Stainless steel and aluminum are generally incompatible when placed in direct contact, especially if moisture is present. This combination leads to accelerated degradation that can compromise the strength and integrity of the aluminum component. Understanding this incompatibility is important for common applications in construction, automotive, and hardware, where these two metals are frequently joined. Knowing the underlying scientific principles allows for proper material selection and design planning.
The Mechanism of Galvanic Corrosion
The accelerated degradation between these two metals is caused by galvanic corrosion, an electrochemical process. This reaction begins when two metals with different electrical potentials are connected and exposed to a conductive liquid, or electrolyte. The setup forms a small, self-powered battery where an electrical current flows between the materials.
Aluminum acts as the anode because it is less “noble” and more chemically reactive than stainless steel. As the anode, aluminum releases electrons, causing its material to dissolve into the electrolyte, which is observed as pitting or flaking.
Stainless steel acts as the cathode, receiving the electrons released by the aluminum. This protects the stainless steel from corroding. The difference in electrochemical potential drives this reaction, and without direct contact, the corrosive process cannot be sustained.
Environmental Factors That Increase Risk
The speed and severity of corrosion depend heavily on external environmental conditions, particularly the presence of an electrolyte. Moisture is a fundamental requirement, as water acts as the medium for ion movement. High humidity, rain, or condensation can provide the necessary electrolyte to sustain the electrical current.
The addition of salt drastically intensifies the corrosive action by lowering the electrical resistance of the water. Marine environments or areas using road salts are especially aggressive, allowing the galvanic current to flow more easily. Higher ambient temperatures also accelerate the rate of the chemical reaction, leading to faster aluminum degradation.
The surface area ratio between the two metals is another critical factor. Corrosion is most severe when a small piece of aluminum (the anode) is connected to a much larger piece of stainless steel (the cathode). Designers must aim for a large anode to small cathode ratio to spread the current and reduce localized attack.
Practical Methods for Safe Integration
When circumstances require stainless steel and aluminum to be joined, the most reliable strategy is to physically isolate the two metals. This is achieved by introducing a non-conductive, dielectric barrier at every point of contact to break the electrical circuit.
Isolation Barriers
Non-absorbent materials are commonly used around stainless steel bolts or between surfaces to break the electrical circuit. Examples include specialized plastic washers, neoprene gaskets, or insulating sleeves. Applying specialized coatings or paint is another effective barrier method, but the barrier must be continuous, as any breach allows localized corrosion to begin.
Sacrificial Protection
A secondary strategy involves using a third metal for sacrificial protection, often employed with fasteners. A metal closer to aluminum on the galvanic series, such as zinc or cadmium plating, can be used on a steel fastener. This less-noble coating becomes the new anode and corrodes away before the underlying aluminum, protecting the main structure.
Design Considerations
Design choices also play a part in minimizing risk. Ensuring good drainage is important so that moisture, the electrolyte, does not pool in the joint and sustain the corrosive reaction. Selecting the proper grade of stainless steel, such as Type 316, can slightly widen the potential difference with aluminum.