Galvanic corrosion is an electrochemical process that occurs when two different metals are in electrical contact and exposed to an electrolyte. This phenomenon causes one metal to corrode preferentially, leading to material degradation. Understanding how to prevent this type of corrosion is important for maintaining the integrity and longevity of metal structures and components across various industries.
Understanding Galvanic Corrosion
For galvanic corrosion to occur, three specific conditions must be present simultaneously. There must be two electrochemically dissimilar metals, meaning they have different tendencies to lose or gain electrons. These metals must be in direct electrical contact, either through physical connection or an external conductive path. Finally, an electrolyte, such as water, moisture, or a salt solution, must complete the electrical circuit between the metals.
When these conditions are met, the more active metal (anode) corrodes at an accelerated rate, sacrificing itself to protect the less active or noble metal (cathode). This occurs because the electrical potential difference drives a current, causing the anodic metal to dissolve into the electrolyte.
Material Selection and Design Strategies
Proactive measures during material selection and design can significantly mitigate the risk of galvanic corrosion. Choosing metals that are close to each other in the galvanic series, a list ranking metals by their electrochemical potential, is a foundational step. This minimizes the potential difference that drives the corrosion process, reducing the likelihood and severity of galvanic attack.
When using dissimilar metals is unavoidable, physical separation can prevent direct electrical contact. Non-conductive spacers or insulators placed between the metals disrupt the electrical pathway required for the galvanic cell to form. This approach allows each metal to corrode at its normal rate.
The relative surface area of the two metals also influences the corrosion rate. A small anodic area connected to a large cathodic area results in highly concentrated and rapid corrosion of the anodic material. Designing systems with a large anodic area relative to the cathodic area helps distribute corrosion over a wider surface, slowing the attack.
Proper design includes ensuring good drainage and avoiding crevices where electrolytes can accumulate. Stagnant moisture and trapped liquids create concentrated corrosive environments that accelerate corrosion, even between compatible metals. Incorporating features that allow easy runoff and prevent liquid pooling helps interrupt the continuous presence of an electrolyte.
Protective Coatings and Barriers
Applying protective coatings and barriers offers another layer of defense against galvanic corrosion by isolating metals from the electrolyte or each other. Paints and non-conductive coatings provide a physical barrier that prevents moisture and other corrosive agents from reaching the metal surfaces. For optimal protection, it is often recommended to coat both metals, particularly ensuring complete coverage of the more noble (cathodic) metal.
Metallic platings can also be employed, where one metal is coated with another that offers better compatibility or acts sacrificially. For instance, galvanizing steel with a zinc coating protects the steel because zinc is more active and will corrode preferentially. Zinc-nickel plating can also reduce galvanic corrosion between dissimilar metals.
Electrical Isolation and Cathodic Protection
Electrical isolation techniques involve introducing non-conductive materials to completely separate dissimilar metals, thereby breaking the electrical circuit. Dielectric fittings, for example, are commonly used in plumbing to isolate pipes made of different metals, such as copper and galvanized steel, preventing current flow between them. Insulating spacers or bushings serve a similar purpose in structural assemblies.
Cathodic protection is a specialized method that involves supplying an external current to make the entire structure cathodic, thus preventing it from corroding. One common type is sacrificial anode cathodic protection, where a more electrochemically active metal, such as zinc, magnesium, or aluminum, is electrically connected to the structure to be protected. This “sacrificial” metal corrodes instead of the desired structure, donating its electrons and preserving the main component.
Another approach is impressed current cathodic protection (ICCP), which uses an external power source to drive a protective electrical current through an inert anode to the structure. This system effectively forces the metal surface to become a cathode, halting the corrosion process. ICCP systems are employed for large structures like pipelines, ship hulls, and underground tanks, where consistent and controlled protective current is necessary.