Propulsion

Corrosion  Types

The majority of  metals are found in a mineralise  oxidised ore state, as oxides, chlorides, carbonates, sulphates, sulphides etc. The metal is extracted form the ore by a reduction process resulting in a high energy state. The metal will then tend to restore  back to the low energy level through interaction with environment and oxidation.


Certain metals form a stable oxide layer which acts to inhibit further corrosion. Others form a much more porous less stable  oxide layer typically have much higher  levels of  diminution ( material loss).


Iron has a number of different oxides depending on the amount of Oxygen present. Magnetite with a low oxygen content is very stable and tenacious and provides a good protective barrier. Haematite has a higher oxide content and tends to be porous and less stable.




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)

Valent Bonds

For the purpose of understanding corrosion atoms can be imagine to consist of a nucleus containing positively charge Protons and neutral Neutrons surrounded by  negatively electrons. These electrons orbit  in distinct discrete layers each of which must be filled before the next highest layer can contain and electron.


The numbers of electrons that must appear in each of these shells is well understood. For example the first layer must have  2 electrons, the second layer 8.


Hydrogen has  one protons and a single electron and is therefore  neutrally charged. But requires an additional ‘Valence’ electron to fill the shell


Oxygen has 8 Protons  and 8 electrons, the first  Shell requiring 2 electrons is therefore full but the second Shell ,requiring 8 electrons is not full as there are only 6 remaining electrons. It therefore requires 2  Valence electrons to complete this shell.


Atoms can share there electrons with other atoms in co-valent bonds. A water molecule  consists on 2 Hydrogen  atoms  with 2 covalent electron sharing bonds with an Oxygen atom.

Atoms may share more than one electrons in multiple bonds

Principle type of Corrosion

Ionic Bonds

Ionic  bond are relatively week bonds were  an atom hands over an electronic to another electron making it  positively charge due to the excess of protons and the receiving atom negatively charged due to the excess of electrons. A typical example of this  is Sodium Chloride Salt

In water a small portion of the Water molecules and split with the electron from one of the hydrogen atoms  being shed to the remaining Hydoxide molecule. The hydrogen ion with one proton but no electrons is positively charged and the Hydroxide ion with 9 protons (8 in oxygen and 1 in hydrogen) and 8 electrons is negatively charged.


The positively charged hydrogen ion will seek to draw and electron to form mono-atomic hydrogen. The mono-atomic hydrogen is highly reactive and will react with anything available including other mono-atomic hydrogen to form the stable diatomic form of Hydrogen.

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.


The Ferrous Hydroxide formed dissolves in the water and therefore there is general wastage

Oxygen Absorption ( High Oxygen Corrosion)

pH between 6- 10, Oxygen present. Leads to pitting. Very troublesome and can be due to ineffective feed treatment prevalent in idle boilers. A scab forms of Ferrous Oxide and  Ferrous Hydroxide which the Hydroxyl ions can penetrate.


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.


 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

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 O
2

The three causes of unprotected metal surfaces are

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.

General Wastage

Common in boilers having an open feed system


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

Corrosion fatigue cracking occurs  were there is a combination of cyclical  loading and stressing within a corrosive environment.

Cases are found in areas associated to restraint or changes of section such as tube mounting arrangements as well as  as a result of aggressive load changes and in particular due to incorrect start up procedures from cold. Rapid re-starting with water thermal stratification  within the Tubes.


Corrosive environment aggravates through removal of protective magnetite layers.


Failure identification is due to thick edge most commonly in the form of Pin homes but also as cracks. Catastrophic rupture may occur in the region or the membrane  tube welds.


Common in wall and superheater 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 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 sootblower lines where vibration stresses are developed , and in blowdown lines.


Cracks are Trans-Crystalline ( trans granular)

Caustic Cracking (embrittlement) or Stress Corrosion Cracking


Steel is formed iron ferrite grains bounded by cementite ( iron carbide). Stress Corrosion Cracking occurs  when there is a high pH environment and sufficient tensile stress.


It was a common failure on older riveted boilers where localised leakages, thermal gradients etc caused increased local densities of water  the Sodium Hydroxide used as the water treatment.


Modern water treatments limiting, or eliminating the use of NaOH and improved control of pH water  has all but been eliminated this as an issue.

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.


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



Generally confined to

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 are often filled with corrosion products that sometimes contain sparkling crystals of magnetite.


Iron oxides being amphoteric (reactive with acids and base chemicals) 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;

Departure form nucleate boiling (DNB)

Deposition

Evapouration


Departure from Nucleate Boiling (DNB)

Under normal conditions steam bubbles are formed in discretely. Boiler water solids develop near the surface of these bubbles and concentrate at their edges.  As the Bubble lifts from the surface of the metal water inrushes  and clears the solids before they can accumulate and stick to the metal.


