A new concept developed by engineers at Farwest Corrosion Control Co. (Gardena, CA), designed particularly for steel water tank applications, uses the energy generated by the magnesium anodes and the tank itself to power an electronic controller that automatically regulates the tank-to-water voltage potential in a galvanic cathodic protection (CP) system.
Drawing inspiration from the latest low power digital communication systems, engineers developed a patent-pending technology that taps into the power generated by a sacrificial anode-cathode cell to operate a controller that automatically adjusts anode current to maintain a constant IR-free tank-to-water potential.
"The digital technology available now for electronics has resulted in components that have very low power requirements," said John Bollinger, professional corrosion engineer with Farwest Corrosion Control Co. "We can operate the control circuit on as little as 3 milliamperes drain from the anode system."
In a galvanic anode system for a water storage tank, magnesium anodes are installed in contact with the water and are also connected to the tank. These anodes have a more active voltage -- a more negative electrochemical potential -- than the steel tank (the cathode). The difference in electrochemical potential between the anode and the cathode causes a galvanic current to flow from the magnesium anode to the cathode (steel tank), thus providing the small electrical current. Meanwhile, the magnesium anodes corrode preferentially and provide corrosion protection to the tank. Because of this generated current, no electricity from an external power supply is used.
Typically, the internal surface of a modern steel potable-water storage reservoir can be protected from corrosion with a galvanic (sacrificial) CP system, explained Bollinger, because the effectiveness and durability of today's internal tank coatings make it possible to provide CP protection using relatively little current (i.e., less than 100 mA for a 3 million gal storage tank).
However, Bollinger noted that one drawback to this type of corrosion protection is that the galvanic current is difficult to regulate. Because the water level in the tanks can vary unpredictably, the tank-to-water voltage potential in a tank may change as the water level in the tank changes. Often times, Bollinger explained, the anode current is higher than required and over protection is possible on the tank's interior.
NACE recommended practices call for tank-to-water potentials to be maintained between -0.850 and -1.100 V. But in some instances, Bollinger noted, exceeding the -1.200 V potential limit can produce excessive hydrogen and may cause the tank's protective coating to disbond or blister.
CP designers are also specifying that all tank-to-water voltage potential measurements be "IR free"; that is, the measurements are corrected for voltage drops (gradients) that can result in measurement errors. Because most magnesium anode systems are "on" continuously, it is very difficult or impractical to capture a true IR-free potential measurement.
Impressed current CP (ICCP) systems, utilize electronic equipment to automatically measure and regulate tank-to-water potentials. An ICCP also can easily facilitate an IR free measurement. However, providing AC power to a water tank to operate the circuitry of an automatic potential control CP system can be difficult in many cases. For example, Bollinger said, water storage tanks are often located in remote areas where an AC power source is simply unavailable. Finding another way to provide power to an automatic control system was needed, he added, and so the PowerMag 1000 was invented.
The PowerMag controller uses a state of the art microprocessor that includes a pulse width modulation current regulator and a sophisticated potential measuring system to monitor and control the various functions. "This technology provides the ability to regulate the potential of a sacrificial CP system and avoid 'over voltage' issues; something that is very unique," commented Bollinger. "By regulating this CP system, the anodes can run at a lower current output, which would result in longer anode life as well."
The PowerMag 1000 requires no external power, or internal batteries. The system has an LCD display that will provide the operator with operational status of the system (i.e., IR-free potential, anode current, set potential, as well as other system conditions). The system is designed to control up to 2 amperes of anode current and can accommodate two reference electrode inputs. It can also be retrofitted to existing magnesium systems and can accommodate most remote potential monitors.
Farwest Corrosion Control Company is a leader in cathodic protection and corrosion materials and equipment, plus cathodic protection engineering services and installation services. For more information, visit http://www.farwst.com.
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