What is the reason for using a protective separator in rivets for dissimilar metals?

Using a protective separator in rivets for dissimilar metals is primarily to prevent electrolytic action. When two dissimilar metals come into contact in the presence of moisture or an electrolyte, galvanic corrosion can occur, which leads to the deterioration of one of the metals. This is particularly critical in aircraft structures, where materials like aluminum and steel are frequently used together. The protective separator acts as a barrier that isolates the two metals, thus preventing any direct contact and mitigating the risk of corrosion due to electrochemical reactions. This is essential for maintaining the integrity and longevity of the aircraft's structural components.