Aluminum is often utilized for its corrosion resistance, primarily due to a naturally forming surface film. However, the short answer to whether aluminum is resistant to galvanic corrosion is no; it is, in fact, highly susceptible. When aluminum is electrically coupled with many other common construction metals in a moist environment, it will rapidly degrade. This vulnerability is rooted in its fundamental position within the electrochemical hierarchy of metals. Understanding this process is the first step toward effective prevention.
Understanding Galvanic Corrosion
Galvanic corrosion is an electrochemical process, similar to the reaction inside a battery, where a difference in electrical potential drives the flow of electrons between two connected metals. For this corrosion to occur, three components must be simultaneously present: two dissimilar metals with different electrical potentials, a direct metallic connection between them, and an electrolyte.
The electrolyte is a conductive liquid, such as saltwater, high humidity, or rain, that facilitates the movement of ions to complete the circuit. The metal with the lower electrical potential is designated the anode and actively corrodes, while the metal with the higher potential becomes the cathode and is protected. The anode dissolves into the electrolyte, releasing electrons that travel through the connection to the cathode. The overall rate of degradation is directly related to the magnitude of the electrical potential difference between the two metals.
Aluminum’s Activity in the Galvanic Series
The tendency of a metal to act as an anode or a cathode is determined by its position in the galvanic series, a ranking of metals based on their electrochemical activity. Aluminum is considered an active, or base, metal and is situated near the anodic end of the series, close to magnesium and zinc. This position means aluminum will function as the anode and corrode when electrically connected to nearly all common structural metals, including stainless steel, copper, nickel, and brass.
The paradox of aluminum’s general corrosion resistance versus its high susceptibility to galvanic corrosion lies in its protective surface layer. When exposed to air, aluminum naturally forms a thin, dense, and stable layer of aluminum oxide (Al2O3), which shields the underlying metal from generalized corrosion. This passive film is the source of aluminum’s durability.
However, when aluminum is coupled with a more noble metal, this passive layer is compromised in the presence of an electrolyte. The electrical current generated by the galvanic couple breaks down the protective oxide, preventing it from reforming and exposing the active aluminum metal. This exposed metal rapidly dissolves as the anode, accelerating the localized corrosion rate at the point of contact with the cathode.
Environmental and Design Factors
The rate at which the aluminum anode corrodes is heavily influenced by the surrounding environment and the connection geometry. The strength of the electrolyte is the most significant environmental factor; high-salinity solutions like seawater are particularly aggressive due to their high electrical conductivity. Even high humidity or freshwater with dissolved pollutants can create a sufficient electrolyte to sustain the corrosion process.
Temperature also plays a substantial role, as higher temperatures generally increase the rate of chemical reactions, accelerating electron flow and aluminum dissolution. Furthermore, the design introduces a factor known as the area ratio.
Corrosion is most severe when the surface area of the noble metal (cathode) is large compared to the surface area of the active metal (anode). For example, using a small aluminum fastener to join a large sheet of copper or stainless steel creates an unfavorable ratio. The large cathodic surface concentrates the corrosion current onto the small anodic surface, leading to localized failure of the aluminum component. Different aluminum alloys also exhibit slight variations in their corrosion potential.
Mitigation and Prevention Techniques
Preventing galvanic corrosion requires breaking the electrical circuit. This can be achieved by interrupting the metallic connection, the electrolyte path, or the potential difference.
Electrical Isolation
The most reliable technique is the electrical isolation of the two dissimilar metals. This is achieved by placing non-conductive barriers, such as plastic or rubber gaskets, spacers, or specialized tapes, between the aluminum and the noble metal at the joint.
Protective Coatings
Another effective strategy involves using protective coatings, such as paint, epoxy, or anodizing, on the aluminum surface. It is beneficial to coat the more noble cathode metal, as this reduces the surface area that can accept electrons and drive the corrosion reaction. If the coating on the aluminum anode is scratched or damaged, corrosion will become intensely localized at that small exposed point, leading to rapid pitting.
Sacrificial Anodes and Material Selection
A third method is the application of cathodic protection through sacrificial anodes. This involves introducing a third metal, such as zinc or magnesium, that is more active than the aluminum. When connected to the aluminum structure, this piece acts as the new anode and corrodes first, protecting the aluminum from degradation. Careful material selection is also important, involving choosing fasteners and components close to aluminum in the galvanic series, such as specific zinc-plated or cadmium-plated steel, and avoiding noble metals like copper entirely in corrosive environments.