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Anode Rods and Cathodic Protection

Corrosion can be defined as the destructive attack of a metal by an electrochemical reaction with its environment. Steel exposed to moisture and oxygen will rust and corrode. Corrosion is defined as the ‘eating away’ of metal by electrochemical means. There are four main factors affecting water’s ability to corrode:

1. Acidity – Water is made acidic by naturally occurring dissolved gases such as carbon dioxide.

2. Temperature – Higher temperatures speeds up the corrosive process.

3. Electrical conductivity – The more dissolved mineral solids in the water, the greater its ability to carry electrical current. When dissimilar metals are in the water:

   1. Electrical current flows between metals;

   2. One of the metals gradually corrodes faster than the other.

4. Amount of dissolved oxygen – Free oxygen dissolved in the water promotes corrosion. 

Dissimilar metals are present in the interior steel tanks surfaces of all water heaters in several forms such as the drain nipples, heating elements, immersion thermostats, inlet and outlet nipples. These metals, and others present in the water itself, combined with the oxygen content of the water and heat, establish an environment conducive to corrosion. The dissimilar metals create a corrosion cell that is enhanced by the conductivity of the water. The anode is put in to overcome (neutralize) the corrosive cell.

How can  water tanks be protected from corrosion?

In a water heater, corrosion is protected by a glass (actually a porcelain enamel) lining in the steel tank or through hot deep galvanizing.   

 However every glasslined or galvanized water tank,  no matter how carefully it is manufactured, has some bare metal exposed. This is due to the inability to effectively cover sharp corners and the radius around the fittings. There is a chance that a crack or chip in the porcelain lining will allow the water to come in contact with the steel tank. Over time, water, a universal solvent that becomes more aggressive with temperature, slowly dissolves the interior tank lining. This will create conditions for corrosion, pin holes in the tank and finally tank failure.

 How does an anode rod protect the water tank from corrosion?

Cathodic protection is accomplished in the glasslined or galvanized  water heater through the use of an auxiliary magnesium anode. Due to the relative position of magnesium to steel in the electromotive series of metals, magnesium will corrode producing an abundance of electrons which flow (much the same an electrical current) to the exposed steel surface and maintaining it in the electro- negative state. As current flow takes place, the anode rod reacts chemically to corrode at a rate faster than the steel inner tank. This process stops tank corrosion by substituting the sacrificial magnesium anode rod in place of the steel tank. As long as the magnesium anode rod remains in the tank, in an active state, there will be no corrosion of the minute areas of exposed steel inside the tank.

The life of the anode, in turn, depends on water temperature, amount of water used, and the quality of the glass lining. However, the most important factor in the life of the anode rod is the water chemistry, the hardness or softness of the water. Also check the manufacture and installation date of the water heater. If the heater is more than five (5) years old, and the anode rod has not been replaced, inspect the anode rod.