Corrosion Behaviour of Carbon Steel Fasteners in Neutral Chloride Solution

doi:10.1007/s40195-021-01284-4

Abstract

Abstract:

An experimental model for simulating the corrosion of carbon steel fasteners (bolt and nut) composed of a contact carbon steel electrode (CCSE) and an exposed bare carbon steel plate electrode (BCSE) was designed. The effect of coupling on the corrosion process of the galvanically coupled carbon steel electrode was evaluated and compared with the self-corrosion process observed independently at the exposed and contact regions. Results obtained indicated that at an equal area ratio and uncoupled conditions, the corrosion rate is accelerated in the surface directly exposed to bulk solution compared to the bolt surface in contact with the nut. A coupling current was recorded when the exposed surface (BCSE) was electrically connected with the contact surface (CCSE); with the CCSE acting as the anode thereby suppressing the corrosion process in the exposed surface. By implication, the galvanic coupling between CCSE and BCSE increased the corrosion rate of CCSE. The difference in oxygen supply was responsible for the coupling effect observed in the system as there was no decrease in the solution pH. Moreover, varying the cathode-to-anode area (S_c/S_a) ratio significantly influenced the corrosion current density as increased S_c/S_a ratio resulted in an accelerated galvanic corrosion process. The corroded surfaces and interfaces were analysed using stereomicroscopy and scanning electron microscopy. X-ray diffractometry was adopted for corrosion product characterization. The results obtained showed supportive evidence of the corrosion behaviour in carbon steel fasteners.

Key words: Bolt and nut, Galvanic current density, Zero resistance ammetry (ZRA), Coupling potential, Differential aeration cell, Carbon steel fasteners

Enobong Felix Daniel, Junhua Dong, Xiaofang Li, Ini-Ibehe Nabuk Etim, Inime Ime Udoh, Rongyao Ma, Lei Chen, Changgang Wang. Corrosion Behaviour of Carbon Steel Fasteners in Neutral Chloride Solution[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(4): 563-576.

Figures/Tables 20

Fig. 1 Schematic illustration of a threaded bolt and nut assembly, b vertical cross-section of the regions of interest immersed in the solution, c electrochemical set-up, d experimental model of the simulated bolt and nut assembly

Fig. 2 SEM micrographs of the contact surface (CCSE) at uncoupled condition with and without corrosion products after a, a' 1 day, b, b' 7 days, c, c' 28 days, respectively

Fig. 3 SEM micrographs of the exposed surface (BCSE) at uncoupled conditions with and without corrosion products after; a, a' 1 day, b, b' 7 days, c, c' 28 days, respectively

Fig. 4 Cross-sectional morphologies of the surfaces after immersion in the test solution for 28 days under coupled condition: a CCSE, b BCSE

Fig. 5 Macroscopic images of a carbon steel bolt and nut assembly before immersion: a coupled bolt with nut, b uncoupled bolt, and after 28 days of immersion in 0.06 mol/L NaCl solution: c coupled bolt with nut, d uncoupled bolt, e region A exposed to the bulk solution, f region B (bolt and nut contact)

Fig. 6 X-ray diffraction patterns of corrosion products formed on the exposed surface of the carbon steel fastener after immersion in 0.06 mol/L NaCl solution for a 1 day, b 7 days, c 28 days

Fig. 7 X-ray diffraction patterns of corrosion products formed on the contact surface of the carbon steel fastener after immersion in 0.06 mol/L NaCl solution for a 1 day, b 7 days, c 28 days

Fig. 8 Schematic representation of the process involved in the formation of the different oxide phases

Fig. 9 Open circuit potential (OCP) of the exposed region (BCSE) and contact region (CCSE) in 0.06 mol/L NaCl solution

Fig. 10 a Time-dependent variation in weight loss and corrosion rate for the CCSE and BCSE, b bilogarithmic plots of thickness loss versus immersion time CCSE and BCSE at uncoupled conditions

Fig. 11 Polarization curves of samples at both uncoupled and coupled conditions after different immersion time: a CCSE, b BCSE, c GCSE

