Aluminum and steel are common construction materials. Steel is an alloy of iron and carbon, known for its high strength and low cost, while aluminum is a light, corrosion-resistant metal. Under typical dry conditions, these two metals do not react chemically because their surfaces are stable. However, when moisture is introduced, the difference in their electrochemical properties creates conditions for a damaging reaction.
The Primary Danger: Galvanic Corrosion
The most significant reaction between aluminum and steel is galvanic corrosion, an electrochemical process that functions like a small battery. This reaction requires four components: an anode, a cathode, an electrical connection, and an electrolyte. Aluminum acts as the anode because it is the less noble metal, meaning it corrodes preferentially. Steel serves as the cathode and is protected from corrosion.
This process is driven by the potential difference between the metals, causing electrons to flow from the aluminum to the steel. This flow accelerates the oxidation and dissolution of the aluminum, which sacrifices itself by losing metal ions. An electrolyte, typically water containing dissolved salts, completes the circuit and facilitates ion movement. The resulting corrosion on the aluminum often appears as pitting, flaking, or a white, powdery residue. The speed of this corrosion is directly related to the magnitude of the electrical potential difference.
Environmental Conditions That Drive Reaction Rate
The rate of galvanic corrosion is dramatically influenced by the surrounding environment. The presence of an electrolyte, such as moisture, is a primary accelerator. Saltwater and road salts are particularly aggressive because chlorides drastically increase the water’s electrical conductivity, making the corrosive reaction much faster.
Higher temperatures also increase the rate of corrosion, intensifying the problem in hot, humid climates. Furthermore, the relative surface area of the two metals plays a critical role. If a small piece of aluminum (the anode) is attached to a large piece of steel (the cathode), the corrosion on the aluminum will be highly concentrated and rapid. This occurs because the large cathode surface drives a greater current density into the small anodic area, leading to swift destruction of the aluminum component.
Strategies for Preventing Metal Contact Reactions
Preventing galvanic corrosion centers on interrupting the electrical circuit created by the dissimilar metals. The most effective strategy is isolation, which involves separating the aluminum and steel using a non-conductive barrier.
Isolation and Barriers
Non-conductive materials are used to physically break the electrical connection at the point of contact. These include plastic washers, rubber gaskets, or nylon bushings. Applying protective coatings to both metals also helps by creating a barrier against the electrolyte. Specialized dielectric coatings or primers can insulate the surfaces and prevent moisture from bridging the gap. When fasteners are required, using coated fasteners or applying a barrier material, such as specialized tape or sealant, between the joint interface is recommended.
Design and Cathodic Protection
Designers should avoid creating small aluminum components fastened to large steel structures, as this is the worst-case scenario for corrosion. In highly corrosive environments, such as marine settings, cathodic protection can be employed. This involves connecting a more reactive metal, such as zinc, to the assembly. This more reactive metal becomes a sacrificial anode, corroding instead of the aluminum or steel and diverting the electrochemical attack.
Reactions Under Extreme Heat
A different reaction occurs when aluminum and steel are exposed to extreme heat, such as during welding or high-temperature fabrication. When steel and molten aluminum mix, they undergo a metallurgical reaction that forms intermetallic compounds (IMCs) at the interface. These compounds are brittle and consist of various iron-aluminum phases (e.g., FeAl3 and Fe2Al5).
The formation of these IMC layers compromises the structural integrity of the joint, leading to reduced strength and ductility. The compounds are significantly harder and more brittle than the parent metals, making the joint prone to fracture. This is a major concern in processes like hot-dip aluminizing or fusion welding, where the heat input is high enough to melt the aluminum. The thickness of the brittle IMC layer increases with both temperature and time, which is why fusion welding is generally not recommended. To mitigate this, specialized solid-state joining techniques or intermediate layers are often used.