POWER May 2011 - 18

It is important to note that these three stages may be sepaStanding
strong,
we've seen the power
industry grow in our
85 years; and with each
branch that reaches
outward, whether gas,
nuclear, renewables
or AQCS, our goal has
always been for
you to flourish,
renew and thrive.
Zachry-
overcoming obstacles,
securing success.
rated by thousands of operating hours. Apart from sampling
problems, it is often difficult to associate the causes of copper
plating on turbines with specific operating conditions because so
much time may pass between cause and effect.
Copper Corrosion. Corrosion typically occurs when a passivating
layer of copper oxide is removed or it changes to a less-stable
oxide. This corrosion can occur during operation or lay-up.
Corrosion during operation results from ammonia, CO2
monium carbonate, or O2 in the feedwater. Each of these may
, amact
separately or together to attack the passivating copper
(I) oxide layer.
Ammonia can be either directly injected to increase the pH
of the feedwater, produced from the breakdown of a neutralizing
amine, or generated when excess hydrazine degrades in the
HP feedwater heaters, economizer, and boiler. Excessive amounts
of ammonia often are in the system during start-up. Previously,
common wet lay-up practices for boilers called for high concentrations
of hydrazine and ammonia or neutralizing amine. This
practically guaranteed high ammonia levels during the first critical
hours of start-up.
Major sources of CO2
are air in-leakage and the degradation
of neutralizing amines and hydrazine alternatives in the steam
cycle. Which source is a greater contributor to corrosion depends
on conditions in each unit. CO2
lowers the pH of the condensate
and may produce some localized, corrosive pH levels. The low-pH
feedwater requires additional ammonia to neutralize it, which in
turn increases the amount of ammonia or amine feed needed to
maintain the desired feedwater pH.
Ammonia and CO2
may combine to form ammonium carbonate
in the feedwater. To understand how corrosive ammonium
carbonate can be on copper and copper oxides, consider that
ammonia and ammonium carbonate are chemicals used to clean
turbines of copper deposits.
Depending on the location and amount of air in-leakage, dissolved
O2 in the condensate may also increase copper corrosion.
The worst type of dissolved O2 problem is the sporadic increase
in dissolved O2 that can be caused by transient conditions, such
as cycling and low-load operation. These operating modes are
especially corrosive, as feedwater conditions repeatedly change
from an oxidizing to reducing state. Copper (I) changes to copper
2. The final stages of the low-pressure section of a
steam turbine are the most vulnerable to corrosion.
Courtesy: David Daniels
www.zhi.com
ENGINEERING | CONSTRUCTION | NUCLEAR | INDUSTRIAL SERVICES
CIRCLE 11 ON READER SERVICE CARD
18
www.powermag.com
POWER | May 2011
A D A P T I V E
http://www.zhi.com http://www.powermag.com
POWER May 2011

Table of Contents for the Digital Edition of POWER May 2011
