B.Tech. I year Notes

1. Corrosion and its control

2. Corrosion and its control
• Definition: The process of decay of metal by environmental
attack is called corrosion.
• Pure metals are extracted from their ores. Metals are used
to manufacturing different articles.
• These articles undergo corrosion, when they are exposed to
environment.

3. Disadvantages with corrosion:
1. Loss of useful metallic properties such as
Malleability, Ductility, Tensile strength and
Conductance.
2. Lifespan of machineries reduced.
3. Failure of machinery working.
4. Economic loss: it is estimated that the amount of
iron wasted by corrosion each year about 25% of its
annual world production.

4. 2016 Kanpur train-accident: Not
conspiracy, corrosion blamed for 152
deaths

5. Using TRIZ to Develop New Corrosion
Protection Concepts in Shipbuilding

7. CBS Gouliguda-Hyderabad

8. Types of corrosion /Theories of corrosion
• Corrosion is two types :
1. Dry corrosion (or) Chemical corrosion.
2. Wet corrosion (or) Electrochemical corrosion.
1.chemical corrosion : The direct chemical reaction
of environmental gases such as O2, Cl2, HCl,..etc. on
metal, in the absence of moisture is called dry
corrosion or chemical corrosion.
• Types chemical corrosion:
• 1. Corrosion due to oxygen.
• 2. Corrosion due to other gases.
• 3. Liquid metal corrosion.

9. Oxidation corrosion
• All the metals have low electronegativity, so metals
readily undergo oxidation and form metal ions and
electrons, atmospheric oxygen absorb these
electrons & converted to oxide ions at low and high
temperature.
• Metal ions and oxide ion combine to form metal
oxides.
• These metal oxides form a uniform layer on the
metal surface.
• Further oxidation depends on nature of metal oxide
formed.
• 1. If the metal oxide layer is unstable: Reverse
reaction takes place, Corrosion does not takes place.

10. • 2.If the metal layer is stable, sticky and fine grain
structure. It prevent the further oxidation or corrosion.
• Examples: Aluminium(Al), Tin(Sn), Lead(Pb),
Copper(Cu), Chromium(Cr), Tungsten(W).
• 3. If metal oxide layer is volatile: Metal oxide formed
and removed from the metal surface to from fresh
metal surface for further oxidation.
• Example: Molybdenum oxide(MoO3).
. 4.If the metal oxide layer having pores and cracks:
Through which atmospheric oxygen penetrate into
metal surface, further oxidation continues.
Example: Lithium(Li), Potassium(K), Sodium(Na),
Magnesium(Mg).

11. • Pilling Bedworth Rule: it says that “smaller the specific
volume ratio greater is the oxidation corrosion”
• i. If VMO > VM, metal oxide layer is tight and non porous
and protect to further oxidation.
• Example: Tungsten(W)- 3.6, Chromium(Cr)- 2.0,
Nickel(Ni)-1.6. So rate of corrosion: Ni > Cr > W.
• ii. If VMO < VM, metal oxide layer is porous and non
protective, corrosion continues till all the metal
undergo oxidation corrosion.
• Example: Lithium(Li), Sodium(Na), Potassium(K).

12. 2. Corrosion by other gases: Chlorine(Cl2), Carbon
dioxide(CO2), Sulphur dioxide(SO2) and hydrogen
sulphide(H2S).
• 2Ag + Cl2 ---- 2AgCl (Stable, Protective)
• Sn + 2Cl2 ---- SnCl4 (Volatile, Non protective)
• it is depends on the nature of metal and metal
oxide formed. Same pilling bedworth rules applied.
3. Liquid metal corrosion: when metal exposure to
liquid metals, react with the liquid metal and
undergo corrosion to form solution.
• Zn + 2HCl ----- ZnCl2 + H2
• Zn + Hg ------ Zn-Hg

13. 2. Wet corrosion / Electro chemical corrosion
• Takes place in presence of moisture.
• More effective than dry corrosion.
• Takes place in acidic and neutral medium.
• Cathodic and anodic areas are formed on same
metal surface , separated by conducting medium.
• Types of wet corrosion:
• i. Evolution of Hydrogen gas type.
• Ii. Absorption of Oxygen gas type.

