Aluminum and steel are two of the most widely used metallic materials, frequently joined together in applications ranging from infrastructure to vehicle assemblies. When these dissimilar metals are placed in direct contact, they undergo a powerful electrochemical process rather than a simple chemical reaction. This process, known as galvanic corrosion, leads to the rapid deterioration of one of the materials. Galvanic corrosion is the primary mechanism governing how aluminum and steel “react” when coupled.
Understanding Galvanic Corrosion
The interaction between two different metals in a conductive environment is governed by galvanic corrosion. This process functions like a small, unintentional battery, requiring three components. The anode is the metal that corrodes by releasing electrons, and the cathode is the metal that receives electrons and is protected from corrosion.
The third necessary component is the electrolyte, a conductive liquid that allows ions to move between the two metals, completing the electrical circuit. When the metals are in electrical contact and an electrolyte is present, a difference in their electrical potential drives the reaction. Electrons flow from the metal with the higher potential (the anode) to the metal with the lower potential (the cathode). This electron transfer causes the atoms of the anodic metal to dissolve into the electrolyte as positively charged ions, resulting in material loss.
The rate of this process relates directly to the difference in electrical potential between the metals, a value referenced in the galvanic series. Metals further apart on this series exhibit a greater driving force for corrosion when coupled. The process continues until the electrical connection is broken or the anodic metal is consumed.
The Electrochemical Pairing of Aluminum and Steel
When aluminum and steel are joined, galvanic corrosion defines the fate of each metal. Aluminum is significantly more electrochemically active than steel, consistently acting as the anode in the pairing. Steel is the more noble, or cathodic, metal, remaining protected while the aluminum corrodes sacrificially.
The consequence of this pairing is that the aluminum rapidly dissolves and degrades at the point of contact with the steel. This anodic dissolution often results in localized material loss, manifesting as pitting and crevice corrosion in the aluminum. The aggressive attack is a direct result of the large potential difference between the metals.
The type of steel used impacts the severity of the reaction. While carbon steel is cathodic relative to aluminum, stainless steel is even more noble on the galvanic series. Coupling aluminum with stainless steel, such as a bolt in an aluminum plate, increases the electrical potential difference, accelerating the corrosion of the aluminum. If a small piece of aluminum is connected to a large surface area of steel, the corrosion current concentrates onto the small anodic area, dramatically increasing the rate of aluminum destruction.
How Environmental Conditions Accelerate Corrosion
The severity and speed of galvanic corrosion are amplified by external conditions. Moisture is the fundamental requirement, as water acts as the necessary electrolyte to enable ion movement and complete the circuit. Even high humidity and condensation can provide the thin film of moisture required to initiate the reaction.
Salinity is a powerful accelerator of the corrosive process. Saltwater, or chloride ions from road salt or marine air, dramatically increases the electrical conductivity of the electrolyte. Higher conductivity allows the electrochemical reaction to proceed faster, leading to a sharp rise in the aluminum’s corrosion rate.
Increased temperature also contributes to faster corrosion rates by generally increasing the speed of chemical reactions, including those involved in the galvanic cell. The surface area ratio of the coupled metals is another factor. If the cathodic metal (steel) has a much larger surface area than the anodic metal (aluminum), the corrosion current generated by the large cathode focuses onto the small aluminum area, causing extremely rapid, localized failure.
Practical Methods for Preventing Reaction
Preventing the galvanic reaction between aluminum and steel centers on interrupting one or more of the three required components of the corrosion cell. The most reliable strategy is electrical isolation, using non-conductive barriers to break the connection between the dissimilar metals. This can be achieved by placing non-metallic materials, such as neoprene gaskets, nylon washers, or plastic bushings, at every contact point.
Applying protective coatings to one or both metal surfaces is an effective method, as the coating acts as a dielectric barrier. High-performance paints, primers, or powder coatings create an insulating layer that prevents the electrolyte from establishing a conductive path. Care must be taken to ensure the coating is intact, as any scratch or pinhole can concentrate the corrosion at that fault.
Modifying the steel surface to introduce a third, more active metal is another approach. For instance, hot-dip galvanized steel is coated with a layer of zinc. Zinc is more anodic than aluminum, so if the coating is breached, the zinc corrodes sacrificially to protect both the underlying steel and the adjacent aluminum. Sealing joints effectively prevents moisture from reaching the junction, eliminating the electrolyte and stopping the process.