USNA Chemistry Department CORROSION LECTURE OK to have attention grabbers at the beginning.

What is Corrosion? Why is it a problem? 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. But they can be distracting if used too much. Poor contrast here. I changed font sizes and size of picture 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 Electrolyte Metal Active metal Water Oxygen (atmospheric corrosion) Acid (chemical corrosion) Salt High temperature Corrosion Triangle Electrolyte Metal Oxidize

Chemical vs. Atmospheric Corrosion (H+ vs. O2) Eo red (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 Ag(s) Ag+(aq) + e- Cu(s) Cu2+(aq) + 2e- Au(s) Au3+(aq) + 3e- +1.50 +0.34 -0.44 +0.80 0.00 +0.40 +1.23 Eocell (V) +0.44 +0.84 +1.67

Why won’t iron corrode in pure (degassed) water? Eo (V) Anodic Reaction: Fe0(s) Fe2+(aq) + 2e- Cathodic Reaction: -0.44 2H2O (l) + 2e- H2 (g) + 2OH-(aq) -0.83 Eocell (V) Overall reaction: Fe0(s) + 2H2O (l) Fe2+(aq) + H2 (g) + 2OH-(aq) -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)

Example of Atmospheric Corrosion Corrosion on wing of Navy aircraft F/A-18C Hornet Contrast. What exactly is shown in the pictures? Too wordy. 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 Hull of ship O2 + 2H2O + 4e- 4OH- Cathodic Region OH- RUST Fe2+ + 2OH- Fe(OH)2 4Fe(OH)2 + O2 2(Fe2O3·H2O) + 2H2O RUST Anodic Region e- Fe2+ Fe Fe2+ + 2e- Electrons Migrate from Anodic to Cathodic Region

STRESS CORROSION 2Fe0(s) + O2 (g) + 2H2O (l) → 2Fe2+(aq) + 4OH-(aq) anode 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 The gel has been prepared with a little phenolphthalein indicator and a little potassium ferricyanide mixed in. As corrosion occurs, the hydroxide ion produced in the cathodic region turns the phenolphthalein pink while the ferrous and ferric ions produced in the anodic region react with the ferricyanide to make Prussian blue (the deep blue color appears almost black on the photographs).

STRESS CORROSION

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

Preventing Corrosion Salt Figure: 20-25 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 Paint Layer Shipboard Power Shipboard Power Controller Reference Electrode Hull e- e- Power Supply Pt Anode Insulation

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

GALVANIC CORROSION

There are some leaks in Sampson Hall 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.