14. i. Evolution of Hydrogen gas type
• This type of corrosion takes place, when anodic
area is large and cathodic area is small.

15. ii. Absorption of Oxygen gas type
• This type of corrosion takes place, when anodic
area is small and cathodic area is large.
• This type of corrosion takes place, when metal have
cracks, rough surfaces, joints, bends…….etc.
• Absorption of oxygen gas takes place at cathodic
area and corrosion takes place at anodic area.
• Rust formed at the junction of cathod and anode.

17. Difference between dry & wet corrosion

18. Types of electrochemical corrosion:
• Galvanic corrosion:
• It is also known as bimetallic corrosion.
• It is an electrochemical process whereby one metal
corrodes in preference to another metal that it is in
contact with through an electrolyte.
• Galvanic corrosion occurs when two dissimilar
metals are immersed in a conductive solution and
are electrically connected.

19. Galvanic Series: When all the metals and their alloys are
arranged according to their increasing order of nobility.
Top metal and their alloys are more susceptible to
corrosion due less reduction potential values.
The bottom elements are more inert to corrosion due to
nobility of metals.

21. • Waterline Corrosion: When water is stored in a
metallic tank, it is observed that the metal below
the waterline gets corroded.
• It is because the water below the waterline is
poorly oxygenated and acts as an anode.
• The metal above the waterline is highly oxygenated
and acts as a cathode.

22. • Pitting corrosion:
• It is a localized form of corrosion by which cavities
or "holes" are produced in the material.
• Pitting is considered to be more dangerous than
uniform corrosion damage because it is more
difficult to detect, predict and design against.
• Corrosion products often cover the pits.

23. Factors effecting the corrosion

24. 1. Position in Galvanic Series
• Metal which has lower reduction potential,
greater is the tendency of corrosion.
• Rate of corrosion α
1
Reduction potential
• If two metals were joined difference in reduction
potential increases, rate of corrosion increases.

25. 2. Overvoltage
• Rate of corrosion α
1
Overvoltage
• Zn + HCl -------------- ZnCl2 + H2 EMF =0.76V
• Hydogen overvoltage is 0.76V
• Zn + CuSO4 -------------- ZnSO4 + Cu EMF= 1.1V
• Hydogen overvoltage is 1.1 - 0.76 = 0.34V

26. 3. Relative Area of Cathodic and
Anodic Surfaces
• Cathodic Area > Anodic Area, Then
_____________ corrosion takes place
• Anodic Area > Cathodic Area, Then
_____________ corrosion takes place

27. 4. Purity of Metal
Percentage of
Aluminium(Al) 99.998 99.97 99.2
Rate of Reaction
with HCl 1 1000 30,000
Rate of corrosion α
1
Purity of metals
Purity of metal decreases i.e. foreign metal content
increases , electrochemical corrosion accelerated. So that
metals easily undergo corrosion.

28. 5. Nature of surface films
• Porous---
• Non porous…
• Unstable..
• 6. Solubility of corrosion products
• 7. Volatility of corrosion products

30. Nature of Environment
• Temperature
• pH
• Humidity
• Nature of ions present
• Conductance of corroding Medium
• Impurities

31. Corrosion control methods
• Cathodic protection: The method of protection
given to a metal by forcefully making it to behave
like a cathode is called cathodic protection.
• This is done by two ways :
• 1. Sacrificial anodic protection.
• 2. Impressed current cathodic method.

32. Sacrificial anodic protection
• In this method the metallic structure used to be
protected is called base metal.
• The base metal is connected to more anodic metal
through a wire.
• The anode metal undergoes corrosion slowly,
while the base metal is protected.
• The corroded sacrificial anode block is replaced by
a fresh one.
• The commonly used anodic metals are Aluminium,
Zinc, Magnesium and their Alloys.

33. Applications

35. Impressed current cathodic method
• In this method an impressive current little more
than corrosion current is applied in the opposite
direction to nullify the corrosion current producing
a reverse cell reaction.
• Thus, the anodic corroding metal become cathodic
and protected from corrosion.
• The impressed current taken from a battery or
alternative current(A.C.) line.
• The anode is usually insoluble anode like graphite,
high silica iron, scrap iron, stainless steel, platinum.

