Energy and Power Engineering, 2009, 17-20
doi:10.4236/epe.2009.11003 Published Online August 2009 (http://www.scirp.org/journal/epe)
Corrosion Prevention of the Generator Stator Hollow
Copper Conductor and Water Quality Adjustment
of Its Internal Cooling Water
Ling PAN 1, Qianqian HAN1, Xuenong XIE2, Xuejun XIE1, Peng XIAO1
1School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China
2Sinohydro bureau 8 Co., LTD
Abstract: On the basis of expounding the corrosion mechanism of the stator hollow copper conductor in the
water-cooling generator, methods of preventing corrosion of the stator hollow copper conductor in the wa-
ter-cooling generator through adjusting water quality of its cooling water have been proposed. For internal
water cooling systems which are airtight, the corrosion of the hollow copper conductor can be prevented
through keeping foreign oxygen and carbon dioxide from entering the system, and the amount of oxygen in
the internal water can be lowered by blowing high purity nitrogen. For systems not airtight, the corrosion of
the hollow copper conductor can be inhibited through lowering the amount of oxygen to some extent by
sealing and increasing pH value by processing part of cooling water with bypass small flow sodium-type
mix-bed.
Keywords: generator, internal cooling-water, hollow copper conductor, corrosion and protection
1 Corrosion Mechanism of the Generator
Stator Hollow Copper Conductor
The corrosion mechanism of the generator stator copper
conductor has been studied by large quantities of re-
searches home and abroad. The main reactions consid-
ered to take place in the process of copper corrosion are
as follows:
1) Corrosion caused by dissolved oxygen:
Anodic reactions:
2Cu +H2O-2e→Cu2O +2H+
Cu -2e→Cu2+
Cu -e→Cu+
Cathodic reactions:
O2 +2H2O +4e→4OH-
Cu +2OH-→Cu(O H)2
Cu(OH)2→CuO +H2O
Other cathodic reactions included:
2Cu+ +H2O +2e→Cu2O +H2
Cu+ +H2O +e→CuO +H2
2Cu+ +1/2O2 +2e→Cu2O
2Cu+ +O2 +2e→2CuO
The result is the corrosion of copper and the formation
of an oxidative membrane (the copper oxidative protec-
tion membrane) with double-layer structure on the sur-
face of the copper at the meantime in normal case.
2) In acidic environment, the copper oxidative protec-
tion membrane will be dissolved. The internal cooling
water which contains carbon dioxide belongs to acidic
medium as the dissolved carbon dioxide can react as the
following reaction.
CO2 +2H+=Cu2+ +H2O
H2CO3H+ +HCO3
-
Furthermore, pH value will decrease under the condi-
tion of existence of carbon dioxide due to the high purity
and poor buffering performance of internal cooling water.

L. PAN, Q. Q. HAN, X. N. XIE, X. J. XIE, P. XIAO
18
For example, pH value of the pure water will be lower
than 6.7 when there is 1mg/L dissociate carbon dioxide.
Therefore, the corrosion is accelerated by carbon di-
oxide, mainly through lowering pH value of internal
cooling water and the following reactions which can
destroy the protective membrane on the surface.
CuO +2H+=Cu2+ +H2O
Cu2O +2H+=2Cu+ +H2O
Cu(OH)2 +2H+=Cu2+ +2H2O
2 Methods of Preventing the Corrosion of
the Generator Stator Copper Conductor
At present, many scientific workers home and abroad are
carrying out a lot of researches on the problem of copper
conductor corrosion and protection, trying to seek for a
method which is more economical, convenient, safe and
reliable to prevent the corrosion of hollow copper con-
ductor.
2.1 Lower the Amount of Dissolved Oxygen and
Carbon Dioxide in the Internal Cooling Water
The corrosion rate peaks in common case when the con-
centration of dissolved oxygen ranges from 0.5mg/L to
2.0mg/L. The concentration of dissolved oxygen in the
water contacting with air at the temperature of 25 is ℃
1.4~3.2mg/L. The operational temperature of cooling
water ranges from 20 to 85, and the temperature of ℃℃
water flowing through hollow copper conductor is usu-
ally above 40. The amount of oxygen decreases when ℃
the temperature increases, but the decreased extent of the
amount of oxygen is not large.
