Experimental and Theoretical Studies of Action Mechanism of an Octadecylamine-based Molecular Assembly on Mild Steel†

doi:10.7503/cjcu20180589

Abstract

Abstract:

A supramolecular complex CDDA was prepared based on β-cyclodextrin(β-CyD) and octadecylamine(ODA). Four possible configurations of CDDA might coexist in the supramolecular system based on molecular dynamics simulation(MD). The corrosion inhibition effect and mechanism of CDDA for Q235 steel in the condensate water was evaluated via dynamic weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy(EIS), coupling with scanning electron microscopy(SEM), atomic force microscopy, contact angles, X-ray photoelectron spectroscopy(XPS) and attenuated total reflection infrared spectroscopy(ATR-FTIR). The inhibition efficiency for mild steel in the condensate water could reach 94.1% with 1 mmol/L CDDA at 35 ℃. The corrosion mechanism of mild steel in the condensate water was not altered in the presence of CDDA; however, both the anodic and cathodic processes were inhibited along with the elevated polarization resistance. CDDA could be treated as a mixed-type inhibitor with predominantly anodic dominant. The results of XPS and ATR-FTIR indicated that ODA could be released from CDDA at on the metal/solution interface and form a protective monolayer film. The adsorption process could be well fitted by the Langmuir adsorption isotherm. Molecular dynamics simulations provided the visual evidence of the assembly mechanism of CDDA on the mild steel surface. The release of ODA from CDDA and assembly mechanism on the steel/solution interface were verified by MD and quantum chemical calculations based on density functional theory.

Key words: Condensate water, Octadecylamine, Molecular recognition and self-assembly, Molecular dynamics, Quantum chemistry

CLC Number:

O646

TrendMD:

MA Yucong,FAN Baomin,HAO Hua,LÜ Jinyu,FENG Yunhao,YANG Biao. Experimental and Theoretical Studies of Action Mechanism of an Octadecylamine-based Molecular Assembly on Mild Steel^†[J]. Chem. J. Chinese Universities, 2019, 40(1): 96.

Figures/Tables 19

Table 1 Water parameters of the sampled condensate water at 25 ℃

pH	Conductivity/ (μS·cm^-1)	Concentration of dissolved oxygen/(mg·L^-1)	Concentration of Na⁺/ (μg·L^-1)	Concentration of Cl^-/ (μg·L^-1)
5.89	108.32	2.11	66.13	129.19

Fig.1 Four possible configurations of CDDA(A) Wide towards narrow; (B) wide towards wide; (C) narrow towards narrow; (D) narrow towards wide.

Fig.2 Calculated energies of CDDA for different configurations(A) Wide towards narrow; (B) wide towards wide; (C) narrow towards narrow; (D) narrow towards wide.

Table 2 Equilibrium values of total energy(Et), potential energy(Ep) and non-bond energy(En) for different spatial configurations of CDDA

Configuration	E_t/(kJ·mol^-1)	E_p/(kJ·mol^-1)	E_n/(kJ·mol^-1)
Wide towards narrow	3820.59	2534.28	2444.70
Wide towards wide	3828.99	2562.72	2420.12
Narrow towards narrow	3830.92	2497.26	2464.99
Narrow towards wide	3815.06	2518.11	2402.59

Table 3 Corrosion rates(g·m-2·h-1) of Q235 steel with various concentrations of CDDA in condensate water under preset temperatures along with the corresponding corrosion efficiencies(ηw, %) in the brackets

Temperature/℃	c(CDDA)/(mol·L^-1)
Temperature/℃	0	0.05	0.1	0.3	0.6	1
35	0.118	0.021(82.2)	0.013(89.0)	0.011(90.7)	0.009(92.4)	0.007(94.1)
45	0.153	0.030(80.4)	0.021(86.3)	0.016(89.5)	0.013(91.5)	0.010(93.5)
55	0.202	0.055(72.8)	0.044(78.2)	0.028(86.1)	0.021(89.6)	0.018(91.1)
65	0.210	0.064(69.5)	0.052(75.2)	0.041(80.5)	0.030(85.7)	0.022(89.5)
75	0.233	0.082(64.8)	0.061(73.8)	0.048(79.4)	0.040(82.8)	0.033(85.8)

Fig.3 Cathodic(A) and anodic(B) polarization curves of Q235 steel in the condensate water at 35 ℃ with various concentrations of CDDA

Table 4 Electrochemical parameters and inhibition efficiencies for Q235 steel derived from cathodic and anodic polarization measurements

c(CDDA)/(mmol·L^-1)	E_corr/mV	J_corr/(μA·cm^-2)	β_a/(mV·dec^-1)	β_c/(mV·dec^-1)	η_p(%)
0	-538.02	29.89	69.28	-504.45	——
0.05	-495.82	4.90	222.47	-462.29	83.6
0.1	-490.06	2.36	253.04	-484.93	92.1
0.3	-492.89	2.01	252.97	-481.25	93.3
0.6	-478.53	1.94	258.49	-478.06	93.5
1	-462.73	1.88	251.29	-479.89	93.7

Fig.4 Impedance spectra for Q235 steel in the condensate water at 35 ℃ without(A) and with(B) various concentrations of CDDA in Nyquist form along with the equivalent electric circuit(C)WE: working electrode; RE: reference electrode.

