Materials_Performance_Magazine_article
>ì ` Vê?à ìiVì êV ìà iàê? üiài`êLTê}> ?> Vê> `iêV?àài ì
A new concept developed by engineers at Farwest Corrosion Control Co. (Gar-dena, California), designed particularly for steel water tank applications, uses a small percentage of the energy generated by magnesium anodes to power an elec-tronic controller that automatically regu-lates the tank-to-water voltage potential in a galvanic cathodic protection (CP) system.
In a galvanic anode system for a water storage tank, magnesium anodes are in-stalled 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 differ-ence in electrochemical potential between the anode and the cathode causes a gal-vanic current to ?ow from the magnesium anode to the cathode (steel tank). Through this process, the magnesium anodes cor-rode preferentially and provide corrosion protection to the tank. No electricity from an external power supply is used. Typi-cally, the internal surface of a modern steel potable-water storage reservoir can be protected from corrosion with a gal-vanic (sacri?cial) CP system, says John Bollinger, professional corrosion engineer with Farwest Corrosion Control Co., because the effectiveness and durability of today’s internal
tank coatings make
it possible to provide
CP using relatively
little current (i.e.,
less than 100 mA for
a 3 million gal [11
million L] storage
tank). However,
Bollinger notes, one
drawback to this
type of corrosion
protection is that
the galvanic current
is dif?cult to regu-
late.
Because the wa-
ter level in the tanks
can vary unpredict-
ably, the tank-to-water voltage potential
in a tank may change as the water level
in the tank changes. Often times, Bol-
linger explains, the anode current is
higher than required and overprotection
is possible on the tank’s interior.
NACE International recommended
practices call for tank-to-water potentials
to be maintained between –0.850 and
–1.100 V. But in some instances, Bollinger
notes, 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 po-
tential measure-
ments be “IR free”;
that is, the mea-
surements are cor-
rected for voltage
drops (gradients)
that can result in
measurement er-
rors. Because most
magnesium anode
systems are “on”
continuously, it is
very dif?cult or im-
practical to capture a true IR free potential
measurement.
Impressed current CP (ICCP) systems
utilize electronic equipment to automati-
cally measure and regulate tank-to-water
potentials. An ICCP also can easily fa-
cilitate an IR free measurement. How-
ever, providing alternating current (AC)
power to a water tank to operate the
circuitry of an automatic potential control
CP system can be dif?cult in many cases.
For example, Bollinger says, water stor-
age tanks are often located in remote
areas where an AC power source is simply
unavailable. Another way to provide
power to an automatic control system is
needed, he adds.
Drawing inspiration from the latest
low-power digital communication sys-
tems, Bollinger developed a patent-pend-
ing technology, PowerMag?, that taps into
the power generated by a sacri?cial an-
ode-cathode cell to operate a controller
that automatically adjusts anode current
to maintain a constant IR free tank-to-
water potential. The controller requires
no external power or internal batteries.
“The digital technology available now for
electronics has resulted in components
Continued on page 22
ê iüê}> ?> Vê *êìiV }Tê`i?i ?i`êv àêü>ìiàê?ì à>}iê
ì> ?ê iêì iê iê? ü ê iàiê??i?ê? üiàê}i ià>ìi`êLTê
>} i? ? ê> `i?êì ê ?ià>ìiê>êV ìà iàêì >ìê>?ì >ì
V> Têài}? >ìi?êì> ì ü>ìiàê? ì>}iê? ìi ì > °ê* ì ê
V ?àìi?Tê vê >àüi?ìê àà ? ê ìà ê °
ê }iàêü à ?ê êì iêV ìà iàê?T?ìi ê? üiài`êLTê>ê
>} i? ? ê?>Và wV > ê> `i V>ì `iêVi °ê* ì êV ?àìi?Tê vê
>àüi?ìê àà ? ê ìà ê °
?Trade name.
September 2008 -!4%2)!,3 0%2&/2-!.#%
/ , ê // ,-
that have very low power requirements,” Bollinger says. “We can operate the con-trol circuit on as little as a 3 mA drain from the anode system.”
The controller uses a state-of-the-art microprocessor that includes a pulse width modulation current regulator and a sophisticated potential measuring sys-tem to monitor and control various func-tions. The system, which can also be
retro?tted into existing magnesium sys-
tems, is designed to control up to 2 A of
anode current and can accommodate
two reference electrode inputs. “This
technology provides the ability to regu-
late the potential of a sacri?cial CP sys-
tem and avoid ‘over-voltage’ issues;
something that is very unique,” com-
ments Bollinger. “By regulating this CP
system, the anodes can run at a lower
current output, which would result in
longer anode life as well.”
Contact John Bollinger, Farwest Corro-
sion Control Co.—e-mail: jbollinger@
https://www.wendangku.net/doc/0714852456.html,.
Continued from page 21
-!4%2)!,3 0%2&/2-!.#% September 2008