Corrosion Chemistry : Its Causes and Remedy

This Conceptual Power Point project consists of a grieve description about the science of Corrosion, perfect for covering whole topic for short period of time. The project consists of basic knowledge regarding corrosion, its type of causes that is dry and wet corrosion. It also summarizes about different types of differential corrosion and its remedies. Its allover a good conceptual ppt for students of chemistry in their prelims. It's good for the ones achieving for engineering degrees.

1. Corrosion chemistry: causes and remedy R epresented By : Arijit Dhali

2. Introduction to Corrosion Corrosion can be defined as the process by which the metals have a tendency to stay in the combined state such as oxides, carbonates, sulphides, chlorides etc. Metallic Ore (Stable State) Pure Metal (Unstable State) Extraction of Metal Corrosion Reaction (with environmental elements ) Change in properties of metal : Weakening of materials due to loss of cross sectional area. Decaying of surface of the materials. Loss of properties like malleability and ductility. Common examples of corrosion : Rusting of iron : Iron is exposed to atmospheric conditions to form reddish brown scale of Fe 3 O 4 . Tarnishing of silver : Metallic silver is exposed to atmospheric conditions to form black coating of Ag 2 O.

3. Theories of corrosion (Dry) Chemical Corrosion It involves direct chemical attack of atmospheric gases like CO 2 , O 2 , H 2 S, SO 2 , halogen, moisture and inorganic acid vapours on metal . Example, tarnishing of silver ware in H 2 S laden air. There are 3 types of (dry) chemical corrosion : Oxidation Corrosion : Takes place by direct action of oxygen at high & low temperature in absence of moisture. Corrosion by Other Gases : The occurrence of corrosion due to oxidation of other gases like CO 2 , SO 2 , Cl 2 , H 2 S, F 2 on metal. Liquid – Metal Corrosion : The corrosion occurs due to action of flowing liquid at high temperature on solid metals or alloys. (Wet) Electrochemical Corrosion It occurs due to setting up of a large number of tiny galvanic cells in metals in presence of an impurity as well as in presence of moisture. Generally impurity (more active metal) act as anode and original metal act as cathode so anode is the area where corrosion occurs. Example, rusting of iron in moist atmosphere. There are 2 types of (wet) electrochemical corrosion reaction : Anodic Reaction. Cathodic Reaction. Evolution of Hydrogen Absorption of Oxygen

4. ( Dry) Chemical Corrosion Occurrence : Due to direct chemical reaction of atmospheric gases. Due to molten metal in contact with metal surface. Types : Oxidation corrosion Corrosion by other gases Liquid-Metal Corrosion Pilling – Bed Worth Rule : A protective and Non-Porous metal oxide layer has volume equal to or greater than the volume of metal from which it is formed. A Non-Protective and Porous metal oxide layer has volume lesser than the volume of metal from which it is formed. Specific Volume Ratio :   Volume of oxide formed Volume of metal Corrosion by Other Gases : Ag + Cl AgCl (Non –Porous Layer) Fe + H 2 S FeS ( Porous Layer) + H 2 Sn + 2Cl 2 SnCl 4 (Volatile Layer) Liquid-Metal Corrosion : Occurs due to the action of flowing liquid metal at high temperature on solid metals or alloys. Observed in nuclear reaction where Na metal used as a coolant leads to corrosion of Cd. (Dry) Chemical Corrosion Visual Representation Oxidation corrosion : Occurrence : Due to direct chemical reaction of atmosphere O 2 with metal surface forming metal oxide. Absence of moisture. Increases with increase in temperature. Mechanism : On exposure to atmosphere , metal gets oxidized to form metal ions M(S) M +n + ne - Electrons lost by metal are taken up by Oxygen to form oxide ions ½O 2 + 2e - O 2- M + n ½O 2 2M +n + nO 2- M 2 O n (Metal Oxide)

5. (Wet) Electrochemical corrosion Occurrence : When a metal is contact with moist air or any liquid medium. When two different metals are partially immersed in a solution. Chemically non- uniform surfaces of metals behave like electrochemical cells in the presence of water containing dissolved O 2 & CO 2 . Always occurs at anodic areas. Mechanism : Involves oxidation- reduction process Depending on the nature of corroding environment, electrons released at anode are consumed at the cathodic area by two ways : Evolution of H 2 . Absorption of O 2 . Absorption of O 2 : Anode : M(S) M +n + ne - (Oxidation) Cathode : O 2 + 4e - + 2H 2 O 4OH - (Reduction) Overall Reaction : 4M(S) + 2nH 2 O 4M +n + 4OH - Evolution of H 2 : Anode : M(S) M +n + ne (Oxidation) Cathode : 2H + + 2e - H 2 (g) (Reduction) Overall Reaction : 2M(S) + 2nH + 2M +n + nH 2 (g)

6. Types of corrosion Galvanic Corrosion When two different metals are present in contact with each other in conducting medium. Concentration Cell Corrosion Occurs due to different in potential aerated areas. Part of metal exposed to air is more oxygenated part & acts as CATHODE . Part of metal immersed in electrolyte is poorly oxygenated & acts as ANODE . Stress Corrosion It is the combined effect of mechanical stress and corrosive environment of material. Pitting Corrosion It is formed as a result of pit and cavities, localized attack and formed by cracking protective coating.

7. Factors effecting corrosion Nature of metal Purity of metal. Physical state of metal. Position of metal in Galvanic series. Nature of Oxide film. Nature of products of corrosion. Relative areas of Cathode & Anode. Nature of Corroding Environment Temperature and Humidity. Effects of pH. Presence of impurities in atmosphere. Nature of electrolyte. Solubility of products of corrosion.

8. Corrosion control The common methods for controlling corrosion are : Protection by proper design and fabrication procedure. Proper material selection. Change in environmental condition. Modifying the properties of metal. Application of inhibitors. Use of protective coatings. Cathodic Protection or Electrochemical Protection : Galvanic Protection : In galvanic protection or sacrificial anodic protection, the metallic structure to be protected is connected by a wire to a more anodic metal. Impressed Current Cathodic Protection : In this method , an impressed current is applied in the opposite direction to nullify the corrosion current, and converting the corroding metal from anode to cathode. Passivation : It is the phenomenon in which a metal exhibits a much higher corrosion resistance than expected from its position in the electrochemical series.

9. Reference : Chemistry I By : Gourkrishna Dasmohapatro Link : https://www.learnpick.in/prime/documents/ppts/details/4116/corrosion