Table 5 Impedance parameters for Q235 steel in the condensate water with various concentrations of CDDA

c(CDDA)/(mmol·L^-1)	R_p/(Ω·cm²)	C_dl/(μF·cm^-2)	n	$R p *$ /(Ω·cm²)	η_E(%)
0	255.21	223.53	0.79	243.93	——
0.05	1589.18	83.55	0.82	1598.41	84.7
0.1	1999.30	72.18	0.77	2003.76	87.8
0.3	2060.62	50.49	0.80	2079.39	88.3
0.6	2265.85	30.42	0.81	2289.16	89.3
1	2330.47	27.40	0.83	2358.32	89.7

Table 5 Impedance parameters for Q235 steel in the condensate water with various concentrations of CDDA

c(CDDA)/(mmol·L^-1)	R_p/(Ω·cm²)	C_dl/(μF·cm^-2)	n	$R p *$ /(Ω·cm²)	η_E(%)
0	255.21	223.53	0.79	243.93	——
0.05	1589.18	83.55	0.82	1598.41	84.7
0.1	1999.30	72.18	0.77	2003.76	87.8
0.3	2060.62	50.49	0.80	2079.39	88.3
0.6	2265.85	30.42	0.81	2289.16	89.3
1	2330.47	27.40	0.83	2358.32	89.7

Fig.5 EDX spectra(A1—C1) and surface morphologies(A2—C2) of Q235 steel before and after immersion in condensate water for 72 h (A1, A2) Without CDDA; (B1, B2) with 1 mmol/L CDDA; (C1, C2) freshly polished.

Fig.6 Two-dimension AFM images(A1—C1) and sectional analyses(A2—C2) of Q235 steel surface before and after immersion in condensate water at 35 ℃ for 72 h(A1, A2) Without inhibitor; (B1, B2) with 1 mmol/L CDDA; (C1, C2) freshly polished.

Fig.7 Variation of contact angles for Q235 steel after immersion in condensate water containing 1 mmol/L CDDA with different time(A) and under different pH values after 72 h immersion(B)

Fig.8 XPS deconvoluted spectra of major elements on Q235 steel after immersion in condensate water with 1 mmol/L CDDA at 35 ℃ for 72 h(A) C1s; (B) N1s; (C) O1s.

Fig.9 XPS deconvoluted spectra of β-CyD focused on C1s(A) and O1s(B)

Fig.10 ATR-FTIR spectra of the freshly polished Q235 steel(a) and the Q235 steel after 72 h immersion in the condensate water with 1 mmol/L CDDA(b)

Fig.11 Langmuir adsorption isotherms for CDDA on Q235 steel at presetting temperatures

Fig.12 Assembly mechanism of CDDA on Fe(110) surface(water molecules are removed for clarity)Simulation time/ps: (A) 0; (B) 20; (C) 50; (D) 80.

Fig.13 Optimized structure(A), electrostatic potential plot(B), HOMO(C) and LUMO(D) distributions of octadecylamine

Table 6 Condensed Fukui indices of ODA*

Atom	$f k +$	$f k -$	Atom	$f k +$	$f k -$
C1	0.019	0.004	C11	0.022	0.005
C2	0.010	0	C12	0.023	0.008
C3	0.019	0.001	C13	0.030	0.011
C4	0.024	0.002	C14	0.031	0.017
C5	0.028	0	C15	0.032	0.025
C6	0.027	0.002	C16	0.059	0.038
C7	0.027	0.001	C17	0.091	0.058
C8	0.026	0.002	C18	0.121	0.223
C9	0.013	0.003	N19	0.015	0.367
C10	0.022	0.004

Table 6 Condensed Fukui indices of ODA*

Atom	$f k +$	$f k -$	Atom	$f k +$	$f k -$
C1	0.019	0.004	C11	0.022	0.005
C2	0.010	0	C12	0.023	0.008
C3	0.019	0.001	C13	0.030	0.011
C4	0.024	0.002	C14	0.031	0.017
C5	0.028	0	C15	0.032	0.025
C6	0.027	0.002	C16	0.059	0.038
C7	0.027	0.001	C17	0.091	0.058
C8	0.026	0.002	C18	0.121	0.223
C9	0.013	0.003	N19	0.015	0.367
C10	0.022	0.004

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