Page 315 - Corrosion Engineering Principles and Practice

286 C h a p t e r 8 C o r r o s i o n b y W a t e r 287

frequently a severe problem mostly caused by dissolved carbon
dioxide (CO ) and aggravated by the presence of DO. Carbon
2
dioxide is produced in the boiler as a result of thermal breakdown
of the natural alkalinity, principally bicarbonate ions, which enter
with the feedwater. The following reactions are produced at boiler
operation temperatures:

g
2NaHCO → Na CO + CO ( ) + H O (8.17)
3
3
2
2
2
Na CO + H O → 2NaOH + CO ( ) (8.18)
g
2
2
2
3

The conversion of bicarbonate alkalinity to gaseous carbon dioxide
in Eq. (8.19) is a function of boiler temperature, pressure, and residence
time. Because it is extremely volatile, the carbon dioxide exits the boiler
with the steam produced. At points of condensation, some fraction of
the carbon dioxide present in the steam dissolves in the condensate,
forming carbonic acid that hydrolyzes into hydrogen ions:
CO ( ) + H O → H CO → H + HCO (8.19)
g
+
−
2
2
3
3
2
The hydrogen ions cause acidic corrosion of both iron and copper
alloy surfaces in the steam condensate system. The simplified corrosion
reaction for iron is
Fe(s) + 2H CO → Fe (HCO ) + H ( ) (8.20)
g
3
2
3 2
2
The Fe(HCO ) formed in this redox reaction is relatively soluble,
3 2
dissociated and its formation is competing with the formation of
insoluble iron(II) oxides such as Fe(OH) or colloidal associations
2
such as Fe (OH) n n−2 [13].
Dissolved oxygen may be another major cause of condensate
system corrosion. Oxygen contamination of steam condensate can
occur due to inefficient or improper feedwater deaeration, air leakage
at pump seals, receivers and flanges, leaking heat exchangers, and
ingress into systems that are under vacuum. In the presence of
oxygen, in addition to providing another possible cathodic reaction
to pair with the iron oxidation, one more oxidative step is possible as
shown in Eq. (8.21). This reaction releases carbon dioxide which
makes the process self-perpetuating.
4Fe(HCO ) + O 2(dissolved ) → 2Fe O (s) + 4H O + 8CO (g) (8.21)
2
3 2
2
3
2
Excessive corrosion of the condensate system can lead not only to
costly equipment failure and increased maintenance costs, but can
also cause deposition of metal oxide corrosion products on boiler
heat transfer surfaces if the condensate is recovered as feedwater.
Metal oxide deposition on boiler heat transfer surfaces will result in
lower fuel to steam efficiency and higher fuel costs. The deposition
may also lead to tube failure due to long-term overheating.