1. CORROSION AND ITS CONTROL UNIT-II

2. CONTENTS INTRODUCTION EFFECTS OF THE CORROSION PROCESS
TYPES OF CORROSION
THEORIES OF CORROSION
FACTORS INFLUENCING CORROSION
CORROSION CONTROL METHODS
CATHODIC PROTECTION
CORROSION INHIBITORS
SURFACE COATINGS
METHODS OF APPLICATION ON METAL COATINGS
ORGANIC SURFACE COATINGS -PAINTS

3. INTRODUCTION RUST ON IRON
The process of decay of metal by environmental attack is called corrosion.
For example
a) formation of reddish brown layer of rust on the surface of iron.
b) formation of green film of basic carbonate on the surface of copper.
The corrosion of metals is measured in the units of milliinches/year or mm/year.
RUST ON IRON

4. CAUSES OF CORROSION
The basic reason for this attack is most of the metals exist in the nature in the form of their minerals or ores, in the stable combined forms as oxides, chlorides, silicates, carbonates, sulphides etc., During extraction of metals these ores are reduced to metallic state by considerable amounts of energy.

5. EFFECTS OR DISADVANTAGES OF CORROSION
The valuable metallic properties like conductivity, malleability, ductility etc., are lost due to corrosion.
The process of corrosion is very harmful and is responsible for the enormous wastage of metal in the form of its compounds.
Life span of the metallic parts of the machineries is reduced.
The failure of the machinery takes place due to lose of useful properties of metals.

6. TYPES OF CORROSION
Corrosion is classified into different forms based on nature of the environment, composition of the metal and existence of stresses.
They are
Uniform or General corrosion
Localised corrosion
a) Pitting corrosion
b) Crevice corrosion
Intergranular corrosion
Environmental assisted corrosion
a) Stress corrosion
b) Season cracking corrosion
c) Caustic embrittlement

7. Galvanic corrosion
Concentration cell corossion or Differential aeration corrosion
Erosion corrosion
Microbiologically influenced corrosion
Except general or uniform corrosion most of the other forms of corrosion are insidous in nature and are considerably more difficult to predict.

8. Uniform or general corrosion
This is usually caused due to chemical and electrochemical attack.
Slow corrosion of some materials in appropriate environment comes under this category.
Localised corrosion
Pitting corrosion
Extreme localized attack, may perforate metal sheet/plate etc.

9. CREVICE CORROSION
Crevice corrosion at a metal-to-metal crevice site formed between components of type 304 stainless steel fastener in seawater
Crevice corrosion at a metal-to-metal crevice site
formed between components of type 304 stainless steel fastener in seawater

10. INTERGRANULAR CORROSION
This corrosion occurs along grain boundaries.
STRESS CORROSION
Preferential attack area under stress in a corrosive environment, where such an environment alone would not have caused corrosion

11. Early steam boilers (19th and early 20th century) of riveted
SEASON CRACKING
SEASON CRACKING OF GERMAN AMMUNITION
Occurs where brass case is crimped onto bullet, i.e., in area of high residual stress.
Common in warm, wet environments (e.g., tropics).
Carbon steel plate from a caustic storage
tank failed by caustic embrittlement.
“CAUSTIC EMBRITTLEMENT”
Early steam boilers (19th and
early 20th century) of riveted
carbon steel.
Both stationary and locomotive engines often exploded.

12. GALVANIC CORROSION
Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when in electrical contact with a different type of metal and both metals are immersed in an electrolyte.
CONCENTRATION CELL CORROSION
Concentration cells can arise when the concentration of one of the species Participating in a corrosion reaction varies within the electrolyte.

13. EROSION CORROSION
Erosion corrosion is caused by the combined effect of the abrading action of turbulent flow of gasses vapours and liquids and the mechanical rubbing action of solids over a metal surface.

14. MICROBIOLOGICAL CORROSION
Deterioration of metal, directly or indirectly, by the metabolic activity of living organism is called microbial corrosion.
These micro- organisms include bacteria, algae etc.

