1) C)Define terms: Hardwater: A sample of water which does not produces lather readily with soap is called hard water.
Soft water: A sample of water which produces lather readily with soap is called soft water.
Priming: During the production of steam in the boiler, due to rapid boiling some particles of liquid water are carried along with steam. steam containing droplets of water is called wet steam. this process of wet steam is called priming.
Foaming: The formation of stable bubbles above the surface of water is called foaming.
2) A)explain various type of Dry corrosion with appropriate reactions.
ans:
(1) Corrosion due to oxygen
Alkali and alkaline earth metals react with oxygen at room
temperature. Other metals except Ag, Au, Pt react with oxygen
at elevated temperature.
2M- 2Mn+ + 2ne-
(n/2) O2 + 2ne- nO2-
2M + (n/2) O2 Mn+ + nO2- MO (Metal Oxide film)
Corrosion product
Further interaction depends on the nature of the oxide film
formed.
Stable film: If the oxide film is stable , it remains adhered to
the surface strongly. This prevents further penetration of oxygen
to the base metal. Egg. In case of oxides of Al, Pb, Cu etc.
Unstable film: If the oxide formed is unstable, it
dissociates back to metal and oxygen. Therefore
corrosion does not take place. E.g., Oxides of Ag, Au, Pt.
Volatile: If the oxide film is volatile, it gets volatized to
expose new surface to interact with oxygen. Here
corrosion continues to eat the whole metallic structure. Therefore corrosion is rapid and continuous, e.g., oxide
of molybdenum.
Porous: Here the oxide film has pores or channels
through which oxygen slowly diffuses to interact with
the base metal. Therefore in this case corrosion is slow
but continuous. E.g., Oxide of iron.
(2) Corrosion due to other corrosive gases:
The extent of corrosion depends on the chemical affinity
between the base metal and the atmospheric
components.
Fe + H2S —–> FeS + H2
Porous
2Ag + Cl2 —–>AgCl (Stable)
DRY CORROSION
(3) Liquid metal corrosion: This type of corrosion occurs
when a molten liquid is passed continuously on a solid
metal/alloy surface.
The corrosion is attributed to either dissolution of the
molten liquid into the solid metal phase.
OR
A)
B)What are the essential requirements of boiler feed water ? explain difference between sludge and scale.
ans: essential requirements for boiler feed water:
- Turbidity, oil and non-scaling dissolved salts to reduce the tendency for priming and foaming.
- Hardness-causing and scale-forming constituents like Ca and Mg salts as the formation of scale would result in wastage of fuel, loss in output and overheating of the boiler tubes leading to explosion.
- Dissolved oxygen and CO2 in order to prevent corrosion in the boiler.
- Caustic alkali to remove caustic embrittlement.
| sludge | scale |
| 1.sludges are soft and non-adherent deposits. | 1.scales are hard deposits which stick very firmly to the inner surface of the boiler. |
| 2.sludges can be removed easily. | 2.scales are very different to remove. |
| 3.sludges can transfer heat to some extent and are less dangerous. | 3.scales are bad conductors of heat and are more dangerous. |
| 4.sludges are formed by substances like Mgcl2 and Cacl2. | 4.scales are formed by substances like CaSO4 and Mg(OH)2. |
OR
B)Explain reverse osmosis method of water purification with suitable diagram and procedure.
ans: When two solutions of different concentrations are separated by a semi-permeable membrane, flow of solvent takes from a region of low concentration to high concentration until the concentration is equal on both the sides. This process is called osmosis. The driving force in this phenomenon is called osmotic pressure. If a hydrostatic pressure in excess of osmotic pressure is applied on the higher concentration side, the solvent flow reverses, i.e. solvent is forced to move from higher concentration to lower concentration (Figure 1.8). This is the principle of reverse osmosis. Thus in the reverse osmosis method pure water is separated from its dissolved solids.
Using this method pure water is separated from sea water. This process is also called super-filtration. The membranes used are cellulose acetate, cellulose butyrate, etc.
Q3) A)Derive Nernst equation for the oxidation reaction in detail.
ans:
For the reaction, aA+bB=cC+dD
Where,
A & B = reactants
C & D = products
a& b = number of moles of reactants
c& d = number of moles of product
• When 1 Faraday electricity is passed through the cell, the forward reaction becomes faster.
• According to thermodynamics, the work done in the electrochemical cell is related with decrease in the energy.
-ΔG = nFΔE ………………………….1
ΔG = Free energy change
n= the number of electrons transferred
F= Faraday constant = 96500 coulombs
ΔE= cell potential in volts When the electrochemical cell is in a standard state,
When the electrochemical cell is in a standard state
Where, ΔGo = standard free energy change
ΔEo = standard cell potential
• When a reaction attains equilibrium,
• According to VontHoff’s equation,
Where,
R = gas constant (8.314 J mol-1 K-1)
K = equilibrium constant
But for electrochemical cell
Substitute this value in equation no. 3, we get:
Dividing above equation both the sides by –nF,
OR
A)
B )Explain in detail the water line corrosion and stress corrosion.
ans: Water line corrosion:
WET CORROSION
When a steel tank is partially filled with water for a long time, the inner portion of the tank below the water line is exposed only to dissolve oxygen, whereas, the portion above the water line is exposed to more oxygen.
Thus the portion below the water line acts as anode and undergoes corrosion. The upper portion acts as cathode and is unaffected.
A distinct brown line is formed just below the water line due to the deposition of rust.
Other example:
Ships which remain partially immersed in sea water for a long
time undergo water line corrosion.
Stress corrosion: In a metallic structure, if there is a portion under stress, it will act as anode and rest part of the structure will act as cathode.
It is now a galvanic system and hence anodic part which is small in area will corrode more.
Example: Caustic embrittlement in boilers
It is a type of boiler corrosion which makes boiler material brittle.
This is caused by using highly alkaline water in the boiler, most commonly in high pressure boiler.
During lime soda process, free Na2CO3 is usually present in small proportion in the softened water.
Na2CO3 in high pressure boilers decomposes to give sodium hydroxide and carbon dioxide.
This makes boiler water caustic.
This causes embrittlement of boiler parts, particularly stressed parts like bends, joints etc.
The water containing NaOH flows into the minute hair-cracks, in the inner wall of boiler, by capillary action.
Here, water evaporates and the concentration of NaOH increases progressively.
When the concentration of NaOH increases to 10%, caustic soda attacks the surrounding areas, thereby dissolving iron of boiler wall as sodium-ferrate.
This causes embrittlement of boiler wall at stressed parts like bends, joints, etc.
C)