P3 5 Corrosion y Degradacion

7/26/2019 P3 5 Corrosion y Degradacion

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Chapter 17 -

IMC207: CIENCIA DE LOSMATERIALES I

Corrosin y degradacin

a

Henry A. Colorado, PhD

Mechanical Engineering Department

Universidad de Antioquia

From, Materials science and engineering an introduction, William D. Callister (Wiley)Chapter 17 -

What is Corrosion?Corrosion is the oxidation of a metal due to anELECTROCHEMICAL reaction. The oxidizing agent ismost often O2 (atmospheric corrosion) or H

+ (chemicalcorrosion) or both.

Why is it a problem?Financial - $350 Billion Dollar Annual Problem in U.S.(4.25% of GNP) Department of Defense spends $68 Billion

Chapter 17 -

Corrosion Mechanism

Cathodic cell

discussion of emf

galvanic series

intergranular corrosion

oxidation-reduction of iron

salt effects

Chapter 17 -

Basics of Corrosion

EMF series is a numeric rating of potentialunder ideal conditions

Galvanic Series is a practical listing

Galvanic Protection

Chapter 17 -

EL COSTO DE LA CORROSION

Chapter 17 -

Recipe for corrosion

Active metal

Water

Oxygen(atmospheric corrosion)

Acid(chemical corrosion)

Salt

High temperature


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Chapter 17 -

Oxidation - Reduction Reduction

Anodic metal gives up electrons(oxidation)

Cathodic metal accepts electrons(reduction)

Or gases accept electrons (reduction)

Fe Fe e 2 2

Al Al e 3 3

Cu e Cu2 2

2 2 2H e H gas

( )

Chapter 17 -

Chemical vs. Atmospheric Corrosion(H+ vs. O2)

Anodic Reaction:

Fe0(s) Fe2+

(aq) + 2e- Deterioration of metal

Cathodic Reaction:

2H+(aq) + 2e- H2 ( g) Chemical

O2 ( g) + 2H2O (l) + 4e- 4OH-(aq) Atmospheric

O2 ( g) + 4H+

(aq) + 4e- 2H2O (l) Combination

Which of these wil l oxidize copper? Silver? Gold?

Overall Reaction:

Fe0(s) + 2H+

(aq) Fe2+

(aq) + H2 ( g) Chemical

2Fe0(s) + O2 (g) + 2H2O (l) 2Fe2+

(aq) + 4OH-(aq) Atmospheric

2Fe0(s) + O2 ( g) + 4H+

(aq) 2Fe2+

(aq) + 2H2O (l) Combination

Eo red(V)

Eocell (V)

-0.44

0.00

+0.40

+1.23

+1.67

+0.84

+0.44

Cu(s) Cu2+

(aq) + 2e-

+0.34

Ag(s) Ag+

(aq) + e-

+0.80

+1.50

Chapter 17 -

Why wont iron corrode in pure (degassed) water?

Anodic Reaction:

Fe0(s) Fe2+

(aq) + 2e-

Cathodic Reaction:

Eo (V)

-0.44

-0.83

Fe0(s) + 2H2O (l) Fe2+

(aq) + H2 ( g) + 2OH-

(aq)

Eocell (V)

-0.39

What metals will corrode in pure (degassed) water?

Any sufficiently active metal Eored < -0.83 V

(alkali metals, alkaline earth metals, aluminum, manganese)

Overall reaction:

2H2O (l) + 2e- H2 ( g) + 2OH

-(aq)

Chapter 17 -

Steel Corrosion

2 2 22 2 2Fe O H O Fe OH ( )

2 1

2

22 2 2 3Fe OH O H O Fe OH( ) ( )

Initial Oxidation Reaction

Secondary Oxidation Reaction

Rust

Chapter 17 -

Example of AtmosphericCorrosion

Corrosion on wing of Navy aircraft

Why does corrosion of anairplane occur primarilywhile the plane is on theground?

F/A-18C Hornet

Chapter 17 -

Example of Chemical CorrosionNuclear Reactor Vessel Head

Degradation February 16, 2002, Davis-Besse Nuclear Power Station in Oak Harbor,

Ohio Boric Acid leak fromcontrol rod drivemechanism led tochemical corrosion ofreactor vessel head

Serious potential for loss ofreactor coolant access


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Chapter 17 -

Corrosion of a Ships Hull

Anodic and Cathodic Regions

O2

O2

+ 2H2

O + 4e- 4OH-

OH-

Fe2+

Fe2+ + 2OH-

Fe(OH)2

4Fe(OH)2 + O2 2(Fe2O3H2O)+ 2H2O Fe Fe2+ + 2e-

Hull ofship

CathodicRegionRUST

AnodicRegion

Electrons Migratefrom Anodic toCathodic Region

e-

Chapter 17 -High pH (> 9)

Preventing Corrosion

Salt

Chapter 17 -

Preventing the Corrosion of Iron

(cathodic protection/sacrificial anode)

