Metals are foundational to modern construction and infrastructure, but combining certain types can lead to galvanic corrosion, which dramatically accelerates degradation. This deterioration occurs when two metals with different electrical potentials are placed in contact within a conductive environment. The result is a highly focused, rapid breakdown of one metal while the other remains largely unaffected. Understanding this incompatibility is necessary to ensure the longevity and structural integrity of assemblies that rely on multiple materials.
The Mechanism of Galvanic Corrosion
Galvanic corrosion is an electrochemical process similar to the reaction that powers a simple battery. For this accelerated decay to take place, three components must be present: two dissimilar metals, a conductive liquid (electrolyte), and a direct electrical connection between the metals. If any one of these three elements is removed, the galvanic reaction ceases.
The two metals assume roles based on their electrochemical potential. The more electrically active metal becomes the anode, and the more noble metal becomes the cathode. Electrons flow from the anode to the cathode through the connection, causing the anodic metal to dissolve into the electrolyte. This process significantly increases the corrosion rate of the anode while protecting the cathode.
Common electrolytes include moisture, condensation, or water, especially when they contain dissolved salts or industrial pollutants. The presence of salt greatly increases the liquid’s conductivity, accelerating the corrosive attack. If the cathode has a much larger surface area than the dissolving anode, the corrosion of the anode will be far more intense.
Identifying Incompatible Metal Pairs
The potential for galvanic corrosion is predicted using the Galvanic Series, which ranks metals according to their electrical activity in a given environment. Metals at the active end, such as magnesium and zinc, are more likely to corrode and act as the anode. Conversely, metals at the noble end, such as gold, platinum, and various stainless steels, are less likely to corrode and typically serve as the cathode.
Compatibility is determined by the distance between two metals on this series, with pairs that are far apart posing the greatest risk. A difference in electrochemical potential greater than approximately 0.25 volts often indicates a problematic combination. A common incompatible pairing is aluminum and copper, often seen in older wiring or HVAC systems. When connected in a damp environment, the aluminum acts as the anode and quickly breaks down.
Other problematic combinations involve zinc or galvanized steel paired with copper or stainless steel fasteners. Galvanized coatings are zinc applied over steel, designed to sacrifice themselves to protect the underlying steel. This protective layer is rapidly consumed when coupled with a much more noble metal.
A small stainless steel fastener driven into a large aluminum structure is also a recipe for accelerated failure. The large aluminum structure acts as the anode and is forced to supply current to the small stainless steel cathode. This concentrated attack leads to the quick breakdown of the aluminum material immediately surrounding the fastener.
Strategies for Preventing Metal Reaction
Electrical Insulation
The most direct method is to introduce electrical insulation or separation between the two metals to break the conductive path. This involves using non-conductive materials like plastic, nylon, rubber, or specialized gaskets and washers to ensure the metals never touch. For example, when connecting different metal pipes, a dielectric union or non-conducting insert can be used at the flange to isolate the two materials.
Protective Coatings
Applying a protective coating to one or both metals is a common method for inhibiting the reaction. Organic coatings, such as paint or powder coating, create a physical barrier that prevents the electrolyte from reaching the metal surface. The coating must be applied perfectly, as any pinhole or scratch will expose the underlying metal, concentrating the corrosive attack at that small point. It is safer to coat the more noble, cathodic metal for a distance beyond the joint, preventing the electrolyte from reaching the connection area.
Sacrificial Protection
A third strategy is to employ sacrificial protection, which is a form of cathodic protection. This method involves intentionally introducing a third, highly active metal, such as zinc or magnesium, into the system. This metal is more anodic than both materials being protected, ensuring it becomes the primary anode in the electrochemical cell. The sacrificial anode corrodes instead of the intended structure, protecting the main assembly. This technique is widely used in marine environments, where zinc or aluminum anodes are attached to steel hulls to protect them from the highly corrosive saltwater electrolyte.