Influence of assembly on corrosion behaviors of bolt/nut connections in a salt-spray environment

Due to the rapid development of naval aviation equipment, naval aircraft will encounter complicated atmospheric corrosion problems resulting from exposure to the oceanic atmosphere. The corrosion of the fasteners in the aircraft, especially due to the galvanic corrosion between fasteners, seriously compromises the safety of a particular component in an aircraft. When an aircraft is used for a long duration in an oceanic atmosphere containing high humidity and salinity, a layer of liquid film having a thickness of less than 1 μm will form on the surface of the structure, which causes electrical conduction between different structures, thereby increasing the risk of galvanic corrosion. Currently, many studies mainly focus on investigating the stress corrosion cracking (SCC) of bolt or galvanic corrosion between the bolt and aluminum alloy plates. However, only a few studies have investigated the galvanic corrosion of bolt/nut in assembly. Therefore, three types of nuts (Cd-plated 30CrMnSiA, Zn-plated 30CrMnSiA, and passivated 0Cr16Ni6) were utilized in this study, which were assembled to a Cd-plated 30CrMnSiA bolt. The salt-spray corrosion simulation, observations from scanning electron microscope (SEM), and electrochemical measurements (open circuit potential, potentiodynamic polarization, and galvanic corrosion current tests) were used to investigate the effect of the galvanic corrosion between the bolt/nut couples on the corrosion behaviors of bolts and nuts. The results depict that the highest potential difference and galvanic current are observed between the Cd-plated 30CrMnSiA bolts and passivated 0Cr16Ni6 nuts, which indicates the most significant galvanic effects. Further, the galvanic corrosion sensitivity rating reaches an E level, which significantly promotes the propagation of the pitting corrosion of the bolt. Additionally, the acceleration factor (AF) becomes 3.4. For the couple including the Cd-plated 30CrMnSiA bolt and Zn-plated 30CrMnSiA nut, the nut acts as an anode. Further, the corrosion rate of the nut increases, and the AF approximately becomes 1.2. Compared to the aforementioned two couples, the Cd-plated 30CrMnSiA bolt and 30CrMnSiA nut exhibit the weakest galvanic effect, and the galvanic corrosion sensitivity rating is observed to be at an A level.