Why are insulating washers installed between steel bolts and connecting aluminium plates?

Corrosion of aluminium can be triggered by putting it in contact with another metal in the presence of water. This is known as bimetallic corrosion or galvanic corrosion. The mechanism of such corrosion is the formation of a cell in moist condition so that an electric current is generated to flow between the two metals in direct contact. The degree of corrosion is influenced by the nature of connecting metals, their electrode potential, their areas, conductivity of fluid etc.

When aluminium plates are connected together by means of steel bolts, bimetallic corrosion may occur. Where there is presence of a good electrolyte like seawater, there may be local attack on aluminium. Therefore, some jointing compound or insulating insert and washer are adopted to insulate electrically the dissimilar metals from one another

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.