However at increased steam generation the rate of bubble formation may exceed the flow of rinsing water , and at higher still rate, a stable film may occur with solid 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. A similar effect may be seen in Vacuum water makers and appear as rings of scale formations


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


 Evaporation at waterline
Where a waterline exists corrosives may concentrate at this point by evaporation and corrosion occurs.



Prevention

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 ;



High temperature corrosion.

Loss of circulation , high temperature in steam atmosphere, or externally on suphtr tubes


Chelant corrosion

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 )





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.


Deposits and scales found in boilers

Definition: material originating elsewhere and conveyed to deposition site; Oxides formed at the site are not deposits.


Water formed and steam formed deposits


Water and steam drums can contain deposits, as these are readily accessed then inspection of the deposition can indicate types of corrosion. e.g. Sparkling black magnetite can precipitate in stm drums when iron is released by decomposition of organic complexing agents.


DEPOSITS

Iron oxides

Magnetite (Fe3O4)
A smooth black tenacious , dense magnetite layer normally grows on boiler water side surfaces. taken to indicate good corrosion protection as it forms in low oxygen levels and is susceptible to acidic attack

Heamatite (Fe2O3)
is favoured at low temperatures and high oxygen levels can be red and is a binding agent and tends to hold over materials in deposition. This is an indication of active corrosion occurring within the boiler/feed system



Other metals

Copper and Copper oxide is deposited by direct exchange with iron or by reduction of copper oxide by hydrogen evolved during corrosion . Reddish stains of copper are common at or near areas of caustic corrosion. Copper Oxide appears as a black deposits. It is considered very serious corrosion risk because of the initiation of galvanic corrosion mechanisms.

Galvanic corrosion associated with copper deposition is very rare in a well passivated boiler. Zinc and nickel are very often found near copper deposition , nickel being a particularly tenacious binder

Rapid loss of boiler metals can occur. Copper can appear in various forms as a deposit in the boiler. As a copper coloured metallic deposit, usually in a corrosion pit, as a bright red/orange tubercules on the boiler metal surface or as a brown tear drop shaped formation.

Copper is generally an indicator of corrosion (or possible wear) occurring in the feed pump whether in the condensate lines or in the parts of a feed pump. A possible cause of this is the excessive treatment of hydrazine which decompose to ammonia carrying over with the steam to attack such areas as the air ejectors on condensers.

Copper oxide formed in boiler conditions is black and non- metallic.



SALTS


The least soluble salts deposit first

Calcium carbonate-effervesces when exposed to HCl acid

Calcium sulphate-Slightly less friable then CaCO3

Magnesium Phosphate-Tenacious binder, discoloured by contaminants

Silicates-Insoluble except in hydroflouric acid E.G. Analcite


Water soluble deposits can only be retained if local concentration mechanism is severe. Presence of NaOH , NaPO3 Na2SO3 should be considered proof of vapouration to dryness.


Calcium and magnesium salts exhibit inverse solubility. As the water temperature rises their solubility reduces, at a temperature of 70'C and above they come out of solution and begin to deposit. Feed water must be condition to remove the hardness salts before the water enters the boiler. The purity of the water is related to the steam conditions required of the boiler.





Hydrolyzable salts such as MgCl can concentrate in porous deposits and hydrolyze to hydrochloric acid

Scaling mechanism examples

Calcium Carbonate
Cacium Carbonate is formed by the thermal decomposition of Calcium BiCarbonate and apperas as a pale cream to yellow scale

Ca(HCO3)2 + Heat = CaCO3 + H2O + CO2

Magnessium Silicate
Tor form requires sufficient amounts of magnessium and silicate ions coupled with a deficiency in OH
- alkalinity

Mg2+ + OH- = MgOH+

H2SiO3 = H+ + HSiO3-

MgOH- + HSiO3- = MgSiO3 + H2SO4

Thus this rough tan scale can be prevented by the maintenace of alkalinity levels

Calcium Phosphate (hydroxyapatite)
Ca
10(PO4)6(OH)2

Found in biolers using the phosphate cycle treatment method this is a tan/cream deposit. This is generally associated with overdosing a boiler but can occur where insufficient disperseing agent reduces the effects of blow down.

In anouther form Ca3(PO4)2Ca(OH)2 it is associated with correct treatment control

Scales forming salts found in the boiler

Calcium Bi-Carbonate 180ppm

Magnesium BiCarbonate 150 ppm

Calcium Sulphate 1200 ppm

Magnesium Sulphate 1900ppm

Magnesium Chloride 3200ppm

Sodium Chloride 32230 to 25600 ppm

Other deposits-

Amorphous Silicon dioxide (SiO2) - trace

Magnessium Silicate 3MgO.2SiO2.2H2O (Serpentine) is formed in water with proper treatment control



Dissolved solids in fresh water


Hard water  

-Calcium and magnesium salts  

 - Alkaline  

 -Scale forming  

 .  

 .  

Soft water  

-Mainly sodium salts  

 - Acidic  

 - Causes corrosion rather than scale