A sacrificial anode

Dr. Neil Canter, Contributing Editor | TLT Tech Beat November 2010

An electronic sensor alerts users when a sacrificial anode fails and needs replacement.

KEY CONCEPTS
• Corrosion of steel surfaces can be reduced through the use of a second metal that can be more readily oxidized and functions as a sacrificial anode.
• A wear indicator or an electronic sensor can more easily monitor the consumption of a sacrificial anode.
• Applications for sacrificial anodes include water heaters, recreational marine products, pipelines and underground storage tanks.

One of the key elements in the corrosion process is the gradual deterioration of the anode as metal is oxidized. If the corrosion process continues uninterrupted, then eventually the entire metal anode will be consumed.

Most of the corrosion problems that we deal with in the lubricant industry involve iron. One approach that slows down corrosion is to use a sacrificial anode composed of a metal different from iron.

A better understanding of the function of a sacrificial anode can be found in remembering the electromotive series, which lists metals in order of how easily they can be oxidized. Minimization of the oxidation of iron can be realized if a second metal is introduced that will oxidize more readily than iron. Examples include aluminum and zinc.

Therefore, introduction of a second metal such as aluminum with iron will alter the electrochemical process so that the aluminum will eventually be consumed over time instead of iron. In this fashion, aluminum assumes the role of a sacrificial anode.

A key application for sacrificial anodes is in water heaters. Eric Cytrynowicz, director of marketing at Performance Metal Products in Bechtelsville, Pa., says, “A sacrificial anode is the backbone in the life of a water heater.”

Water heaters are prepared by fusing glass to a steel surface at 870 C, giving an inner lining of an enameled porcelain referred to as a glass lining. However, like the glass interior of a thermos bottle, this lining can be fractured. Cytrynowicz explains, “The glass can crack when transporting or shaking the heater, leaving the water with a direct path to the metal. With this exposure, the steel surface will eventually wear away reducing the operating life of the water heater, unless a sacrificial anode is used.”

A sacrificial anode will last on average between two and six years, according to Cytrynowicz. Once the sacrificial anode is consumed, the water heater might not last more than two years. In fact, Cytrynowicz indicates that, depending on the water, the heater and the usage, the water heater may fail as soon as a few months after the sacrificial anode is used up.

A number of factors influence how long the sacrificial anode will last in a water heater. Cytrynowicz says, “The rate of corrosion will double for every 10 C increase in temperature. For an average water heater, the temperature will climb from 4 C to 55 C every time the unit is used. This increase can lead to a tenfold increase in the rate of corrosion.”

Water hardness also plays a role as higher levels of calcium and magnesium ions accelerate corrosion. But Cytrynowicz maintains that softened water can be a problem, too. He says, “Under softened water conditions, the presence of extra salts in the water will accelerate the corrosion of the steel surface.”

Sacrificial anodes come in differing diameters, so the use of a thicker rod can delay the failure of a water heater. Use of more than one sacrificial anode also can delay the process.

Aluminum, magnesium and zinc are the typical metals used in sacrificial anodes. Cytrynowicz says, “Most sacrificial anodes used in residential applications are prepared with an alloy containing 94%-96% magnesium. An aluminum/zinc alloy is employed in those applications, which involve water of inferior quality.”

Once a sacrificial anode is consumed, it is only a matter of time before the water heater fails. Cytrynowicz says, “The only warning for a user is the presence of water on the floor.”

Replacement of a sacrificial anode is recommended to prevent water heater failure. The problem is when to install a second anode. Cytrynowicz indicates that a sacrificial anode needs to be removed when it has been depleted by 50%-60%. The challenge is to figure out when that occurs in order to maximize the operating life of the sacrificial anode, particularly because it is inside the water heater and cannot be seen by the user.

A technology needs to be developed that will alert the user about when to replace the sacrificial anode. Such a technology has not been available until now.

ELECTRONIC SENSOR
Performance Metal Products has developed several approaches to inform the user that the sacrificial anode is due to fail and needs to be replaced. Cytrynowicz says, “In the recreational marine products, we have embedded a wear indicator that is brightly colored orange into the sacrificial anode. This is at a specific depth from a surface of the sacrificial anode and will be easily seen by the user. The orange-colored indicator does not become visible to the end-user until the sacrificial anode has lost between 60% and 70% of its metal.”

The sacrificial anodes can be attached to the hull, drives and other metal components in recreational marine boats.

The indicator is composed of an inert plastic resin that does not react with the steel surface or the metal used to prepare the sacrificial anode. Typically, polypropylene is used.

Another version includes an electronic sensor that can either be hooked up to a monitoring box or send a wireless signal to a specific site on the Internet. The sensor can trigger a visual signal and send out a beeping noise when the sacrificial anode needs to be replaced.

Figure 3 shows an image of a standard water heater sacrificial anode to the right of one containing an electronic sensor.


Figure 3. Use of an electronic sensor in a sacrificial anode (shown on the left) enables the user to determine when it is nearly depleted and should be replaced. When used in a water heater, the standard sacrificial anode (shown on the right) will fail without warning, leading to water leakage. (Courtesy of Performance Metal Products)

Cytrynowicz indicates that the probe is placed in the center of the anode but not too close to the middle. He says, “We conducted a series of calculations and trials to determine where to best position the probe to maximize the life of the probe.”

Sacrificial anodes are used in other applications such as pipelines and underground storage tanks. In addition, sacrificial anodes are used to prevent corrosion in the lock gates of canals and other waterways.

Cytrynowicz indicates that his company is also looking at other applications for sacrificial anodes. He says, “We are currently using the anodes in fire engine pumps and are exploring the possibility of using these sensors in fire suppression pumps used in hotels and in marine vessels.”

Further information can be found in a recent U.S. Patent (1) or by contacting Cytrynowicz at e.cytrynowicz@performancemetals.com. 

REFERENCE
1. Wigg, M., Leipert, H., Elder, J. and Dixon, K. (2005), “Wear Indicator for Sacrificial Anode,” U.S. Patent 6,932,891 B2.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.