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 Professor Govt. 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 toenvironment.Chemicals are chemically turned to new substances such asoxides, hydatedoxides, carbonates, chlorides, sulphides, sulphates etc havingentirely new properties.Once the metals begin to decay or get corroded, they suffer aloss in electrical conductivity, tensile strength, color and luster.
Definition of CorrosionCorrosion is the slow process of decay of themetal, due to the attack of the atmosphere gaseson the surface of the metal, resulting in theformation of metallic compounds such oxides,hydroxides, carbonates, sulphides etc. ORCorrosion is the process of destruction ordeterioration of the metal and alloys by unwantedor unintentional chemical or electrochemicalattack by its environments staring its surface.
Type of Corrosion1. Dry or Chemical Corrosion:It is due to the direct chemical action ofenvironmental atmosphere gases such as O2.H2S, SO2, N2, halogens or anhydrousinorganic liquids with metal surfaces.2. Wet or Electrochemical Corrosion: It is due to the existence of separate“anodic” and “cathodic” areas in the systembetween which current flows through theconducting liquid and the anode gets oxidizedand wasted.
Electrochemical Theory of Corrosion (Mechanism Of Corrosion)Corrosion is an electrochemical process.Galvanic cells are setup between dissimilar metals in contact with eachother or between dissimilar parts of he same metal when surrounded bymoist air or liquid.Anodic area oxidation takes placeCathodic area reduction takes place.Metallic ions formed at the anodic part and ions formed at thecathodic part diffuses towards each other through conductingmedium and the corrosion product is formed somewhere betweenthe 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 Metal SRP, Eo Lithium----------------- -3.05 VMore oxidation Less Reduction Potassium Calcium Sodium Decreasing Magnesium Increasing tendency Aluminum order of std to loose Zinc reduction electrons Nickel potential Tin Hydrogen--------------- 0.00 Copper Silver Platinum Gold---------------------- +1.15 V
When two metals are in contact with each other, inpresence of an electrolyte, the more active metalbecomes the anode and easily undergoes oxidationie, corrosion. More active Less activeGreater the difference between the reductionpotentials of the metals, more severe will be the rate ofcorrosion.
2. Relative areas of anodic and cathodic partsWhen two dissimilar metals or alloys are incontact and the anodic area is smaller than thecathodic area, the corrosion of the anode becomerapid and severe. Cathode Anode
3. Purity of the metalA pure metal is more corrosion resistant than animpure metal. The rate and extent of corrosionincreases with the amount of impurities present.
4.Physical state of the metalSmaller the size of the metal, more the area understress and greater is the corrosion.
5. Solubility and volatility of corrosion productIf the product of corrosion is soluble in thecorroding medium and also is volatile, corrosionoccurs 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)3A galvanic cell is set up between two dissimilar parts of the samemetal iron.a) The portion of iron which is in contact with water acts asanode 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 thecathodic site.
c) Fe2+ and OH- ions combine to form Fe(OH)2 which getsoxidized to Fe(OH)3.Fe2+ + OH- Fe(OH)24 Fe(OH)2 + O2 + 2H2O 4Fe(OH)3Overall reaction2 Fe + O2 + 4H+ 2Fe2+ + 2H2O
Conditions for rusting1.Impurities in iron2.Presence of oxygen3.Presence of moisture4.Presence of electrolyte5.Presence of Cl2 or SO2 in the atm.
Prevention of rusting1.Minimizing the impurities in iron2. Giving suitable coating of Zn/Sn/Cr3.Painting the iron surface
Prevention of Corrosion
1. Maintaining the purity of the metalPure metals are generally corrosion resistant.
2. Alloying with other elementsIt to found that alloying with suitableelements increases corrosion resistance ofmost metals.
3. Modification of environmentDeaeration of aqueous environment anddehumidification decreases the rate ofcorrosion.
i. Metallic Coatinga) Using less active metalb) Using a more active metalGalvanization is the process ofcoating iron or steel sheets with athin layer of zinc by dipping inmolten zinc.
The iron or steel article is first cleaned withdil. H2SO4 at 60-90oC (Pickling).Then the article is treated with 5% HF to removegrains of sand, washed with water and dried.It is dipped in molten zinc (425-450oC).A layer of zinc gets coated on the article, whichis then pressed through a roller to remove excesszinc.
ii. Non-Metallic Coatinga) Phosphate coating by alkaline solution of phosphate.b) Chromate coating using chromate solutions.c) Anodizing on non-ferrous metals.
iii. Organic CoatingPlastics, polythene, rubber etc, are used for coating toprevent corrosion.Mainly on articles like, ship, submarines. Etc.