1. The document discusses various types of corrosion that can occur in boiler systems, including oxygen corrosion, ammonia attack, caustic corrosion, and hydrogen damage. 2. It also covers factors that influence corrosion rates like dissolved oxygen content, pH, temperature gradients, and chemical treatments. 3. Deposit buildup on evaporator tubes is discussed as a cause of overheating failures. Maintaining proper water chemistry and regular chemical cleanings are recommended to prevent issues from deposits.

1. Chemist / Mohamad Abd Elmenem Farghaly 1
Upper Egypt Electricity production Company
Assiut thermal power plant
300MW Unit
‫اﻟﻜﯿﻤﯿﺎﺋﯿﺔ‬ ‫اﻟﻌﺎﻣﺔ‬ ‫اﻹدارة‬
‫اﻟﻤﻌﺎﻟﺠﺔ‬ ‫إدارة‬
Chemist / Mohamade Abd Elmenem Farghaly
Water quality control system

2. Chemist / Mohamad Abd Elmenem Farghaly 2
Oxygen Corrosion
Basic Concept : - On the metal surface in solution , there exist places different
in electrical potential due to various causes : -
(1) Metal side : - A - Internal stress .
B - Temperature gradient .
C – Surface condition of metal .
(2) Water side : - A – Dissolved oxygen concentration .
B – PH- value
C – Dissolved ions
D – Temperature gradient
E – Flow velocity
(3) Ions CL and So4 act as corrosion accelerator .
* As a result of this potential difference , a local short circuit cell is formed . The
cell due to the difference in oxygen concentration in the solution is called an
oxygen concentration cell .
 This difference in oxygen concentration produce a potential difference
and yields as anode and cathode in the following way : -
Anode : Fe → Fe+2
+ 2e-
Cathode : 2H+
+ ½O2 + 2e-
→ H2O
- This iron dissolution at anode is corrosion .
- The anodic reaction is rapid but the cathodic reaction is slow .
- Dissolved ferrous iron (Fe+2
) through anodic reaction forms ferrous
hydroxide Fe(OH)2 which converted into ferric hydroxide Fe(OH)3 . when
dissolved oxygen exists in the water .
- Ferric hydroxide Fe(OH)3 converted into Fe2O3.H2O ( red rust ) where
dissolved oxygen content in the solution .
- Fe3O4.H2O ( black colored magnetite ) formed where dissolved oxygen
content is low in the solution .
 When dissolved oxygen is very high or the solution is very acidic ,
corrosion will take place at a higher rate .
 When dissolved oxygen is relatively high and solution is alkaline , pitting
corrosion or water droplet exist locally on metal surface .
Metal
H+
Cathode
Fe+2
Fe+2
Fe(OH)2 → Fe(OH)3
Cathode
Water
H+
Anode
Electric current flow
e-
Fe
O2
Metalic
Oxide filme
Water
+OH
│
Fe+2
│
Fe(OH)2
↓
Fe(OH)3
Precipitation
H+
H+
H+

