Galvanic corrosion is an electrochemical process that occurs when two dissimilar metals are in direct contact and exposed to an electrolyte, such as moisture, saltwater, or humid air. This condition forms a corrosion cell where one metal, the anode, actively gives up electrons and corrodes at an accelerated rate, while the other, the cathode, is protected from degradation. The presence of the electrolyte provides the necessary path for ion migration, completing the circuit and driving the corrosive reaction forward. Stopping this destructive process requires breaking one of the three necessary components: the electrical connection, the metal difference, or the electrolyte path.
Strategic Material Selection
Preventing galvanic corrosion begins by minimizing the potential difference between coupled metals, often by consulting the Galvanic Series, which ranks metals based on their electrochemical nobility. Metals that are closer together on this series exhibit a smaller potential difference, which translates directly to a slower rate of corrosion when they are connected.
Designers should select metal combinations with an electrical potential difference of less than 0.25 volts for long-term compatibility. For instance, pairing stainless steel with carbon steel presents a high risk, whereas coupling different grades of stainless steel is often acceptable because they are close on the series. When using materials with a large potential difference is unavoidable, introducing a third, more active material as a transition piece can help distribute the corrosion.
The unfavorable area effect, where a small anode is coupled to a large cathode, is particularly harmful. The large cathode concentrates the total corrosive current onto the small anodic area, leading to extremely rapid penetration and failure. For example, a small fastener made of a less noble metal will quickly dissolve when used to secure a large plate of a more noble metal. Conversely, if the anodic area is significantly larger than the cathode area, the corrosion current is spread out, resulting in a much slower rate of material loss.
Isolation and Barrier Methods
The next preventative layer involves physically isolating the two metals and shielding them from the electrolyte. Breaking the electrical path is an effective strategy because it eliminates the electron flow necessary to sustain the galvanic cell. This is commonly achieved by inserting non-conductive materials, known as dielectric spacers, between the dissimilar metal surfaces.
These insulators include gaskets, washers, bushings, and sleeves made from materials like neoprene, nylon, or specialized glass-reinforced epoxy (GRE). When used in a bolted connection, dielectric washers separate the bolt head and nut from the metal surface, while a sleeve isolates the bolt shank from the hole wall. It is important that these isolation barriers are robust and resistant to moisture absorption to prevent a conductive bridge from forming over time.
Protective surface coatings, such as paints, epoxies, or metal plating, create a barrier against the electrolyte. For this approach to be successful, both the anodic and cathodic surfaces should be fully coated to prevent the electrolyte from reaching either metal. A partial coating on only the anodic metal can be counterproductive, as any small scratch will concentrate the corrosion current onto a tiny, exposed spot, severely accelerating local material loss. Specialized dielectric sealants and corrosion-inhibiting pastes can be applied to joints and threads to seal out moisture and provide electrical resistance.
Cathodic Protection Techniques
When material selection and passive barriers are insufficient, active electrochemical methods, known as cathodic protection, are employed to suppress the corrosive reaction. It involves forcing the entire structure to become a cathode, thus preventing metal loss. There are two primary techniques used to achieve this protection.
The first method utilizes sacrificial anodes, blocks of a highly active metal (such as zinc, aluminum, or magnesium) electrically connected to the protected structure. Because these materials are far more active than the structural metal, they preferentially corrode, sacrificing themselves to supply the protective electrical current. This technique is self-regulating and requires no external power source, but the anodes must be regularly replaced as they are consumed over time.
The second method is the Impressed Current Cathodic Protection (ICCP) system, typically reserved for large-scale assets like ships, storage tanks, and extensive pipelines. ICCP uses an external direct current (DC) power source, often a transformer-rectifier, to drive a protective current from an inert anode to the protected structure. ICCP anodes are not consumed like sacrificial anodes, offering a long-term, controlled solution. This active system uses reference electrodes to continuously monitor the structure’s electrical potential, allowing the rectifier to automatically adjust the current output to maintain an optimal level of corrosion prevention.
Environmental Mitigation
A final layer of prevention focuses on controlling the environment to make the electrolyte less conducive to corrosion. Since water is the necessary medium for the electrochemical reaction, controlling moisture levels is a straightforward approach. In enclosed spaces, such as storage facilities or equipment housings, dehumidification systems can reduce the relative humidity below the threshold required for the electrolyte to be sufficiently conductive.
For fluid-carrying systems, such as cooling loops or pipelines, chemical inhibitors reduce the liquid’s corrosiveness. These compounds work by forming a thin, protective film on the metal surfaces, or by neutralizing corrosive components in the fluid, such as dissolved oxygen. Examples of effective inhibitors include nitrites, molybdates, and volatile corrosion inhibitors (VCIs), which vaporize to protect metals in the surrounding atmosphere.
Regular maintenance and cleaning mitigate the environment’s impact on corrosion. Removing deposits of salt, dirt, or other contaminants that can act as strong, localized electrolytes is important for long-term integrity. This practice reduces the conductivity of the medium and prevents the formation of concentrated corrosion cells that can accelerate the degradation of the metal structures.