Get help from Pros. Remove ads. Promote your CV or LinkedIn profile.

Protection of a ship's hull against corrosion by the use of impressed electric current or by sacrificial anodes.
Cathodic protection has been credited with considerable success in this field. Ships equipped with this type of protection may have some or all of their tanks fitted with a number of sacrificial metal anodes. These anodes consist of magnesium or other suitable metals. To obtain the best results the tanks containing the anodes have to be cleaned so that the steelwork is free of grease and oil residue. After cleaning, clean salt water ballast is pumped into the tank until it is full and the tank is left for several days.

The salt water sets up a galvanic couple. Galvanic current attacking the sacrificial anodes conveys a protective coating from the anode to the steelwork around it. The anodes waste away and eventually have to be replaced.

Unfortunately, cathodic protection has a number of drawbacks. It is only effective when the tanks containing the anodes are ballasted for a reasonable period of time. Obviously, this is not always possible. In certain trades, the ballast passage may be of limited duration and in any case, work has to go on, tanks have to be cleaned, dirty ballast changed, and so on. Because of this, some ship-owners and tanker companies only fit cathodic protection in the ballast tanks which fall within one of the vessel's regular ballast patterns. To protect the other tanks alternative measures are taken. Sometimes the interior of the tank is sandblasted or shot blasted to clear the interior surfaces of all scale and the special protective paints are applied.

Another factor that must not be overlooked when considering corrosion in oil tankers, is the effect of relatively high humidity or dampness in empty cargo tanks on the ballast passage. Corrosion from this source is considered to be sufficient to justify the installation of a ventilation system, the purpose of which is to pass warm dry air into the cargo tanks. The reduction in humidity automatically results in the reduction of corrosion. Vessels fitted with this type of system report that it is also an excellent means of keeping empty tanks gas free.

cathodic protection

Impressed current systems have been moving gradually towards better materials and increased efficiency. Diodes of silicon have superseded selenium in some applications and although these must be protected against static discharge, the changeover is generally justified in terms of cost. One advantage with these devices, of course, is that they can be used at high temperatures and are therefore, suitable for process plants of different kinds.

Cathodic protection is applied to most major structures where materials (metals) are exposed to adverse conditions where electrolytic action is likely to take place. Although they require a power supply, the demand is low and compared with other methods of protection the maintenance cost is small.

But, there is one potential drawback which follows as a natural corollary of the system and that is, it is as harmful to supply too large a current as it is to supply too little. Changes in temperatures, surface conditions of the metals and conductivity of the electrolyte demand that the current, and therefore the potential difference, should be changed.

It is for this reason that there has been a move towards automation in recent years, where conditions are automatically monitored and the power supply regulated to match them exactly through, for example, thyristor control system.

Automated systems of this kind are particularly applied to ships and power stations and are being used on a modest scale in these fields.

But recently cathodic protection has moved beyond the narrowly defined areas of metal corrosion and has moved into the protection of metals against other forms of attack.

Related Terms

SEEBECK EFFECT

When two different adjacent metals are heated, an electric current is generated between the metals.

SEEBECK EFFECT

When two different adjacent metals are heated, an electric current is generated between the metals.

DIELECTRIC STRENGTH

A measure of the ability of an insulating material to withstand electric stress (voltage) without failure. Fluids with high dielectric strength (usually expressed in volts or kilovolts) are good electrical insulators. (ASTM Designation D 877.)

AMMETER

An instrument for measuring the magnitude of electric current flow.

AMMETER

An instrument for measuring the magnitude of electric current flow.

ARC

A flash, caused by an electric current ionizing a gas or vapor.

ARC

A flash, caused by an electric current ionizing a gas or vapor.

ALIVE

A term referring to a circuit in which a current is flowing. Also referred to as live.

ASME PART PEB SECTION I

Requirements for electric boilers.

ALTERNATOR

A device which converts mechanical energy, into alternating current.
Get help from Pros. Remove ads. Promote your CV or LinkedIn profile.

Related questions

MarineProHelp 2018 - 2022