Corrosion Behavior of Epoxy-Coated Rebar with Pinhole Defect in Seawater Concrete

doi:10.1007/s40195-018-0755-z

Abstract

Abstract:

Experiments were carried out to investigate the corrosion behavior of epoxy-coated rebar (ECR) with pinhole defect (diameter in hundreds of microns) immersed in the uncarbonated/carbonated simulated pore solution (SPS) of seawater concrete. Corrosion behavior was analyzed by electrochemical impedance spectroscopy. The composition and morphology of corrosion products were characterized by X-ray diffraction, energy-dispersive spectrometry and scanning electron microscopy. Meanwhile, oxide film produced by preheating before spray coating was investigated by X-ray photoelectron spectroscopy and Mott-Schottky technology. Results indicated that corrosion behavior of ECR with pinhole defect exhibited three stages when immersed in the uncarbonated/carbonated SPS. In the initial stage, steel in defect was passivated when exposed in the uncarbonated SPS and corroded when exposed in the carbonated SPS, due to competitive adsorption between chloride and hydroxyl ions. In the second stage, the oxide film under coating reconstituted (the thickness and defects density decreasing) in the uncarbonated SPS, which was caused by the synergy between high hydroxide and chloride activity, while in the carbonated SPS, crevice corrosion happened under the coating around pinhole, because of the different oxygen concentrations cell at the coating/steel interface. In the third stage, localized corrosion occurred under the coating around the pinhole in the uncarbonated SPS, which was probably induced by ion diffusion at the nano-scale coating/steel interface. The corrosion products adjacent to the defects were re-oxidized from FeCl₂·4H₂O and Fe₂(OH)₃Cl to Fe₂O₃·H₂O, and the corrosion area was expanded outward in the carbonated SPS.

Key words: Corrosion, Epoxy-coated, rebar, (ECR), Seawater, concrete, Pinhole, defects

Yao-Zong Mao, Ying-Hua Wei, Hong-Tao Zhao, Chen-Xi Lv, Hai-Jiao Cao, Jing Li. Corrosion Behavior of Epoxy-Coated Rebar with Pinhole Defect in Seawater Concrete[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(11): 1171-1182.

Figures/Tables 16

Table 1 Composition and pH of simulated pore solution

Solution	KOH (mol/L)	NaOH (mol/L)	NaHCO₃ (mol/L)	Na₂CO₃ (mol/L)	NaCl (mol/L)	pH
Uncarbonated	0.6	0.2	0	0	0.6	13.4
Carbonated	0	0	0.06	0.04	0.6	9.8

Fig. 1 Schematic illustration of three-electrode cell for electrochemical test (RE: reference electrode; CE: counter electrode; WE: work electrode)

Fig. 2 OCP-time curves of defected ECR in seawater concrete: a uncarbonated SPS; b carbonated SPS

Fig. 3 Nyquist (a, c) and Bode (b, d) plots of defected ECR and corresponding fitting results of EIS data after immersion for 24 h in uncarbonated (a, b); carbonated (c, d) SPS (Zr: real part of impedance; Zi: imaginary part of impedance)

Fig. 4 Equivalent circuit models used to fit the EIS data of defected ECR at different stages: a first stage; b second stage; c third stage (Rs: solution resistance; R0 and Cf: pore resistance in pinhole defect and coating capacitance, respectively; Rct and CPEdl: charge transfer resistance and double-layer capacitance, respectively; Rfi and CPEfi: oxide film resistance and capacitance, respectively; Zw: Warburg impedance)

Fig. 5 Nyquist (a) and Bode (b) plots of defected ECR immersed in uncarbonated SPS for 3 days and corresponding fitting results of EIS data

Fig. 6 XPS graphs of substrate surface under different conditions immersed in uncarbonated SPS: a XPS depth etching diagram; b XPS spectra of Fe 2p on steel surface for preheating 20 min by Ar+ sputtering for different time; c-e XPS spectra of Fe 2p on surface of F-0h, F-2d, L-2d, respectively

Fig. 7 Mott-Schttoky curves for preheated steel at different immersion time

Fig. 8 Nyquist (a) and Bode (b) plots of defected ECR immersed in carbonated SPS for 3 days and corresponding fitting results of EIS data

Fig. 9 SEM micrographs of corrosion products under coating: a 140 days after immersed in uncarbonated SPS; b enlarged view of corrosion product of a; c 10 days after immersed in carbonated SPS; d 100 days after immersed in carbonated SPS

Fig. 10 XRD patterns of corrosion products: a black products; b yellow-brown products

Fig. 11 Schematic of two behavior modes for defected ECR at the second stage: a uncarbonated SPS; b carbonated SPS

Fig. 12 Nyquist (a, c) and Bode (b, d) plots of defected ECR in uncarbonated (a, b) and carbonated (c, d) SPS and corresponding fitting results of EIS data at third stage

Fig. 13 Schematics of corrosion behavior modes for defected ECR at the third stage: a uncarbonated SPS; b carbonated SPS

Fig. 14 |Z|0.01 Hz of different size of defected ECR with immersion time in uncarbonated SPS: a 200 μm; b 500 μm; c 800 μm

Table 2 Parameters of power exponent equation

Size (μm)	A (Ω)	B (Ω)	p	r ²
200	1.14?×?10⁷	1.43?×?10⁸	127	0.998
500	3.24?×?10⁷	2.64?×?10⁸	38	0.980
800	1.44?×?10⁷	1.46?×?10⁶	12	0.985

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