For the purpose of studying the effect dissolved oxy-
gen had on the copper corrosion, the operational envi-
ronment of internal cooling water was simulated and the
relationship between dissolved oxygen amount and cor-
rosion behavior of copper at 50 was investigated. The ℃
result of experiment carried out to study the relationship
between oxygen concentration and copper corrosion at
50 in simulated internal cooling water operational e℃n-
vironment is shown in Table 1. The test was conducted
using pure copper specimen as test material and deion-
ized water as test medium at the temperature of 50l ℃
and the test time was 72h. The concentration of dissolved
oxygen was controlled through blowing nitrogen into the
water during the test. Pre-test pH value and conductivity
is measured at 11 before starting to blow nitrogen. It ℃
can be learned from Table 1 that the corrosion of copper
in deionized water can be inhibited effectively when the
concentration of dissolved oxygen is controlled ap-
proximately at the level of 10g/L.
Table 1. Result of test on the relationship between dissolved oxygen concentration and copper corrosion at the temperature of 50℃
Pre-test After test
Dissolved Oxygen
Content,g/L
Conductivity,
s/cm pH Conductivity, s/cm pH Cu2+ content, g/L
Surface condition of
test specimen
Below 5 0.64 6.2 1.85 7.15 3.9 bright
8～12 0.65 6.2 2.12 7.20 8.9 bright
40～60 0.67 6.2 2.10 7.15 45.1 light dead red
90～110 0.65 6.2 2.43 7.14 103.7 partially black
450～550 0.64 6.2 2.38 7.00 300.8 partially black
950～1050 0.65 6.2 3.05 6.90 269.2 partially black
4800～5200 0.65 6.2 2.30 6.75 190.4 mostly black
11500 0.65 6.2 1.92 6.80 182.2 dark dead red

L. PAN, Q. Q. HAN, X. N. XIE, X. J. XIE, P. XIAO
19
For a 300MW water-hydrogen-hydrogen cooled unit
with airtight internal water cooling systems and an in-
ternal cooling water tank whose volume is 2m3, the con-
ductivity and pH value of the internal cooling water is
monitored continuously at the sample flow ranges from
500 ml/min to 700ml/min. Thus, the amount of daily
make-up water, which is the effluent of high-speed
mixed bed, is 0.72t. The effluent water quality of
high-speed mixed bed is that the conductivity is no more
than 0.2s/cm, and pH value ranges from 7.03 to 7.10,
and the concentration of dissolved oxygen is 20~30g/L,
and there is no NH4
+. As a result, the operation of inter-
nal cooling system is in good condition and the corrosion
of hollow copper conductor is inhibited significantly, as
the internal cooling water has conductivity no more than
0.2s/cm, pH value ranging from 7.03~7.10, and low
copper concentration which is 9.8516.4g/L [2].
For a 500MW supercritical pressure unit using con-
densate whose pH value is 8.6~8.8 approximately as
make-up of the internal cooling water, due to the con-
tinuous dissolving of carbon dioxide, pH value of the
internal cooling water is above 8.2 in common case, and
the conductivity of 1.5~3.0s/cm is relatively high [3].
After blowing high purity nitrogen into the internal
cooling water, pH value of the internal cooling water is
above 8.0, and the concentration of copper is lowered to
less than 10g/L, and the conductivity can be below
1.0s/cm.
2.2 pH Adjustment for Preventing Corrosion of
Hollow Copper Conductor
Figure 1 is the reduced potential-pH diagram of cop-
per-water system, which is ploted taking Cu、Cu2O、CuO
and Cu2O3 as the balanced solid-phases and the value of
balanced concentration of Cu2+ and other relative ions at
the level of 10-6mol/L (i.e.64μg/L) or 10-6.2mol/L
(i.e.40μg/L) as the threshold concentration which can
indicate weather the copper is corrupted or not [1][4].
