrosion at v = 7 m/s.

Fig. 13. Absolute pressure (Pa) of defect local magnification under different flow velocities with flow direction from left to right.

Fig. 14. SEM of the SAD removed corrosion scale.

Fig. 15. 3D surface morphology and profile of area (c) of the SAD removed corrosion scale.

Fig. 16. Localized corrosion rate and the depth of localized corrosion pits at downstream of defect (the error bar represents the standard deviation of five deepest localized corrosion pits).

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