Galvanic corrosion is an electrochemical process that occurs when two dissimilar metals are electrically connected in the presence of an electrolyte, such as moisture or saltwater. The pairing of aluminum and stainless steel is high-risk due to the significant difference in their electrical potentials. When coupled, the less noble metal, aluminum, rapidly deteriorates. Preventing this decay relies on strategies that break the electrical circuit, block the electrolyte, or modify the materials. This article provides methods to ensure the longevity of assemblies combining aluminum and stainless steel.
Understanding the Galvanic Reaction
Galvanic corrosion functions like a small battery, requiring an anode, a cathode, and a conductive electrolyte to complete the circuit. The Galvanic Series ranks metals by their electrochemical activity. Aluminum is significantly more active, or anodic, than stainless steel, which is more noble and acts as the cathode.
In this coupled environment, aluminum acts as the sacrificial anode, giving up electrons and corroding rapidly, while the stainless steel cathode remains protected. The electrochemical potential difference between the two metals provides the driving force for the reaction. The presence of an electrolyte, especially chloride-rich saltwater, accelerates this electron flow, causing aluminum to pit, flake, and fail prematurely.
Barrier Isolation Techniques
The most direct way to stop galvanic corrosion is to physically interrupt the electrical pathway between the two metals. This isolation must be achieved using non-conductive, or dielectric, materials that maintain complete separation even under load. Specialized insulating materials are placed directly at the interface where the metals meet.
For bolted connections, full isolation kits composed of multiple non-conductive components are necessary. Dielectric gaskets or shims, often made from high-strength phenolic, PTFE, or glass-reinforced epoxy, are placed between the flat mating surfaces. The bolts themselves must also be isolated to prevent electrical continuity through the fastener.
Insulating sleeves, commonly made from Mylar or PTFE, slide over the bolt shank to prevent contact with the bolt hole walls. Non-conductive washers, such as nylon or phenolic, are used under the metal washers and nut. This meticulous separation eliminates the conductive path, effectively breaking the galvanic circuit.
Protective Surface Coatings
Applying a specialized coating serves as a second defense by creating a non-conductive barrier that blocks the electrolyte from reaching the metal surfaces. Coatings must be applied uniformly to ensure moisture cannot bridge the gap between the two metals. Proper surface preparation, including cleaning and roughening, is paramount for the coating’s long-term adhesion.
One highly effective treatment for aluminum is anodizing, an electrochemical process that creates a thick, hard layer of aluminum oxide. This oxide layer is thousands of times thicker than the natural passive film and acts as an excellent electrical insulator. For general barrier protection, a high-performance epoxy or polyurethane primer is often utilized on both metals.
Specialized primers, such as zinc-rich coatings, can be employed on steel fasteners, where the zinc acts as a sacrificial anode to both the steel and the aluminum. Thick barrier compounds like anti-corrosion pastes or specialized tapes can also be applied to aluminum joint areas before assembly. This physical blocking of the electrolyte path is essential in marine or high-humidity environments.
Environmental and Design Considerations
Preventative measures should begin in the design phase, focusing on managing the environment and the assembly geometry. A key principle is controlling the cathode-to-anode surface area ratio, which dictates corrosion severity. If isolation fails, coupling a small area of anodic aluminum with a large area of cathodic stainless steel causes extremely rapid, localized corrosion.
Design elements should promote fast drainage and prevent water pooling, which acts as the necessary electrolyte. Proper ventilation allows surfaces to dry quickly, minimizing the time the galvanic circuit is active. When selecting fasteners, hot-dip galvanized steel bolts are preferred because the thick zinc coating serves as a sacrificial anode to both the aluminum and the steel bolt.
For large or complex assemblies in harsh environments, such as marine structures, a dedicated sacrificial anode made of zinc or magnesium may be incorporated. This highly active metal is intentionally connected to the system, ensuring it corrodes instead of the aluminum. Integrating these strategic material choices and environmental controls extends the service life of the structure.