USNA Chemistry Department CORROSION LECTURE
Corrosion
Why is Corrosion Important? Corrosion is the destructive attack of a metal due to an ELECTROCHEMICAL reaction. Attack of Elements on Structural Metals Financial - $350 Billion Dollar Annual Problem (4.25% of GNP) Department of Defense spends $6 8 Billion Your future Military career USMC USN
DoD Strategic Refueling Airframe Circa 1960 Circa 2040
General Corrosion Anodic Reaction: Fe 0 (s) Fe 2+ (aq) + 2e - Deterioration of metal Cathodic Reaction: 2H + (aq) + 2e - H 2 O (l) Chemical O 2 (g) + 2H 2 O (l) + 4e - 4OH - (aq) Atmospheric Overall Reaction: Fe 0 (s) + 2H + (aq) Fe 2+ (aq) + H 2 O (l) Chemical 2Fe 0 (s) + O 2 (g) + 2H 2 O (l) 2Fe 2+ (aq) + 4OH - (aq) Atmospheric
Activity Series Reduction Half Reactions E 0 (V) Au 3+ (aq) + 3e - Au (s) +1.46 Ag + (aq) + e - Ag (s) +0.80 O 2 (g) + 2H 2 O +4e - 4OH - (aq) +0.40 Cu 2+ (aq) + 2e - Cu (s) +0.34 2H + (aq) + 2e - H 2 (g) 0.00 Sn 2+ (aq) + 2e - Sn (s) -0.14 Fe 2+ (aq) +2e - Fe (s) -0.44 Zn 2+ (aq) + 2e - Zn (s) -0.76 Al 3+ (aq) + 3e - Al (s) -1.66 Mg 2+ (aq) + 2e - Mg (s) -2.37
Corrosion to a Ship s Hull O 2 Hull of ship O 2 + 2H 2 O + 4e - 4OH - Fe 2+ + 2OH - Fe(OH) 2 OH - Cathodic Region RUST Fe(OH) 2 + xh 2 O Fe 2 O 3 xh 2 O e - Anodic Region Fe 2+ Fe Fe 2+ + 2e - Electrons Migrate from Anodic to Cathodic Region
Factors of General Corrosion 1. ph 2. Type of Metal Rate of Cathode Reaction 4.3 12 ph 3. Temperature 4. Oxygen Content
Localized Corrosion Problems Galvanic Electrical contact of dissimilar metals underwater accelerate anode corrosion and decelerates cathode corrosion Exfoliation Active metal behind surface causes corrosive products Stress Ex. B-52 wing bend Corrosion due to accelerated oxidation to metals that are subjected to strain Pitting and Crevice Atoms too far Atoms too close Primary cause of Boiler Explosion Metal Corrosion at specific sites (low flow area) Less Active Metals Pitting Anodic Region Corrosion More Active Metals Bent Metal Water Flow Crevice
Galvanic Corrosion Electrical contact of dissimilar metals underwater accelerate anode corrosion and decelerates cathode corrosion
Galvanic Corrosion Active metal behind surface causes corrosive products
Stress Corrosion Corrosion due to accelerated oxidation to metals that are subjected to strain
Pitting and Crevice Primary cause of Boiler Explosion. Metal Corrosion at specific sites (low flow area) Pitting Corrosion Crevice Corrosion
Pitting Corrosion Horizontal grain attack Vertical grain attack Narrow, deep Subsurface Shallow, wide Undercutting Elliptical
Prevention of Corrosion Separate reactants with coatings Paint, grease, oils, and polymers Coat with less active metals (tin cans, noble metals) Exclude Water Exclude Oxygen O 2 Self Protective Coatings Aluminum Al 2 O 3 Magnetite Fe 3 O 4 Stainless Steel Cr oxides
Galvanic 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 Shipboard Power Paint Layer Controller Reference Electrode Power Supply e - e - Hull Pt Anode Insulation
Real world application e + - -
Real World Examples Aloha Flight 243 (28 APR 1988) Flight 243 was a series of Hawaiian inter-island flights. First class cabin walls and ceiling were ripped from the airplane. Multiple site damage around (row 5) Corrosion of the metallic interior frame of the Boeing 737 (Stress 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 Aircraft Skin Corrosion on wing of Navy aircraft F/A-18C Hornet Because of the adverse environments Naval and Marine Aircraft operate in, corrosion of all materials is a major problem that experts are actively combating
Various Forms of Corrosion in High Strength Aluminum Alloy Structures Localized corrosion: Pits, Crevices, IGC, Exfoliation, Filiform Al-Cu-Mg Al-Cu-(Fe,Mn) Aluminum 2024-T3 4% Cu Topcoat Primer Oxide Coating Perpendicular cross section of corroded area from lap joint in KC-135
Steel Fasteners on the Wing of a KC-135 Results in Severe Corrosion of Al Alloy Fe 2+ (aq) + 2e- Fe (s) E = -0.44 V vs. Al 3+ (aq) + 3e- Al (s) E = -1.66 V O 2 (g) + 2H 2 O + 4e- 4OH- (aq) E = 0.40 V 4Al (s) + 3O 2 (g) + 6H 2 O 4Al 3+ (aq) + 12OH- (aq) E = 2.06 V!
New Mg-Rich Primers* Nano-science to the Rescue! New Mg (20 200 mm) powders with robust nano-scale oxide coating* No Corrosion in Scratch! Aircraft Aluminum Alloy Mg 2+ (aq) + 2e- Mg (s) E = -2.37 V vs. Mg powder in Primer O 2 (g) + 2H 2 O + 4e- 4OH- (aq) E = 0.40 V Al 3+ (aq) + 3e- Al (s) E = -1.66 V 2Mg (s) + O 2 (g) + 2H 2 O 2Mg 2+ (aq) + 6OH- (aq) E = 2.77 V! *currently under testing. Potential for application on JSF
Tips for Reducing Corrosion Keep the Ship/Sub/Aircraft Clean Allows for detection of corrosion in early stages Lengthens service life of finished surfaces Reduction of all types of corrosion types by eliminating the electrical path (no dirt, no electrolyte, no corrosion) Keep the bilges dry Removal of electrolyte reduces the rate of all major corrosion types Keep on top of the rust Properly prepared surfaces last longer If a fastener is corroding in place, probably due to dissimilar metal corrosion
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