CHAPTER 16: CORROSION AND DEGRADATION

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?CHAPTER 16:CORROSION AND DEGRADATION

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2 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**THE COST OF CORROSION

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3 Two reactions are necessary: -- oxidation reaction: -- reduction reaction:

Other reduction reactions:-- in an acid solution-- in a neutral or base solution

CORROSION OF ZINC IN ACID

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4 Two outcomes:--Metal sample mass

--Metal sample mass

--Metal is the anode (-)--Metal is the cathode (+)

(relative to Pt)

(relative to Pt)

Standard Electrode PotentialSTANDARD HYDROGEN (EMF) TEST

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5 EMF series Metal with smaller V corrodes. Ex: Cd-Ni cellmetalo

AuCuPbSnNiCoCdFeCrZnAlMgNaK+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- 2.924metalVmetalo

DV = 0.153VoSTANDARD EMF SERIES

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CORROSION IN A GRAPEFRUIT

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7 Ex: Cd-Ni cell with standard 1M solutions Ex: Cd-Ni cell with non-standard solutions

n = #e-per unitoxid/redreaction(=2 here)F = Faraday'sconstant=96,500C/mol. Reduce VNi - VCd by --increasing X --decreasing YEFFECT OF SOLUTION CONCENTRATION

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Ranks the reactivity of metals/alloys in seawater

PlatinumGoldGraphiteTitaniumSilver316 Stainless SteelNickel (passive)CopperNickel (active)TinLead316 Stainless SteelIron/SteelAluminum AlloysCadmiumZincMagnesium8GALVANIC SERIES

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Uniform AttackOxidation & reductionoccur uniformly oversurface. Selective LeachingPreferred corrosion ofone element/constituent(e.g., Zn from brass (Cu-Zn)). IntergranularCorrosion alonggrain boundaries,often where specialphases exist. Stress corrosionStress & corrosionwork togetherat crack tips. GalvanicDissimilar metals arephysically joined. Themore anodic onecorrodes.(see Table17.2) Zn & Mgvery anodic. Erosion-corrosionBreak down of passivatinglayer by erosion (pipeelbows). PittingDownward propagationof small pits & holes. Crevice Between twopieces of the same metal.

FORMS OF CORROSION

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Stress & Saltwater... --causes cracks!

Heat treatment: slows crack speed in salt water!

4mm--material: 7150-T651 Al "alloy" (Zn,Cu,Mg,Zr)10

DETERIORATIVE

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Uniform Corrosion: Rust!Prevention:PaintPlateSacrificial anode

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Galvanic CorrosionCauses:Dissimilar metalsElectrolyteCurrent PathDescribed by Galvanic SeriesSolutions:Choose metals close in galvanic seriesHave large anode/cathode ratiosInsulate dissimilar metalsUse Cathodic protection

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Pitting and Creviced CorrosionPrevention:Weld dont rivetUse non-absorbing gasketsPolish surfacesAdd drains avoid stagnant waterAdjust composition; e.g., add Mo to SSCauses: concentration gradients in electrolyte cause some areas high in ion concentrations that accelerate oxidation

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Intergranular CorrosionOccurs in specific alloys precipitation of corrosive specimens along grain boundaries and in particular environmentse.g. : Chromium carbide forming in SS, leaving adjacent areas depleted in Cr

Solutions: High temp heat treat to redissolve carbides Lower carbon content (in SS) to minimize carbide formationAlloy with a material that has stronger carbide formation (e.g., Ti or Nb)

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Erosion CorrosionCauses: abrasive fluids impinging on surfacesCommonly found in piping, propellers, turbine blades, valves and pumpsSolutions: Change design to minimize or eliminate fluid turbulence and impingement effects.Use other materials that resist erosionRemove particulates from fluids

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Selective LeachingOccurs in alloys in which one element is preferentially removed e.g., in Brass, Zinc is electrically active and is removed, leaving behind porous CopperOccurs in other metals, such as Al, Fe, Co, CrSolutions:Use protective coating to protect surfacesUse alternative materials

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Stress CorrosionAka: stress corrosion crackingCracks grow along grain boundaries as a result of residual or applied stress or trapped gas or solid corrosion productse.g., brasses are sensitive to ammoniaStress levels may be very lowSolutions: Reduce stress levelsHeat treatmentAtmosphere control

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Hydrogen EmbrittlementMetals loose strength when Hydrogen is absorbed through surface, especially along grain boundaries and dislocationsOften occurs as a result of decorative platingHigh strength steels particularly susceptibleCan be removed by baking the alloy

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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.Adapted from Fig. 17.14, Callister 6e.CONTROLLING CORROSION

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Corrosion preventionSacrificial AnodeApplied Voltage

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Surface coatings & Passivation

Some materials, such as Aluminum or Stainless Steel, form oxide barrier coatings that prevent oxidation at active surface this is called passivationSurface can be coated with protective layers: painted, anodized, plated (Caution!!! Cracks in plating or paint can lead to crevice corrosion!)

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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

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