Corrosion

Dr. SURENDRAN PARAMBADATH (M.Sc, M.Phil, M.Tech)

Formerly: Post Doctoral Research Associate,Nano-Information Materials Research Laboratory,

Pusan National University, Busan-South Korea

Currently: Assistant ProfessorGovt. Polytechnic College, Perinthalmanna

Nature keep all metals in their ore form.

Metals are usually extracted from their ores.

Nature tries to convert the metals again into their ore form.

The surface of metals are attacked when exposed to environment.

Chemicals are chemically turned to new substances such as oxides, hydated oxides, carbonates, chlorides, sulphides, sulphates etc having entirely new properties.

Once the metals begin to decay or get corroded, they suffer a loss in electrical conductivity, tensile strength, color and luster.

Definition of Corrosion

Corrosion is the slow process of decay of the metal, due to the attack of the atmosphere gases on the surface of the metal, resulting in the formation of metallic compounds such oxides, hydroxides, carbonates, sulphides etc.

ORCorrosion is the process of destruction or deterioration of the metal and alloys by unwanted or unintentional chemical or electrochemical attack by its environments staring its surface.

Type of Corrosion

2. Wet or Electrochemical Corrosion: It is due to the existence of separate “anodic” and “cathodic” areas in the system between which current flows through the conducting liquid and the anode gets oxidized and wasted.

1. Dry or Chemical Corrosion:It is due to the direct chemical action of environmental atmosphere gases such as O2. H2S, SO2, N2, halogens or anhydrous inorganic liquids with metal surfaces.

Electrochemical Theory of Corrosion(Mechanism Of Corrosion)

Corrosion is an electrochemical process.

Galvanic cells are setup between dissimilar metals in contact with each other or between dissimilar parts of he same metal when surrounded by moist air or liquid.

Anodic area oxidation takes place

Cathodic area reduction takes place.

Metallic ions formed at the anodic part and ions formed at the cathodic part diffuses towards each other through conducting medium and the corrosion product is formed somewhere between the anodic and cathodic areas.

Anodic iron gets oxidised to Fe2+

M Mn+ + ne-

The oxygen at the cathode changes to OH- by reduction. ½ O2 + H2O +2e- 2 OH-

Factors Affecting Corrosion

Rate & Extent of corrosion

1. The position of metals in the electrochemical Series

Lithium----------------- -3.05 VPotassiumCalciumSodiumMagnesiumAluminumZincNickelTinHydrogen--------------- 0.00CopperSilverPlatinumGold---------------------- +1.15 V

Metal SRP, Eo

Decreasing tendency to loose electrons

Increasing order of std reduction potential

Less ReductionMore oxidation

When two metals are in contact with each other, in presence of an electrolyte, the more active metal becomes the anode and easily undergoes oxidation ie, corrosion.

Greater the difference between the reduction potentials of the metals, more severe will be the rate of corrosion.

More active

Less active

2. Relative areas of anodic and cathodic parts

Cathode

Anode

When two dissimilar metals or alloys are in contact and the anodic area is smaller than the cathodic area, the corrosion of the anode become rapid and severe.

3. Purity of the metal

A pure metal is more corrosion resistant than an impure metal. The rate and extent of corrosion increases with the amount of impurities present.

4.Physical state of the metal

Smaller the size of the metal, more the area under stress and greater is the corrosion.

5. Solubility and volatility of corrosion product

If the product of corrosion is soluble in the corroding medium and also is volatile, corrosion occurs faster.

6. Nature of the corroding environment

A.Temperature: Corrosion generally increases with rise in temperature of environment.

B.Humidity of the air: Rate of corrosion increases with presence of moisture in the atmosphere.

C.Impurities: Presence of impurities like CO2, H2S, SO2, acid fumes etc increases corrosion rate.

D.Influence of pH: In acid medium corrosion is more and in alkaline medium it is less.

Rusting of Iron

Rust: Fe2O3.XH2O or Fe(OH)3

A galvanic cell is set up between two dissimilar parts of the same metal iron.

a) The portion of iron which is in contact with water acts as anode and the other portion in contact with air acts as cathode.

b) Anodic iron gets oxidised to Fe2+

AnodeFe Fe2+ + 2e- CathodeThe oxygen at the cathode changes to OH- by reduction. ½ O2 + H2O +2e- 2 OH-

The electron released at the anode move through the metal to the cathodic site.

c) Fe2+ and OH- ions combine to form Fe(OH)2 which gets oxidized to Fe(OH)3.

Fe2+ + OH- Fe(OH)2

4 Fe(OH)2 + O2 + 2H2O 4Fe(OH)3

Overall reaction

2 Fe + O2 + 4H+ 2Fe2+ + 2H2O

Conditions for rusting

1.Impurities in iron

2.Presence of oxygen

3.Presence of moisture

4.Presence of electrolyte

5.Presence of Cl2 or SO2 in the atm.

Prevention of rusting

1.Minimizing the impurities in iron

2. Giving suitable coating of Zn/Sn/Cr

3.Painting the iron surface

Prevention of Corrosion

1. Maintaining the purity of the metal

Pure metals are generally corrosion resistant.

2. Alloying with other elements

It to found that alloying with suitable elements increases corrosion resistance of most metals.

3. Modification of environment

Deaeration of aqueous environment and dehumidification decreases the rate of corrosion.

4. Barrier protection or Protective coatings

1.Metallic Coating2.Non-metallic coatings3.Organic coatings

i. Metallic Coating

a) Using less active metalb) Using a more active metal

Galvanization is the process of coating iron or steel sheets with a thin layer of zinc by dipping in molten zinc.

The iron or steel article is first cleaned with dil. H2SO4 at 60-90oC (Pickling).

Then the article is treated with 5% HF to remove grains of sand, washed with water and dried.

It is dipped in molten zinc (425-450oC).

A layer of zinc gets coated on the article, which is then pressed through a roller to remove excess zinc.

ii. Non-Metallic Coating

a) Phosphate coating by alkaline solution of phosphate. b) Chromate coating using chromate solutions.c) Anodizing on non-ferrous metals.

iii. Organic Coating

Plastics, polythene, rubber etc, are used for coating to prevent corrosion.

Mainly on articles like, ship, submarines. Etc.

CATHODIC PROTECTION OR

ELECTRICAL PROTECTION

Zn Zn2+ + 2e-

½ O2 + H2O + 2e- 2OH-

Anode

Cathode

Cathode

Anode

Zn

Thank You