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 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
Recipe for corrosion
Active metal
Water
Oxygen
(atmospheric corrosion)
Acid
(chemical corrosion)
Salt
High temperature
Chemical vs. Atmospheric Corrosion
(H+ vs. O2)
Eo red (V)
Anodic Reaction:
Ag
Fe
Au0(s)
Cu
(s)
(s)
- -3+ ++e3e
Ag
Fe+2+
Au
Cu
(aq)
(aq) + 2e Deterioration of metal
(aq)
-0.44
+1.50
+0.34
+0.80
Cathodic Reaction:
2H+(aq) + 2e-
H2 (g) Chemical
O2 (g) + 2H2O (l) + 4eO2 (g) + 4H+(aq) + 4e-
4OH-(aq) Atmospheric
2H2O (l) Combination
0.00
+0.40
+1.23
Which of these will oxidize copper? Silver? Gold?
Eocell (V)
Overall Reaction:
Fe0(s) + 2H+(aq)
Fe2+(aq) + H2 (g) Chemical
2Fe0(s) + O2 (g) + 2H2O (l)
2Fe0(s) + O2 (g) + 4H+(aq)
2Fe2+(aq) + 4OH-(aq) Atmospheric
2Fe2+(aq) + 2H2O (l) Combination
+0.44
+0.84
+1.67
Why won’t iron corrode in pure (degassed) water?
Eo (V)
Anodic Reaction:
Fe0(s)
Fe2+(aq) + 2e-
-0.44
Cathodic Reaction:
2H2O (l) + 2e-
H2 (g) + 2OH-(aq)
Overall reaction:
Fe0(s) + 2H2O (l)
Fe2+(aq) + H2 (g) + 2OH-
-0.83
Eocell (V)
-0.39
(aq)
What metals will corrode in pure (degassed) water?
Any sufficiently active metal Eored < -0.83 V
(alkali metals, alkaline earth metals, aluminum, manganese)
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?
Example of Chemical Corrosion
Nuclear 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
Corrosion of a Ship’s Hull
Anodic and Cathodic Regions
O2
Hull of ship
O2 + 2H2O + 4e-
Cathodic Region
4OHOH-
Fe2+ + 2OH-
4Fe(OH)2 + O2
RUST
Fe(OH)2
2(Fe2O3·H2O) + 2H2O
e-
Anodic Region
Fe2+
Fe
Fe2+ + 2e-
Electrons Migrate from
Anodic to Cathodic Region
STRESS CORROSION
2Fe0(s) + O2 (g) + 2H2O (l)
anode
→
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.
cathode
STRESS CORROSION
Example of Stress Corrosion
Aloha Flight 243 (28 APR 1988)
Lesson:
Frequent inspections
to detect signs of stress!
Preventing Corrosion
Salt
High pH (> 9)
Preventing the Corrosion of Iron
(cathodic protection/sacrificial anode)
Sacrificial anode
Applications of Cathodic Protection
• Galvanized Steel
Zinc coating
• Sacrificial Anodes
Ship Hulls
Subs (free flooding areas)
Los Angeles Class Sub
Arleigh-Burke Destroyer
Impressed Current Cathodic
Protection
Shipboard Power
Paint
Layer
Shipboard Power
Controller
Reference
Electrode
Hull
Power Supply
e-
ePt Anode
Insulation
Galvanic Corrosion
(contact between unlike metals;
opposite of cathodic protection)
O2
Copper
Cathode:
O2 + 2H2O + 4e- → 4OH-
H2O
Iron
Anode:
Fe → Fe2+ + 2e-
GALVANIC CORROSION
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.