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Journal of Chinese Society for Corrosion and protection  2016, Vol. 36 Issue (4): 295-305    DOI: 10.11902/1005.4537.2015.142
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Electrochemical Behavior of Solvent-free Epoxy Coating during Erosion in Simulated Flowing Sea Water
Hongtao ZHAO,Weizhong LU(),Jing LI,Yugui ZHENG
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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Abstract  

The electrochemical behavior of three solvent-free epoxy coatings on Q345E carbon steel during corrosion-erosion induced by flowing slurry of simulated sea water with 1% (mass fraction) of sand at 60 ℃ was investigated by means of electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM) and confocal laser scanning microscopy (CLSM), respectively. The results showed that the flowing slurry induced erosion aggravated the deterioration of coatings. The amine-cured epoxy powder coating and phenolic aldehyde amine-cured liquid epoxy coating were cured with slow speed and thus much densified, accordingly, their deterioration process experienced three stages: firstly, the inward migration of corrosive medium; secondly, the corrosion initiation of the substrate metal; thirdly, the corrosion propagation of the substrate metal beneath the coating and finally the failure of coatings. On the contrary, the phenolic aldehyde-cured epoxy powder coating was cured with fast speed and thus much porous, its deterioration process experienced two stages with a shortened second stage. Therefore,the existence of pores in the coating is one of the important factors for inducing failure of coatings during corrosion-erosion.
Key words:  solvent-free epoxy coating      erosion      sea water      electrochemical behavior      pore     
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Hongtao ZHAO
Weizhong LU
Jing LI
Yugui ZHENG

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Hongtao ZHAO,Weizhong LU,Jing LI,Yugui ZHENG. Electrochemical Behavior of Solvent-free Epoxy Coating during Erosion in Simulated Flowing Sea Water. Journal of Chinese Society for Corrosion and protection, 2016, 36(4): 295-305.

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https://www.jcscp.org/EN/10.11902/1005.4537.2015.142     OR     https://www.jcscp.org/EN/Y2016/V36/I4/295

