Aluminum and steel are both classified as metal, but it doesn’t mean they have the same responses when bonded together in automotive repairs and manufacturing. Complicating the process it that the two metals are alike in some aspects, but different to an extent that the reactive chart lists them as dissimilar metals.
Complicating it still further are the differences in the adhesives used for bonding these dissimilar metals. These adhesives, too, are not all identical. They are made up of different mixes of minerals and chemicals and only some will be suitable for bonding dissimilar metals, and particularly aluminium, without impacting negatively on the surface of the bond.
The wrong adhesive choice for bonding steel and aluminum can increase the chances of galvanic corrosion and oxidization, and electrochemical process which causes one dissimilar metal to corrode onto the other, particularly when both the steel and aluminum are in electrical contact and an electrolyte is involved.
The electrolyte allows ions to pass between the two metals, giving rise to the galvanic process and when bonding steel and aluminum, the aluminum is the biggest challenge and most likely to corrode onto the steel because Aluminum oxidizes faster, quickly producing a white powder which rapidly corrodes onto the steel.
Corrosion can also occur as a result in the presence of carbon, whether on un-primed bare metal, in moldings, clips, plastic parts and gaskets, or even be included in the adhesives used for bonding if bare aluminum comes in contact with it. While carbon has no impact on bare steel, carbon properties may react with the aluminum and cause corrosion not only between the aluminum and the steel, but also between the adhesive and the aluminum, because of their dissimilarity.
Automotive manufacturers have shown that epoxy priming followed with paint on steel can help to control the galvanic corrosion between it and aluminum by insulating it from corrosion and improving the adhesion by separating aluminum from contact with moisture and from coming in contact with other substances or chemicals which would allow for ions passing between the aluminum and the steel.