1.10 Corrosion

 Corrosion:

In nature every thing wants to attain stable state (low energy state). We spend lot of energy to reduce metal ore to pure metal. Metallica form is higher energy state and less stable than its ore. Therefore metal have a tendency to go back to its ore form  due to corrosion.

Fe => Fe++ + 2e-        Anode

4e- + O2 + H20  =>  4OH-   Cathode

Fe++  + 2OH-   => Fe(OH)2

Polarization:

Initially corrosion starts with higher rate but gradually corrosion product buildup at anode and cathode. The potential drift towards each other, slowly down the corrosion rate. This change in anode and cathode potential is called polarization.

The forms of corrosion:

1.      Uniform corrosion:

·       When loss of metal take place from entire surface.

·       Example - High temperature oxidation, rusting of iron, metal in contact with acid.

2.      Galvanic corrosion:

·       Potential difference between two metals.

·       Example – steel rivets (anodic) on  copper plate (cathode).

3.      Pitting:

·       When a small anode is surrounded by large cathode.

·       Example – stainless steel if exposed in chlorine environment. Chlorine ion break down the passive film and gives pitting.

4.      Crevice  corrosion:

·       Occurs in crevice or area which is shielded from the bulk environment.

·       This gives rise to differential concentrations cell.

5.      Inter granular corrosion:

·       Attacks at metal grain boundaries giving inter crystalline cracks.

·       Example – stainless steel improperly heat treated, some high strength aluminum alloy.

6.      Selective leaching:

·       One element of an alloy has preferentially dissolution.

·       Dezincification  of brass (less than  85% copper), graphite corrosion of cast iron.

7.      Erosion corrosion:

An oxide layer is formed on super heater tube surface due to high  temperature oxidation and then rate of corrosion decreases. High fluid velocity turbulence or high solid content in flue gas will erode this oxide layer. Then base metal will be exposed and again oxide layer will form.

8.      Stress corrosion cracking:

When a metal under stress exposed to a specific corrosion environment for that metal.

 

9.      Corrosion fatigue:

A metal under cyclic stress, if exposed to corrosion environment will definitely fail, whatever stress may be. This failure is due to corrosion fatigue.

How to combat corrosion:

1.      Change the metal.

2.      Use of suitable paint, protective coating etc.

3.      Change of design.

4.      Cathodic protection.

5.      Change of environment.

·       Eliminate oxygen => by proper mechanical deaeration  and chemical oxygen scavenger.

·       Removal of CO2 => CO2 form carbonic acid in water,  results in lowering the pH and acid corrosion. CO2 can be neutralized by suitable volatile alkaline chemicals like ammonia, morpholene, cyclohexyle ammines etc.

·       Maintaining proper pH => below pH 4.5 acid corrosion and above pH 13 alkaline corrosion.

·       Humidity => humidity makes the environment conducting.

·       Reduce liquid speed and temperature.

·       Inhibitor => they are inorganic and organic compounds, retards the corrosion rate by increasing the polarization rate.

Anodic inhibitor – chromate, nitrite, phosphate etc.

Cathodic inhibitor – arsenic, nickel, salt etc.

 

Corrosion and Deposition on fire side:

Inorganic matter in fossil fuels, coal and oil.

External corrosion:

1.      High temperature corrosion – presence of alkalies and appreciable amount of SO2, a liquid film of K3Fe(SO4)3 can cause catastrophic failure.

(I)               Corrosion of the water wall tube in the boiler furnace take place in the zone of flame core because of the attack of the tube metal by sulphurous gases present in the combustion product.

Corrosion area => in the tube section of the burner level or immediate above it.

Reason => pulverized coal anthracite, semi anthracite, lene coal having very low yield of volatiles and fuel oil with high sulphur content.

Fe + H2S  => FeS + H2

FeS + 2O2  => FeSO4

 

(II)            Super heater tube and their fastening elements experience damage of metal by the vanadium oxide present in the flue gases. Sodium vanadate (Na2.5V2O5.V2O4) – a low fusing compound ( MP ~600°C) forming a thin film of corrosive liquid on super heater tubes at 610 - 620 °C. This thin film attack carbon, low alloy and austenitic Steel.

Remedy => keeping superheated temperature below 600 °C, using alkaline additives to fuel oil (MgCl2).

 

2.      Low temperature corrosion – Dew point corrosion due to SO3. It occurs on the heating surface of the air preheater due to attack of the metal by sulphuric acid vapor.

SO3 in flue gas (ppm)	Dew point (°C)
0.1	104
1.0	120
10	135
100	155
1000	190

             – physical transport of fused or partially fused particles on walls and tube surfaces.

3.      High temperature bonded deposits – on super heater or reheater tubes above 600 °C.

4.      Alkali bonded deposits

5.      Sintered deposits

What is Dew Point?

It refers to the temperature at which moisture condenses on solid surface. It is also called thermodynamic dew temperature.

Dew point corrosion:

If the sulphuric acid vapor in partial pressure in flue gas is reached to its dew point, a film of highly corrosive sulphuric acid will settle on the tube surfaces and corrode the metal. This corrosion is called dew point corrosion. The dew point of sulphuric acid vapor is 140 – 160 °C (415 – 433 K).

How Sulphuric Acid vapor form in flue gas stream:

The sulphur present in fuel is oxidized to sulphurous anhydride during the combustion of fuel.

S     +    O2    <=>    SO2

The sulphur dioxide so produce is further oxidized to sulphur trioxide ( sulphuric anhydride) by atomic oxygen.

SO2   +   O   <=>   SO3

In the zones, where temperature drops below 500°C, the sulphur trioxide reacts with water vapor and produces sulphuric acid vapor which is carried off by gas stream.

SO3   +   H2O    <=>   H2SO4

 

Rate of corrosion Vs Temperature Curve:

Preventing due point corrosion during boiler shut down:

When the temperature inside the boiler drop below the dew point during shut down, moisture condenses on the tube surfaces and react with sulphurous ash deposits, forming a highly localized acid solution (pH <2). This is followed by rapid attack of the underlying metal surfaces. As a consequence the tube fails relatively short time.

 Following steps can be adopted to prevent due point corrosion while boiler is shut down.

• 1.      Remove ash, soot and other deposits from fire side tube and tube sheet surface immediately after boiler shut down.
• 2.      Flush ash free surfaces using water jet and then air dry.
• 3.      Coat fire side tube and tube sheet surfaces with light grade oil to prevent rusting.
• 4.      Stack a tray of quick lime in the ash pit during shut down to help absorb moisture in the air, lower the relative humidity thereby minimizing the probability of condensate forming. Make periodic checks. Replace the lime if it is no longer dry and powdery.