Aluminum is a versatile, lightweight material used in countless applications, from aircraft wings to building frames. Its low density and natural resistance to atmospheric corrosion make it highly desirable for structural use. Ensuring the long-term integrity of an aluminum structure requires careful consideration of what other metals it touches. Compatibility refers to the ability of two different metals to exist in physical contact without one suffering rapid degradation. Selecting the correct accompanying metals is a prerequisite for structural longevity.
Understanding Galvanic Compatibility
The primary threat to aluminum’s compatibility is galvanic corrosion. This electrochemical process occurs when two dissimilar metals are electrically connected and immersed in a conductive liquid, or electrolyte, such as saltwater or humid air. The assembly acts like a tiny battery, causing the more chemically active metal to sacrifice itself.
Engineers use the galvanic series to predict corrosion, ranking metals based on their electrochemical potential from most active (anodic) to most noble (cathodic). Aluminum sits toward the active end, meaning it often corrodes when placed in contact with many common structural metals. The greater the separation between two metals on the series, the greater the potential difference, leading to more severe and rapid corrosion. For galvanic corrosion to occur, three conditions must be present: two dissimilar metals, electrical contact, and the presence of an electrolyte.
Metals That Pair Well with Aluminum
Compatible metals share a similar electrochemical potential, minimizing the voltage difference that drives galvanic corrosion. Zinc is highly compatible, as it is only slightly more active than aluminum on the galvanic series. This makes zinc-plated steel fasteners a safer choice for aluminum assemblies, since the zinc coating corrodes preferentially and protects the aluminum.
Magnesium is also highly compatible and is often alloyed directly with aluminum to create the 5xxx series used in marine environments. Their close proximity on the galvanic scale means their potential difference is negligible, making them structurally sound partners. Certain grades of stainless steel, specifically the 300 series (like 304 and 316), are commonly used with aluminum in transportation and architecture. Although stainless steel is more noble than aluminum, the pairing is often acceptable in mild environments, especially when the stainless steel component is small, such as a fastener.
Metals That Cause Severe Corrosion
Metals positioned far from aluminum on the noble end of the galvanic series present a high risk of rapid corrosion. Copper and its alloys, such as brass and bronze, are among the most incompatible materials to pair with aluminum. Copper is significantly more noble, creating a large potential difference that drives electrons from the aluminum, leading to its accelerated degradation. Even trace amounts of copper ions washed onto an aluminum surface can initiate severe pitting corrosion.
Iron and carbon steel are also substantially more noble than aluminum, making direct contact problematic, particularly in environments exposed to moisture. When a steel component, like a large bracket, is coupled with a smaller aluminum part, the aluminum (the small anode) will corrode at an extremely fast rate. The corrosive potential is so high that even small steel rivets or bolts can initiate a failure point where they meet the aluminum structure.
Strategies for Joining Dissimilar Metals
Since it is not always possible to avoid pairing incompatible metals, several engineering strategies exist to mitigate the risk of galvanic corrosion.
Physical Separation
The most effective approach is to physically separate the two metals using a non-conductive barrier, which breaks the electrical connection. This insulation can be achieved with non-metallic materials like neoprene gaskets, plastic washers, or nylon sleeves placed between the surfaces of the joints.
Protective Coatings
Applying protective coatings is another common method to prevent the necessary conditions for corrosion. This involves painting or coating one or both materials to shield them from the electrolyte and to act as an electrical insulator. A highly effective technique is to coat the more noble metal (the cathode), which reduces the surface area available to drive the corrosion reaction.
Sacrificial Metals (Cathodic Protection)
This strategy involves the use of sacrificial metals, also known as cathodic protection. This method intentionally introduces a metal that is even more active than aluminum, such as pure zinc, into the electrical circuit. Zinc washers or plates corrode preferentially, sacrificing themselves to protect the aluminum structure nearby. This technique requires periodic inspection and replacement of the sacrificial component, but it provides robust protection in harsh, electrolyte-rich environments.