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Chinese Journal of Materials Research  2020, Vol. 34 Issue (7): 545-553    DOI: 10.11901/1005.3093.2019.571
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In Situ SECM Observation of Corrosion Behavior of Carbon Steel at Defects of Epoxy Coating under AC Current Conditions
GONG Weiwei1, YANG Bingkun2(), CHEN Yun2, HAO Wenkui2, WANG Xiaofang2, CHEN Hao1
1.Inner Mongolia Power Research Institute, Hohhot 010020, China
2.State Key Laboratory of Advanced Transmission Technology, Global Energy Interconnection Research Institute, Beijing 102211, China
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Abstract  

The carbon steel corrosion behavior at defects of epoxy coating in the effect of different AC current densities was investigated by means of in situ micro-area electrochemistry and traditional macro-electrochemical techniques, namely electrochemical scanning microscopy (SECM) and electrochemical impedance spectroscopy techniques. SECM observation can directly reveal the change of electrochemical active points during the local corrosion process of carbon steel at coating defects. The results show that when AC current is present, the number of corrosion active points at the coating defects is significantly more than that in the absence of AC current, correspondingly, the inhibitory effect of the corrosion products is significantly weaker than that in the case without AC current. The corrosion at the coating defect at the initial stage of immersion is an electron transfer control process, and it is converted to a diffusion control process after soaking for 10 hours. With the increase of AC current intensity and immersion time the degree of coating peeling increases, and the depth and width of the pits increase.

Key words:  materials failure and protection      in situ      carbon steel      corrosion      AC current     
Received:  05 December 2019     
ZTFLH:  TG174.4  
Fund: Science and Technology Project of Inner Mongolia Power Company(2019-102)
Corresponding Authors:  YANG Bingkun     E-mail:  ybk_511@163.com

Cite this article: 

GONG Weiwei, YANG Bingkun, CHEN Yun, HAO Wenkui, WANG Xiaofang, CHEN Hao. In Situ SECM Observation of Corrosion Behavior of Carbon Steel at Defects of Epoxy Coating under AC Current Conditions. Chinese Journal of Materials Research, 2020, 34(7): 545-553.

URL: 

https://www.cjmr.org/EN/10.11901/1005.3093.2019.571     OR     https://www.cjmr.org/EN/Y2020/V34/I7/545

Fig.1  Schematic diagram of SECM scanning device
Fig.2  SECM image of samples soaked for different time without AC current (a) 0 h; (b) 5 h; (c) 10 h; (d) 24 h
Fig.3  SECM images of samples soaked for different time at current density of 50 A/m2 (a) 0 h; (b) 5 h; (c) 10 h; (d) 24 h
Fig.4  SECM images of samples soaked for different time at current density of 100 A/m2 (a) 0 h; (b) 5 h; (c) 10 h; (d) 24 h
Fig.5  SECM images of samples soaked for different time at current density of 300 A/m2 (a) 0 h; (b) 5 h; (c) 10 h; (d) 24 h
AC current density/A·m-2Maximum anode current peak/pA
0 h5 h10 h24 h
061.467.698.8125.2
50 A/m2114.0236.0197.0308.0
100 A/m259.2151.0187.0385.0
300 A/m263.0178.0192.0437.0
Table 1  Maximum anode current peak for samples soaked for different time at different AC current densities
Fig.6  Probe scanning current for samples soaked for different time at different AC current densities (a) 50 A/m2; (b) 100 A/m2; (c) 300 A/m2
Fig.7  Nyquist plots and corresponding Bode plots for samples soaked for deferent time at different AC current densities (a, b) 50 A/m2; (c, d) 100 A/m2; (e, f) 300 A/m2
Fig.8  Equivalent circuit for the sample defect
AC current densityImmersion time

Cf

/μF·cm-2

Qd

Rf

/Ω·cm2

Rp

/Ω·cm2

Cd

/μF·cm-2

n
50 A/m20 h2.71×10-36.660.79164.38.95×104
5 h7.87×10-355.50.75116.96.17×104
10 h8.59×10-331.50.9545.9420.55
24 h3.72×10-3103.10.70119.35.06×104
100 A/m20 h2.71×10-36.660.79164.38.95×104
5 h3.90×10-38.090.80136.54.95×104
10 h1.12×10-22730.7036.551.51×104
24 h1.18×10-246.60.78114.65.65×104
300 A/m20 h8.33×10-39.650.77138.96.77×104
5 h1.49×10-294.90.7468.733.09×104
10 h15.968.40.6667.691.30×104
24 h9.06×10-371.80.75252.83.34×104
Table 2  Fitted values of the elements of the equivalent circuit
Fig.9  Change of Rp for samples soaked for deferent time at different AC current densities
AC current densityImmersion time
5 h10 h24 h
0 A/m215.525.450.8
50 A/m278.8115.2143.2
100 A/m285.6135.5154.1
300 A/m292.7153.1238.4
Table 3  Corrosion pit depth for samples soaked for different time at different AC current densities (μm)
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