Effects of bentonite content on the corrosion evolution of low carbon steel in simulated geological disposal environment

doi:10.1016/j.jmst.2020.04.071

Abstract

Abstract:

The effects of bentonite content on the corrosion behavior of low carbon steel in 5 mM NaHCO₃ + 1 mM NaCl + 1 mM Na₂SO₄ solution were investigated by electrochemical measurements combined with X-ray diffraction (XRD) and scanning electron microscopy (SEM). In the initial immersion stage, the cathodic process of low carbon steel corrosion was dominated by the reduction of dissolved oxygen, while it transformed to the reduction of ferric corrosion products with the immersion time. The presence of bentonite colloids could suppress the cathodic reduction of oxygen due to their barrier effect on the diffusion of oxygen. However, the barrier performance of bentonite layer was gradually deteriorated due to the coagulation and separation of bentonite colloids caused by the charge neutralization of iron corrosion products dissolved from the steel substrate. More bentonite colloids could maintain the barrier effect for a long time before it was deteriorated by the accumulation of corrosion products. Conversely, it could lose the performance completely, and the corrosion behavior of low carbon steel reverted to the same as that in the blank solution.

Key words: Low carbon steel, Bentonite, Electrochemical measurements, Corrosion products, Corrosion evolution

Xin Wei, Junhua Dong, Nan Chen, Amar Prasad Yadav, Qiying Ren, Jie Wei, Changgang Wang, Rongyao Ma, Wei Ke. Effects of bentonite content on the corrosion evolution of low carbon steel in simulated geological disposal environment[J]. J. Mater. Sci. Technol., 2021, 66: 46-56.

Figures/Tables 20

Table 1 Conductivity and chemical composition of different environments.

Bentonite content	Conductivity (S m^-1)	Chemical compositions (mmol L^-1)
		HCO₃^-	Cl^-	SO₄^2-
0%	0.328	5	1	1
2%	0.379	5.78	1.05	1.05
9.1 %	0.540	8.9	1.24	1.23
33.3 %	0.915	24.5	2.2	2.15

Fig. 1. Potentiodynamic polarization curves of low carbon steel in the simulated environments containing different bentonite contents.

Table 2 Tafel slopes of the polarization curves in different environments.

Bentonite content (wt.%)	0%	2%	9.1 %	33.3 %
β_a (V/dec)	0.097	0.078	0.049	0.050
β_c (V/dec)	0.350	0.210	0.196	0.148

Fig. 2. Open circuit potential curves of low carbon steel in the simulated environments containing different bentonite contents.

Fig. 3. Cross-sectional morphology and EDS results of low carbon steel in the blank solution after immersion for 30 days (a) and 107 days (b).

Fig. 4. Cross-sectional morphology and EDS results of low carbon steel in 2% bentonite solution after immersion for 30 days (a) and 107 days (b).

Fig. 5. Cross-sectional morphology and EDS results of low carbon steel in 9.1 % bentonite slurry after immersion for 30 days (a) and 107 days (b).

Fig. 6. Cross-sectional morphology and EDS results of low carbon steel in 33.3 % bentonite mud after immersion for 30 days (a) and 107 days (b).

Fig. 7. XRD pattern of the corrosion products formed in different simulated environments after immersion for 30 days and 107 days: (a) blank solution, (b) 2% bentonite solution, (c) 9.1 % bentonite slurry, (d) 33.3 % bentonite mud.

Fig. 8. EIS of low carbon steel in the blank solution: (a) impedance modulus vs. frequency plots (b) phase angle vs. frequency plots.

Fig. 9. Equivalent circuits for the EIS of low carbon steel in the blank solution (Re - solution resistance, Qdl - constant phase element (CPE) of electric double layer, Rct - charge transfer resistance of low carbon steel, W - Warburg impedance of oxygen diffusion, Qr - CPE of rust reduction, Rr - Faraday resistance of rust reduction).

Table 3 Fitting results of EIS in the blank solution.

