1.What is galvanic corrosion?
When two metals with different reactivity (such as steel and aluminum) come into direct contact in an electrolyte solution (such as water or humid air), a galvanic cell is formed. The more reactive metal (with a more negative electrode potential) acts as the anode and is corroded more rapidly, while the less reactive metal (with a more positive electrode potential) acts as the cathode and is protected.
2.Who is the victim in the steel-aluminum combination?
In most common environments, aluminum is far more reactive than steel. Therefore, in this galvanic couple:
Anode (being corroded): Aluminum coil
Cathode (being protected): Cold-rolled steel coil
The corrosion current flows from the aluminum to the steel, causing the aluminum to be accelerated and locally corroded near the connection point, typically manifesting as white aluminum oxide powder or pits on the aluminum parts.
3.What are the specific processes and risk points involved in corrosion?
Special characteristics of riveting points:
Direct metal-to-metal contact: Rivets (whether steel or aluminum) create a tight contact between the steel and aluminum at the hole.
Gap: Riveting inevitably creates tiny gaps that easily absorb moisture and salt, forming a crevice corrosion environment and accelerating galvanic corrosion.
Coating damage: Whether it's the galvanized layer of cold-rolled steel or the oxide film of aluminum coils, the drilling and riveting process can damage them, exposing the base metal and providing a starting point for corrosion.
Environmental factors:
Humid environments, salt spray, and industrial atmospheres provide excellent electrolytes, greatly accelerating galvanic corrosion. This is the main reason why such structures fail in outdoor or marine environments.
4.How effective is the electrical insulation?
Use insulating gaskets/washers: Place non-conductive gaskets between the steel and aluminum contact surfaces, and between the rivet head/nut and the metal. Materials typically include:
Plastics: such as nylon, polyethylene, and polytetrafluoroethylene.
Rubber: such as neoprene rubber.
Impregnated gaskets: such as fiber gaskets impregnated with sealant.
Use insulating coatings: Spray or adhere insulating materials, such as powder coatings, asphalt varnish, or specialized insulating compounds, onto the contact surfaces.
5.How to choose the right rivets and sealants?
Rivet Material: Aluminum or stainless steel rivets are preferred (note that there is still a potential difference between stainless steel and aluminum, but it is smaller than that of carbon steel). Avoid using ordinary carbon steel rivets.
Sealant/Structural Adhesive: Before riveting, apply sealant or a low-conductivity elastic adhesive to the mating surfaces. This prevents electrolyte from penetrating the gaps and provides a certain degree of insulation and mechanical strength.