Near-neutral pH corrosion of mill-scaled X-65 pipeline steel with paint primer

doi:10.1016/j.jmst.2020.01.016

Abstract

Abstract:

The corrosion behaviour of mill-scaled X65 pipeline steel with and without a primer layer was studied in a simulated near-neutral pH soil solution. Results revealed a three-stage corrosion process of the mill-scaled pipeline steel surface. The first stage included an initial preferential dissolution of goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) in mill scale. The second stage was marked by enhanced localized corrosion and pit-formation because of either galvanic corrosion or acidic dissolution in areas enclosed by mill scale. The final stage was general corrosion after the mill scale flaked off the steel surface. When the primer layer was applied, localized corrosion was significantly enhanced on the steel surface and persisted for an extended period as compared to the mill-scaled condition. The precipitation of siderite (FeCO₃) was observed at flawed locations of mill scale, although the bulk chemistry is not favorable for its formation on the steel surface free of mill scale. The local precipitation of siderite formed a capped mill scale enclosure where localized corrosion can be further enhanced.

Key words: Pipeline steel, Primer paint, Mill scale, Localized corrosion, Siderite precipitation

Shidong Wang, Lyndon Lamborn, Karina Chevil, Erwin Gamboa, Weixing Chen. Near-neutral pH corrosion of mill-scaled X-65 pipeline steel with paint primer[J]. J. Mater. Sci. Technol., 2020, 49: 166-178.

Figures/Tables 22

Table 1 Chemical composition of the primer paint used in the current investigation (wt%).

Ingredient	Concentration	CAS number
Acetone	24.23	67-64-1
Propane	13.86	74-98-6
N-butane	8.14	106-97-8
VM&P Naphtha	5.58	64742-89-8
TiO₂	5.13	13463-67-7
Toluene	5.07	108-88-3
Talc (Mg₃Si₄O₁₀(OH)₂)	4.49	14807-96-6
Xylene (mix)	4.08	1330-20-7
Ethyl alcohol	3.98	64-17-5
N-butyl acetate	2.78	123-86-4
Mineral spirits	2.55	64742-47-8
Isobutyl acetate	1.62	110-19-0
Isopropyl alcohol	1.02	67-63-0
Other additives	17.47	N/A

Table 2 Chemical composition of the C2 solution used in the current investigation (g/l).

KCl	NaHCO₃	CaCl₂·2H₂O	MgSO₄·7H₂O	CaCO₃
0.0035	0.0195	0.0255	0.0274	0.0606

Fig. 1. Schematic illustration of the samples used in the current investigation for (a) mill-scaled and (b) primer pre-coated samples.

Fig. 2. Microstructural characterization of samples before test: (a, b) secondary electron imaging of the surfaces of mill-scaled and primer pre-coated samples, respectively; (c, d) backscattered electron imaging of the cross-sectional surfaces of mill-scaled and primer pre-coated samples, respectively; (e) Raman spectra of the outer and inner layers of mill scale.

Fig. 3. Mass loss of the mill-scaled and primer pre-coated samples immersed in C2 solution for up to 90 d. Standard deviation of the measured data was provided. The symbols and lines denote the experimental and fitted data, respectively.

Fig. 4. pH variation of the testing solution for up to 90 d.

Fig. 5. XRD patterns of samples after exposure to C2 solution for up to 90 d: (a) mill-scaled, (b) primer pre-coated and (c) polished samples. JCPDS standard cards are provided, and the strongest lines are marked correspondingly (Fe, No. 65-4899; γ-FeOOH, No. 74-1877; α-FeOOH, No. 08-0097; Fe3O4, No. 79-0419; FeCO3, No. 29-0696; TiO2, No. 87-0710; Mg3Si4O10(OH)2, No. 83-1768).

Fig. 6. Backscattered electron imaging to surfaces of mill-scaled samples exposed to C2 solution for (a) 0 d, (b) 5 d, (c) 30 d and (d) 90 d; (e) secondary electron observation of the image (d); (f) high-magnification imaging of the marked area “f” in image (e). The inset in image (f) is a Raman spectrum from the corresponding marked area in image (f).

Fig. 7. EDS analysis results obtained within the marked areas in Fig. 6. Images (a), (b), (c) and (d) are EDS results of the corresponding marked areas “a”, “b”, “c” and “d” in Fig. 6 (a, b), respectively. Note that C element was removed from EDS results, as it might come from the contamination.

Fig. 8. Backscattered electron imaging to surfaces of primer pre-coated samples exposed to C2 solution for (a) 0 d, (b) 5 d, (c) 30 d and (d) 90 d; (e) secondary electron observation of the image (d); (f) high-magnification imaging of marked area “f” in image (e). The inset in image (f) is the backscattered electron observation of the marked area “g” in image (f).

Fig. 9. Representative backscattered electron images of cross-sectional surfaces of samples after exposure to C2 solution for various time: (a, c) mill-scaled and primer pre-coated samples after 30 d of immersion, respectively; (b, d) mill-scaled and primer pre-coated samples after 90 d of immersion, respectively.

Fig. 10. Pit-depth distribution measured on 1 cm long cross-section of each sample after corrosion exposure for 90 d. Error bars show the standard deviation of the measured data.

