Crack grows incrementally typ. 1 to 6 increase in crack length per loading cycle Failed rotating shaft --crack grew even though K max < K c --crack grows faster as  increases crack gets longer loading freq. increases. crack origin Fatigue Mechanism

Fatigue limit, S fat : --no fatigue if S < S fat Adapted from Fig. 8.19(a), Callister 7e. Fatigue Design Parameters S fat case for steel (typ.) N = Cycles to failure unsafe safe S = stress amplitude Sometimes, the fatigue limit is zero! Adapted from Fig. 8.19(b), Callister 7e. case for Al (typ.) N = Cycles to failure unsafe safe S = stress amplitude

Improving Fatigue Life 1. Impose a compressive surface stress (to suppress surface cracks from growing) N = Cycles to failure moderate tensile  m Larger tensile  m S = stress amplitude near zero or compressive  m Increasing  m --Method 1: shot peening put surface into compression shot --Method 2: carburizing C-rich gas 2. Remove stress concentrators. Adapted from Fig. 8.25, Callister 7e. bad better Adapted from Fig. 8.24, Callister 7e.

Corrosion: -- the destructive electrochemical attack of a material. -- Al Capone's ship, Sapona, off the coast of Bimini. Cost: -- 4 to 5% of the Gross National Product (GNP)* -- this amounts to just over $400 billion/yr** * H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc., **Economic Report of the President (1998). Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission. THE COST OF CORROSION

What is CORROSION? Corrosion is a natural event It represents a return of metals to their more natural state as minerals (oxides)

Metal “Wants” to be Dirt ENERGY METAL ORE

Basics of Corrosion Corrosion is essentially the oxidation of metal Need: 1.An Anode (where oxidation is taking place) 2.A Cathode (where reduction is taking place) 3.Conductive electrolyte 4.Electrical contact between the Anode and Cathode Source: Moore, J.J. Chemical Metallurgy

Electrochemistry  Corrosion is an electrochemical reaction –½ reaction at the anode : M M n+ + ne- –Possible ½ reactions at the cathode: 2H + + 2e - H 2 2H + + 2e - H 2 Acid Solutions: H 2 O + e - ½ H 2 + OH - ½ O 2 + 2H + + 2e - 2OH - ½ O 2 + 2H + + 2e - 2OH -  Important thing to note is the flow of electrons

Thermodynamic Driving Force  Like all chemical reactions – Thermodynamics  What is the driving force for the reaction? (otherwise stated as what is the electrochemical potential for the reaction) –Dissimilar metals –Different cold work states –Different grain sizes –Difference in local chemistry –Difference in the availability of species for a reaction (concentration cells) –Differential aeration cells

Derivation of Nernst Equation For: 1 1

Derivation of Nernst Equation…  Introduce: The total electropotential is  G = -nFE Where: F = Faraday’s constant (total charge on Avogadro’s number of electrons) n = the number of electrons transferred E = The electrode potential

Thermodynamics Continued Nernst Equation: THE Basic equation which describes ALL corrosion reactions For Our Example: Note: pH = -log 10 [H+]

Pourbaix Diagram  Potential vs pH  pH is the measure of [H + ] ions in solution  Map regions of thermodynamic stability for metal’s aqueous chemical species Source:

STANDARD EMF SERIES EMF series Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K V metal V o more anodic more cathodic Metal with smaller V o metal corrodes. Ex: Cd-Ni cell  V = 0.153V o M Ni 2+ solution 1.0 M Cd 2+ solution + 25°C NiCd

Galvanic Series HUNTINGTON CITY WATER, 25  C Volts: Saturated Calomel Half-Cell Reference Electrode Magnesium Manganese Cast Iron Zinc Aluminum Aluminum Alloy 5052 Mild Steel Tin Lead Nickel - Silver Copper Alloy 20Cb3 Alloy Brass Alloys Alloy 3RE Copper-Nickel Copper-Nickel Alloy EFE62 Bronze Alloys Alloy 6X Alloy 17-4PH Alloy 255 (ferrallium) Alloy 230 (Coronel) Alloy 26-1, /4 Alloys C276, G, X Alloy 254 SLX MONEL alloys 400, R405, K500 Alloy B, P, PD (Illium) INCOLOY alloy 800, 825, 840 Nickel 200, 270 Stainless Steel 304, 316, 317, 403 INCONEL alloys 600, 617, 618, 625, 671, 690, 702, X750 Titanium Alloy 700 (Jessop) V Platinum Source: Crum and Scarberry, Corrosion of Nickel Base Alloys Conference Proceedings - ASM 1985

Kinetics Describes Rate of Reaction Evan’s Diagram CORR E i

Area Effects M M + +e - iaia O2O2 AcAc ioio H+H+ AaAa ½ O 2 + H 2 O + 2e - 2OH - AaAa ioio O2O2 Crevice Effect No Crevice Effect PLUS Crevice Effect i corr in very aggresive environment Log E E M/M + E O 2 /OH + = Area Inside Crevice (Anodic) = Area Outside Crevice (Cathodic) A a << A c AaAa AcAc i + o i H A c

Effect of Oxidizer Concentration (e.g., Oxygen) on the Electrochemical Behavior of an Active - Passive Metal Log i M M + [Fontanna and Greene, Corrosion Engineering, McGraw-Hill, 1967] Increasing Oxidant Concentration

Effect of Temperature and Dissolved O 2

Types of Aqueous Corrosion Cells –General Corrosion –Localized Corrosion  Pitting  Crevice Corrosion  Under-deposit Corrosion  MIC –Tuberculation –Galvanic Corrosion

General Corrosion

  Random Creation and Destruction of Anodes and Cathodes   Movement of Anodes and Cathodes   Near Uniform Thinning   Weight Loss is a Useful Measure O O M+M+ e e - - O O M+M+ e e - - O OH M+M+ - e e - - O M+M+ M+M+ - e e e Source: Corrosion, ASM Handbook, Volume 13, 1987

General Corrosion Original Surface Penetration due to Corrosion

Localized Corrosion Carbon Steel

Localized Corrosion  Stationary Electrodes  All of the dissolution occurs in one location  Weight loss measurement – not useful  Local Penetration –Sometimes local weakening –May or may not jeopardize structural integrity –Determines “failure” M n+ CI - OH - M(OH) n OH - ee-e- Source: Corrosion, ASM Handbook, Volume 13, 1987

Potential and Current Fields in Electrolyte in the Vicinity of a Localized Corrosion Site Potential Anodic + - Localized Anodic Site Metal Cathodic Current Density