Posted by : sanjay swain Saturday, October 26, 2013

CORROSION FOUND IN BOILER AND FEED WATER SYSTEM


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)

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

General Wastage

Common in boilers having an open feed system.

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Pitting
-Most serious form of corrosion on the waterside

-Often found in boiler shell at w.l.

-Usually due to poor shape

-In HP blrs found also in screen and generating tubes and in suphtr tubes after priming.

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)

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

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.

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.



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.

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.

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 )

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 )

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.  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

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

Ammonium corrosion
Ammonium formed by the decompositin of hydrazine
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,

The copper travels to the boiler and leads to piting.

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