Difference Between Anodic and Cathodic Protection

The key difference between anodic and cathodic protection is that in anodic protection, the surface to be protected acts as the anode whereas, in cathodic protection, the surface to be protected acts as the cathode.

Anodic and cathodic protection are two electrochemical processes we use to prevent surfaces from corrosion or rusting. In an electrochemical process, we use an electrochemical cell with two electrodes as anode and cathode. In anodic and cathodic protection processes, we use the surface to be protected (substrate) as either anode or cathode, which leads to name those processes as such. Sacrificial protection is a type of cathodic protection in which we use a metal as a sacrificial anode. In this process, this sacrificial metal corrodes while avoiding the corrosion of the cathode.

CONTENTS

1. Overview and Key Difference
2. What is Anodic Protection
3. What is Cathodic Protection
4. Side by Side Comparison – Anodic vs Cathodic Protection in Tabular Form
5. Summary

What is Anodic Protection?

Anodic protection is a type of electrochemical process in which we can protect a metal surface by making it the anode in the electrochemical cell. We can denote this as AP. However, this method is possible only for material-environment combinations that show fairly wide passive regions. i.e. steel and stainless steel in 98% sulfuric acid.

In AP, we need to bring the metal to a high potential. Then, the metal becomes passive due to the formation of a protective layer. However, AP is not widely used as cathodic protection because it is limited to metals which has a sufficiently reliable passive layer on the surface; for example, stainless steel.

There are two major considerations for the application of AP. First, we need to ensure that the whole system is in the passive range. Second, we need to have precise knowledge of ions, which can lead to extensive pitting.

What is Cathodic Protection?

Cathodic protection is a type of electrochemical process in which we can protect a metal surface by making it the cathode in the electrochemical cell. We can denote it as CP. CP can prevent metal surfaces from corrosion. There are different types of CP; for example, galvanic protection or sacrificial protection, impressed current systems and hybrid systems.

In this method, the sacrificial metal corrodes instead of the protected metal. If we use cathodic protection for large structures such as long pipelines, galvanic protection technique is not enough. Therefore, we need to provide sufficient current using an external DC electrical power source.

Moreover, we can use this technique to protect fuel or water pipelines made of steel, storage tanks, ships and boat hulls, galvanized steel, etc.

What is the Difference Between Anodic and Cathodic Protection?

Anodic protection is a type of electrochemical process in which we can protect a metal surface by making it the anode in the electrochemical cell, while cathodic protection is a type of electrochemical process in which we can protect a metal surface by making it the cathode in the electrochemical cell.  So, the key difference between anodic and cathodic protection is that, in anodic protection, the surface to be protected acts as the anode whereas, in cathodic protection, it is the cathode.

Furthermore, anodic protection involves the suppression of reactivity of a metal by adjusting the potential of the more reactive metal; however, cathodic protection involves the reversing of the flow of current between two dissimilar electrodes. Therefore, we can consider this too as a difference between anodic and cathodic protection.

Summary – Anodic vs Cathodic Protection

Anodic protection is a type of electrochemical process in which we can protect a metal surface by making it the anode in the electrochemical cell, while cathodic protection is a type of electrochemical process in which we can protect a metal surface by making it the cathode in the electrochemical cell. The key difference between anodic and cathodic protection is that, in anodic protection, the surface to be protected acts as the anode whereas, in cathodic protection, it is the cathode.

Reference:

1. “What Is a Sacrificial Protection? – Definition from Corrosionpedia.” Corrosionpedia, Available here.

Image Courtesy:

1. “Cathodic Protection diagram” By Cafe Nervosa – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Sacrificial anode” By Zwergelstern (CC BY-SA 3.0) via Commons Wikimedia