The synergy between cementite spheroidization and Cu alloying on the corrosion resistance of ferrite-pearlite steel in acidic chloride solution

doi:10.1016/j.jmst.2020.12.031

Abstract

Abstract:

In this work, the corrosion behavior of medium-carbon steels (45, 45Cu and 45Cuq steels) in acidic chloride environment was investigated. The results indicated that the micro-galvanic effect between the anodic ferrite matrix phase and the cathodic cementite secondary phase notably affected the corrosion resistance of the three steels. For 45 steel, serious pitting corrosion happened in and around the pearlite regions, and a large number of lamellar cementite was fixed in the corrosion pits. Meanwhile, the continuously increasing superficial area of cathodic cementite enhanced the micro-galvanic corrosion, resulting in a rapidly increase in corrosion rate with time. While for 45Cu and 45Cuq steels, macroscopic uniform corrosion occurred, and the cementite accumulation was markedly reduced as compared with 45 steel, thus the micro-galvanic effect was weakened and the corrosion rate was decreased accordingly. Among these, 45Cuq steel showed the most stable and excellent corrosion resistance during long-term corrosion, indicating the occurrence of a synergistic effect between cementite spheroidization and Cu alloying, thereby significantly improving the corrosion resistance of 45 steel.

Key words: Cementite spheroidization, Cu alloying, Ferrite-pearlite steel, Micro-galvanic effect

Hu Liu, Jie Wei, Junhua Dong, Yangtao Zhou, Yiqing Chen, Yumin Wu, Subedi Dhruba Babu, Aniefiok Joseph Umoh, Wei Ke. The synergy between cementite spheroidization and Cu alloying on the corrosion resistance of ferrite-pearlite steel in acidic chloride solution[J]. J. Mater. Sci. Technol., 2021, 84: 65-75.

Figures/Tables 21

Table 1 The chemical composition (wt. %) of the experimental steels.

Samples	C	Si	Mn	P	S	Cu
45 steel	0.427	0.308	0.558	0.007	0.006	--
45Cu steel	0.436	0.286	0.532	0.004	0.003	0.325

Fig. 1. Schematic illustration of spheroidization process.

Fig. 2. The SEM images of 45 (a, a1), 45Cu (b, b1) and 45Cuq (c, c1) steels before immersion.

Fig. 3. The TEM BF micrographs of the three steels before immersion. (a) 45 steel, (b) 45Cu steel, (c) 45Cuq steel.

Fig. 4. The XRD patterns of the three steels before corrosion and after 72 h of immersion. (a) 45 steel, (b) 45Cu steel, (c) 45Cuq steel.

Fig. 5. The macroscopic corrosion morphologies of the three steels after rust removal with 216 h of immersion. (a) 45 steel, (b) 45Cu steel, (c) 45Cuq steel.

Fig. 6. The BSE images of cross-section morphologies of the three steels after different time of immersion.

Fig. 7. The SEM images of corroded surface morphologies of the three steels after different time of immersion.

Fig. 8. The TEM micrographs of the diluted surface residues of 45Cu steel after 24 h of immersion. (a) Spherical particles, (b) invert contrast SAED pattern of the spherical precipitations taken along the [001] direction of Cu.

Fig. 9. The EPMA mapping results of 45Cu (a) and 45Cuq (b) steels before immersion.

Fig. 10. The EPMA mapping results of 45Cu (a) and 45Cuq (b) steels after 72 h of immersion.

Fig. 11. The residual cementite weights and the average corrosion rates of the three steels as a function of immersion time in simulated solution.

Fig. 12. The PD curves of the three steels after different time of immersion. (a) 45 steel, (b) 45Cu steel, (c) 45Cuq steel.

Fig. 13. The PD plots of the three steels immersed in simulated solution for 24 h (a) and 216 h (b).

Fig. 14. The Icorr-t histogram of three steels.

Fig. 15. The Bode plots and fitting curves of 45 (a, a1), 45Cu (b, b1) and 45Cuq (c, c1) steels in simulated solution after different time of immersion. (a-c) Bode impendence module plots, (a1-c1) Bode phase angle plots.

Fig. 16. The equivalent circuits for EIS fitting of the three steels in simulated solution after different time of immersion. (a) 45 steel (24 h-72 h), (a') 45 steel (120 h-216 h), (b) 45Cu and 45Cuq steels.

Table 2 The fitting parameters of 45 steel after different time of immersion.

t (h)	L₁×10⁷ (H cm²)	R_s (Ω cm²)	Y_c×10³ (Ω^-1 cm^-2 s-ⁿ)	n_c	R_c (Ω cm²)	L₂ (H cm²)	R₀ (Ω cm²)	Y_a×10³ (Ω^-1 cm^-2 s-ⁿ)	n_a	R_a (Ω cm²)	Chi-squared, χ²
24	4.566	4.439	0.4558	0.9406	84.20	-	-	1.873	0.7682	86.91	1.617 × 10^-4
48	5.677	4.084	0.6104	0.9048	47.45	-	-	1.936	0.8378	45.60	6.522 × 10^-5
72	5.264	4.225	0.8828	0.8047	21.43	-	-	8.158	0.8132	22.30	3.200 × 10^-4
120	7.542	3.681	2.552	0.7839	13.16	410.4	4.022	6.620	0.6606	10.24	4.776 × 10^-4
216	4.065	4.075	4.234	0.7353	3.277	167.8	3.580	65.98	0.6609	4.980	7.592 × 10^-4

Table 3 The fitting parameters of 45Cu steel after different time of immersion.

t (h)	L₁×10⁷ (H cm²)	R_s (Ω cm²)	Y_c×10³ (Ω^-1 cm^-2 s-ⁿ)	n_c	R_c (Ω cm²)	Y_a×10³ (Ω^-1 cm^-2 s-ⁿ)	n_a	R_a (Ω cm²)	Chi-squared, χ²
24	4.821	4.397	0.1750	1.000	145.6	0.2898	0.8335	139.5	3.936 × 10^-4
48	8.618	3.916	0.2847	0.9128	134.2	2.136	0.7754	135.6	2.910 × 10^-4
72	12.02	3.833	0.4944	0.9012	146.8	5.012	0.6613	145.2	7.783 × 10^-4
120	8.818	4.654	0.4700	0.9380	54.34	7.161	0.7422	58.20	1.885 × 10^-5
216	6.960	4.548	0.8707	0.9374	62.37	7.283	0.7327	61.22	1.077 × 10^-4

Table 4 The fitting parameters of 45Cuq steel after different time of immersion.

t (h)	L₁×10⁷ (H cm²)	R_s (Ω cm²)	Y_c×10³ (Ω^-1 cm^-2 s-ⁿ)	n_c	R_c (Ω cm²)	Y_a×10³ (Ω^-1 cm^-2 s-ⁿ)	n_a	R_a (Ω cm²)	Chi-squared, χ²
24	9.906	4.252	0.1878	0.9953	132.6	0.3839	0.8067	132.7	5.251 × 10^-4
48	10.54	4.742	0.4071	0.9946	134.5	1.459	0.8002	138.3	7.142 × 10^-5
72	10.10	4.533	0.3323	0.9325	97.92	1.930	0.8331	98.60	8.536 × 10^-5
120	9.340	4.380	0.3540	0.9384	92.54	0.9206	0.8454	88.40	1.246 × 10^-4
216	10.11	4.531	0.3332	0.9369	94.60	1.837	0.8271	102.2	8.007 × 10^-5

Fig. 17. The Rc and Ra values of the three steels as a function of time.

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