Galvanic corrosion is a destructive electrochemical process that occurs when dissimilar metals are in direct or indirect contact with each other with an electrolyte present. During this process, one metal experiences severe corrosion, while the other remains relatively unaffected. (Learn more about dissimilar metals in the article Why Do Two Dissimilar Metals Cause Corrosion?)
The driving force for galvanic corrosion is the electrode potential that exists between the two metals. This potential difference causes electrons to migrate from the more anodic metal to the more cathodic metal. The continuous flow of electrons from one metal to another fuels the corrosive electrochemical process as well as the resulting oxidation/reduction reactions.
The measures employed to prevent galvanic corrosion are aimed at eliminating the basic components necessary for initiating the electrochemical process. These include impeding the electrical path, removing oxygen from the electrolyte, and reducing the overall potential difference between the contacting metals.
Some of the most common techniques for avoiding galvanic corrosion include:
- Electrical Insulation – Insulation serves to restrict the flow of electrons between the two metals (anode and cathode). Insulation can be achieved by placing a non-conductive material (such as polymer or elastomer-based washers, bushings, coatings or gaskets) between the contact points of the connected metals.
- Isolating the Electrolyte – This method involves separating the electrolyte from the coupling metals. The electrolyte is the medium that consists of ions that facilitate the redox reactions in the galvanic cell. In practice, paints, coatings, oils, greases and other water-repellent compounds can be used to shield metal substrates from the electrolyte.
- Appropriate Metal Selection – Galvanic corrosion prevention can also be achieved by minimizing the primary driver of this electrochemical reaction – the potential difference between the contacting metals. The higher the difference in electrode potential between the two metals, the more severe the rate of corrosion. Selecting contacting metals that are close to each other in the galvanic series can help reduce the potential difference and the possibility of galvanic corrosion. (See An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion for more information.)
- Apply Corrosion Inhibiting Compounds – Corrosion inhibitors are fluid or gaseous compounds that, when added to the electrolyte, will initiate processes that suppress the electrochemical reactions responsible for galvanic corrosion. The inhibitors that are most effective are those that remove dissolved oxygen from the electrolyte solution. The removal of oxygen hinders the oxidation/reduction reactions, thus halting the galvanic corrosion process.
- Minimize the Cathode to Anode Area Ratio – Galvanic corrosion is also influenced by the ratio of the cathode to the anode area. The larger this ratio, the greater the rate and severity of the resulting galvanic corrosion. Therefore, it is worthwhile to ensure that the area of the anodic metal is made as large as possible in relation to the cathodic metal in the early stages of design.