Metallic Corrosion: Dry Corrosion Mechanism
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1. Dry or chemical corrosion occurs via direct chemical reaction between a metal surface and gases in the atmosphere like oxygen, carbon dioxide, hydrogen sulfide, and sulfur dioxide. It involves adsorption of gas molecules on the metal surface. 2. The mechanism of dry corrosion involves oxidation of the metal surface and reduction of gas molecules, forming a metal oxide layer. The nature and properties of this oxide layer determines whether it is protective or porous. 3. According to the Pilling-Bedworth rule, a protective oxide layer has a volume equal to or greater than the original metal, while a porous layer has a smaller volume, allowing further corrosion. Metals like chromium and aluminum form protective passive layers contributing to corrosion resistance.
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Metallic Corrosion: Dry Corrosion Mechanism
- 1. Prepared by: Prof Sarala Prasanna Pattanaik Assistant Professor Department of Chemistry
- 3. • Metallic Corrosion: Definition, types and examples. • Introduction to Dry or Chemical corrosion • Mechanism of Dry or Chemical corrosion. • Pilling – Bedworth Rule • Concept of Passivity or Passivation
- 4. Corrosion can be defined as a destructive chemical and electrochemical reaction of a metal with its environment (like O2, moisture, CO2 etc.) which disfigures metallic products leading to reduction in their thickness, strength and also causes loss of useful properties such as malleability, ductility, electrical conductivity and optical refractivity. Or Corrosion can be defined as “Any process of deterioration or destruction and consequent loss of a solid metallic material through an unwanted or unintentional chemical or electrochemical attack by its environment at its surface is called corrosion”. Thus corrosion is a reverse process of extraction of metals. Except a few metals such as Gold, Silver and Platinum (called noble metal), other metals are prone to corrosion. Typical examples of Corrosion are • Rusting of iron due to formation of hydrated ferric oxide (Fe2O3. 3H2O). • Tarnishing of silver wares in H2S laden air due to formation of silver sulphide. • Formation of green film of basic carbonate- [CuCO3 + Cu(OH)2] on the surface of copper when exposed to moist air containing CO2.
- 5. Formation of Iron oxide: Fe → Fe2+ + 2e– (oxidation) 1/2O2 + 2e– → O2– (reduction) Overall reaction:- Fe + 1/2O2 → Fe2+ + O2– Or, 2Fe + O2 → 2Fe2+ + 2O2– → 2FeO Similarly, in excess supply of oxygen, 4Fe + 3O2 → 2Fe2O3 Rusting of Iron in presence of Electrolyte:- Fe → Fe2+ + 2e– (oxidation) 1/2O2 + H2O + 2e– → 2OH– (reduction) Overall: Fe + 1/2O2 + H2O → Fe2+ + 2OH– → Fe(OH)2 Or, 2Fe + O2 + 2H2O → 2Fe2+ + 4OH– → 2Fe(OH)2 In the presence of excess Oxygen: 4Fe(OH)2 + O2 + 2H2O → 4 Fe(OH)3 or 2Fe2O3. 3H2O General formula of Rust: Fe2O3. xH2O, x = 2 to 5 Corrosion of Copper when exposed to moist air containing CO2 2Cu + O2 + H2O + CO2 → CuCO3.Cu(OH)2 (Green protective coating)
- 6. Metals are electropositive in nature. Except a few metals like Gold, Silver and Platinum (noble metal), other metals are found in nature as their compounds (such as oxides, hydroxides, carbonates, bicarbonates, chlorides, nitrates, sulphates, sulphides, phosphates, silicates etc.) which are called as their ore. Metals are thus obtained by extraction from their ores by reduction process. In nature, when metals exists as their compounds (or ore) they are stable and they are in the low energy states. However, during extraction of metals from their ores, free metals are become less stable and are in the higher energy state than in the ionic state or ore. So, metals have a tendency to converted back to the ionic state (ore) and hence metal atoms are prone to get attacked by environment. This is the main reason for corrosion of metals. The mechanism of Corrosion of metals involve the concept of Redox reaction. Metal (M) Mineral or ore (Mn+ ) Corrosion product (Mn+ ) Extraction by reduction + ne Corrosion -ne
- 7. Types of Corrosion Dry or Chemical Corrosion Wet or Electrochemical Corrosion
- 8. Dry or Chemical Corrosion: It involves direct chemical attack of atmospheric gases like CO2, O2, H2S, SO2, halogens, and inorganic acid vapours on exposed metallic surface. Example:- Tarnishing of silver ware in H2S laden air. Wet or 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 or an electrolytic medium. Generally, impurity (more active metal) acts as anode and original metal acts as cathode. So anode is the area where corrosion occurs. Example:- Rusting of iron in moist atmosphere.
