Corrosion
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Transcript of Corrosion
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USNA Chemistry Department
CORROSIONCORROSIONLECTURELECTURE
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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
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Recipe for corrosionActive metal
Water
Oxygen (atmospheric corrosion)
Acid (chemical corrosion)
Salt
High temperature
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Chemical vs. Atmospheric Corrosion(H+ vs. O2)
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
Eo red
(V)
Eocell (V)
-0.44
0.00+0.40
+1.23
+1.67
+0.84
+0.44
Cu(s) Cu2+(aq) + 2e- +0.34Ag(s) Ag+
(aq) + e- +0.80Au(s) Au3+(aq) + 3e- +1.50
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Why won’t iron corrode in pure (degassed) water?
Anodic Reaction:
Fe0(s) Fe2+
(aq) + 2e-
Cathodic Reaction:
Eo (V)
-0.44
-0.83
Fe0(s) + 2H2O (l) Fe2+
(aq) + H2 (g) + 2OH-(aq)
Eocell (V)
-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)
Overall reaction:
2H2O (l) + 2e- H2 (g) + 2OH-(aq)
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Example of Atmospheric Corrosion
Corrosion on wing of Navy aircraft
Why does corrosion of an airplane occur primarily while the plane is on the ground?
How might this corrosion be minimized?
F/A-18C Hornet
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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
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Corrosion of a Ship’s HullAnodic and Cathodic Regions
O2
O2 + 2H2O + 4e- 4OH-
OH-
Fe2+
Fe2+ + 2OH- Fe(OH)2
4Fe(OH)2 + O2 2(Fe2O3·H2O) + 2H2O
Fe Fe2+ + 2e-
Hull of ship
Cathodic Region
RUST
Anodic Region
Electrons Migrate from Anodic to Cathodic Region
e-
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STRESS CORROSION
Nail is embedded in a gel.
Gel contains phenolphthaleinand ferricyanide ion.
Phenolphthalein turns pink inbasic solution.
Ferricyanide reacts with Fe2+
to produce a blue color.
2Fe0(s) + O2 (g) + 2H2O (l) → 2Fe2+
(aq) + 4OH-(aq)
anode
cathode
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STRESS CORROSION
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Example of Stress CorrosionAloha Flight 243 (28 APR 1988)
Lesson:
Frequent inspectionsto detect signs of stress!
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High pH (> 9)
Preventing Corrosion
Salt
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Preventing the Corrosion of Iron Preventing the Corrosion of Iron (cathodic protection/sacrificial anode)(cathodic protection/sacrificial anode)
Sacrificial anode
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Applications of Cathodic Protection
• Galvanized SteelZinc coating
• Sacrificial AnodesShip HullsSubs (free flooding areas)
Los Angeles Class Sub
Arleigh-Burke Destroyer
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Impressed Current Cathodic Protection
Power Supply
Shipboard Power
Controller
Shipboard Power
Insulation
Pt Anode
Reference Electrode
PaintLayer
Hull
e-e-
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Galvanic Corrosion Galvanic Corrosion (contact between unlike metals;(contact between unlike metals;opposite of cathodic protection)opposite of cathodic protection)
Copper Iron
H2OO2
Cathode:O2 + 2H2O + 4e- → 4OH-
Anode:Fe → Fe2+ + 2e-
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GALVANIC CORROSION
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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.