Time (day)	R_e (Ω cm²)	Y_0-r (mS sⁿ cm^-2)	n_r	R_r (Ω cm²)	Y_0-dl (mS sⁿ cm^-2)	n_dl	R_ct (Ω cm²)	Y_0-W (mS s^0.5 cm^-2)
initial	470	-	-	-	0.81	0.75	891	32
4	408	0.7	0.73	173.8	1.7	0.51	1687	-
16	295	1.5	0.75	228	2.3	0.56	1298	-
30	247	1.9	0.71	105.8	2.0	0.39	1553	-
58	188	3.2	0.72	47.4	1.1	0.3	1489	-
86	167	2.7	0.63	201.5	1.4	0.84	1101	-
107	163	1.0	0.67	37.6	2.9	0.74	1251	-

Fig. 10. EIS of low carbon steel in 2% bentonite solution: (a) impedance modulus vs. frequency plots, (b) phase angle vs. frequency plots.

Fig. 11. Equivalent circuits for the EIS of low carbon steel in bentonite environments (Qm - CPE of attached bentonite layer, Rm - resistance of attached bentonite layer).

Table 4 Fitting results of EIS in 2% bentonite solution.

Time (day)	R_e (Ω cm²)	Y_0-m (mS sⁿ cm^-2)	n_m	R_m (Ω cm²)	Y_0-r (mS sⁿ cm^-2)	n_r	R_r (Ω cm²)	Y_0-dl (mS sⁿ cm^-2)	n_dl	R_ct (Ω cm²)	Y_0-W (mS s^0.5 cm^-2)
initial	402.7	0.41	0.85	29.3	-	-	-	0.56	0.73	822	20.7
4	384.4	0.94	0.85	28.5	-	-	-	0.34	0.79	4711	18.3
16	279.4	0.1	0.42	15.3	-	-	-	0.28	0.85	7965	14.4
30	230.2	6.4	0.78	23.6	3.5	0.42	4.2	2.3	0.91	2549	-
58	166.8	-	-	-	4.8	0.63	114.6	1.7	0.86	1093	-
86	150.2	-	-	-	4.1	0.64	151.1	2.9	0.84	1102	-
107	136.7	-	-	-	4.1	0.62	237.9	3.1	0.89	1122	-

Fig. 12. EIS of low carbon steel immersed in 9.1 % bentonite slurry: (a) impedance modulus vs. frequency plots, (b) phase angle vs. frequency plots.

Table 5 Fitting results of EIS in 9.1 % bentonite slurry.

Time (day)	R_e (Ω cm²)	Y_0-m (mS sⁿ cm^-2)	n_m	R_m (Ω cm²)	Y_0-r (mS sⁿ cm^-2)	n_r	R_r (Ω cm²)	Y_0-dl (mS sⁿ cm^-2)	n_dl	R_ct (Ω cm²)	Y_0-W (mS s^0.5 cm^-2)
initial	270.5	0.16	0.41	53.4	-	-	-	0.27	0.7	2182	19.5
4	235.5	0.17	0.42	52.5	-	-	-	0.45	0.82	10250	10.1
16	193.4	0.9	0.27	25.7	-	-	-	0.48	0.86	5730	13.4
30	149.4	0.22	0.41	16.7	-	-	-	0.81	0.81	5470	14.1
58	113.4	0.5	0.5	8.7	4.4	0.78	135.3	3.0	0.78	3276	-
86	91.4	4.7	0.23	19.9	7.3	0.72	142	3.7	0.81	3384	-
107	103.5	5.5	0.44	14.2	10.0	0.7	140.6	4.1	0.83	3320	-

Fig. 13. EIS of low carbon steel immersed in 33.3 % bentonite mud: (a) impedance modulus vs. frequency plots, (b) phase angle vs. frequency plots.

Table 6 Fitting results of EIS in 33.3 % bentonite mud.

Time (day)	R_e (Ω cm²)	Y_0-m (mS sⁿ cm^-2)	n_m	R_m (Ω cm²)	Y_0-dl (mS sⁿ cm^-2)	n_dl	R_ct (Ω cm²)	Y_0-W (mS s^0.5 cm^-2)
initial	153.3	1.59	0.23	74.8	0.28	0.75	2414	11.6
4	147.4	2.55	0.37	39	0.36	0.81	17610	1.7
16	117.0	0.89	0.65	28.8	0.38	0.86	21670	1.2
30	83.1	11.1	0.23	52.9	0.50	0.85	16100	3.9
58	70.7	20.0	0.35	33.6	0.68	0.86	9752	6.6
86	53.1	16.8	1	27.7	0.79	0.86	10860	6.3
107	57.1	11.8	0.99	28.1	0.98	0.88	6891	6.8

Fig. 14. Diagram of the corrosion processes of low carbon steel in the simulated environments containing different bentonite contents.

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