Corrosion

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Corrosion found in the Boiler and feed systems
Corrosion and tube failure caused by water chemistry
Metals obtained from their oxide ores will tend to revert to that state. However, if on
exposure to oxygen the oxide layer is stable, no further oxidation will occur. If it is
porous or unstable then no protection is afforded.

(porous)

Iron+O2 --- magnetite (stable and protective) + O2----ferrous oxide

Two principle types of corrosion
Direct chemical-higher temperature metal comes into contact with air or other
gasses (oxidation, Sulphurisation) Electrochemical-e.g. Galvanic action, hydrogen
evolution, oxygen absorption

Hydrogen Evolution (low pH attack)

Valency = No of electrons required to fill outer shell

Pure water contains equal amounts of hydrogen and hydroxyl ions.
Impurities change the balance. Acidic water has an excess of hydrogen ions which
leads to hydrogen evolution

For hydrogen absorption to occur no oxygen needs to be present, a pH
of less than 6.5 and so an excess of free hydrogen ions is required.
The Protective film of hydrogen gas on the cathodic surface breaks down as the
hydrogen combines and bubbles off as diatomic hydrogen gas.

Oxygen Absorption (high O2 corrosion)

pH between 6- 10, Oxygen present. Leads to pitting. Very troublesome
and can be due to ineffective feed treatment prevalent in idle boilers. Once started
this type of corrosion cannot be stopped until the rust scab is removed, either by
mechanical means or by acid cleaning. One special type is called deposit attack, the
area under a deposit being deprived of oxygen become anodic. More common in
horizontal than vertical tubing and often associated with condensers.

Boiler corrosion
Common in boilers having an open feed system.

General Wastage

.
.
-Most serious form of corrosion on the waterside

Pitting

-Often found in boiler shell at w.l.
-Usually due to poor shape
-In HP boilers found also in screen and generating tubes
and in superheater tubes after priming.
Corrosion fatigue cracking

Cases found in water tube boilers where due to alternating cyclic stresses set up in
tube material leading to a series of fine cracks in wall. Corrosive environment
aggravates. Trans crystalline
more in depth: Occurs in any location where cyclic stressing of sufficient
magnitude are present
Rapid start up and shut down can greatly increase susceptibility.
Common in wall and superheater tubes, end of the membrane on water
wall tubes, economizers, desecrators. Also common on areas of rigid constraint such
as connections to inlet and outlet headers
Other possible locations and causes are in grooves along partially full
boiler tubes (cracks normally lie at right angle to groove ), at points of intermittent
steam blanketing within generating tubes, at oxygen pits in waterline or feed water
lines, in welds at slag pockets or points of incomplete fusion , in soot blower lines
where vibration stresses are developed , and in blow down lines.

Caustic cracking (embrittlement) or stress corrosion cracking

Pure iron grains bound by cementite (iron carbide).
Occurs when a specific corrodent and sufficient tensile stress exists
Due to improved water treatment caustic stress- Corrosion cracking (or
caustic embrittlement) has all but been eliminated.
It can however be found in water tubes, superheater and reheat tubes
and
in
stressed
components
of
the
water
drum.
The required stress may be applied ( e.g. thermal, bending etc. ) or residual ( e.g.
welding)
Boiler steel is sensitive to Na OH , stainless steel is sensitive to NaOH and chlorides
A large scale attack on the material is not normal and indeed uncommon. The
combination of NaOH , some soluble silica and a tensile stress is all that is required
to form the characteristic inter-granular cracks in carbon steel.
Concentrations of the corrodent may build up in a similar way to those caustic corrosion
i.e.





DNB
Deposition
Evaporation at water line
And also by small leakage
Caustic corrosion at temperatures less than 149oC is rare

NaOH concentration may be as low as 5% but increased susceptibility
occurs in the range 20- 40 %
Failure is of the thick walled type regardless of ductility.
Whitish highly alkaline deposits or sparkling magnetite may indicate a
corrosion sight.

To eliminate this problem either the stresses can be removed or the
corrodent. The stresses may be hoop stress( temp', pressure) which cannot be
avoided bending or residual weld stresses which must be removed in the design/
manufacturing stage.
Avoidance of the concentrations of the corrodents is generally the most
successful. Avoid DNB , avoid undue deposits prevent leakage of corrodents, prevent
carryover.
Proper water treatment is essential.

