What is a sacrificial anode

The key difference between sacrificial anode and impressed current is that in sacrificial anodes, a metal or alloy is placed in order to act as the anode instead of the metal to be protected whereas, in impressed current method, a DC current is provided to the metal to be protected in order to make it the cathode.

The sacrificial protection and impressed current system are two types of cathodic protection (CP). Moreover, cathodic protection is a method of protecting the metal surfaces from corrosion by supplying it with an external cathodic current.

CONTENTS

1. Overview and Key Difference
2. What is Sacrificial Anode 
3. What is Impressed Current
4. Side by Side Comparison – Sacrificial Anode vs Impressed Current in Tabular Form
5. Summary

What is Sacrificial Anode?

A sacrificial anode is a highly active metal that can save the less active metal surfaces from corrosion. It can be a metal or a metal alloy. Another name for this type of anodes is galvanic anode. These anodes supply cathodic protection. However, as anodes are consumed during the protection process, protection has to be replaced and maintained.

When considering the materials used for sacrificial anodes, they are most relatively pure metals; i.e. zinc and magnesium. Sometimes, we use alloys of magnesium or aluminium too. Besides, these anodes provide protection by being more electronegative or much more anodic than the protected metal. In the process of protection, a current will pass from the sacrificial anode to the protected metal, and the protected metal becomes a cathode. Thus, this creates a galvanic cell.

Figure 01: Corrosion of Sacrificial Anodes

In putting on the sacrificial anodes, we can use either lead wires (attached to the metal surface we are going to protect via welding) or use cast-m straps (either by welding or can use the straps as locations for attachment). The applications of sacrificial anodes include the protection of hulls of ships, water heaters, pipelines, underground tanks, refineries, etc.

What is Impressed Current?

Impressed current is a type of cathodic protection utilizing electrochemical means to obtain protection against corrosion. And, this method is important for the protection of large structures such as long pipelines because the sacrificial anodes cannot protect such structures. We can denote this method as ICCP, which stands for impressed current cathodic protection.

In this method, we apply an impressed current to convert the corroding metal from anode to cathode. Here, we have to apply the impressed current in the opposite direction to that of the corrosion current. Generally, the current source is a DC power supply. We can give this current to graphite, stainless steel, etc. which do not dissolve upon current supply. So, these materials are the anodes we are going to convert into cathodes during this method. Besides, the negative end of the current source has to connect to the structure that we are going to protect.

Figure 02: Method of ICCP

Moreover, the applications of this method include protection of steel in seawater or soil, subsea pipelines, hull, oil platform in steel and concrete, concrete bridges placed in seawater, pipelines buried in soil, underground tanks, etc.

What is the Difference Between Sacrificial Anode and Impressed Current?

A sacrificial anode is a highly active metal that can protect less active metal surfaces from corrosion. In contrast, an impressed current is a type of cathodic protection utilizing electrochemical means to obtain protection against corrosion. In sacrificial anodes, a metal or alloy is placed in order to act as the anode instead of the metal to be protected whereas, in impressed current method, a DC current is provided to the metal to be protected in order to make it the cathode. Hence, this is the key difference between sacrificial anode and impressed current.

Below infographic shows more information related to the difference between sacrificial anode and impressed current.

Summary – Sacrificial Anode vs Impressed Current

A sacrificial anode is a highly active metal that can prevent the less active metal surfaces from corrosion. Meanwhile, an impressed current is a type of cathodic protection utilizing electrochemical means to obtain protection against corrosion. Thus, this is the key difference between sacrificial anode and impressed current.

Reference:

1.“Sacrificial Anode.” Sacrificial Anode – an Overview | ScienceDirect Topics, Available here.

Image Courtesy:

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

Filed Under: Physical Chemistry