Fig.1  Schematic diagram of device of erosion experiment
Fig.2  Modulus (a) and phase angle (b) diagrams of epoxy powder coating A after immersion in 3.5%NaCl solution at 60 ℃ for different time under erosion condition
Fig.3  Modulus (a) and phase angle (b) diagrams of epoxy powder coating A after immersion in 3.5%NaCl solution fordifferent time at 60 ℃ under static immersion condition
Fig.4  Modulus (a) and phase angle (b) diagrams of epoxy powder coating B after immersion in 3.5%NaClsolution at 60 ℃ for different time under erosion condition
Fig.5  Modulus (a) and phase angle (b) diagrams of epoxy powder coating B after immersion in 3.5%NaClsolution at 60 ℃ for different time under static immersion condition
Fig.6  Modulus (a) and phase angle (b) diagrams of solvent-free epoxy liquid coating C after immersion in 3.5%NaCl solution at 60 ℃ for different time under erosion condition
Fig.7  Modulus (a) and phase angle (b) diagrams of solvent-free epoxy liquid coating C after immersion in 3.5%NaCl solution at 60 ℃ for different time under static immersion condition
Fig.8  Nyquist plots of epoxy powder coating A immersed in 3.5% NaCl solution at 60 ℃ for 0~6.5 d (a), 9 d (b), 12.5 d (c) and 18.5~26.5 d (d) under erosion condition
Fig.9  Nyquist plots of epoxy powder coating B immersed in 3.5%NaCl solution at 60 ℃ for 0~6.5 d (a), 9 d (b) and 12.5~26.5 d (c) under erosion condition
Fig.10  Nyquist plots of solvent-free epoxy liquid coating C immersed in 3.5%NaCl solution at 60 ℃ for 0~9 d (a), 12.5~18.5 d (b) and 22~26.5 d (c) under erosion condition
Fig.11  Equivalent circuits for solvent-free epoxy coating system: (a) model A, (b) model B, (c) model C, (d) model D, (e) model E
Timed RcΩcm2 n1 QcFcm-2 RctΩcm2 n2 QdlFcm-2 RdiffΩcm2 n3 QdiffFcm-2 LHcm2 RLΩcm2
0 1.9×1011 1.00 7.5×10-11 --- --- --- --- --- --- --- ---
1.5 4.8×1010 0.91 2.5×10-10 --- --- --- --- --- --- --- ---
4 1.3×109 0.89 3.4×10-10 --- --- --- --- --- --- --- ---
9 2.6×108 0.89 3.7×10-10 1.8×108 0.79 1.5×10-8 --- --- --- --- ---
12.5 6.5×107 0.89 2.9×10-10 --- --- --- --- --- --- 1.2×109 9.6×108
18.5 3.9×105 1.00 9.1×10-11 3.4×105 1.00 8.5×10-11 4.7×105 0.33 2.1×10-7 --- ---
22 1.3×105 1.00 1.1×10-10 2.1×105 0.37 1.8×10-8 3.8×105 1.00 1.1×10-10 --- ---
26.5 7.9×104 0.20 6.8×10-8 2.1×105 1.00 1.3×10-10 1.3×105 1.00 3.4×10-10 --- ---
Table 1  Fitted parameters of EIS of epoxy powder coating A immersed in 3.5%NaCl solution at 60 ℃ for different time under erosion condition
Time / d Rc / Ωcm2 n1 Qc / Fcm-2 Rct / Ωcm2 n2 Qdl / Fcm-2 Rdiff / Ωcm2 n3 Qdiff / Fcm-2 Zw
0 1.2×1012 1.00 1.0×10-10 --- --- --- --- --- --- ---
1.5 5.0×1010 0.96 1.5×10-10 --- --- --- --- --- --- ---
4 4.2×1010 0.94 2.8×10-10 --- --- --- --- --- --- ---
9 8.9×106 0.96 2.1×10-10 8.50×107 0.37 2.6×10-9 --- --- --- 1.1×10-7
12.5 8.0×105 0.94 3.1×10-10 5.20×106 0.38 7.0×10-8 2.9×107 0.66 9.1×10-8 ---
15 1.0×106 0.93 3.1×10-10 6.30×106 0.49 4.1×10-8 4.6×106 1.00 2.0×10-6 ---
18.5 5.4×105 1.00 1.4×10-10 1.60×106 0.46 3.1×10-7 1.0×108 1.00 1.2×10-7 ---
22 1.7×105 0.86 9.2×10-10 2.30×106 0.57 4.1×10-7 7.7×1010 1.00 5.9×10-4 ---
26.5 4.8×105 0.89 5.8×10-10 2.55×106 0.46 4.4×10-7 3.4×106 1.00 2.0×10-7 ---
Table 2  Fitted parameters of EIS of epoxy powder coating B immersed in 3.5%NaCl solution at 60 ℃ for different time under erosion condition
Time / d Rc / Ωcm2 n1 Qc / Fcm-2 Rct / Ωcm2 n2 Qdl / Fcm-2 Rdiff / Ωcm2 n3 Qdiff / Fcm-2
0 7.5×1010 1.00 6.7×10-11 --- --- --- --- --- ---
1.5 3.3×1010 0.92 2.1×10-10 --- --- --- --- --- ---
4 7.7×109 0.91 2.4×10-10 --- --- --- --- --- ---
9 3.8×108 0.95 1.7×10-10 3.1×108 1.00 1.8×10-10 --- --- ---
12.5 3.5×107 0.95 1.7×10-10 8.3×106 0.66 1.1×10-9 --- --- ---
15 2.1×106 0.96 1.4×10-10 1.6×107 0.05 3.2×10-8 --- --- ---
18.5 2.9×108 0.95 1.8×10-10 1.0×109 0.26 3.2×10-9 --- --- ---
22 1.6×105 0.92 2.8×10-10 1.6×105 0.45 3.2×10-6 2.1×106 1.00 2.1×10-6
26.5 8.5×104 0.83 9.5×10-10 4.6×105 0.52 3.3×10-6 7.7×1014 1.00 2.1×10-6
Table 3  Fitted parameters of EIS of solvent-free epoxy liquid coating C immersed in 3.5%NaCl solution at60 ℃ for different time under erosion condition
Fig.12  Evolutions of Qc (a) and |Z |0.01 Hz (b) of epoxy powder coating A during immersion under static and erosion conditions at 60 ℃
Fig.13  Evolutions of Qc (a) and |Z |0.01 Hz (b) of epoxy powder coating B during immersion under static and erosion conditions at 60 ℃
Fig.14  Evolutions of Qc (a) and |Z |0.01 Hz (b) of epoxy powder coating C during immersion under static and erosion conditions at 60 ℃
Fig.15  CLSM section morphologies of epoxy powder coatings A (a) and B (b), and solvent-free epoxy liquid coating C (c)
Fig.16  SEM surface morphologies of epoxy powder coating B (a, b), epoxy powder coating A (c) and solvent-free epoxy liquid coating C (d) after erosion test for 26.5 d
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