When aluminum and steel are placed in direct contact, they can react under certain environmental conditions, a process known as galvanic corrosion. This occurs because the two metals possess different electrochemical potentials. In the presence of a conductive liquid, the pairing of aluminum and steel effectively creates a small, unintended battery. This electrochemical difference drives the accelerated degradation of one metal at the expense of the other. The outcome is an electrochemical reaction requiring specific circumstances to initiate damage.
The Mechanism of Galvanic Corrosion
The interaction between aluminum and steel forms a galvanic cell, similar to a simple battery. This cell requires two dissimilar metals, an electrical connection, and an electrolyte. Aluminum is the more “active” metal and acts as the anode, while steel is the more “noble” metal and serves as the cathode.
The difference in nobility establishes a voltage potential, driving electron flow from the active aluminum anode to the noble steel cathode. This electron transfer requires an electrolyte, a conductive fluid like water containing dissolved ions.
At the aluminum anode, oxidation occurs, causing the metal to dissolve into the electrolyte as positively charged ions (corrosion). The steel cathode is protected because it receives the electrons, a process known as cathodic protection. The aluminum effectively sacrifices itself to protect the steel.
Environmental Factors That Accelerate Degradation
The rate of galvanic corrosion is highly dependent on environmental conditions, specifically the quality of the electrolyte. Increased conductivity dramatically accelerates the flow of electrons between the metals. For example, saltwater or coastal environments create a highly conductive electrolyte due to dissolved chloride ions, leading to much faster corrosion rates than in freshwater.
Temperature and humidity also play a significant role. Higher temperatures increase the speed of chemical reactions involved in galvanic corrosion. High humidity ensures that a microscopic film of moisture, which acts as a weak electrolyte, is present on the metal surfaces for longer periods, sustaining the corrosive cell.
Another factor controlling the corrosion rate is the surface area ratio between the anode and the cathode. A small aluminum component, such as a fastener, connected to a very large steel plate will degrade exceptionally fast. The large cathodic area concentrates the corrosive current onto the small anodic area, leading to rapid and localized attack. Conversely, a large aluminum surface attached to a small steel fastener experiences a much slower, more distributed rate of corrosion.
Mitigation and Isolation Techniques
To prevent the accelerated corrosion of aluminum coupled with steel, the primary goal is to break the electrical circuit required for the galvanic cell. Physical isolation is a direct method, involving separating the two metals with a non-conductive barrier. Materials such as neoprene gaskets, plastic washers, or insulating sleeves prevent direct electrical contact between the aluminum and steel components.
Applying protective coatings to one or both metals is another effective strategy, as the coating acts as a shield, excluding the electrolyte. Common coatings include paints, epoxies, or anodizing treatments on the aluminum, creating an inert, non-conductive layer. For steel, a coating of zinc (galvanized steel) can be applied, where the zinc itself becomes the sacrificial anode, protecting both the steel and the coupled aluminum.
Specialized primers or compounds that incorporate sacrificial materials offer another approach. While aluminum is typically the sacrifice, certain primers contain zinc dust, which is more active than both steel and aluminum. The zinc in the primer corrodes preferentially, offering cathodic protection to both metals. Additionally, properly designing the joint to allow for drainage prevents water accumulation, effectively eliminating the electrolyte.