- 9. Dry or Chemical Corrosion Occurs • Due to direct chemical reaction of atmospheric gases. • It involves the process of adsorption. • Corrosion occurs uniformly on the entire exposed metallic surface. • It occurs on both homo and heterogeneous metallic surface. • Both oxidation and reduction occurs at the exposed metallic surface without the formation of anodic and cathodic area. • Extent of dry corrosion depends on the nature of the layer formed on the metallic surface and also on the attraction or affinity between the gas and the exposed metal. Types • Oxidation corrosion due to Oxygen • Corrosion by other atmospheric gases • Liquid Metal Corrosion
- 10. Types of dry corrosion Oxidation corrosion Occurs Due to direct chemical reaction of atm. O2 with metal surface forming metal oxide. It involves the process of adsorption of Oxygen on metallic surface. It occurs in the absence of moisture or any electrolytic medium. Increases with increase in temperature and may take place at low or high temperature. Extent of Oxidation corrosion depends on the nature of oxide layer formed on the metallic surface. Mechanism On exposure to atm., metal gets oxidized to form metal ions. (i) 2M(s) → 2M+n + 2ne- (Oxidation) Electrons lost by the metal are taken up by oxygen to forms oxide ions. (ii) n/2O2 (g) + 2ne- → nO2- (Reduction) Overall reaction: 2M + n/2O2 → 2M+n + nO2- → M2On Metal Oxide
- 11. Mechanism of Dry or Chemical Corrosion
- 12. Mechanism of Dry or Chemical Corrosion
- 13. Nature of metal oxide layer Types of Oxide Layers formed Stable layer: - Al, Pb, Cu, Cr, Ni, Mn, Sn etc. (Negligible corrosion) Unstable layer:- Ag, Au, Pt etc. (No oxidation corrosion) Volatile layer:- Mo (as MoO3). (Excess corrosion) Porous layer:- Alkali metals & Alkaline earth metals. (Excess corrosion) Pilling – Bedworth 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 (R) = Volume of oxide layer formed Volume of parent metal exposed If, R ≥ 1, the oxide layer is said to be protective and non porous. If, R < 1, the oxide layer is said to be non - protective and porous.
- 14. Types of dry corrosion Corrosion by Other Gases 2Ag + Cl2 → 2AgCl (Non-Porous layer) Fe + H2S → FeS + H2 (Porous Layer) Sn + 2Cl2 → SnCl4 (Volatile Layer) Liquid Metal Corrosion • Occurs due to the action of flowing liquid metal at high temp on solid metals or alloys and the solid metal or alloy usually gets weakened. • Observed in nuclear reactors where Na metal used as a coolant leads to corrosion of Cd. • In such type of corrosion, either the liquid metal dissolves the solid metal surface or penetrates into the solid surface and weakens the bond.
- 15. Passivity or Passivation Passivity or Passivation is the phenomenon in which a metal or an alloy exhibits much higher corrosion resistance than expected from its position in the electrochemical series. Passivity is the result of the formation of a highly protective but very thin (about 0.0004 mm thick) and quite invisible film on the surface of metal or an alloy which make it more noble. This thin film is invisible, non porous and of such a Self healing nature that when broken, it repairs itself on exposure to the oxidising environment. Examples of such passive metals and alloys are: Ti, Al, Cr, Ni, Co, Mn, Pb, Cu and stainless steel alloys containing Cr (about 13 – 25%) etc. Outstanding corrosion resistance exhibited by various surgical instruments is due to the presence of Cr (about 13%) in it. Similarly, Al is not attacked by Conc. HNO3 due to passivity while Fe is easily attacked by even dilute HNO3.
- 16. Text books references 1. Jain P C and Jain M: Engineering Chemistry (15th Edition) 2006 Dhanpat Rai Publishing Company, NewDelhi. 2. Dara S.S. & Umare S.S. A Text Book of Engineering Chemistry(12th Edition ) 2008 S.Chand Publishing Company, New Delhi 3. Chawla Shashi: A text book of Engineering Chemistry (3rd Edition) 2010 Dhanpat Rai Publishing Company, New Delhi. 4. Palanna O G : A text book of Engineering Chemistry(4th Reprint) 2012 McGraw Hill, New Delhi 5. Sharma BK, Industrial Chemistry (16th Edition), 2014, Krishna Prakashan Media (P) ltd. Meerut.