How to Separate Dissimilar Metals

The introduction of Littoral Combat Ships in the last decade was expected to revolutionize the US Navy's operations due to their reconfigurable designs, large carrying capacities, and impressive speed. Two years into the deployment of the second vessel, the USS Independence, the unthinkable happened. The ship began to disintegrate in real-time. And far from being the result of hostile action, the culprit was simple galvanic corrosion caused by the use of dissimilar metals.

The USS Independence, the Santa Barbara Oil spill, and 1980s corrosion at the Statue of Liberty all highlight the importance of understanding how dissimilar metals affect one another. Engineers and equipment operators must devise ways to separate them. So, what are dissimilar metals?

Understanding dissimilar metals

Metals come with different inherent properties. One such trait, their nobility, plays an important role in determining how metals are ranked on the galvanic scale. Here, nobility refers to a metal's resistance to corrosion. For example, noble metals that are higher up the galvanic scale retain their electrons better compared to base metals at the lower end of the scale when placed within the same environment.

A base metal such as aluminum loses more of its electrons than a noble metal such as stainless steel. Hence, when two different or dissimilar metals from opposite ends of the galvanic scale are in contact and are subjected to a corrosive environment, electrons are transferred from the base metal to the noble metal. The transfer of electrons (also expressible as the current flow) accelerates the base metal's corrosion rate and further reduces the corrosion rate of the noble metal.

For example, using aluminum and stainless steel to build structures within an aircraft will result in dissimilar corrosion rates. The corrosion rate of aluminum will increase while that of stainless steel slows. This imbalance is accelerated in highly corrosive environments with excessive moisture and changing temperatures and will cause the aircraft's structure to crumble and fail if left unattended.

Separating dissimilar metals

In theory, limiting the adverse effects of dissimilar metals coming into contact with one another may seem like an easy task. Utilizing all the same metal or galvanically similar metals should solve the problem. However, in the real world, some dissimilar metal use is unavoidable thanks to the properties they bring to the table. In such a scenario, four major preventive measures can be applied to separate dissimilar metals and slow corrosion.

1. Using non-absorbent insulating materials — When dissimilar metals must be placed side-by-side, applying a two-sided polyurethane tape such as AV-DEC HI-TAK® between both metals reduces the effects of galvanic corrosion. The use of a nonabsorbent insulating material slows the transfer of electrons and minimizes the corrosion rate by denying electrons an electrolyte through which to travel. Nonabsorbent tapes also protect the metal surfaces from non-galvanic corrosion modes. Keeping out moisture and inhibiting the loss of electrons will reduce corrosive reactions.
2. Apply protective coatings or prime the metals — Protective coatings and paints are known for their ability to insulate metals against corrosion. Emphasis should be placed on painting or priming joints that consist of dissimilar metals. Pay special attention to properly coating the more galvanically active metal in the configuration.
3. Utilize nonmetal washers and gaskets — Threaded fittings and fasteners are often made of different metals than the parts they secure. Eliminating them or physically separating them from contact with the metal part can reduce corrosion rates. Plastic washers and gaskets are among the simplest tools for this task. These simple protective components separate dissimilar metal surfaces from corrosive agents and galvanic reactions.
4. Use tape instead of caulk — Maintenance and manufacturing service providers regularly utilize caulk to join metals or reject moisture. Over time, however, caulk is likely to get squeezed out of dissimilar-metal joints, allowing both metals to contact one another or water to creep in. In place of caulk, polyurethane tape is the best option for joints. It sticks to the surfaces themselves and does not get squeezed out. Quality tape is also easy to remove when further maintenance work is required.

Stop corrosion when using dissimilar metals

Utilizing dissimilar metals is a notorious cause of corrosion within the aviation, naval, heavy trucking, and manufacturing industries. All that equipment also tends to wind up in environments with diverse corrosive agents. Proper use of durable sealants, gaskets, and tapes can limit the damaging effects of dissimilar metals and keep moisture out. Get in touch with an Av-DEC representative today to learn more about how to separate dissimilar metals.