To effectively prevent long-term corrosion of mild steel in seawater, which coating is most suitable?

The most suitable method for preventing long-term corrosion of mild steel in seawater is plating with a relatively unstable metal, such as zinc or aluminum. This approach leverages the principles of cathodic protection, where the more reactive (or less stable) metal serves as a sacrificial anode. In this case, zinc or aluminum will corrode preferentially before the underlying mild steel which protects it from corrosion.

When these metals are used as a coating, they will corrode in place of the steel, thus extending the life of the steel structure significantly. This method is particularly effective in marine environments where galvanic corrosion can occur due to the presence of saltwater.

While the other options involve corrosion-resistant materials, they do not offer the same sacrificial benefits. For instance, plating with a highly stable metal like nickel or silver would not provide the necessary protection for mild steel, as these metals do not corrode easily and therefore would not prevent corrosion of the underlying steel. Similarly, organic paints with copper oxide or carbon-epoxy laminates may provide some protection, but they do not possess the sacrificial attributes necessary for effective long-term mitigation of corrosion in a seawater environment.