15. CORROSION REMOVAL Cleaning and stripping of the corroded area
Complete corrosion treatment involves the following:
Cleaning and stripping of the corroded area
Removing as much of the corrosion products as practicable
Neutralizing any residual materials remaining in pits and crevices
Restoring protective surface films
Applying temporary or permanent coatings or paint finishes

16. THEORIES OF CORROSION
Broadly the corrosion process proceeds in two types by direct chemical action of environment on the surface of metal in absence of moisture and in presence of a conducting liquid by the formation of electrochemical cells.
DRY CORROSION
WET CORROSION

17. DRY CORROSION OXIDATION CORROSION CORROSION BY OTHER GASES
LIQUID METAL CORROSION

18. OXIDATION CORROSION
Direct attack of oxygen at high or low temperature on metals in the absence of moisture is called oxidation corrosion. 2M→2M+ +2e- O2+2e-→2O2- 2M+O2→2M++2O2- 2MO

19. PILLING BED WORTH RULE
The smaller the specific volume ratio ,the greater is oxidation corrosion. If the volume of the metal oxide layer is at least as great as the volume of the metal from which it is formed is non porous and becomes protective layer by tightly adhering to the base metal from which it is formed. If the volume of the metal oxide is less than the volume of the metal, the oxide layer is porous, non continuous and non-protective and faces strains. Hence cracks and pores are developed in the layer, creating access to atmospheric oxygen to reach the underlying metal. In this case corrosion is continuous and rapidly increase.

20. CORROSION BY OTHER GASES
The gases like SO2,Cl2,H2S and F2 etc., also cause chemical corrosion and the extent of corrosion mainly depends on the reactivity to the gas on the metal surface.
The corrosion product is metal compound.
The nature of the metal compound determines the corrosion.

21. LIQUID METAL CORROSION
The chemical action of a flowing liquid metal at high temperature, on a solid metal or alloy produces liquid metal corrosion.
The reason for corrosion is the dissolution of the solid metal by liquid metal or internal penetration of the liquid metal into the solid phase, weakening the solid metal.

22. WET OR ELECTROCHEMICAL CORROSION
Electro chemical corrosion involves
Evolution of hydrogen
Absorption of oxygen

23. EVOLUTION OF HYDROGEN
This type of corrosion occurs usually in acidic environments. The rusting of iron takes place in acidic environment.
Anodic Reaction (Oxidation):
Production of Electrons
Fe Fe e
Cathodic Reaction (Reduction)
Consumption of Electrons
2 H e H Acid
2 H2O e H2 + 2 OH Alkaline
Fe OH Fe(OH)2
4 Fe(OH)2 + O2 + 2H2O Fe(OH) Fe2o3.3H2O
(ferrous hydroxide) (ferric hydroxide) (rust)

24. ABSORPTION OF OXYGEN
In presence of neutral, aqueous solution of electrolytes like NaCl, in the presence of atmospheric oxygen and some cracks developed in iron oxide film causes this type of electrochemical corrosion
Fe Fe e-
O2 + 2H e OH -
Fe OH Fe(OH)2
4 Fe(OH)2 + O2 + 2H2O Fe(OH) Fe2o3.3H2O
(ferrous hydroxide) (ferric hydroxide) (rust)

25. FACTORS INFLUENCING CORROSION
The rate of extent of corrosion depends on the following factors
Nature of metal
Nature of corroding atmosphere

26. NATURE OF THE METAL Position in the galvanic series Over voltage
Purity of metal
Nature of oxide film
Nature of corrosion product

27. NATURE OF THE ENVIRONMENT
Effect of temperature
Effect of pH
Humidity
Effect of oxidant

28. CORROSION CONTROL METHODS
Proper designing
Using pure metal
Using metal alloys
Modifying the environment
Use of inhibitors
Cathodic protection
Application of protection coatings

29. CATHODIC PROTECTION
The method of protection given to a metal by forcibly making it to behave like a cathode is called cathodic protection.
There are two types of cathodic protection.
Sacrificial anodic protection
Impressed current cathodic protection

30. SACRIFICIAL ANODIC PROTECTION
In this method of protection, the metallic structure to be protected called “base metal” is connected to more anodic metal through a wire.
The anodic 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 Magnesium, Zinc and Aluminium and their alloys.

31. IMPRESSED CURRENT CATHODIC PROTECTION
In this method an impressed 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 becomes cathodic and protected from corrosion.
The impressed current is taken from a battery or rectified on A.C. line.
The anode is usually insoluble anode like graphite, high silica iron, scrap iron, stainless steel, or platinum.

