1.How exactly does electrochemical corrosion manifest itself?
Electrode Formation:
Anode (Sacrificial Electrode): Aluminum (Al). Aluminum is chemically more reactive than iron, resulting in a lower standard electrode potential.
Cathode (Protected Electrode): Cold-rolled coil (steel, primarily composed of Fe).
Corrosion Process:
In a humid environment, an electrolyte (such as water) connects the two metals.
Aluminum, acting as the anode, actively loses electrons and is oxidized into aluminum ions (Al³⁺), which dissolve into the electrolyte.
Electrons flow to the steel, acting as the cathode, where a reduction reaction occurs on the steel surface (usually oxygen and water gain electrons to form hydroxide ions).
The result is that the aluminum coil corrodes much faster than it would alone, producing white powdery alumina or more complex aluminum rust. The steel (cold-rolled coil), however, is protected, and corrosion is slowed.
2.What specific phenomena might occur?
Aluminum coils: White, off-white, or powdery corrosion products (aluminum hydroxide, aluminum oxide, etc.) form on the surface. In severe cases, this can cause the aluminum material around the riveting point to thin, lose strength, and even loosen the rivets.
Cold-rolled steel coils: Although protected as a cathode, their own oxidation (rusting, reddish-brown Fe₂O₃) can still occur if the environment is harsh or the coating is damaged, although the rate is usually slower than when they exist alone.
Contact surfaces: Corrosion is likely to be most severe inside the seam where the two metals meet. It is difficult to detect visually but will seriously affect the connection strength.
3.How to achieve electrical insulation/isolation?
Use insulating gaskets/blades: Between aluminum and steel coils, and between rivets and either material, use non-conductive gaskets (such as rubber, nylon, or engineering plastic gaskets) or coated bushings to completely block direct metal-to-metal contact and electrical pathways.
Use insulating coatings: Apply insulating coatings, epoxy primers, or zinc-chromium coatings (Dacromet) to the contact surfaces.
4.How to choose the right rivets?
Avoid using carbon steel rivets. Stainless steel rivets, which have a potential close to that of aluminum (note: some stainless steels still have a potential difference with aluminum) or aluminum rivets can be used. Ideally, aluminum rivets are preferred because they are made of the same material as aluminum coils and will not cause galvanic corrosion.
5.How to perform surface treatment and coating?
The two materials are protected separately: for example, cold-rolled coils are galvanized (zinc has a small potential difference with aluminum, and zinc will first sacrifice and protect iron), phosphate treated, or sprayed with anti-corrosion paint. Aluminum coils are anodized or sprayed.
Overall coating: After riveting and assembly, the entire assembly is uniformly sprayed for protection, isolating the gaps from the environment. Ensure the coating fully penetrates the joints.