Ch 17 pt 2

Ch 17 pt 2 - ANNOUNCEMENTS - 1 Reading: Chapter 17, all...

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Unformatted text preview: ANNOUNCEMENTS - 1 Reading: Chapter 17, all sections except 17.10 Week 7 Recitations: Quiz on Chapter 14 + Review of Midterm 1 Problems: 17.1, 17.4, 17.6,17.8, 17.11, 17.33 No Quiz on these in Recitation - included in Midterm 2 Midterm 2: Tuesday, May 18, 8:00-8:50 am, Main Auditorium Accommodations: LeBow 348; 8:00-9:15 (1.5X) or 8:00-9:40 am (2X) Chapters 12, 14, 17 + sprinkling of earlier topics TA Help Session: Wednesday (Today!) 5-12-10, 4-6 pm, CAT 061 Chapter 17 - 15 GALVANIC SERIES more anodic (active) more cathodic (inert) • Ranks relative reactivity of various metals/alloys in seawater More realistic & practical than standard emf series Platinum Gold Graphite Titanium Silver 316 Stainless Steel Nickel (passive) Copper Nickel (active) Tin Lead 316 Stainless Steel Iron/Steel Aluminum Alloys Cadmium Zinc Magnesium/Mg Alloys Based on Table 17.2, Callister 7e. (Source of Table 17.2 is M.G. Fontana, Corrosion Engineering, 3rd ed., McGrawHill Book Company, 1986.) Chapter 17 - 16 CORROSION RATES • EMF & Galvanic Series: Reflect systems and rank materials under equilibrium conditions Thermodynamic data…driving force Can indicate spontaneous reaction directions But not corrosion rate • Real-World Engineering: Generally not in equilibrium e- flow from anode to cathode via external circuit Chapter 17 - 17 CORROSION PENETRATION RATE • CPR: Practical consideration…empirical formula Rate of material removal Useful for design… KW CPR = At W = weight loss after time t; and A = density and exposed sample area, respectively K = a unit-dependent constant CPR either mils per year (mpy) or millimeters per year (mm/yr) CPR mpy mm/yr K 534 87.6 W mg mg g/cm3 g/cm3 A in2 cm2 t hrs hrs Chapter 17 - 18 PASSIVITY • Some metals lose chemical reactivity and become very inert: Cr, Fe, Ni, Ti + many of their alloys Why? Formation of highly adherent, v. thin oxide film on metal surface Protects against further corrosion e.g. S-Steel…~11% Cr, minimizes rusting, forms passive surface film in oxidizing environments Caution…change the environment and ??? Al also passivates…film reforms v. rapidly if damaged Chapter 17 - 19 FORMS OF CORROSION • Stress Corrosion • Uniform Attack Oxidation & reduction occur uniformly over surface (general rusting) Stress & corrosion work together at crack tips • Intergranular Corrosion along grain boundaries, often where special phases (Cr23C6) exist g.b. prec. attacked zones Fig. 17.18, Callister 7e. Mech. abrasion of passivating layer + chemical attack (e.g. at pipe elbows) • Pitting • Selective Leaching Preferred corrosion of solid solns - 1 element/constituent (e.g., Zn from brass (Cu-Zn)) • Erosion-Corrosion Forms of Corrosion • Galvanic Dissimilar metals are physically joined. The more anodic one corrodes.(see Table 17.2) Zn & Mg very anodic Downward propagation of small pits & holes Fig. 17.17, Callister 7e. (Fig. 17.17 from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.) • Crevice Conc. diffs. bet. 2 pieces of the same metal Rivet holes Fig. 17.15, Callister 7e. (Fig. 17.15 is courtesy LaQue Center for Corrosion Technology, Inc.) Chapter 17 - 20 CONTROLLING CORROSION Metal oxide Metal (e.g., Al, stainless steel) • Self-protecting metals! Metal ions combine with O to form a thin, adherent oxide layer that slows corrosion • Reduce T (slows kinetics of oxidation and reduction) • Add Inhibitors: Slow oxidation/reduction reactions by removing reactants (e.g., remove O2 gas by reacting it w/an inhibitor) Slow oxidation reaction by attaching species to the surface (e.g., paint it!). • Cathodic (or sacrificial) protection: Attach a more anodic material to the one to be protected Adapted from Fig. 17.23, Callister 7e. e.g., Zinc-coated nail Zn 2+ zinc zinc 2 e - 2e steel e.g., Mg Anode steel pipe e- Cu wire Mg Mg 2+ anode Earth Adapted from Fig. 17.22(a), Callister 7e. (Fig. 17.22(a) is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Co., 1986.) Chapter 17 - 21 CATHODIC PROTECTION • V. effective against ~ all forms of corrosion: Supply, from ext. source, e- to metal being protected, making it the cathode, forcing normal oxidation reaction M Mn+ + nein reverse (reduction) direction • Galvanic Couple: • Electrically connect item to a more reactive (anodic) metal • Latter is oxidized (corroded)…sacrificial anode • commonly Zn and Mg…anodic end of galvanic series • galvanizing…dip into molten Zn – examples? • Zn does corrode, but v. slowly due to high anode-to-cathode surface area ratio • Passive technique Chapter 17 - 22 CATHODIC PROTECTION • Active Protection: Supply e-, from ext. DC power supply -ve connected to structure to be protected +ve connected to inert anode, e.g. e.g. a piece of graphite, buried in the ground. Current path between cathode & anode via the soil. Impressed Current Cathodic (ICC) protection: o Underground storage tanks o Buried metal pipes o Marine structures Chapter 17 - 23 POLYMER DEGRADATION Polymers: Do interact with environment Known as “degradation” rather than corrosion Different from metals…electrochemical Physiochemical in nature for polymers Wider range of reactions: – Swelling and dissolution – Covalent bond rupture…heat, chemical, radiation – Loss of mechanical integrity – More complex – e.g. polyethylene, when heated in O2 atm., becomes brittle… Chapter 17 - 24 SWELLING & DISSOLUTION Polymers exposed to liquids Liq. solute diffuses into polymer: small solute molecules occupy positions along chain macromolecules (chains) forced apart… material expands or swells consequences of increased chain separation? – reduced secondary bonding between chains – material becomes softer and more ductile – lowers Tg…rubbery & weak Swelling = partial dissolution w. limited solubility Dissolution = complete solubility Inc. similarity bet. solvent & polymer inc. swelling – e.g. rubbers absorb H-C liquids (gasoline) Chapter 17 - 25 BOND RUPTURE Degradation of Polymers by Scission: breakage/rupture of bonds within molecular chain usually due to heat, radiation or chemical reaction Consequences? chain separation reduction in MW reduction of mechanical properties Chapter 17 - 26 RADIATION EFFECTS Radiation types? e- beams, X-rays, particles, rays, UV… Interact with atoms (& e-) in polymer chains: Ionization Removal of orbital e-; converts atom to +ve ion Breaks 1 covalent bond + rest of atoms/bonds rearrange: – scission, or – cross-linking Stabilizers added to reduce radiation damage sensitivity Most damage due to UV. Polymers become: brittle (polymer sheets) discolored (windows) c-r-a-c-k-e-d (dashboards) but not always bad news: inc. crosslinking ( rays) can improve mech. props. Chapter 17 - 27 CHEMICAL EFFECTS O2, O3 Exposure: can cause/accelerate chain scission e.g. vulcanized rubber: – O3 exposure, = bond in backbone chain – – – – chain severed at = bond. cracks form if material under stress rupture e.g. bicycle tire walls; old rubber bands ozone + other free radicals in atmosphere - smog Chapter 17 - 28 THERMAL EFFECTS Increased Temperatures: can also cause/accelerate chain scission gases may be evolved shows up as mass loss thermal stability relates to? – magnitude of bond energy between atoms in polymer – C-F bond energy > C-H > C-Cl – Fluorocarbons (C-F) = most thermally resistant polymers – PVC heated above ~200 °C HCl…not good! – Add stabilizers (e.g. ZnO) to react w. HCl Weathering: Combined effects of # processes: oxidation, UV, water absorption… Chapter 17 - 29 SUMMARY • Corrosion occurs due to: the natural tendency of metals to give up electrons electrons are given up by an oxidation reaction these electrons then used in a reduction reaction • Metals with a more negative Standard Electrode Potential are more likely to corrode relative to other metals • The Galvanic Series ranks the reactivity of metals in seawater • Increasing T speeds up oxidation/reduction reactions • Corrosion may be controlled by: using metals which form a protective oxide layer reducing T adding inhibitors painting using cathodic protection Chapter 17 - 30 ...
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