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USNA Chemistry Department

USNA Chemistry Department. CORROSION LECTURE. What is Corrosion?. Corrosion is the oxidation of a metal due to an ELECTROCHEMICAL reaction. The oxidizing agent is most often O 2 (atmospheric corrosion) or H + (chemical corrosion) or both. Why is it a problem?.

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USNA Chemistry Department

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  1. USNA Chemistry Department CORROSION LECTURE

  2. What is Corrosion? Corrosion is the oxidation of a metal due to an ELECTROCHEMICAL reaction. The oxidizing agent is most often O2 (atmospheric corrosion) or H+ (chemical corrosion) or both. Why is it a problem? Financial - $350 Billion Dollar Annual Problem in U.S. (4.25% of GNP) Department of Defense spends $6 – 8 Billion

  3. Electrolyte Corrosion Triangle Metal Oxidize Recipe for corrosion Active metal Water Oxygen (atmospheric corrosion) Acid (chemical corrosion) Salt High temperature

  4. Chemical vs. Atmospheric Corrosion(H+ vs. O2) Eored(V) Anodic Reaction: Fe0(s) Fe2+(aq) + 2e-Deterioration of metal Cathodic Reaction: 2H+(aq) + 2e- H2(g)Chemical O2(g) + 2H2O (l) + 4e- 4OH-(aq)Atmospheric O2(g) + 4H+(aq) + 4e- 2H2O (l)Combination Which of these will oxidize copper? Silver? Gold? Overall Reaction: Fe0(s) + 2H+(aq) Fe2+(aq) + H2(g)Chemical 2Fe0(s) + O2 (g) + 2H2O (l) 2Fe2+(aq) + 4OH-(aq)Atmospheric 2Fe0(s) + O2(g) + 4H+(aq) 2Fe2+(aq) + 2H2O (l)Combination -0.44 Ag(s) Ag+(aq) + e- +0.34 +1.50 +0.80 Au(s) Au3+(aq) + 3e- Cu(s) Cu2+(aq) + 2e- 0.00 +0.40 +1.23 Eocell (V) +0.44 +0.84 +1.67

  5. 2H2O (l) + 2e- H2(g) + 2OH-(aq) Fe0(s) + 2H2O (l) Fe2+(aq) + H2(g) + 2OH-(aq) Why won’t iron corrode in pure (degassed) water? Eo(V) Anodic Reaction: Fe0(s) Fe2+(aq) + 2e- Cathodic Reaction: -0.44 -0.83 Eocell (V) Overall reaction: -0.39 What metals will corrode in pure (degassed) water? Any sufficiently active metal Eored < -0.83 V (alkali metals, alkaline earth metals, aluminum, manganese)

  6. Example of Atmospheric Corrosion Corrosion on wing of Navy aircraft F/A-18C Hornet Why does corrosion of an airplane occur primarily while the plane is on the ground? How might this corrosion be minimized?

  7. Example of Chemical CorrosionNuclear Reactor Vessel Head Degradation • February 16, 2002, Davis-Besse Nuclear Power Station in Oak Harbor, Ohio Boric Acid leak from control rod drive mechanism led to chemical corrosion of reactor vessel head Serious potential for loss of reactor coolant access

  8. O2 + 2H2O + 4e- 4OH- Fe2+ + 2OH- Fe(OH)2 Anodic Region RUST 4Fe(OH)2 + O2 2(Fe2O3·H2O) + 2H2O Corrosion of a Ship’s HullAnodic and Cathodic Regions O2 Hull of ship Cathodic Region OH- RUST e- Fe2+ Fe Fe2+ + 2e- Electrons Migrate from Anodic to Cathodic Region

  9. anode cathode STRESSCORROSION 2Fe0(s) + O2 (g) + 2H2O (l) →2Fe2+(aq) + 4OH-(aq) Nail is embedded in a gel. Gel contains phenolphthalein and ferricyanide ion. Phenolphthalein turns pink in basic solution. Ferricyanide reacts with Fe2+ to produce a blue color.

  10. STRESSCORROSION

  11. Example of Stress CorrosionAloha Flight 243 (28 APR 1988) Lesson: Frequent inspections to detect signs of stress!

  12. Preventing Corrosion Salt High pH (> 9)

  13. Preventing the Corrosion of Iron (cathodic protection/sacrificial anode) Sacrificial anode

  14. Applications of Cathodic Protection • Galvanized Steel Zinc coating • Sacrificial Anodes Ship Hulls Subs (free flooding areas) Los Angeles Class Sub Arleigh-Burke Destroyer

  15. Controller Impressed Current Cathodic Protection Paint Layer Shipboard Power Shipboard Power Reference Electrode Hull e- e- Power Supply Pt Anode Insulation

  16. Galvanic Corrosion (contact between unlike metals; opposite of cathodic protection) H2O O2 Iron Copper Anode: Fe →Fe2+ + 2e- Cathode: O2 + 2H2O + 4e-→ 4OH-

  17. GALVANIC CORROSION

  18. There are some leaks in Sampson Hall. Upon inspection it turns out that copper (Cu) was attached to lead (Pb) on the roof. I guess it turns out when they are mixed with water (H2O) or air (N2, O2, CO2, etc) there is some kind of reaction that breaks down the properties of the copper and lead.

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