Aluminum reacts with galvanized steel through a process known as galvanic corrosion, a form of accelerated material degradation. Galvanized steel consists of a steel base protected by a surface layer of zinc, a highly reactive metal. When aluminum and this zinc-coated steel are brought into electrical contact in a wet environment, they form a natural electrochemical cell, similar to a battery. This reaction is a significant concern in construction and automotive projects where these materials are frequently coupled together. If left unchecked, this contact results in the preferential dissolution of one metal, leading to joint failure and structural compromise.
Understanding Galvanic Corrosion
The underlying science driving this reaction is the difference in electrochemical potential between the two metals, established by the galvanic series. This series ranks metals based on their nobility, indicating which metal will sacrifice itself to protect the other when a conductive path is formed. For galvanic corrosion to occur, three components must be present: two dissimilar metals, direct electrical contact, and an electrolyte (a conductive liquid, most commonly water or moisture).
Initially, the zinc coating on the steel acts as the anode because it is the most active metal. The zinc will readily give up electrons and corrode first, protecting both the underlying steel and the aluminum in a phenomenon called cathodic protection. Once the sacrificial zinc coating is fully depleted, the aluminum then becomes the anode relative to the exposed steel’s cathode.
The corrosion then shifts its focus to the aluminum, which begins to rapidly degrade at the point of contact to protect the more noble steel. This dissolution of the aluminum anode is a consequence of the electrochemical cell driving a flow of electrons. A small aluminum fastener connected to a large piece of steel is particularly vulnerable because the concentrated corrosion occurs over a small anodic surface area. The rate of aluminum loss is dramatically accelerated due to the unfavorable anode-to-cathode surface area ratio.
Environmental Factors Driving the Reaction
External conditions determine the speed and severity of the corrosion process. The most significant factor is the presence of an electrolyte, as corrosion is negligible in perfectly dry, indoor environments. Any form of moisture, from high humidity to rain or condensation, can act as the necessary electrolyte to complete the electrical circuit between the metals.
The conductivity of this moisture is drastically increased by dissolved ions, particularly chloride ions found in salt. Coastal environments or areas where road salts are used experience dramatically higher rates of galvanic corrosion. The higher concentration of salt allows electrons to flow much more freely, accelerating the consumption of the anodic metal.
Temperature also plays a role, as warmer conditions increase the kinetic energy of the ions in the electrolyte, speeding up the chemical reaction. Atmospheric pollutants, such as sulfur dioxide, can mix with water vapor to create acid rain. This acidic electrolyte further lowers the pH of the moisture, creating an aggressive corrosive environment that quickly strips away the protective oxide layer on the aluminum surface.
Strategies for Preventing Contact Corrosion
The most effective approach to mitigating galvanic corrosion is to interrupt one of the three required elements of the electrochemical cell. Since it is often impossible to eliminate moisture or substitute the materials, the primary strategy involves electrical isolation. Using non-conductive barriers to physically separate the aluminum from the galvanized steel prevents the flow of electrons between the dissimilar metals.
Electrical Isolation
Common isolation materials include durable, non-absorbent items placed between the surfaces and around fasteners. These insulators must be effective at breaking the metallic path and resistant to moisture absorption to prevent them from becoming a saturated, conductive wick.
- Neoprene rubber gaskets
- Plastic washers or nylon sleeves
- Specialized, heavy-duty barrier tapes
Protective Coatings
Another highly effective method is the application of protective coatings to prevent the electrolyte from reaching the joint. Painting or priming both surfaces creates a physical barrier, but it is particularly beneficial to coat the cathode (the more noble metal). If the coating on the cathode is accidentally scratched, the small exposed area will not significantly accelerate corrosion on the large aluminum anode.
Specialized dielectric greases or sealants, which are non-conductive and water-resistant, can be liberally applied to the joint interface and fasteners before assembly. These materials fill crevices and repel moisture, effectively eliminating the electrolyte from the critical contact area.
In situations where fasteners must be used, substituting galvanized steel bolts with stainless steel alternatives can significantly reduce the potential difference. Stainless steel is closer to aluminum on the galvanic series than zinc.