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How The Oxide Layer Deposit Is Formed In Various Heat Transfer Regions?

by ircengg on June 4, 2015

Oxide of high porosity (>50%) is found to deposit in drum as well as once-through boilers under both low and high oxygen water chemistry conditions. The deposition rate is approx. proportional to the concentration of particulate iron oxide and the square of the heat flux. The best approximation to the real situation is given by

D = k q2 c t

Where,
D = amount of magnetite deposited (kg/m2)
q = the heat flux (W/m2)
c = concentration of iron in water (kg/m3)
t = time (hour)
k = constant ( approx. 5 X 10-13 / W2 m2/s

In a wick boiling mechanism, salts dissolved in the boiler water can be concentrated by factors > 104 as shown in the figure below.

Generally, the protective magnetite scale thickness is 10-15 microns in the waterwall tube. When the corrosion rate increases due to upset of water chemistry parameters in boiler, (due to salt ingress and concentration), the deposit formation also increases due to corrosion of metal and precipitation of contaminants whose water solubility decreases at higher temperature on the evaporator tube surface. To maintain the pH in boiler water, in case of reduction of pH due to salt ingress, addition of more Tri Sodium Phosphate (TSP) is required. In this process, at some places on the internal surface of waterwall tubes, deposit thickness increases and the protective iron oxide scale becomes non protective and porous in nature. Porous, insulating types of deposits allow boiler water to diffuse into the deposit where the water becomes trapped and boils.

The boiling of deposit in entrapped water produces relatively pure steam which tends to diffuse out of the deposit, leaving behind super heated non-boiling equilibrium solution of caustic, which is responsible for caustic corrosion or acidic solution, which is responsible of hydrogen damage in waterwall tubes as discussed below.

IRC is regularly carrying out such studies to find out the reason of the failures and are also giving remedial actions to prevent it, if required we can also carry out chemical cleaning of boiler.
IRC is a service provider having expertise in ndt, residual life assessment, fitness for service, advanced ndt, failure investigation, chemical cleaning, certification of storage tanks as per chief controller of explosives guidelines , consultancy for boiler water chemistry and training

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ircenggHow The Oxide Layer Deposit Is Formed In Various Heat Transfer Regions?

How Mild Steel Corrodes In Boiler Water?

by ircengg on June 4, 2015

Waterwall tubes in most of the fossil-fired boiler are made of carbon steel. In pure DM water, or in very dilute acid or alkaline solutions at boiler temperature, it normally corrodes very slowly to form the black iron oxide known as magnetite (Fe3O4). The overall reaction is:

3 Fe + 4 H2O ——-> Fe3O4 + 4 H2

The corrosion rate is dependent by the rate at which the reactant (water) can reach the metal surface and the reaction product can leave the surface. In nearly neutral solutions, magnetite is very slightly soluble and it deposits as coherent and tenacious surface film, which greatly impedes this two-way chemical traffic. The transport processes are dominated by slow state diffusion through the oxide layer and the corrosion rate is virtually independent of solution composition. The corrosion rate is also diminishes with time as the oxide thickness grows. Even after years of exposure, the layer is no more than a few microns thick.

In more alkaline or more acid solutions, magnetite becomes increasingly soluble and precipitates in a different physical form. Instead of yielding a strongly coherent film, it has a more porous structure. Soluble species can now diffuse relatively rapidly through the film and the corrosion rate is much faster, although it still falls off the time as the oxide accumulated.

In many countries there are mainly three types of Medical Laboratories as per the types of investigations carried out. 1. Clinical Pathology 2. Clinical Microbiology & 3. Clinical Biochemistry laboratories. 1. Clinical Pathology: Haematology, Histopathology, Cytology, Routine Pathology2. Clinical Microbiology: Bacteriology, Mycobacteriology, Virology, Mycology, Parasitology, Immunology, Serology. The deposition of salts and corrosion products observed in different waterwall tubes has shown in the photograph no.1-4.


Photograph no.1&2 showing waterwall tubes of 60 MW boilers having a very thick deposit ranging thickness 1.0-1.5 mm.

Photograph no.3 & 4 showing a waterwall tube of 200 & 110 MW boilers having a thick iron oxide as well as hardness salt deposition which is non uniform in nature.


IRC is regularly carrying out such studies to find out the reason of the failures and are also giving remedial actions to prevent it, if required we can also carry out chemical cleaning of boiler.

IRC is a service provider having expertise in ndt, residual life assessment, fitness for service, advanced ndt, failure investigation, chemical cleaning, certification of storage tanks as per chief controller of explosives guidelines , consultancy for boiler water chemistry and training

read more
ircenggHow Mild Steel Corrodes In Boiler Water?

How Caustic Corrosion And Acidic Corrosion Are Formed In Case Of Boilers?

by ircengg on June 4, 2015

Caustic Corrosion

If the salt concentrated under the deposit is having high pH due to concentration of caustic from TSP dosing, it start dissolution of protective magnetite (Fe3O4) layer on the evaporator tube wall inner surface and form sodium ferrite (NaFeO2) and sodium ferroate (Na2FeO2) as shown in the equation.

Fe3O4 + 4 NaOH ——-> NaFeO2 + Na2FeO2 + 2 H2O

ACIDIC CORROSION

Solution of low pH is generated in high pressure boilers in two different ways:

  1. pH of the entire boiler water is reduced when contaminants which are acidic or becomes acidic when heated in to the boiler.
  2. The bulk boiler water remains alkaline but acidic solutions are generated within corrosion pits by the action of dissolved oxygen and chloride. The most common acid forming contaminant is sea water or a river water which is low in carbonate and sulphate. In the boiler, the acidity is increased locally to corrosive concentrations by boiling.

In the acidic or highly alkaline conditions, iron reacts and hydrogen is liberated.

Fe + 2 NaOH = Na2FeO2 + H2

Fe + 2 HCl = FeCl2 + H2

If the hydrogen is liberated in an atomic form, it is capable of diffusing into the steel. Some of this diffused, atomic hydrogen will combine at metal grain boundaries or inclusions to produce molecular hydrogen, or it will react with iron carbides in the metal to produce methane.

Fe3C + 4 H = CH4 + 3 Fe

Because neither molecular hydrogen nor methane is capable of diffusing through the steel, these gases accumulate, primarily at grain boundaries. Eventually, the gas pressure created will cause separation of the metal at its grain boundaries, forming discontinuous, intergranular micro cracks as shown in the micrograph.

As these micro cracks accumulate, tube strength diminishes until stresses imposed by the internal pressure exceed the tensile strength of the remaining, intact metal. At this point a thick-walled, longitudinal burst may occur depending on the extent of hydrogen damage as shown in photographs below:

Photographs of 210 and 110 MW boilers, waterwall tubes show the failure due to hydrogen damage due to localised acidic condition.

IRC is regularly carrying out such studies to find out the reason of the failures and are also giving remedial actions to prevent it, if required we can also carry out chemical cleaning of boiler.
IRC is a service provider having expertise in ndt, residual life assessment, fitness for service, advanced ndt, failure investigation, chemical cleaning, certification of storage tanks as per chief controller of explosives guidelines , consultancy for boiler water chemistry and training

read more
ircenggHow Caustic Corrosion And Acidic Corrosion Are Formed In Case Of Boilers?