The Galvanic Corrosion Research Between Fastener Materials And Coupled Materials On The Condition Of Cyclic Immersion

Various fasteners connected to other parts are often used in the mechanical structure of great marine projects, for example ships, sea-crossing bridge and so on. Under marine environment, such as seawater and wet air, these dissimilar materials will emerge varied galvanic corrosion. Currently, the fastener materials used commonly are carbon steel, high strength steel and so on. Practically, anti-corrosion coatings are often applied to protect steel substrate which is poor corrosion resistance in seawater.Composite coating consists of metallic coating and organic coating, its protective performance plays a central role during galvanic corrosion in real system. The water absorption process of coating, which may reduce the physical shielding effect, plays an important role in coating degradation process. The reduction of oxygen occurred in the coating/steel interface is important factor to coating stripping process. In practical engineering, coatings are often in wet/dry alternative environment. Moreover, wet/dry alternative environment will strength the water absorption process and oxygen reduction process.The galvanic corrosion law between fastener material and coupled material was studied under the seawater wet/dry alternating environment by the means of electrochemical impedance, galvanic current technology.Therefore, the main researches in this paper are listed as following:(1) The galvanic corrosion behavior of coupling system consisted of fastener materials(45steel, galvanized coating and Zn-Al coating)and coupled materials(316L stainless steel,907hull steel and5083aluminum alloy) was studied by corrosion potential, galvanic potential, galvanic current and potentiodynamic polarization curves under cyclic immersion in seawater. It is found that the galvanic corrosion behaviors of45steel with three coupled materials are more serious. The galvanic corrosion behaviors of coupling system (e.g., galvanized coating/316L, Zn-Al coating/316L, galvanized coating/907, etc) are also more serious. The galvanic corrosion rates of galvanic system, galvanized coating/5083, Zn-Al coating/5083and Zn-Al coating/907, are small relatively.The anti-corrosion performance of Zn-Al coating and Zn-Al coating coupled with5083was studied by linear polarization technique. The results showed that, after seven wet/dry alternative cycles, Zn-Al coating has good corrosion resistance and coupled Zn-Al coating can protect the5083alloy effectively.(2) The galvanic corrosion behavior between45steel used for fastener and B10Cu-Ni alloy under cyclic immersion in seawater was investigated as compared with that on full immersion condition by the means of electrochemical impedance, potentiodynamic polarization, galvanic potential and galvanic current. The results showed that the galvanic potential and the galvanic current under two conditions is basically same trend in each cycle. The couple potential became more negative and the couple current decreased with time. Thus, the couple potential became more positive with wet/dry cycling, the couple current the couple current decreased with time under the cyclic immersion. Compared with the full immersion, the constant value of couple current under cyclic immersion is greater. Finally, the galvanic effect and the galvanic corrosion rate are greater than full immersion.(3) The galvanic corrosion behavior between composite coating and5083aluminum alloy under cyclic immersion (wet/dry ratio are1d:1d and1d:2d, immersion frequency are2d:2d and5d:5d) in seawater was investigated by the means of electrochemical impedance, galvanic potential, galvanic current and XRD. The results showed that the polarity reversal of coupling system occurred in wet to dry ratio of1d:2d and the degradation degree of composite coating is great as compared with that wet to dry ratio of1d:1d. Moreover, the galvanic potentials of coupling system with two wet to dry ratio became more positive compared with full immersion, the state of electrode surface changed to a certain extent. Similarly, the polarity reversal of coupling system occurred in two immersion frequency and the absolute value of current density|ig|(5d:5d)<|ig|(2d:2d). Consequently, two kinds of immersion frequency can accelerated the degradation of composite coating. When the immersion frequency was2d:2d, the degradation of composite coating was greater, more speed.