32. METHODS OF APPLICATION OF METAL COATINGS
HOT DIPPING
GALVANISATION
TINNING
METAL CLADDING
ELECTROPLATING OR ELECTRODEPOSITION
METAL SPRAYING
CEMENTATION

33. HOT DIPPING
This is a method of application of coating a low melting metal such as Zn, Sn, Pb,Al, etc., on iron, steel and copper which have relatively higher melting points.
The base metal is dipped in a molten bath of the coating metal, which is covered by a molten flux layer which cleans the base metal surface and prevents the oxidation of the coating metal.
For good adhesion of the coating metal on the surface of base metal, the base metal surface must be very clean.

34. GALVANISATION
The process of coating Zn on the surface of iron or steel is called galvanisation.
Zinc is anodic to the base metal iron or steel.
Zinc protects iron sacrificially.
After galvanisation, the galvanised sheet is subjected to annealing process.
Galvanised containers cannot be used for storing food stuffs, because Zinc reacts with the acids that are present in the food materials and produces toxic compounds.
Galvanised articles are good engineering materials.

35. TINNING
The process of coating tin on the surface of iron or steel is called tinning.
Tin is cathodic to the base metal iron or steel.
Tin protects iron due to its noble nature.
No annealing process.
Tin coated containers are used for storing food materials because tin produces non toxic protective layer on the surface of metal and avoids any food poisoning.
Tinned articles are used majorly in food processing industries.

36. METAL CLADDING
In this process a dense,homogeneous layer of coating metal is bonded firmly and permanently to the base metal on one or both sides. The metal cladded is called cladding metal.
The choice of cladding metals depends on the corrosion resistance required for any particular environment.
The corrosion resistant metals like Nickel, Copper, Lead, Silver,Platinum etc., and alloys like stainless steel,nickel alloys,copper alloys,lead alloys are used as cladding materials.
This method is widely adopted in aircraft industry and automobile industry.

37. ELECTROPLATING OR ELECTRODEPOSITION
The process of depositing the coating metal on the surface of base metal /non metal by electrolysis is called electroplating.
Electroplating is a most important and frequently used technique in industries to produce metallic coatings.
In metals the electroplating increase resistance to corrosion, chemical attack, hardness, water resistance and surface properties.
In nonmetals the electroplating increases strength and decorates the surface of non metals like plastics, wood, glass etc.,

38. PAINTS
Paint is a mechanical dispersion mixture of one or more pigments in a vehicle or drying oil.
CONSTITUENTS OF PAINTS:
Pigment
Vehicle or drying oil
Thinners
Driers
Fillers
Plasticizers
Antiskinning agents

39. Pigment is a solid substituent and an essential constituent of paint.
FUNCTIONS:
Constitute the body of the paint.
Give strength to paint.
Provide desired colour to paint.
VEHICLE OR DRYING OIL
The main film forming function.
It acts as a vehicle or medium for the dispersion of the various constituents in it.

40. Ex: Linseed oil, tung oil,rosin oil,soyabean oil etc.,
THINNERS
FUNCTIONS:
Reduce viscosity of the paint
Increase the elasticity of the paint film
Ex: Turpentine, mineral oil, xylol, kerosene etc.,
DRIERS
Driers are oxygen carrier catalysts.
The main function of the driers is to improve the drying quality of the oil film.
Ex: Resinates, tungstates,Naphthates of Co,Mn,Pb and Zn.

41. FILLERS FUNCTIONS: Increase durability of the paint.
Help to reduce the cracking of dry paint film
Ex: Talc,asbestos, ground silica,gypsum etc.,
PLASTICIZERS
To provide elasticity to the film.
To minimise its cracking.
Ex: Tricresyl phosphate,triphenyl phosphate,tributyl phosphate.

42. The main function is prevent gelling and skinning of the
ANTISKINNING AGENTS
FUNCTIONS:
The main function is prevent gelling and skinning of the
paint film.
Ex: Poly hydroxy phenols.

43. REQUIREMENTS OF A GOOD PAINT
It should be fluid enough to be spread easily over the protected surface.
It should posses high covering power.
It should form a quite tough, uniform, adherent and impervious film.
Its film should not get cracked on drying.
It should protect the painted surface from corrosion effects of environment.
It should form film, the colour of which is quite stable to the effect of atmosphere and other agencies.
Its film should be glossy.
Its film should be stable.

44. THANK YOU