Stainless steel and aluminum are popular metals used in construction, transportation, and consumer goods due to their strength-to-weight ratios and corrosion resistance. When placed together, they can react under specific circumstances involving an electrochemical process known as galvanic corrosion. This reaction occurs when both metals are in electrical contact and exposed to a conductive liquid. If preventive measures are not taken, this phenomenon can rapidly degrade the aluminum component.
Understanding Galvanic Corrosion
Galvanic corrosion is a form of accelerated degradation requiring three simultaneous conditions. First, there must be two dissimilar metals, like aluminum and stainless steel, with different electrical potentials. Second, the metals must be in direct electrical contact, allowing electrons to flow between them. Third, a conductive liquid, or electrolyte, must bridge the contact area to complete the circuit.
This pairing creates a natural electrochemical cell, functioning like a tiny battery. The metal that is less noble on the Galvanic Series scale becomes the anode, and the more noble metal becomes the cathode. Aluminum is significantly less noble than stainless steel, giving it a more negative electrochemical potential.
The difference in potential drives electrons from the anodic aluminum to the cathodic stainless steel through the electrical connection. This flow causes aluminum atoms to dissolve into the electrolyte, resulting in corrosion and material loss. The stainless steel, acting as the cathode, remains protected and intact. The aluminum is rapidly consumed at the point of contact, accelerating its breakdown far beyond its normal atmospheric corrosion rate.
Environmental Factors and Corrosion Rate
While the mechanism relies on the metal pairing, the speed and severity of the attack are dictated by external environmental factors. The most significant factor is the presence and composition of the electrolyte. Chloride-rich solutions, such as saltwater spray or de-icing road salts, are the most aggressive. These solutions increase the electrolyte’s conductivity dramatically, enabling electrons to flow faster and accelerating the corrosion current.
The ratio of the surface area between the cathode (stainless steel) and the anode (aluminum) controls the rate of material loss. If a small piece of aluminum, such as a thin panel, is fastened with a large stainless steel bracket, the corrosion current focuses intensely on the small aluminum surface. This unfavorable arrangement causes rapid and localized deterioration of the aluminum component.
Increased ambient temperature contributes to a higher corrosion rate by speeding up the chemical reactions within the galvanic cell. Designs that allow moisture to pool and remain stagnant create a persistent electrolyte film. These areas will experience more cumulative damage than those that allow for quick drainage and drying. This explains why concealed joints and crevices are often the first places to show signs of corrosion.
Practical Strategies for Isolation
Preventing galvanic corrosion centers on interrupting one of the three required components of the electrochemical cell. The most reliable method is to physically isolate the two metals from one another to break the electrical path. This is achieved by using non-conductive insulating materials wherever the metals meet.
Specialized shims, gaskets, or washers made from materials like neoprene, nylon, or PTFE (Teflon) should be inserted between the aluminum and stainless steel surfaces. If stainless steel fasteners are used, the bolt or screw should be isolated from the aluminum material using a non-conductive sleeve or bushing. This eliminates electrical contact, even under pressure.
Another effective strategy involves applying barrier coatings to seal the metal surfaces from the electrolyte. Non-conductive paints, primers, or specialized sealants can be applied to the contact areas, especially those that are difficult to access. For instance, a layer of zinc-chromate primer, which has corrosion-inhibiting properties, can be applied to the aluminum before assembly.
A further solution involves using a third, sacrificial metal that is more electrically active than aluminum. Fasteners coated with zinc or cadmium, such as hot-dip galvanized steel, will preferentially corrode before the aluminum, temporarily protecting the joint. Designing the assembly to ensure proper drainage is also a simple measure, as eliminating standing water prevents the formation of a persistent, conductive electrolyte.