Corrosion

Corrosion results when two dissimilar metals are in contact with each other and are immersed in a conductive solution (i.e., seawater). Corrosion can cause structural failure of materials or cause sealing surfaces to leak; thus, it is a very important engineering concern. The rate of corrosion is determined by several factors, including:
• Valence of the metals
• Salinity of the water
• Temperature
• Oxygen level
Several techniques are used to minimize or eliminate corrosion. These include:
• Replace one of the metals with a plastic material. For machined parts, Delrin (acetyl) is a good choice. For pipes, PVC is available in many sizes.
• Isolate the metals from each other. Plastic washers are often used with metallic hardware.
• Use sacrificial anodes. Zinc anodes are available in an assortment of sizes and shapes in most marine hardware stores.
• Passify the metal. Various processes are available for different metals that help protect it from corrosion. For aluminum it’s anodize, for titanium it’s Tiodize, for stainless steel it’s passification.
• Paint—coating the entire surface of an object eliminates contact with seawater and hence prevents corrosion. However, any scratch or pin hole in the paint will put the metal in contact with water, so it is usually advisable to have the metal passified before painting.
• Most corrosion occurs as an oxidation of a metal, and thus freely available dissolved oxygen is required. Crevice corrosion is an anaerobic process that occurs in low-oxygen-level waters or in cracks and crevices where there is little exchange of water.