3. Chemist / Mohamad Abd Elmenem Farghaly 3
 Oxygen corrosion found on boiler tube is pitting corrosion
when dissolved oxygen control in feed – water is inadequate . This
corrosion proceeds locally and might finally the tube . so pitting corrosion
is the worst type .
Caution in operation and system application : -
1 – performance of a condenser and dearator should periodically checked .
2 – Hydrazine should be injected into condensate line and PH of feed water
should be controlled .
** reaction between dissolved oxygen , hydrogen and iron oxide in the cycle.
4Fe3O4 + O2 → 6Fe2O3
magnetite red rust
6Fe2O3 + N2H4 → 4Fe3O4 + H2O + N2
 Oxygen depolarization rate at the cathode is decreased by the presence of
ferrous hydroxide Fe(OH)2 formed on the metal surface .
 When PH-value of water is high the solubility of ferrous hydroxide
Fe(OH)2 is reduced and protective hard film is formed on the iron surface
, this film resist dissolved oxygen from contacting with base metal . Thus
corrosion rate is decreased .
 Hydrazine reacts with dissolved oxygen and ferric oxide (red rust )
(Fe2O3) , which produce a harmless products for boiler . At high
temperature ( more than 200c ) the hydrazine is decomposed into
ammonia (NH3 ) .
1- N2H4 + O2 → N2 + 2H2O
-This reaction is recognized as chemical deaeration which applied in boiler
system as a supplemental measure for mechanical deaeration by dearator .
2- 6Fe2O3 + N2H4 → 4Fe3O4 + 2H2O + N2
- This reaction is recognized as corrosion inhibition which plays an important
role in the lay up period of boiler .
3- 5N2H4 → 2N2 + 6NH3 + H2
- This reaction proceed in the evaporation circuit of a boiler and ammonia is
produced . Produced ammonia raises the PH-value which called ( Volatile
treatment ) .

4. Chemist / Mohamad Abd Elmenem Farghaly 4
Ammonia Attack
Basic concept : -
Ammonia attack is found on condenser tube . Which leads to condenser
leakage . Copper is ionized through oxidation reaction and react with ammonia
forming complex salt .
Cu + O2 → Cu+
or Cu++
Cu++
+ NH3 → Cu(NH)4
++
 The oxygen depolarization rate at the cathode is decreased by the
presence of ferrous hydroxide Fe(OH)2 formed on the metal surface .
 When PH-Value of water is high the solubility of ferrous hydroxide
Fe(OH)2 is reduced and protective hard film is formed on the iron surface
this film resist dissolved oxygen from contacting with the base metal .
Thus corrosion rate is greatly decreased .
 Excessive dosing of chemicals for PH-Value control will cause the
dissolution of copper ( ammonia attack ) .
Cu + 4NH3 + ½O2 + H2O → Cu(NH3)4(OH)2
** Countermeasures against ammonia attack : -
1 – Make sure that air ( oxygen ) does not enter into condenser .
2 – Plan the construction of the condenser .
3 – Water treatment should be made so that PH-Value of steam is less than
(9.5) at 25Cº
.

5. Chemist / Mohamad Abd Elmenem Farghaly 5
Caustic Corrosion – (High – PH Damage )
Basic concept : -
Caustic corrosion is caused by free caustic (NaOH) concentration known
as " High–PH damage" .This concentrated caustic corrodes iron as following : -
Fe + 2 NaOH → Na2FeO2 + H2↑
3Na2FeO2 + 4H2O → 6NaOH + Fe3O4 + H2
 The temperature of the fluid touching to a tube surface is superheated and
if a local spot exists along tube , superheating degree might reach to 30 Cº
and more , where free caustic will be considerably concentrated and it
corrodes iron easily.
** Possible causes of caring caustic (acid) into boiler : -
(1) – Condenser leakage : - Raw water cooling that leaks into condenser
ends up in the boiler water .
# Notes : - ☼ If fresh water from lakes or rivers which provides dissolved
solid that hydrolyze in boiler –water to form caustic compound
such as (NaOH) .
☼☼ If fresh water from sea – water and water from recirculation
cooling water system this contain dissolved solids that
hydrolyze to form acidic compounds .
(2) – Make – up deminerlaizer in a source of acidic or caustic contaminants
regenerate chemical used in make – up demineralizer such as HCL and
NaOH may inadvertently enter the feed water system .
(3) – Chemicals incorrectly applied during water treatment also can be
corrosive .
** Cautions in operation : -
◘ It is expected that caustic corrosion occurs when the following conditions
are present : -
1 – Free caustic exists in boiler water .
2 – The hot spot exists in boiler where boiler water is concentrated .
3 – The concentrated boiler water stagnates .
◘◘ It is difficult to eliminate the hot spot in boiler , but free caustic in boiler
is easily eliminated by proper control of water chemistry and this control
gives a practical solution .
◘◘◘ To eliminate caustic corrosion in boiler , molar ratio Na/Po4 in boiler
water should be kept 2.8 and less when phosphate treatment is applied .
◘◘◘◘ It is recommended to inject one part of Di-Sodium phosphate and one
part of Tri-Sodium phosphate in combination into the boiler water , which
result in the molar ratio Na/Po4 = 2.5 in boiler water .
◘◘◘◘◘ Free caustic is neutralized with Di-Sodium phosphate through the
following reaction :-
NaOH + Na2HPo4 → Na3Po4 + H2O

