Galvanic corrosion is a degenerative electrochemical process that occurs when two dissimilar metals are in contact with each other while exposed to an electrolyte. During this type of corrosion, one metal undergoes accelerated deterioration while the other metal remains unaffected. In other words, one metal sacrifices itself by corroding preferentially, thereby protecting the other metal.
Although galvanic corrosion causes millions of dollars in damage annually worldwide, there are instances where this electrochemical process can be used to benefit specific applications. In this article, we will briefly go over the galvanic corrosion process and describe the situations where this corrosive process is advantageous.
Why Does Galvanic Corrosion Occur?
To understand how galvanic corrosion can be used for good, it is essential to first gain a solid understanding of the underlying mechanics responsible for its formation. When a metal is exposed to an electrolyte, it adopts a property known as an electrode potential. This property defines a metal’s ability to either gain electrons (reduce) or lose electrons (oxidize).
When two metals with different electrode potentials are in contact with each other in the presence of an electrolyte, they form a galvanic cell, where the potential difference between the two metals causes electrons to flow from the more reactive metal (the anode) to the less reactive metal (the cathode). This flow of elections gives rise to a current being generated in the cell. Furthermore, as electrons are transferred from the anode to the cathode, the anode oxidizes and deteriorates while the cathode undergoes a reduction reaction and remains protected. (This process is examined further in Why Do Two Dissimilar Metals Cause Corrosion?)
The fact that galvanic corrosion causes (1) the flow of electrons from one point to another, and (2) one metal to corrode preferentially to the other, has proven to be useful in a number of applications.
Benefits of Galvanic Corrosion
Listed below are a few of the applications in which the galvanic process is beneficial.
Voltage Generation in Batteries
The reactions that occur during galvanic corrosion are the driving force by which batteries generate electricity. The typical dry cell or wet cell battery is essentially an electrochemical cell with an anode, a cathode and an electrolyte solution.
Conventional dry cell batteries used in most electronics consist of a zinc inner case surrounding a graphite rod immersed in a moist electrolyte paste comprised of ammonium chloride (NH4Cl), manganese dioxide (MnO2) or another suitable chemical solution. This electrolyte paste bridges the connection between the graphite rod and the zinc casing to form an indirect electrical connection. The zinc casing, being comprised of the more reactive metal, becomes the anode, as electrons move from it to the cathodic graphite rod. The continuous flow of electrons from the anode to the cathode results in the generation of electricity that is used to power numerous electrical appliances and devices.
The battery continues to supply electricity until the anodic zinc casing is fully consumed, at which point the battery is considered to have died.
Galvanic corrosion is used to protect metal components by intentionally forming a galvanic cell with another sacrificial metal. This process is called cathodic protection. There are two types of cathodic protection: passive cathodic protection and impressed current cathodic protection.
- Passive cathodic protection
In passive cathodic protection systems, a sacrificial anodic metal is connected either directly or indirectly to the metal to be protected. The connection between the two systems is adequate to generate enough electricity to form an effective electrochemical cell.
Passive cathodic protection systems are commonly used on offshore platforms to protect structural steel members. Aluminum bars (sacrificial anodes) are directly mounted on tubular steel sections to protect them from corrosion. Other applications that use passive systems are water heaters, storage tanks and steel piles.
Passive cathodic protection may also be observed in galvanized steel. In this process, steel is thoroughly coated by immersing the base metal in a molten zinc bath. When the steel member becomes scratched or otherwise damaged such that the steel substrate is exposed, the adjacent zinc coating prevents corrosion of the underlying steel by acting as a sacrificial anode. The surrounding zinc coating continues to protect the underlying steel until the coating is depleted.
- Impressed current cathodic protection
In longer or larger structures, it is not possible for galvanic anodes to deliver enough current to provide adequate protection. In this case, impressed current cathodic protection (ICCP) is used. In these systems, the anode is connected to an external power source to assist in driving the electrochemical process. (Learn more in the video When to Use Impressed Current Cathodic Protection.)
ICCP systems are frequently used to protect hundreds of miles of oil and gas pipelines. Electrical insulation is often provided between the flanges of connecting pipelines by using flange isolation kits to disrupt the electrical connectivity between adjacent pipes. This insulation improves the efficiency of ICCP systems by electrically separating the pipeline into smaller, more manageable sections.
Although known for its detrimental effects, the galvanic corrosion process possesses specific characteristics that make it ideal for:
- Generating electricity in portable power systems such as batteries, and
- Protecting vulnerable metal components
A sound understanding of this process is essential to ensure that these benefits are utilized effectively in their intended application.