Caustic corrosion



Takes place at high pressure due to excessive NaOH
In high temperature, high evaporation rates leading to local concentrations
nearly coming out of solution and form a thin film near heating surface.
Magnetite layer broken down
Soluble compound formed which deposits on metal as a porous oxide
Local concentrations may cause a significant overall reduction in alkalinity.
If evaporation rate reduced alkalinity restored.






More in depth:
Generally confined to
1.
2.
3.
4.

Water cooled in regions of high heat flux
Slanted or horizontal tubes
Beneath heavy deposits
Adjacent to devices that disrupt flow ( e.g. backing rings)

Caustic ( or ductile ) gouging refers to the corrosive interaction of
concentrated NaOH with a metal to produce distinct hemispherical or elliptical
depressions.
Depression is often filled with corrosion products that sometimes contain
sparkling crystals of magnetite.
Iron oxides being amphoteric are susceptible to corrosion by both high
and low pH environments.

High pH substances such as NaOH dissolve the magnetite then attack

the iron.

The two factors required to cause caustic corrosion are;



the availability of NaOH or of alkaline producing salts. ( e.g. intentional by
water treatment or unintentional by ion exchange resin regeneration.)
Method of concentration, i.e. one of the following;
i.
Departure form nucleate boiling (DNB)
ii.
Deposition
iii.
Evaporation

i)
Departure
form
nucleate
boiling
(DNB)
Under normal conditions steam bubbles are formed in discrete parts. Boiler water
solids develop near the surface . However on departure of the bubble rinsing water
flows in and re-dissolves the soluble solids

However at increased rates the rate of bubble formation may exceed the
flow of rinsing water , and at higher still rate, a stable film may occur with corrosion
concentrations
at
the
edge
of
this
blanket.
The
magnetite
layer
is
then
attacked
leading
to
metal
loss.
The area under the film may be relatively intact.

ii),
Deposition
A similar situation can occur beneath layers of heavy deposition where bubbles
formation occur but the corrosive residue is protected from the bulk water
iii)
Evaporation
at
waterline
Where a waterline exists corrosives may concentrate at this point by evaporation and
corrosion occurs.

prevention's








Rifling is sometimes fitted to prevent DNB by inducing water swirl.
Reduce free NaOH by correct water treatment
Prevent inadvertent release of NaOH into system (say from an ion exchange column
regenerator )
Prevent leakage of alkaline salts via condenser
Prevent DNB
Prevent excessive waterside deposits
Prevent creation of waterlines in tubes- slanted or horizontal tubes are particularly
susceptible to this at light loads were low water flows allow steam water stratification.

Hydrogen attack
If the magnetite layer is broken down by corrosive action, high temperature
hydrogen atoms diffuse into the metal, combine with the carbon and form methane.
Large CH-3 molecules causes internal stress and cracking along crystal boundaries
and sharp sided pits or cracks in tubes appear.
more in depth: Generally confined to internal surfaces of water carrying
tubes that are actively corroding. Usually occurs in regions of high heat flux, beneath
heavy deposits, in slanted and horizontal tubes and in heat regions at or adjacent to
backing rings at welds or near devices that disrupt flow .

Uncommon in boilers with a W.P. of less than 70 bar
A typical sequence would be ;








NaOH removes the magnetite
free hydrogen is formed ( hydrogen in its atomic rather than diatomic state) by either the
reaction of water with the iron reforming the magnetite or by NaOH reacting with the iron
This free hydrogen can diffuse into the steel where it combines at the grain boundaries to
form molecular hydrogen or reacts with the iron carbide to form methane
As neither molecular hydrogen or methane can diffuse through the steel the gasses build
up , increasing pressure and leading to failure at the grain boundaries
These micro cracks accumulate reducing tensile stress and leading to a thick walled
failure. Sections may be blown out.
This form of damage may also occur in regions of low pH
For boilers operating above 70 bar , where high pH corrosion has occurred the possibility
of hydrogen damage should be considered

High temperature corrosion.
Loss of circulation , high temperature in steam atmosphere, or externally on
superheater tubes

Chelant corrosion
Concentrated chelants ( i.e. amines and other protecting chemicals) can attack
magnetite
,
steam
drum
internals
most
susceptible.
A surface under attack is free of deposits and corrosion products , it may be very
smooth
and
coated
with
a
glassy
black
like
substance
Horse shoe shaped contours with comet tails in the direction of the flow may be
present.
Alternately deep discrete isolated pits may occur depending on the flow
and turbulence
The main concentrating mechanism is evaporation and hence DNB
should be avoided
Careful watch on reserves and O2 prescience should be maintained

