Cathodic Protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell.
The prevention of corrosion deals with the provision of protective coating for the ship structure steel and its continued maintenance. Also, a means of preventing electrochemical wastage is required, which is known as Cathodic protection.
In this article, I’ll share all the basics you should know about Cathodic Protection. Such as types of cathodic protection and how do they work, cathodic protection companies, etc.
Let’s get started…
Corrosion and Protection
Different chemical structures, paint thickness’, aeration, etc. can lead to one area of the hull becoming more cathodic than another.
Electrons will flow from the anodic area through the hull to the cathodic area. By hydrolysis negatively charged hydroxyl ions will form. At the anode, electron depletion leads to positively charged Iron ions. Hydroxyl ions migrated through the water to the anode, here combining with the iron ions to form Fe(OH)2 which combines with dissolved oxygen to form Fe(OH)3 or rust. In this way, the anodic area will corrode.
To prevent this it would be necessary to make the entire hull cathodic.
Types of Cathodic Protection
There are two types of cathodic protection methods,
- Sacrificial anode system
- Impressed current system.
Sacrificial anode system
This is a simple method of cathodic protection in which a sacrificial anode is welded to the ship’s hull. Sacrificial anodes are metal or alloy attached to the hull, which has a more anodic potential than steel when immersed in seawater. So sacrificial metal corrodes instead of the protected metal.
Because in simple words, if you place the steel and brass metals in a saline solution, a galvanic reaction takes place. And externally current flow cathode to anode makes the electrons flow from anode to cathode.
So sacrificial anodes are more electronegative metals than a ship’s hull. They act as anodes and get corroded instead of the ship’s hull. Hence protecting the hull from corrosion.
Advantages of Sacrificial Anode method
- The external current supply is not required.
- No need for such monitoring.
- This system will keep on working even the ship is laid up.
Disadvantages of Sacrificial Anode method
- Hull Resistance going to increase because of welded sacrificial anodes all over the hull.
- The efficiency of the system keeps on decreasing because of the corrosion of sacrificial anodes with time.
- Breakage of continuity of anode will make the system fail.
Impressed Current Cathodic Protection(ICCP) system
The basic principle of both methods remains the same. Making the metal act as a cathode. So that, vulnerable metal is supplied with a surplus of electrons.
In the Impressed Current Cathodic Protection method, an external source of power is used to supply current for the reaction to take place.
Advantages ICCP Method
- No need for welding Anode to Ship’s Hull
- The Ship’s Hull Resistance will reduce.
- Simple and easy protection method.
- No need for going drydock quite often.
Disadvantages of ICCP System
- No protection to the hull during the laid-up period.
- Continuous monitoring is required.
- Continuous electric supply has to be given.
- Care must be taken while connecting the terminals. Because wrong connections may end up in losing the metal instead of protecting the metal.
You may like: Important points on Pumps and Pumping system.
The anode is insulated from the hull, electrical connection is via cable and ships side gland box. It may be made of lead or Platinised Titanium. With the lead anodes, the hydroxyl ions turn the surface of the lead a rich brown color (PbO2).
A D.C. voltage is applied to just overcome the natural galvanic voltage. If the current is allowed to become too great then the increased Hydroxyl release causes sponginess and flaking of the paint.
The cathodic system should make the hull 200mV more cathodic i.e. 2oomV negatively charged. The system measures this by checking the hull voltage against an insulated reference anode which has a known value of galvanic voltage with the hull material.
Typically this may be Zinc which is normally at a voltage 450mV more negative than the hull, or Silver which is 600 mV more positive than the hull.
The Cathodic protection system will try to make the potential difference between the hull and the zinc reference anode 250 mV (Zinc anode 250mV more negative than the hull), and the silver anode 600mV (Silver anode 800mV more positive than the hull).
Different Types of Corrosion
Few other different types of corrosion are,
- Selective Phase Corrosion
- Cavitation Erosion
- Erosion Corrosion
- Crevice Corrosion
- Fatigue Corrosion
- Fretting Corrosion
- Spark Corrosion
Cathodic Protection Companies
You can get the list of Cathodic Protection Companies in India from Here
Few of the Cathodic Protection Companies in USA are,
So, this was a brief guide about Cathodic Protection Methods. Have any queries? Drop them in the comments.