What is an anode? How does it work?
What is an anode and why is it so important?
Water heaters have anodes in them to protect them against rust.
All mains pressure, storage water heaters, other than those made from stainless steel, are constructed from 2 to 3 mm thick mild steel which has been coated on the inside with vitreous enamel similar to that which is used to rust-proof barbeques. They are never made from glass or fibreglass. Any misunderstanding here stems from the industry term "glass-lined which is commonly used to describe this type of tank.
This enamel coating is subject to extremes of temperature and pressure which causes it to deteriorate over time. Therefore, all glass-lined tanks are also fitted with one or more sacrificial anodes to protect the steel tank against corrosion as the glass lining loses its effectiveness. Even in places like Sydney and Melbourne, where the water quality is quite good, (TDS*) readings of less than 100 parts per million) anodes in small water heaters can last for as little as 4 or 5 years before being expended and thus allowing corrosion to commence. NB: Refer to the section - "How long should an anode last before it needs replacement" to see how long an anode should last for each size of heater.
(*Total Dissolved Solids)
By simply replacing a heater's anode when required, (the period varies depending on the size and model of the heater) the same protection is provided that keeps ships afloat, underground steel pipelines and tanks corrosion free and even Australia's and the World's sea-based oil drilling platforms intact. - That's why the heater manufacturer put the anode there in the first place.
How can corrosion take place in an enclosed tank full of water?
Corrosion in water heaters is associated with the passage of minute electrical currents which travel through the metal and the water. Corrosion takes place at the anodic areas (the anode) which are the places where the current releases ions to dissolve in the water. Cathodic areas develop at other sites (the tank walls) where the circuit is completed and at these places, provided there is an effective anode working inside the tank, no corrosion can, or will, take place. Anodes corrode - cathodes do not. Once the original anode stops working, the steel tank will react with other incompatible metals such as the copper pipes, brass fittings and the element/s. Once this occurs, the tank wall becomes anodic and corrosion (rust) commences and the tank will very quickly rust through.
How do anodes work?
Anodes are manufactured from a special grade of magnesium and they protect steel by a sacrificial electrochemical action. Magnesium is electro-negative relative to steel. When a magnesium rod is fitted to a steel tank filled with fresh water, a current will constantly flow through the water between the rod and any exposed steel area on the tank wall. The circuit is completed through the tank back to the magnesium rod. This protective current is produced by the magnesium releasing ions, and this results in corrosion in the anodic area. The magnesium (the anode) corrodes instead of the steel tank (the cathode). This principle of electrolytic corrosion control is called cathodic protection. Because cathodic surfaces cannot rust, the steel tank is protected.
This principle is nothing new. No experienced boat owner would risk the propeller, rudder, shafts or other exposed metal equipment on his boat, when all it requires to ensure that they do not corrode, is to keep an effective sacrificial anode in place on the hull of the craft or on the motor itself in the case of outboards. The same system is used on all commercial shipping, oil drilling platforms as well as underground pipelines and storage tanks. It's far more economical to replace the anode than to allow the vessel itself to corrode. This logic also applies to domestic and commercial water heaters.