Low pH attack
Pure water contains equal amounts of hydrogen and hydroxyl ions . Impurities
change the balance . Acidic water has an excess of hydrogen ions which leads to
hydrogen evolution. See previous notes on Hydrogen Evolution
For hydrogen absorption to occur no oxygen needs to be present, a pH
of less than 6.5 and so an excess of free hydrogen ions is required.
The Protective film of hydrogen gas on the cathodic surface breaks down as the
hydrogen
combines
and
bubbles
off
as
diatomic
hydrogen
gas.
May occur due to heavy salt water contamination or by acids leaching into the
system from a demineralisation regeneration.

Localized attack may occur however where evaporation causes the
concentration of acid forming salts . The mechanism is the same as for caustic attack.
The corrosion is of a similar appearance to caustic gouging
Prevention is the same as for caustic attack . Proper maintenance of
boiler water chemicals is essential
Vigorous acid attack may occur following chemical cleaning .
Distinguished from other forms of pitting by its being found on all exposed areas
Very careful monitoring whilst chemical cleaning with the temperature being
maintained below the inhibitor breakdown point. Constant testing of dissolved iron
and non ferrous content in the cleaning solution should be carried out.
After acid cleaning a chelating agent such as phosphoric acid as
sometimes used . This helps to prevent surface rusting , The boiler is then flushed
with warm water until a neutral solution is obtained.

Oxygen corrosion
Uncommon in operating boilers but may be found in idle boilers.
Entire boiler susceptible , but most common in the superheater tubes (reheater
tubes especially where water accumulates in bends and sags )
In an operating boiler firstly the economizer and feed heater are effected.
In the event of severe contamination of oxygen areas such as the steam
drum water line and the steam separation equipment
In all cases considerable damage can occur even if the period of oxygen
contamination is short
Bare steel coming into contact with oxygenated water will tend to form
magnetite
with
a
sound
chemical
water
treatment
program.
However , in areas where water may accumulate then any trace oxygen is dissolved
into the water and corrosion by oxygen absorption occurs( see previous explanation )

Oxygen Absorption
in addition to notes above pH between 6- 10, Oxygen present.
Leads to pitting. Very troublesome and can be due to ineffective feed treatment
prevalent in idle boilers. Once started this type of corrosion cannot be stopped until
the rust scab is removed , either by mechanical means or by acid cleaning.
One special type is called pitting were metal below deposits being
deprived of oxygen become anodic . More common in horizontal than vertical tubing
and often associated with condensers.
The ensuing pitting not only causes trouble due to the material loss but
also acts as a stress raiser
The three critical factors are

i.
ii.
iii.

the prescience of water or moisture
prescience of dissolved oxygen
unprotected metal surface

The corrosiveness of the water increases with temperature and dissolved
solids
and
decreases
with
increased
pH
Aggressiveness generally increases with increased O2
The three causes of unprotected metal surfaces are
i.
ii.
iii.

following acid cleaning
surface covered by a marginally or non protective iron oxide such as
Hematite (Fe2O3)
The metal surface is covered with a protective iron oxide such as magnetite
(Fe3O4 , black) But holidays or cracks exist in the coating, this may be due
to mechanical or thermal stressing.

During normal operation the environment favours rapid repair of these
cracks. However, with high O2 prescience then corrosion may commence before the
crack is adequately repaired.

FEED SYSTEM CORROSION.
Graphitization
Cast iron , ferrous materials corrode leaving a soft matrix structure of carbon flakes

Dezincification
Brass with a high zinc content in contact with sea water , corrodes and the copper is
redeposited. Inhibitors such as arsenic , antimony or phosphorus can be used , but
are
ineffective
at
higher
temperatures.
Tin has some improving effects

Exfoliation (denickelfication)
Normally occurs in feed heaters with a cupro-nickel tubing ( temp 205oC or higher)
Very
low
sea
water
flow
condensers
also
susceptible.
Nickel oxidized forming layers of copper and nickel oxide

Ammonium corrosion
Ammonium formed by the decomposition of hydrazine
Dissolve cupric oxide formed on copper or copper alloy tubes
Does not attack copper, hence oxygen required to provide corrosion, Hence only
possible at the lower temperature regions where the hydrazine is less effective or
inactive,
The copper travels to the boiler and leads to pitting.

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