POWER May 2011 - 19

(II) oxide, which is less stable and more easily transported into
the feedwater, exposing fresh surface area to corrosion. Cycling
from reducing to oxidizing conditions allows the oxide layer to
be removed and reformed.
Improper lay-up practices, particularly on copper alloy feedwater
heaters and condensers, are responsible for massive
amounts of copper corrosion. When they come off-line, many
units simply vent the shell side of the feedwater heaters and
the condenser to atmosphere, leaving copper, O2
, and CO2
in
direct contact with the passive copper (I) oxide layer and condensate.
This corrosive condition may exist for hours to weeks,
during which the copper (I) oxide is converted to copper (II)
oxide. It is well-documented that the amount of copper corrosion
product transport associated with the start-up of units
that have poor lay-up practices is thousands of times greater
than copper transport associated with normal operation.
Copper Transport. Copper is typically transported from the
feedwater as copper oxides or soluble copper complexes. Corrosion
products may be physically washed off piping by high
steam or water flows, exfoliated because of thermal shock, or
dissolved because of the chemistry of the feedwater or steam.
Depending on operating conditions, the copper either remains
in solution until it reaches the turbine or plates out long
before. Some units experience a thin layer of copper precipitation
on the inside of the HP feedwater heater tubing that's
called " snakeskins. "
In one notable study, one station experienced some degree of
copper deposits on all turbines but one. On that unit, mechanics
often needed to clean out boiler feedwater pump screens to
remove copper and copper oxides. In this case, it appears that
because of the precipitation of copper on the screens, the corrosion
products never made it to the turbine.
If they do not precipitate in the feedwater equipment, copper
corrosion products often accumulate in the boiler. The earlier
survey showed a very high correlation between boilers with
significant amounts of copper removed by chemical cleaning
and copper deposits on the turbine. It's not clear whether this
is because the boiler plays a significant role in the collection
and volatilization of copper into the steam or if it is simply indicative
of a general copper corrosion and transport problem.
In either case, if boiler tube deposits contain a significant
percentage of copper, concern is warranted. Even before they
can be volatilized into steam, copper deposits in the boiler
can create problems. At least one utility found that copper
deposits restricted orifices on the suction side of the boiler
circulating pumps.
Transport of boiler copper into steam may be affected by a
number of factors, including mechanical carryover because of
drum internal problems or inaccurate drum level monitors. In
addition to mechanical carryover, chemical conditions in the
boiler may volatilize some species of copper into the steam.
Copper may also enter the steam via the attemperation
sprays, though most large boilers use very little attemperation
spray during normal operation. Some units, however, use tremendous
amounts of attemperation spray during start-up, when
copper concentrations in the feedwater are at their peak.
Copper Deposition. Once it's in the steam, copper may
plate out in the superheater piping or the HP turbine. Copper
deposits in the superheater can create a reservoir of copper
that may migrate over time into the turbine. One plant found
zinc on the HP turbine eight years after the only source of
zinc was removed and after the boiler had been chemically
cleaned.
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