Fig. 11. Open circuit potential (OCP) variation of samples in C2 solution for up to 90 d.

Fig. 12. Electrochemical impedance spectra of mill-scaled samples with different immersion time: (a) Nyquist plots, (b) enlarged graph of (a), (c) Bode plots of log |Z| vs. log f and (d) Bode plots of phase angle (θ) vs. log f. The symbols and lines in images (a-d) denote the experimental and fitted data, respectively.

Fig. 13. Electrochemical impedance spectra of primer pre-coated samples with different immersion time: (a) Nyquist plots, (b) enlarged graph of (a), (c) Bode plots of log |Z| vs. log f and (d) Bode plots of phase angle (θ) vs. log f. The symbols and lines in images (a-d) denote the experimental and fitted data, respectively.

Fig. 14. Equivalent circuit models used for fitting the impedance spectra of samples: (a) mill-scaled samples and (b) primer pre-coated samples. (Rs: solution resistance; CPEo: CPE of the reduction process of oxides; Ro: resistance of the reduction process of oxides; CPEi: CPE of the dissolution of iron; Ri: resistance of the dissolution of iron; CPEc: CPE of the primer layer; Rc: resistance of the primer layer).

Table 3 Fitting results of the EIS for mill-scaled samples with various immersion time.

Time	R_s (Ω cm²)	Q_o (μΩ^-1 cm^-2 sⁿ)	n_o	R_o (Ω cm²)	Q_i (μΩ^-1 cm^-2 sⁿ)	n_i	R_i (Ω cm²)	χ²×10⁴
5 d	582	412	0.76	223	658	0.76	4535	1.01
30 d	561	484	0.79	690	173	0.80	3078	0.54
60 d	539	624	0.80	449	293	0.79	2278	0.43
90 d	529	671	0.82	407	373	0.80	2268	0.39

Table 4 Fitting results of the EIS for primer pre-coated samples with various immersion time.

Time	R_s (Ω cm²)	Q_c (μΩ^-1 cm^-2 sⁿ)	n_c	R_c (Ω cm²)	Q_o (μΩ^-1 cm^-2 sⁿ)	n_o	R_o (Ω cm²)	Q_i (μΩ^-1 cm^-2 sⁿ)	n_i	R_i (Ω cm²)	χ²×10⁴
5 d	588	0.5	0.67	923	170	0.59	1392	426	0.65	7875	1.49
30 d	569	0.8	0.65	911	183	0.63	1131	354	0.77	8833	1.43
60 d	541	1.2	0.61	631	189	0.77	808	357	0.81	6740	1.71
90 d	512	1.4	0.60	555	191	0.77	405	380	0.82	5910	1.70

Table 5 Thermodynamic data of used species in E-pH calculations at 298.15 K.

Species	ΔG⁰ (kJ mol^-1)	Ref.	Species	ΔG⁰ (kJ mol^-1)	Ref.
H₂	0	[52]	CO₃^2-	-527.9	[52]
H⁺	0	[52]	Fe²⁺	-78.9	[48]
Fe	0	[52]	H₂CO₃	-623.2	[52]
FeCO₃	-666.67	[48]	HCO₃^-	-586.85	[52]
Fe₃O₄	-1015.4	[48]	H₂O	-237.141	[49]
α-FeOOH	-485.3	[50]	Fe₂(OH)₂CO₃	-1169.3	[48]
γ-FeOOH	-480.1	[51]	Fe₆(OH)₁₂CO₃	-3650	[48]

Fig. 15. E-pH diagrams for the Fe-H2O-CO2 system at 25 °C, pressure =1 bar: (a) [Fe2+] = 10-6, 10-5, 10-4 and 10-3 mol/l and the concentrations of the anions are assumed to be 10-2 mol/l, (b) [Fe2+] = 10-6 mol/l, [anions] = 10-3, 10-2 and 10-1 mol/l. The superposed red and blue triangles respectively represent OCP values of mill-scaled and primer pre-coated samples at various immersion time.

Table 6 Possible electrode reactions and corresponding expressions for their equilibrium potential used in the currently investigation (pH = 6.29, [Fe2+] = 10-6 mol/l).

No.	Electrode reaction	Equilibrium potential equation	E (V/SCE)
(8)	Fe→Fe²⁺+2e^-	E_Fe2+/Fe=-0.6527+0.0296log[Fe²⁺]	-0.830
(9)	γFeOOH+3H⁺+e^-→Fe²⁺+2H2O	E_{γFeOOH/ Fe2+}=0.5136-0.0592log[Fe²⁺]-0.178pH	-0.251
(10)	αFeOOH+3H⁺+e^-→Fe²⁺+2H2O	E_{αFeOOH/ Fe2+}=0.4597-0.0592log[Fe²⁺]-0.178pH	-0.305
(11)	Fe3O4+8H⁺+2e^-→3Fe²⁺+4H2O	E_Fe3O4/_Fe2+=0.6364-0.0888log[Fe²⁺]-0.237pH	-0.322
(12)	2H++2e^-→H2	E_H+/H2=-0.2438-0.0592pH	-0.616

Fig. 16. Schematic illustration for the corrosion process on cross-sections of (a, c, e, g) mill-scaled and (b, d, f, h) primer pre-coated samples in C2 solution.

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