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.