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Corrosion

Corrosion. Objectives Corrosion process Environmental factors Common forms of corrosion Methods of corrosion control and prevention. Dr Suban K Sahoo MSc, NET (JRF), PhD. Metal Corrosion. “The destruction of a material by chemical or electrochemical reaction to its environment”

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Corrosion

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  1. Corrosion Objectives Corrosion process Environmental factors Common forms of corrosion Methods of corrosion control and prevention Dr Suban K Sahoo MSc, NET (JRF), PhD

  2. Metal Corrosion • “The destruction of a material by chemical or electrochemical reaction to its environment” • typically a transfer of electrons from one metal to another through an Oxidation-Reduction Reaction. • Slow process but spontaneous

  3. Oxidation - Reduction • Anodic metal gives up electrons (oxidation) • Cathodic metal accepts electrons (reduction) • Or gases accept electrons (reduction) -- in an acid solution -- in a neutral or base solution

  4. Corrosion may be… • dry/chemical corrosion…direct action of gases such as oxygen, halogen, H2S, H2SO4, liq. metal etc..generally in the absence of moisture • Metal oxide formed on the metal surface decide the further action: stable, unstable, volatile, porous etc Pilling-Bedworth rule: An oxide is protective or non-protective porous, if the volume of the oxide is at-least as great as the volume of the metal from which it is formed’ Li, K, Na, Mg etc formed oxide volume less than metal volume but Al formed higher oxide volume

  5. Wet/Electrochemical corrosion • Corrosion involves : (i) presence of a conducting medium, (ii) the formation of anodic and cathodic areas or parts may be contact with each other, (iii) corrosion occurs at anodic areas Rust

  6. Galvanic corrosion…. • Occurs when two dissimilar metals (exa. Zn and Cu) are connected and exposed to an electrolyte: the metal higher in electrochemical series undergoes corrosion Concentration cell corrosion This type of corrosion is due to electrochemical attack on the metal surface, exposed to an electrolyte of varying concentrations or of varying aeration.

  7. STANDARD EMF SERIES • Metal with smaller V corrodes. • Ex: Cd-Ni cell • EMF series o V o metal metal metal Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K +1.420 V +0.340 - 0.126 - 0.136 - 0.250 - 0.277 - 0.403 - 0.440 - 0.744 - 0.763 - 1.662 - 2.262 - 2.714 - 2.924 o DV = 0.153V 5

  8. Acceleration of Corrosion • Physical Characteristics • exposed area (less, increases corrosion rate) • time of exposure (more time, more corrosion) • Environmental Characteristics • acidic environment • sulfur gas environment • temperature (high temps, more corrosion) • moisture (oxygenated moisture)

  9. Passivation or Passivity • A protective film in oxidizing atmospheres • chromium,nickel, titanium, aluminum • Metal oxide layer adheres to parent metal • barrier against further damage • self-healing if scratched • Sensitive to environmental conditions • passivated metal may have high corrosion rates

  10. FORMS OF CORROSION • Stress corrosion Stress & corrosion work together at crack tips. • Uniform Attack Oxidation & reduction occur uniformly over surface. • Erosion-corrosion Break down of passivating layer by erosion (pipe elbows). • Selective Leaching Preferred corrosion of one element/constituent (e.g., Zn from brass (Cu-Zn)). • Pitting Downward propagation of small pits & holes. Localized accelerated attack • Intergranular Corrosion along grain boundaries, often where special phases exist. • Galvanic Dissimilar metals are physically joined. The more anodic one corrodes. Zn & Mg very anodic. • CreviceBetween two pieces of the same metal. Waterline and microbiological corrosion 9

  11. Corrosion of Metals in ConcreteReinforcing Steel & Prestressing Steel • Concrete is Normally Highly Alkaline • Protects Steel from Rusting if Properly Embedded • If Corrosion Occurs, the Reaction Products are greater in Volume Than the Original Steel • Corrosion Initiation and Rate Depends On • Amount of Concrete Cover, Quality of Concrete • Details of Construction, & Exposure to Chlorides

  12. Avoiding Corrosive Situations • Choose couple metals close on the galvanic series • Use large anode, and small cathode areas • Electrically insulate dissimilar metals • Connect a more anodic metal to the system • Avoid turbulent flow and impingements in pipe systems

  13. Prevention of Corrosion Basic goal   protect the metal  avoid localized corrosion • When possible chose a nobler metal • Avoid electrical / physical contact between metals with very different electrode potentials (avoid formation of a galvanic couple) • If dissimilar metals are in contact make sure that the anodic metal has a larger surface area / volume • In case of microstructural level galvanic couple, try to use a course microstructure (where possible) to reduce number of galvanic cells formed • Modify the base metal by alloying • Protect the surface by various means • Modify the fluid in contact with the metal Remove a cathodic reactant (e.g. water) Add inhibitors which from a protective layer • Cathodic protection Use a sacrificial anode (as a coating or in electrical contact) Use an external DC source in connection with a inert/expendable electrode

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