It can be learned from Figure 1 that the balanced po-
tential-pH diagram of copper-water system is divided
into three parts by the isoline of solubility of 10-6mol/L
or 10-6.2mol/L for copper and its oxides: corrosion region,
no-corrosion region and passivation region. The state of
copper, in another word, which region the copper falls
into, is jointly determined by the potential of cop-
per-water system and its pH value. As the potential of
hollow copper conductor in the internal cooling water
system is difficult to measure, the pH interval where
copper and its oxides can exist stably is 6.94 to 10.81 or
7.04 to 10.31 when 10-6mol/L (i.e. 64μg/L) or 10-6.2mol/L
(i.e. 40μg/L) is taken as the threshold concentration
which indicates the copper is corrupted weather or not.
Figure 1. Potential-pH diagram for copper-water system

L. PAN, Q. Q. HAN, X. N. XIE, X. J. XIE, P. XIAO
20
It can be learned through calculation that the conduc-
tivity of newly-prepared deionized water is 2.1s/cm
when its pH value is adjusted to 8.9 using analytically
pure ammonia water, and is 2.48s/cm when its pH
value is adjusted to 9 using analytically pure sodium
hydroxide solution. In view of the requirement of the
conductivity no more than 2s/cm in the currently stan-
dard for internal cooling water quality, pH value of in-
ternal cooling water is controlled between 7 and 8.9, and
the corrosion of hollow copper conductor can be inhib-
ited significantly. It is true in the practical internal cool-
ing water system. Among the various methods which can
be employed to increase pH value, the safest and most
reliable one is processing part of cooling water with by-
pass small flow sodium-type mix-bed. When part of in-
ternal cooling water flows through the sodium-type small
mix-bed, the cations such as the small amount of Cu2＋
and Fe3＋are converted into Na+ and the anions are con-
verted into OH- before mixing with the rest internal
cooling water which does not pass through the So-
dium-type small mix-bed. Thus, it corresponds to adding
approximately pure sodium hydroxide to the internal
cooling water. PH value of internal cooling water can be
adjusted to be 7~8.9 as long as there is trace amount of
impurity ions such as iron and copper existing in the
internal cooling water.
For a 300MW water-hydrogen-hydrogen cooled unit
using deionized water as make-up water, the internal
cooling water is processed by small sodium-type mix-
bed, whose ratio of cation resin and anion resin is 2.7:1.
Its water quality is as follows: The conductivity is below
0.5 s/cm, hardness is 0, pH is between 7 and 8, and the
concentration of copper is below 10μg/L [5].
3 Conclusions
1) The corrosion of generator stator hollow copper
conductor is caused by oxygen and accelerated by car-
bon dioxide.
2) For internal water cooling systems which are air-
tight, the corrosion of the hollow copper conductor can
be prevented through keeping foreign oxygen and carbon
dioxide from entering the system, and the amount of
oxygen in the internal water can be lowered by blowing
high purity nitrogen.
3) For systems not airtight, the corrosion of the hollow
copper conductor can be inhibited through lowering the
amount of oxygen to some extent by sealing and in-
creasing pH value by processing part of cooling water
with bypass small flow sodium-type mix-bed.
Of course, not only hollow copper conductor can be
prevented from corrosion, but also the internal cooling
water quality can be adjusted better when the internal
water cooling systems are airtight, and pH of the cooling
water is increased by processing part of cooling water
with bypass small flow sodium-type mix-bed.
REFERENCES
[1] Gong Xunjie, “The corrosion and protection of the thermal
equipment,” The Chinese Electrical Power Press, Beijing, 1998.
[2] Zhang Enkui, “The cause of generator’s inner cooling wa-
ter-quality worsening in Shuangliao power plant and corre-
sponding treatment,” Jilin Electric Power Technology, Vol. 1, pp.
40-41, 2000.
[3] Zhang Jianli and Li Baoguo, “The optimization of the inner
cooling-water of the generator’s stator,” North-China Electric
Power Technology, Vol. 4, pp. 1-3, 2003.
[4] Xie xuejun, Pan Ling, et al., “Potential-pH chart for cop-
per-water system and controlling pH of internal cooling water of
generator to prevent corrosion,” Corrosion Science and Protec-
tion Technology, Vol. 19, No. 3, 2007.
[5] He Shaohong, “Studies on tubular copper conductor corrosion of
the inner-cooling water system and corresponding protection,”
Sichuan Electric Power Technology, Vol. 6, pp. 6-8, 2000.