Saturday, October 26, 2013
CORROSION FOUND IN BOILER AND FEED WATER SYSTEM
CORROSION AND TUBE FAILURE CAUSED BY WATER CHEMISTRY
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)
CORROSION FATIGUE CRACKING
CAUSTIC CRACKING (EMBRITTLEMENT) or STRESS CORROSION CRACKING
Boiler steel is sensitive to Na OH , stainless steel is sensitive to NaOH and chlorides.
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 redissolves 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.
iii),
Evaporation at waterline
Where a waterline exists corrosives may concentrate at this point by evaporation and corrosion occurs.
Dissolve cupric oxide formed on copper or copper alloy tubes
Does not attack copper, hence oxygen required to provide corrosion,Hence only possibel at the lower temperature regions where the hydrazine is less effective or inactive,
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.
Iron+O2 --- magnetite(stable and protective) + O2----ferrous oxide (porous)
Iron+O2 --- magnetite(stable and protective) + O2----ferrous oxide (porous)
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
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General
Wastage
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Common in
boilers having an open feed system.
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Pitting
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-Most
serious form of corrosion on the waterside
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-Often
found in boiler shell at w.l.
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-Usually
due to poor shape
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-In HP
blrs found also in screen and generating tubes and in suphtr tubes after
priming.
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CORROSION FATIGUE CRACKING
Cases found in water
tube blrs 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 supht tubes, end of the membrane on waterwall tubes, economisers,
deaerators . 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 stm
blanketing within generating tubes, at oxygen pits in waterline or feed water
lines, in welds at slag pockets or points of incomplete fusion , in sootblower
lines where vibration stresses are developed , and in blowdown 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 , suphtr 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)
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 intergranular 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 are 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.
Water cooled in regions of high heat
flux
2.
Slanted or horizontal tubes
3.
Beneath heavy deposits
4.
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
are 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
enviroments.
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. Evapouration
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 redissolves the soluble solids.
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
A similar situation can occur beneath layers of heavy deposition where bubbles formation occur but the corrosive residue is protected from the bulk water
Where a waterline exists corrosives may concentrate at this point by evaporation and corrosion occurs.
PREVENTIONS
·
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 stm water stratification.
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
Loss of circulation , high temperature in
steam atmosphere, or externally on suphtr tubes
Concentrated chelants ( i,e. amines and
other protecting chemicals) can attack magnetite , stm 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.
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
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.
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.
Localised attack may occur however where evaporation causes the
concentration of acid forming salts . The mechanism are 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 )
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
economiser and feed heater are effected.
In the event of severe contamination of
oxygen areas such as the stm drum water line and the stm 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 )
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.
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. the prescience of water or moisture
ii. prescience of dissolved oxygen
iii. 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. following acid cleaning
ii. surface covered by a marginally or non protective iron oxide such
as Hematite (Fe2O3)
iii. 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 structur 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
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 oxidised forming layers of copper and nickel oxide
Very low sea water flow condensers also susceptible.
Nickel oxidised forming layers of copper and nickel oxide
Ammonium corrosion
Ammonium formed by the decompositin of hydrazineDissolve cupric oxide formed on copper or copper alloy tubes
Does not attack copper, hence oxygen required to provide corrosion,Hence only possibel at the lower temperature regions where the hydrazine is less effective or inactive,
The copper travels to the boiler and leads to piting.
