USNA Chemistry Department

CORROSIONCORROSIONLECTURELECTURE

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 corrosionActive metal

Water

Oxygen (atmospheric corrosion)

Acid (chemical corrosion)

Salt

High temperature

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

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)

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

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

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-

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

STRESS CORROSION

Example of Stress CorrosionAloha Flight 243 (28 APR 1988)

Lesson:

Frequent inspectionsto detect signs of stress!

High pH (> 9)

Preventing Corrosion

Salt

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

Sacrificial anode

Applications of Cathodic Protection

• Galvanized SteelZinc coating

• Sacrificial AnodesShip HullsSubs (free flooding areas)

Los Angeles Class Sub

Arleigh-Burke Destroyer

Impressed Current Cathodic Protection

Power Supply

Shipboard Power

Controller

Shipboard Power

Insulation

Pt Anode

Reference Electrode

PaintLayer

Hull

e-e-

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-

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.