Technical Service Bulletin

Anode Rods, Cathodic Protection and the Porcelain (glass) Lining

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 waters 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. Amount of dissolved oxygen - Free oxygen dissolved in the water promotes corrosion.
  4. 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:
    • Electrical current flows between metals
    • One of the metals gradually corrodes faster than the other

Dissimilar metals are present in the interior steel tank surfaces of all water heaters in several forms such as the drain nipples, heating elements, inlet and outlet nipples, and immersion thermostats. 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 installed to overcome (neutralize) the corrosive cell.

The Porcelain (glass) Lining

In a water heater, corrosion is protected by a glass (actually a porcelain enamel) lining in the steel tank, and the use of auxiliary anode rods. Porcelain enamel begins as a blend of minerals mixed in a manner to form liquid slurry. This slurry, resembling a thin mud, is sprayed onto the inner surfaces of the water heaters. During the manufacturing process, the inside water tank and both the top and bottom heads are sprayed with a slurry of glass material. After the tanks are sprayed, they are fired at very high temperatures - generally ranging from 1500 to 1600 degrees Fahrenheit. While in the furnace the porcelain bonds with the metal to create more than a coating. It forms an inseparable compound merging the chemical makeup of the porcelain glass lining and metal resulting in a new, chemically unique finish.

This glass provides a long life to the steel tank; otherwise, the tank would fail or corrode in a relatively short time. Every glass lined water tank, no matter how carefully it is manufactured, has some microscopic bare metal areas exposed to the water inside the tank. 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, pinholes in the tank and finally tank failure.

(Microscopic view of steel and porcelain lining after firing)