36. • Applications:
• This method is applicable to large structures, for
long term use.
• Water tanks,
• buried water or oil pipelines,
• Marine pipes,
• Transmission line towers.

39. Surface coating or protective coatings
• This is the oldest method of protection of metals.
• The coated surface isolated from the metal from its
corroding environment or surroundings.
• https://www.youtube.com/watch?v=Qi-tK1jwO-k
• Requirements for surface coating:
• Coating compound must be chemically inert.
• Resistant to environmental conditions such as
temperature, pressure, humidity…….etc.
• Coating must be prevent the passage of
environmental gases onto the metal suface.

40. Methods of cleaning of surface of metal
• Solvent cleaning:
• Before applying any protective coating the metal
surface must be thoroughly cleaned to remove
dirt, old protective coatings such as oils,
greases…..etc.
• Cleaning of metal surface by the application of
organic solvents like Naphthalene, chlorinated
hydrocarbons, toluene, xylene or Acetone.
• Then the metal surface is cleaned with steam and
hot water containing reagents like alkali.

41. • Alkaline cleaning: Metal surface cleaned with alkali
agents like sodium phosphate(Na3PO4) along with
soaps and caustic soda(NaOH).
• This method removes old paint coatings from metal
surface.
• Alkali treatment followed by cleaning with 0.1%
chromic acid to remove traces of alkali.
• Mechanical cleaning: This method used to to
remove loose rust, scales, dirt ..etc. from the metal
surface.
• This is done by using metal brushes, sand papers,
detergents followed by steam or hot water
treatment.
• https://www.youtube.com/watch?v=uCZxr9IzRi0

42. • Flame cleaning: In this method material is
heated followed by vibration wire brushing.
• https://www.youtube.com/watch?v=ACGSzBXKONo
• Sandblasting:
https://www.youtube.com/watch?v=AKrg2cRNxkM
• This method used to remove scales, rust from metal
surface and make it smooth and ready to coating.
• Pickling and etching:
• In this process metals are immersed in acid solutions.
• Generally dilute H2SO4 or dilute HCl or dilute HNO3 and
their mixtures are used.
• This method provides clean and smooth surface for
applying protective coating.

43. Metallic coatings
• In this method, coating of a metal on the surface of
other metal
• The metal which is protected is called base metal,
and the metal which is coated on the surface of
the base metal is called coating metal.
• Metallic coatings are two types:
• Anodic coatings:
• Coating of inferior metal on base metal.
Example: Galvanization
• Cathodic coating:
• Coating of superior metal on base metal.
Example: Tinning

44. Methods of application
• Hot dipping.
Ex. i. Galvanization, Tinning.
• Metal cladding.
Ex. Metal sandwiching.
• Electroplating and
• Electroless plating.

45. Galvanization

49. Electroless plating / chemical plating
/autocatalytic plating
• The deposition of a metal from its salt solution on a
catalytically active surface by a suitable reducing
agent, without using electrical energy is called
electroless plating.

50. The following are the requirements for electroless
plating
• Soluble electroactive metal in the form of metal
chloride or metal sulphate.
• The reducing agents like formaldehyde,
hypophosphite.
• Complexating agents like citrate, tartate and
succinate.
• Exaltants like succinate, glycinates and fluorides to
improve the rate of plating.
• Stabilizers which prevent decomposition of bath.
• To control pH of the bath buffer solution was
added.

51. Electroless plating of nickel
• Base material: Acrylonitrile-Butadiene-Styrene(ABS)
polymer.
• Coating metal : Nickel (Ni)
• The base material is activated by SnCl2 (stannous
chloride) followed by PdCl2 (palladium chloride )to
get a layer of palladium(Pd) and the surface is dried.
• The base metal dipped in a solution of nickel
chloride(NiCl2), sodium dihydrogen phosphite
(NaH2PO2), Buffer solution(CH3COOH + CH3COONa)
and Sodium succinate as complexating agent and
exalt, pH of bath maintained at 4.5 and
temperature at 930 C.

52. •

53. Applications
• Electroless plated nickel objects are better corrosion
resistance , pore free, hard and wear assistant.
• Used in electronic applications providing a non
magnetic underlay in magnetic components.
• ABS plastics are used for decorative applications
such as car finishing's, perfume bottle caps,
costume jewellery and Hi-Fi equipments.