Understanding Corrosion: Causes, Types, and Prevention, Slides of Mechanics of Materials

Download Understanding Corrosion: Causes, Types, and Prevention and more Slides Mechanics of Materials in PDF only on Docsity! CORROSION docsity.com ISSUES TO ADDRESS... • Why does corrosion occur? 1 • What metals are most likely to corrode? • How do temperature and environment affect corrosion rate? • How do we suppress corrosion? CORROSION AND DEGRADATION docsity.com 4 • Two outcomes: --Metal sample mass --Metal sample mass --Metal is the anode (-) --Metal is the cathode (+) Vmetal o  0 (relative to Pt) Vmetal o  0 (relative to Pt) Standard Electrode Potential STANDARD HYDROGEN (EMF) TEST docsity.com 5 • EMF series • Metal with smaller V corrodes. • Ex: Cd-Ni cell metal o Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K +1.420 V +0.340 - 0.126 - 0.136 - 0.250 - 0.277 - 0.403 - 0.440 - 0.744 - 0.763 - 1.662 - 2.262 - 2.714 - 2.924 metal V metal o DV = 0.153V o STANDARD EMF SERIES docsity.com Cu Zn Zn2+ 2e- oxidationreduction Acid H+ H+ H+ H+ H+ H+ H+ -+ AnodeCathode 6 2H   2e H2(gas) O2  4H   4e  2H2O CORROSION IN A GRAPEFRUIT docsity.com • Stress & Saltwater... --causes cracks! • Heat treatment: slows crack speed in salt water! 4mm --material: 7150-T651 Al "alloy" (Zn,Cu,Mg,Zr) 10 DETERIORATIVE docsity.com Uniform Corrosion: Rust! Prevention:  Paint  Plate  Sacrificial anode docsity.com Galvanic Corrosion Causes: Dissimilar metals Electrolyte Current Path Described by Galvanic Series Solutions: Choose metals close in galvanic series Have large anode/cathode ratios Insulate dissimilar metals Use “Cathodic protection” docsity.com Erosion Corrosion Causes: abrasive fluids impinging on surfaces Commonly found in piping, propellers, turbine blades, valves and pumps Solutions: •Change design to minimize or eliminate fluid turbulence and impingement effects. •Use other materials that resist erosion •Remove particulates from fluids docsity.com Selective Leaching  Occurs in alloys in which one element is preferentially removed – e.g., in Brass, Zinc is electrically active and is removed, leaving behind porous Copper  Occurs in other metals, such as Al, Fe, Co, Cr Solutions: • Use protective coating to protect surfaces • Use alternative materials docsity.com Stress Corrosion stress corrosion cracking Cracks grow along grain boundaries as a result of residual or applied stress or trapped gas or solid corrosion products e.g., brasses are sensitive to ammonia Stress levels may be very low Solutions: Reduce stress levels Heat treatment Atmosphere control docsity.com 11 • Self-protecting metals! --Metal ions combine with O to form a thin, adhering 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. CONTROLLING CORROSION docsity.com Sacrificial Anode Applied Voltage Corrosive environment Zinc coatir 2 oe Cathode region 2n2* Zn2+ Lt Surface coatings & Passivation Some materials, such as Aluminum or Stainless Steel, form oxide barrier coatings that prevent oxidation at active surface – this is called “passivation” Surface can be coated with protective layers: painted, anodized, plated (Caution!!! Cracks in plating or paint can lead to crevice corrosion!) docsity.com Table 16.5 Resistance to Degradation by Various Environments for Selected Elastomeric Materials® Weather- Alkali Acid Chlorinated Aliphatic Animal, Sunlight Ozone Dilute/ Dilute/ Hydrocarbons, Hydrocarbons, Vegetable Material Aging Oxidation Cracking Concentrated _ Concentrated Degreasers Kerosene, Etc. Oils Polyisoprene (natural) D B NR ACB ACB NR NR D-B Polyisoprene (synthetic) NR B NR C-BIC-B C-BIC-B NR NR D-B Butadiene D B NR CBIC-B C-BIC-B NR NR D-B Styrene- butadiene D ¢ NR C-BIC-B C-BIC-B NR NR D-B Neoprene B A A AIA AIA D c B Nitrile (high) D B Cc BIB BB CB A B Silicone (polysiloxane) A A A AIA BIC NR D-c A “A = excellent, B = good, C = fair, D = use with caution, NR = not recommended, Source: Compound Selection and Service Guide, Seals Eastern, Inc., Red Bank, NJ, 1977. Metal Degradative concerns  High release of ionic metallic debris  Toxicity: Metal-on-metal bearings are not recommended for patients with poorly functioning kidneys because metal ions excreted through the kidneys can build up in the blood.  osteolysis and implant loosening in total hip patients with metal-on- metal bearings may be associated with hypersensitivity to metallic debris  Surface replacement with metal on metal is a new technology that has gained a great deal of recent interest. Hip surface replacement preserves more bone in the patient than conventional hip replacement. This has the potential of being a first-line treatment of end-stage arthritis in younger, active patients. docsity.com 12 • Corrosion occurs due to: --the natural tendency of metals to give up electrons. --electrons are given up by an oxidation reaction. --these electrons then are part of 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. SUMMARY docsity.com  On March 6, 2002, Workers repairing one of five cracked control rod drive mechanism (CRDM) nozzles at Davis–Besse nuclear plant discovered extensive damage to the reactor vessel head. The reactor vessel head is the dome-shaped upper portion of the carbon steel vessel housing the reactor core. It can be removed when the plant is shut down to allow spent nuclear fuel to be replaced with fresh fuel. The CRDM nozzles connect motors mounted on a platform above the reactor vessel head to control rods within the reactor vessel. Operators withdraw control rods from the reactor core to startup the plant and insert them to shut down the reactor.  The reactor core at the Davis–Besse nuclear plant sits within a metal pot designed to withstand pressures up to 17 MPa. The reactor vessel has carbon steel walls nearly 15 cm thick to provide the necessary strength. Because the water cooling the reactor contains boric acid, which is highly corrosive to carbon steel, the entire inner surface of the reactor vessel is covered with 0.6-mm thick stainless steel. But water routinely leaked onto the reactor vessel‟s outer surface. Because the outer surface lacked a protective stainless steel coating, boric acid ate its way through the carbon steel wall until it reached the backside of the inner liner. High pressure inside the reactor vessel pushed the stainless steel outward into the cavity formed by the boric acid. The stainless steel bent, but did not break. Cooling water remained inside the reactor vessel not because of thick carbon steel, but due to the thin stainless steel layer. The plant‟s owner ignored numerous warning signs spanning many years, which lead to this highly visible incident. Nuclear Reactor with a Hole in the Head docsity.com Irradiation Effect on Corrosion  Corrosion on nuclear materials in aqueous media tends to accelerate.  Mechanism: a) Radiolytic or Radiation Decomposition of Water and Salts. b) Disruption of the thin protective film on metal surface. c) Change on physical properties due to irradiation and enhancement of corrosion rate. Stress corrosion cracking (SCC) or Fatigue corrosion cracking (FCC) can also become dominant. docsity.com Irradiation Effect on Corrosion Corrosion Rate Increases exponentially with temperature. The rate increase sensitivity is higher in Irradiation environment than in normal conditions docsity.com