Fig. 12 Polarization curves showing the effect of coupling on the corrosion mechanism of test samples after a 1 day, b 28 days of immersion in the test solution

Table 1 Electrochemical and galvanic parameters of the simulated carbon steel fastener under coupled conditions with an equal-area ratio

Test sample	Time (day)	- E_couple (V vs Ag/AgCl)	i_couple (μA/cm²)	CR (mm/y)
GCSE (Theoretical)	1	0.709	0.768	0.01
GCSE (Theoretical)	28	0.649	2.181	0.03
GCSE (Experimental)	1	0.688	4.345	0.05
GCSE (Experimental)	28	0.641	2.513	0.03

Fig. 13 Time-dependent plots of: a galvanic current density, b galvanic potential observed for 1 h with respect to CCSE

Table 2 Calculated data for galvanic current density and dissolution rate obtained from ZRA measurement at an equal-area ratio

Immersion time (day)	i_g^A (μA/cm²)	i_d^A (μA/cm²)	r_d (mm/y)
1	5.28	10.56	0.12
3	5.17	10.33	0.12
5	4.21	8.41	0.10
7	3.24	6.49	0.08
14	4.29	8.58	0.10
21	2.49	4.98	0.06
28	1.09	2.20	0.03

Fig. 14 Effect of area ratio on the: a galvanic current density, b surface morphology

Fig. 15 Bode plots for the uncoupled steel samples after immersion in the test solution for different days: a CCSE, b BCSE, c coupled sample (GCSE)

Fig. 16 Equivalent circuit models for the EIS of: a CCSE, b BCSE, c GCSE

Table 3 Electrochemical parameters obtained from the fitting of EIS data for the carbon steel samples in 0.06 mol/L NaCl solution under uncoupled condition

Immersion time (day)	R_s (Ω cm²)	Q_c (Ω^-1 cm^-2 S^-ⁿ)	n_c	R_c (Ω cm²)	Q_a (Ω^-1 cm^-2 S^-ⁿ)	n_a	R_a (Ω cm²)	Z_W (Ω^-1 cm^-2 S^-0.5)
CCSE (Contact region)
1	411.5	3.30 × 10^-4	0.81	386.5	4.81 × 10^-4	0.85	4667	-
7	395.8	2.86 × 10^-4	0.78	465.0	3.55 × 10^-4	0.85	7328	-
14	425.8	2.45 × 10^-4	0.78	541.4	3.00 × 10^-4	0.85	10,090	-
28	437.9	1.90 × 10^-4	0.73	842.3	2.39 × 10^-4	0.87	12,610	-
BCSE (Exposed region)
1	25.9	2.30 × 10^-2	0.72	20.3	1.63 × 10^-2	0.91	198.1	3.92 × 10^-1
7	26.3	1.77 × 10^-2	0.81	4.6	2.41 × 10^-2	0.97	609.6	5.49 × 10^-2
14	30.8	3.45 × 10^-2	0.87	8.6	1.89 × 10^-2	0.80	933.4	-
28	38.1	2.52 × 10^-2	1.00	38.69	1.91 × 10^-2	0.77	1040.0	-

Table 4 Electrochemical parameters obtained from the fitting of EIS data for the carbon steel sample (GCSE) in 0.06 mol/L NaCl solution under coupled condition

Immersion time (day)	R_s (Ω cm²)	Q_c (Ω^-1 cm^-2 S^-ⁿ)	n_a	R_c (Ω cm²)	Q_a (Ω^-1 cm^-2 S^-ⁿ)	n_a	R_a (Ω cm²)
1	21.20	5.87 × 10^-4	0.78	34.41	2.48 × 10^-4	0.82	1370
7	24.80	1.15 × 10^-3	0.78	58.6	1.14 × 10^-3	0.77	948.0
14	37.69	1.87 × 10^-3	1.00	28.38	3.36 × 10^-4	0.74	924.3
28	40.24	1.55 × 10^-3	0.80	11.01	9.18 × 10^-4	0.72	901.1

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