Engineering Chemistry Lecture by USNA

1. USNA Chemistry Department
CORROSIONCORROSION
LECTURELECTURE

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. Recipe for corrosion
Active metal
Water
Oxygen
(atmospheric corrosion)
Acid
(chemical corrosion)
Salt
High temperature

4. 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)
Eo
cell (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

5. 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)
Eo
cell (V)
-0.39
What metals will corrode in pure (degassed) water?
Any sufficiently active metal Eo
red < -0.83 V
(alkali metals, alkaline earth metals, aluminum, manganese)
Overall reaction:
2H2O (l) + 2e-
H2 (g) + 2OH-
(aq)

6. 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

7. 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

8. Corrosion of a Ship’s Hull
Anodic 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-

9. STRESS CORROSION
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.
2Fe0
(s) + O2 (g) + 2H2O (l) → 2Fe2+
(aq) + 4OH-
(aq)
anode
cathode

10. STRESS CORROSION

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

12. High pH (> 9)
Preventing Corrosion
Salt

13. Preventing the Corrosion of IronPreventing the Corrosion of Iron
(cathodic protection/sacrificial anode)(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. Impressed Current Cathodic
Protection
Power Supply
Shipboard Power
Controller
Shipboard Power
Insulation
Pt Anode
Reference
Electrode
Paint
Layer
Hull
e-
e-

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

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.