Corrosion in Water Tanks


Corrosion 

Corrosion is another word for rust, the breakdown of metal.  In a water tank, corrosion is due to an electrochemical reaction.  An electrical current flows through the water from one point on the water tank's inner surface to another.  The flow of the current results in corrosion of the tank's surface as some of the metal dissolves into the water.  An ampere of current flow in a water tank over a year's time can result in 20 to 24 pounds of steel being taken into solution.  

A water tank is like a battery because it has a cathode, an anode, and a closure circuit.

A corroding water tank works in the same way as a battery.  A battery has an anode and a cathode.  The anode is an area which gives off electrons (negatively charged particles which make up a current.)  The electrons flow from the negatively charged anode (-) to the positively charged cathode (+).  In order to flow from the anode to the cathode, electrons must pass through what is called a closure circuit or an electrolyte, a substance which forms a bridge between the anode and cathode.
The electrons in a steel tank move into the water, causing corrosion.


In a water tank, the steel wall of the tank is the anode.  It gives off electrons which flow into the water.  The water is the cathode and the surface of the tank is the closure circuit which connects the anode and cathode.  As the electrons flow out of the steel wall of the tank, the tank corrodes.  Over time, this corrosion can discolor the water and can result in leaks in the tank wall.  

Corrosion of a tank is prevented in two ways.  The inner wall of the tank can be coated with paint, which forms a physical barrier between cathode and anode and prevents the electrons from moving out of the steel wall.  Corrosion can also be prevented by cathodic protection which introduces electrical currents from external sources to counteract the normal corrosion reactions.  Both methods of corrosion control will be explained below.


Coatings  

The most common method of corrosion control is coating the tank wall with paint.  The coating of paint forms a physical barrier between the tank wall and the water.  As a result, electrons cannot flow from the tank wall into the water and the tank wall does not corrode.

A coating forms a physical barrier between the anode and the cathode.


Coatings will perform well in the majority of atmospheres. However, the coating should match the design of the facility in type and application and should be properly inspected.  The American Water Works Association (AWWA) has standard specifications for maintenance and painting of storage facilities, including a list of coatings for interior and exterior surfaces.  

The coating must be repaired at regular intervals as part of the regular maintenance of the tank.  Coatings fail when the protective barrier is broken or when the dielectric strength is not sufficient to withstand the electrical-driving force of the corrosion cell.  

Breaks may be caused by a variety of circumstances.  The paint may be too thin or may have degraded through proximity to the environment.  The coating may be porous and may not form a complete barrier.  Or the paint may have been applied incompletely or improperly.  If the surface was not properly cleaned before applying the paint, then breaks will occur since the paint will not adhere well to contaminated surfaces or to blisters.  The procedure used to repair breaks in the coating will be discussed later in the section on maintenance.  


Cathodic Protection

The other type of protection from corrosion, cathodic protection, has been in use since the mid-1930s.  Cathodic protection systems now protect hundreds of thousands of miles of pipe and acres of steel buried in the ground or immersed in water.  

Since the first cathodic protection system was invented, fully automatic and error-free cathodic protection systems have been developed. These systems both control corrosion and increase the life of the coating systems.  The modern, automatic systems do not require the operator to adjust the system for changing conditions, such as varying water levels, temperature, and coating effectiveness.  Instead, the automatic system will automatically compensate for these changes.

Diagram of a cathodic protection system.


A cathodic protection system consists of a power unit, an anode, and wiring between the power unit and anode and between the tank wall and the power unit, as shown above.  

The power unit is a direct current source, meaning that it converts electricity from alternating current to direct current, producing a continuous flow of electrons.  The electrons are fed along a wire to an anode in the water tank.  The anode disperses the electrons into the water, changing the potential of the water from positive to negative.

Electrons flow out of the anode and into the water.


Since the water now has a negative potential, just like the metal of the tank walls, electrons do not flow out of the tank walls into the water.  Instead, the electrons from the water flow into the tank walls and then along a wire back to the power source, completing the circuit.  

In essence, the cathodic protection system has created a new corrosion battery.  With the addition of the anode attached to the power source, the tank walls have become cathodes which take up, rather than lose, electrons.  If the cathodic protection system is properly controlled, the surface of the tank will remain corrosion-free.  Corrosion occurs only in the new anode, which is easily replaceable and relatively inexpensive.  

Commonly used materials for the anodes in the cathodic protection system are cast iron alloyed with 14.5% silicon, aluminum and platinum wire, and ribbon forms.  When water tanks are not prone to freezing, cast iron and other long-life anodes are most commonly used.      As with most other components of water systems, many factors must be considered when deciding on a cathodic protection system. The dimensions of the structure to be protected, the coating, and the water characteristics all factor into the type of system to be used. The manufacturer's representative will consider these factors and choose a system which will achieve full corrosion control without providing costly over-protection.  A reliable company should be used which specializes in cathodic protection and which has a service organization within one day's travel time of the tank's location.  When buying a cathodic protection system, a service agreement should also be considered.  



Advantages and Disadvantages

Cathodic protection systems are an economic alternative to periodic repainting and the associated downtime for repair.  The systems are being used quite extensively and have proved to be dependable when appropriately sized, installed, and maintained.  

However, cathodic protection systems will only protect the area of the tank interior where the water comes in direct contact with the metal tank wall.  The area of the tank above the water line will not be protected and can still corrode.  In addition, the bottom of tanks are usually covered with silt, debris, and sand of varying depths.  The tank bottom below this debris covering will not be protected unless the covering is removed before installing the cathodic protection system.  

Coatings and cathodic protection have often stood on opposite sides of a fence as exclusive and opposing approaches to corrosion protection.  Proponents of coatings often discount the advantages of cathodic protection and claim that a good, well-applied coating is the only necessary protection for steel.  On the other hand, proponents of cathodic protection systems claim that any immersed or buried metal structure can best be protected by a well-engineered cathodic protection installation.  Under many conditions, both sides may be correct in their assertions.  However, under many more commonly occurring conditions, the ideal corrosion protection is actually a combination of both protection concepts.