Q. Insulating washer between steel bolts and aluminium plates?

Corrosion of aluminium can be activated by putting it in contact with another metal in the presence of water. This is called galvanic corrosion or bimetallic corrosion. The mechanism of such corrosion is formation of a cell in moist condition so that an electric current is produced to flow between two metals in direct contact. Degree of corrosion is influenced by the nature of connecting metals, theirareas, their electrode potential, conductivity of fluid etc. When aluminium plates are connected together through steel bolts, bimetallic corrosion can take place. Where there is presence of a good electrolyte such as seawater, there can be local attack on aluminium. So some jointing compound or insulating insert and washer are adopted to insulate electrically the dissimilar metals from one another.