6. Chemist / Mohamad Abd Elmenem Farghaly 6
Hydrogen damage ( Low – PH damage )
 Basic concept : -
In case that relatively dense deposit exists on the inner surface
boiler tube and PH of boiler water is too low , the metal iron is corroded
rabidly under the said deposit and nascent hydrogen is evolved as a result
of cathodic reaction . This nascent hydrogen diffuse into steel and results
in less of strength and ductility of steel .
Fe → Fe+2
+ 2e-
4H + C → CH4
**‫اﻟﻜﺮﺑﻮن‬‫إﻟﻰ‬ ‫ﯾﺘﺤﻮل‬ ‫وﻋﻨﺪﻣﺎ‬ ‫وﻗﻮي‬ ‫ﺻﻠﺐ‬ ‫اﻟﺤﺪﯾﺪ‬ ‫ﯾﺠﻌﻞ‬( CH4 )‫وﺿﻌﯿﻒ‬ ‫ھﺶ‬ ‫اﻟﺤﺪﯾﺪ‬ ‫ﯾﺠﻌﻞ‬.
☼ Hydrogen damage take place :-
1 – Presence of dense deposit .
2 – PH – decrease of boiler water .
◙ The relation between condensate chloride ion increase and boiler water PH
decrease . In case that chloride level in condensate reaches to 2 ppm qwing to
condenser leakage , the initial PH (9.0) , comes down less than 7 and the
possibility of dangerous damage increase .
☺ Countermeasure against sea water leakage in condenser : -
1 – Cation conductivity should be observed at the condenser outlet .
2 – PH and conductivity of boiler water should be observed .
3 – When condenser leakage is observed , injection quantity of sodium
phosphate to keep the PH – Value of boiler water as specified .
4 – Po4 concentration in boiler water monitored and check the consumption of
sodium phosphate .
5 – Increase the blowing down of boiler water to keep the boiler water quality .
6 – cation conductivity should be checked on each section of condenser hot well
to find the leaked part .
7 – When the PH – Value of boiler water is still less than (7) even though the
considerable amount of sodium phosphate is injected the unit shall be shut
down.
◊ Note : - The above mentioned countermeasures are introduced only for sea
water leakage in condenser .

