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Acta Metall Sin  2017, Vol. 53 Issue (11): 1445-1452    DOI: 10.11900/0412.1961.2017.00076
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Study on the Galvanic Current of Corrosion Behavior for AH32 Long-Scale Specimen in Simulated Tidal Zone
Lin ZHAO, Xin MU, Junhua DONG(), Liping WU, Changgang WANG, Wei KE
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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Abstract  

The environment of the tidal zone is very complex. The interactions of dry-wet alternation and sea erosion lead to serious corrosion of steel structures, which makes it difficult to adopt protective methods. Therefore, it is of great significance to study the corrosion and protection methods of steel in tidal zone. For long-scale steel through the tidal zone and immersion zone, there is a big difference in corrosion behavior with complete immersion condition, the potential of the steel surface changes due to the influence of oxygen concentration difference and tidal fluctuations or other factors. In this study, the galvanic current and open circuit potential of the long-scale AH32 steel were monitored in simulated tidal zone. The results shows that the potential at different tide levels and different immersion depths for a long-scale AH32 specimen is not unified, with the macro cell was formed by the difference of oxygen supply, which caused internal galvanic current. The essence of the galvanic current is the net current that was generated by the sum of anode and cathode current. Galvanic current at different positions on the long-scale AH32 specimen varies with the tidal movement periodically in tidal zone. When tide is at the highest level, the galvanic current of all parts accesses a maximum value, and among these maximum values, the largest one is at the middle part of specimen, which causes the biggest anodic dissolution current density. According to the variation of the galvanic current, the time distributions of the drying, wetting and immersion states were calculated, and the results showed that the corrosion scale of the long-scale AH32 specimen at different positions depends on the time all location of wetting and immersion in tidal zone. The macro cell caused the galvanic current when all parts of the specimen were immersed. At wetting state, the solution resistance of the thin liquid film is very large, which leads to the change of the driving potential of the macro cell into the potential drop. Thus, macro cell is ineffective in the wetting state and cannot produce the galvanic current. According to the relation between wetting time and quantity of electricity at wetting state, the maximum wetting time of the long-scale AH32 specimen is shown above average mean tide level in tidal zone, which indicates that the corrosion loss of this part is maximum due to wetting state. In addition to weight loss measurements, maximum of it for long-scale AH32 specimen was obtained at the average mean tide level caused by immersion state. It can be indicated the maximum weight loss of the long-scale AH32 specimen should appear upper the average mean tide level part in tidal zone. These results were consistent with measurements of corrosion rates.

Key words:  AH32 steel      tidal zone      long-scale specimen      galvanic current      potential     
Received:  09 June 2017     
ZTFLH:  TG172.5  
Fund: Supported by National Natural Science Foundation of China (No.51671200), High Technology Research and Deve-lopment Program of China (No.2015AA034301) and National Corrosion Platform Fund and Key Laboratory of Marine Environment Corrosion and Biofouling Fund (No.MCKF201611)

Cite this article: 

Lin ZHAO, Xin MU, Junhua DONG, Liping WU, Changgang WANG, Wei KE. Study on the Galvanic Current of Corrosion Behavior for AH32 Long-Scale Specimen in Simulated Tidal Zone. Acta Metall Sin, 2017, 53(11): 1445-1452.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00076     OR     https://www.ams.org.cn/EN/Y2017/V53/I11/1445

Fig.1  Schematic of galvanic current measurement for long-scale specimen (WE—working electrode, CE—counter electrode, SCE—reference electrode; HTL—high tide line, LTL—low tide line)
Fig.2  Change of open-circuit potentials Eocp and galvanic current Ig of positions a (a), b (b), c (c) and d (d), and tidal level change (e) with corrosion time in 5 d
Position in Fig.1 I II III
a 0.345~11.655 11.655~12 12~12.345
b 2.55~9.45 9.45~12 12~14.55
c 4.725~7.275 7.275~12 12~16.725
d 6~12 12~18
Table 1  Time nodes of different positions during tide (h)
Fig.3  Bar graphs of dryness-wetness time calculated by galvanic current
Fig.4  Changes of quantity of electricity for galvanic current with tide level under various conditions
Fig.5  Corrosion rates of AH32 steel in simulated marine environment after 15 d immersion
Fig.6  Diagram of corrosion in the vicinity of the measuring position during tide (A1—relative cathode area; A2—relative anode area)
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