Corbec, Hot-dip Galvanizing
Corbec news on hot-dip galvanizing
What is cathodic protection?

What is cathodic protection?

May 2014

Explaining the benefits of hot-dip galvanized steel to a customer may raise more questions than answers. For example, discussing the benefits of the protective barrier as a corrosion controller proves to be a much simpler task than explaining the benefits of cathodic protection, a complex subject that requires a more technical answer. This article concisely answers the question "What is cathodic protection and how does it apply to galvanized steel?" We hope it will help inform prescribers and customers on the processes and benefits of cathodic protection provided by galvanized steel

 Corroded end

Galvanic reactions cause most of the observed corrosion of materials in the environment. The presence of four components - an anode, a cathode, an electrolyte, and a return path for an electrical current - create a galvanic cell via natural thermodynamic processes. Finding the four necessary components together appears to be an almost impossible task, but in reality, these cells are very common and cause most of the corrosion that we see around us. A simple example can be seen in a drawing made of two dissimilar metals in contact. When this occurs, one acts as an anode and the other as a cathode. The position of each metal in the galvanic series (Figure 1) determines whether it will be the cathode or the anode in this system. The corrosion process consumes the anode’s metal. The overlap area creates a return current path, and the air surrounding the metal acts as an electrolyte. Therefore, the four elements of the galvanic cell are in place and only the anode begins to corrode at an accelerated pace.

In applications such as these, the separation of the two metals with an insulating material, such as rubber, solves the problem of galvanic corrosion. However, not all galvanic cells are destructive. Galvanic cells may be deliberately created to successfully protect a material against corrosion. Once again, in this system, only the anode is consumed by corrosion. This fact is advantageously used to control corrosion and protect our structures and investments. Creating a new galvanic cell, in which an anode material more anodic than the material to be protected is introduced, ensures that corrosion will occur on the new anode and not on the cathode to be protected.

This new material is called a "sacrificial anode" and is the key to "cathodic protection." As with other forms of galvanic corrosion, the four essential elements mentioned above must be present and functional to properly obtain cathodic protection. When black steel is exposed to the atmosphere, small areas on the surface of the steel act as anodes, and the surrounding areas act as cathodes. This process causes the whole surface to rust and corrode. The introduction of a sacrificial anode in the system stops the process and forces the steel to act as a large corrosion-free cathode, rather than a system of small anodes and cathodes. The sacrificial anode consumed by this controlled corrosion protects the steel and your investment. In the case of hot-dip galvanized steel, a sacrificial anode is made by coating steel with zinc. Zinc acts as an anode, steel as a cathode, and the metallurgical bond between the two generates a return current. A deep scratch on the zinc surface will expose the bare underlying steel. When a deep scratch exposes the steel, and the atmosphere, soil, or submerged conditions provide an electrolyte between the zinc and the newly exposed steel, the loop is closed by adding the fourth element to have a complete galvanic cell. In this galvanic cell the base steel is protected and the zinc coating is consumed. Even when the coating is scratched, and up to 1/4 inch of steel is exposed, zinc protects the steel against rust.

While the paint may blister after being scratched, hot-dip galvanized steel has the additional advantage of cathodic protection, which allows the steel to be corrosion-free for a long time. Cathodic protection also protects the ends of parts which can be exposed when cutting the steel after galvanizing. While this article does not answer all questions related to cathodic protection, there is sufficient information presented to explain how galvanized steel benefits from cathodic protection. Combined with the zinc layer barrier, cathodic protection keeps your steel structurally sound and helps provide many years of service.

Source: AGA - American Galvanizers Association

Return to news