7. Chemist / Mohamad Abd Elmenem Farghaly 7
Evaporation tube deposit trouble
 Mechanism of deposition : - As operating time of boiler increase , it will
increase the chance of tube failures due to overheating by deposit buildup
. The deposit consists of hardness component ( Calcium , Magnesium ) ,
Iron , Copper , Nickel .
 Hardness component : - When the hardness components in feed water is
high , white calcium carbonate is formed and deposited on a heating
surface through the following reaction : -
Ca(HCo)2 → CaCo3 + Co2 ↑ + H2o
- When ( Sea water ) leakage in condenser occurs , sodium phosphate is to
be injected for the purpose of boiler water quality , which form calcium
phosphate deposit Ca(Po4)2 .
- Iron copper nickel : - The suspended solids such as iron , copper and
nickel dissolved into the feed water in the preboiler system will be
deposited on a tube surface as an evaporation residue deposit .
☼ Over heat trouble due to deposit : -
The deposit laid on the evaporator tube of the drum type unit contains all
kinds of contaminant contained in the feed water . The deposit contains
Fe3O4 as a main component , and Cu , Zno , Nio . In same cases the deposit
contains 5 – 15 % Ca3(Po4)2 which is formed through the reaction of sodium
phosphate with the hardness component in sea water leaked in the condenser.
☻ In previous days : - Non-demineralized water was used as make – up
water . At that time the over heat trouble of boiler tube due to the high heat
resistance of ( Calcium carbonate ) deposit on tube surface .
☺ Nowadays : - The demineralized is used in the make-up water line and
the deposit on evaporator tube consists of mainly Fe3O4 , Cu , Zno Which are
not considered to be heat resistant .
☼ Overheat trouble of boiler tube due to deposit is caused for
the following reasons : -
{1} Soft and porous deposit of Fe3O4 is deposited on tube surface , steam and
eater are stagnated in the cavities of this deposit , which provides the heat
resistance and lead to the overheat trouble of the tube .
{2} When high amount of deposit is formed on a tube surface , chemistry
change of boiler water and rapid change of boiler water temperature might
cause the formation of cavity in deposit . Stagnated steam and water in boiler
cavity provides the heat resistance and leads to the overheat trouble of the
tube .
{3} Alternation of feed water chemistry causes stratified composition of
deposit . Such deposit is portly flaked of where high amount of Cu , Zno ,
Cao are localized . Film boiling will occur at the port where the deposit is
flaked off and the tube will be overheated .
{4} Cavities which exist between the base metal and deposit remained
through chemical cleaning , hot spot is formed . When tube is subject to heat

8. Chemist / Mohamad Abd Elmenem Farghaly 8
flux , the stagnated steam and water provides the heat resistance and leads
to overheat trouble on the tube .
« This trouble is mostly experienced in start-up period of the plant of the
chemical cleaning operation » .
☼ Countermeasures against the overheat trouble on evaporator
tubes due to deposits : -
[1] Make up water treatment : - The make up water treatment equipment
should be controlled through the monitoring of treated water . When something
abnormal is observed , it is required to check the regeneration process or ion
exchange resin for capacity it self .
[2] water treatment : - The proper water chemistry control on condensate and
feed water is essential to attain the clean and quality feed water and this can be
achieve by : -
(a) decrease the dissolved oxygen content in the dearator inlet .
(b) Injecting sufficient quantity of hydrazine " 2 moles and above of hydrazine
should be injected per 1 mol of dissolved oxygen in feed water " .
(N2H4 + O2 → N2 + 2H2o )
(c) Keeping (PH) value of condensate and feed water .(8.8 – 9.2 ) .
[3] Tube sampling and chemical cleaning : - Since the amount of deposit on
boiler tube increases with elapse of operating time thus we have to notes make
up water treatment and water quality of feed water . And the need of chemical
cleaning will become evident through the inspection on the sample tube .
☼ The sample tube is subject to inspection on : -
(1) Density and stat of deposit .
(2) Components and their distribution in deposit .
(3) The thickness of deposit .
☺The need of chemical cleaning on boiler is determined through the above
inspection results .
(( We notes that fuel is oil firing and unit pressure 180 kg /cm2
and
the thickness of deposit about ( 0.2 mm ) and quantity of deposit is
about (75 mg/cm2
) . If the deposit which formed on the sample
tube has this inspection result no doubt we have to make chemical
cleaning . ))
☼ In general the chemical cleaning operation every (2) years for supercritical
pressure unit and every (3) years for the drum type unit will be sufficient .
☼ It is to be noted that the interval of the chemical cleaning operation should
note exceed (5) years , because the deposit will be hardened and could not be
removed by normal chemical cleaning operation .
Thank you all
Chemist & Environmental Engineer
Mohamad Abd Elmenem Farghaly Helal
Assiut Thermal Power Station
Email / moha_hela@yahoo.com
Mobil / 0102369495

9. Chemist / Mohamad Abd Elmenem Farghaly 9