## 3 Results and Discussion
### Galvanic Corrosion  
To prevent corrosion, the cap and pin are coated with more than 70-\(\upmu\)m-thick zinc. The role of zinc is galvanic protection and oxide film formation. Galvanic corrosion occurs when two different metals in an electrolyte come into contact with each other, which causes a current to flow between them. As a result, the corrosion of metals with high corrosion resistance (cathode) is suppressed and the corrosion of metals with high activity (anode) is promoted. When iron and zinc are in contact, if the iron is exposed due to a defect or scratch, it gradually becomes cathodic.  
\[\text{Fe}^{3+}+\text{e}^{-}\rightarrow\text{Fe}^{2+}\]  
Meanwhile, the Zn becomes increasingly anodic.  
\[\text{Zn}^{2+}+2\text{e}^{-}\rightarrow\text{Zn}\]  
Therefore, the iron is protected until the zinc is fully corroded by water and oxygen. In a neutral state where oxygen is dissolved, zinc first causes the following corrosion reactions.  
Anode reaction:  
\[\text{Zn}\rightarrow\text{Zn}^{2+}+2\text{e}^{-}\]  
Cathode reaction:  
\[\text{1/2O}_{2}+\text{H}_{2}\text{O}+2\text{e}^{-}\rightarrow\text{2 OH}^{-}\]  
Zn(OH)\({}_{2}\), which is not soluble in water, is formed by binding a zinc ion (Zn\({}^{2+}\)), which is generated in the anode reaction, and hydroxide ion (OH\({}^{-}\)), which is generated in the cathode reaction. This zinc hydroxide film is stable in the pH range of 6 to 12 and acts as a barrier against the diffusion of oxygen, which suppresses the corrosion of zinc. Figure 4 shows the PH dependence of the zinc corrosion rate. In an actual fresh water environment, a complicated surface film--composed of zinc oxide (ZnO) and basic carbon zinc (2 ZnCO\({}_{3}\bullet\)3 Zn (OH)\({}_{2}\))--as well as zinc hydroxide are formed by the action of various dissolved ions. The corrosion rate of zinc in distilled water shows a relatively high value of approximately 0.25 mm/y at a pH of 7. However, the surface coating formed in actual fresh water serves as a protective film for suppressing the zinc corrosion rate to a much lower level [7,8].  
Figure 4: Zinc corrosion rate dependence on pH.  
The movement of oxygen to the zinc surface is a very important factor in the film formation. A sufficient supply of oxygen to the zinc surface is required to maintain the film. If oxygen is not sufficiently transferred to the zinc surface, a defect presents in the coating and the zinc formula begins. The surrounding area covered with the film not only becomes a cathode with a higher electric potential than that of the zinc itself, but also expands the area compared to the zinc anode, which prompts the anode elution of the exposed zinc. Eventually, the iron base material is exposed. At this time, the galvanized layer covered with the surrounding coating fails to serve as a sacrificial anode because the polarity is reversed with respect to the iron, and the valuable potential of the zinc plating surface accelerates the corrosion of the exposed iron matrix.  
As the pH changes from neutral to acidic, the zinc corrosion rate increases. This is because the basic materials--such as zinc oxide, zinc hydroxide, and basic carbonate salts--constituting the protective coating on the zinc surface are neutralized in the acidic environment and disappear.