POWER April 2013 - 17

3. Corrosion begins. White rust on a galvanized panel is caused
when the protective zinc coating is attacked. When the galvanized surface
is penetrated, the steel base metal will begin to corrode. Courtesy:
Nalco Power ITC
zinc carbonate and zinc hydroxide. The difference between
the two is the rate of formation and the density of the layer.
Rapid formation of an oxide film produces a porous deposit
that allows continued corrosion of the nonpassive surface, if
left unchecked. A slower formation of the zinc layer provides
a nonporous, protective layer.
A common misconception is that once the layer of white
rust has formed, no further corrosion can take place. Not only
is this not true, but the corrosion byproduct also can create
an oxygen concentration cell, creating an accelerated localized
corrosion reaction. This situation is further complicated
with the addition of high levels of sulfates and chlorides.
As stated in the Nalco Guide to Cooling Water System Failure
Analysis (1993), " Once the zinc layer is breached, the underlying
steel becomes susceptible to attack and is severely
wasted locally " (Chapter 5, p. 108). The reference book also
notes: " On galvanized steel, tubercles may develop rapidly at
breaches in the zinc layer. Attack is frequently highly localized
if aggressive ions such as chloride or sulfate concentrate
beneath deposits " (Chapter 4, p. 72).
White rust has a porous, soft, gelatinous, or waxy appearwill
be removed, exposing the unprotected steel, producing
the familiar rusty-brown or black appearance. In less-severe
cases, the zinc coating can be corroded to form what is known
as " white rust " (Figure 3).
Both the protective oxide film and white rust are forms of
ance and is relatively easy to identify. The galvanized zinc
coating can be severely damaged when white rust is formed,
which in turn can shorten the life of the cooling tower. Also,
because white rust formation can be highly localized, once
the zinc layer has been consumed, corrosion of the mild steel
may progress rapidly, as with any penetration of a metal coating.
White rust is typically caused when operating outside of
the typical guidelines for the cooling water pH and alkalinity.
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April 2013 | POWER
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POWER April 2013

Table of Contents for the Digital Edition of POWER April 2013

Contents
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POWER April 2013 - Contents
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