Sacrificial anode

Chapter 17 -

Applications of Cathodic Protection

Galvanized Steel

Zinc coating

Sacrificial Anodes

Ship Hulls

Subs (free flooding areas)

Los Angeles Class Sub

Arleigh-Burke Destroyer

Chapter 17 -

Impressed Current CathodicProtection

Power Supply

Shipboard Power

Controller

Shipboard Power

Insulation

Pt Anode

ReferenceElectrode

Paint

Layer

Hull

e-e-

Chapter 17 -

Galvanic Corrosion

(contact between unlike metals;

opposite of cathodic protection)

Copper Iron

H2OO2

Cathode:O2 + 2H2O + 4e

- 4OH-

Anode:Fe Fe2+ + 2e-


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Chapter 17 - 19

Corrosion:-- the destructive electrochemical attack of a material.--Al Capone's

ship, Sapona,off the coastof 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 Introductionto Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc.,1985.**Economic Report of the President (1998).

Photos courtesy L.M. Maestas, SandiaNational Labs. Used with permission.

EL COSTO DE LA CORROSION

Chapter 17 - 20

Se requieren dos reacciones:-- oxidacion:-- reduccion:

Zn Zn2

2e

2H

2e H2(gas)

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

O2 4H

4e 2H2O O2 2H2O4e

4(OH)

Adapted from Fig. 17.1, Callister 7e.(Fig. 17.1 is from M.G. Fontana,Corrosion Engineering, 3rd ed.,McGraw-Hill Book Company, 1986.)

COROSION DE ZINC EN ACIDO

Zinc

Oxidation reactionZn Zn2+

2e-Acidsolution

reduction reaction

H+H+

H2(gas)

H+

H+

H+

H+

H+

flow of e-

in the metal

Chapter 17 -

STANDARD HYDROGEN (EMF) TEST

Chapter 17 - 22

STANDARD HYDROGEN (EMF) TEST

Two outcomes:

0ometalV (relative to Pt)

Standard Electrode Potential

--Metal sample mass

--Metal is the cathode (+)

Mn+

ions

ne-

e- e-

25C1M Mn+ soln 1M H+ soln

Platinum

metal,M

H+

H+2e-

Adapted from Fig. 17.2, Callister 7e.

0ometalV (relative to Pt)

--Metal sample mass

--Metal is the anode (-)

Platinum

metal,M Mn+

ions

ne-H2(gas)

25C1M Mn+ soln 1M H+ soln

2e-

e-e-

H+

H+

Chapter 17 - 23

STANDARD EMF SERIES EMF series

Au

CuPbSnNiCoCdFeCrZnAlMgNaK

+1.420 V

+0.340- 0.126- 0.136- 0.250- 0.277- 0.403- 0.440- 0.744- 0.763- 1.662- 2.363- 2.714- 2.924

metal Vmetalo

Data based on Table 17.1,Callister 7e.

moreanodic

morecathodic

metalo

Metal with smallerV corrodes.

Ex: Cd-Ni cell

DV =0.153V

o

Adapted from Fig. 17.2, Callister 7e.

-

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

+

25C NiCd

Chapter 17 -


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Chapter 17 -

Potentiodynamic Polarization

Curve

Chapter 17 -

Corrosion potential calculation

Reduction Reaction must have higherpotential than the oxidation reaction orthey will not form a cathodic cell

Fe e Fe

2

2 -0.440 V

Zn e Zn

2

2 -0.763 V

DV V . . .440 763 0 323 Relative measure ofcorrosion

Chapter 17 - 27

GALVANIC SERIES Ranks the reactivity of metals/alloys in seawater

Based on Table 17.2, Callister7e. (Source of Table 17.2 isM.G. Fontana, CorrosionEngineering, 3rd ed., McGraw-Hill Book Company, 1986.)

PlatinumGoldGraphiteTitaniumSilver316 Stainless SteelNickel (passive)CopperNickel (active)TinLead316 Stainless SteelIron/Steel

Aluminum AlloysCadmiumZincMagnesium

moreanodic

(active)

morecathodic

(inert)

Chapter 17 -

Chapter 17 - Chapter 17 -


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Acceleration of Corrosion

Physical Characteristics

exposed area (less, increases corrosion rate)

time of exposure (more time, more corrosion)

Environmental Characteristics acidic environment

sulfur gas environment

temperature (high temps, more corrosion)

moisture (oxygenated moisture)

Chapter 17 -

Passivation

A protective film in oxidizing atmospheres

chromium,nickel, titanium, aluminum

Metal oxide layer adheres to parent metal

barrier against further damage

self-healing if scratched

Sensitive to environmental conditions

passivated metal may have high corrosionrates

Chapter 17 - 34

Uniform AttackOxidation & reductionoccur uniformly oversurface.

Selective LeachingPreferred corrosion ofone element/constituent(e.g., Zn from brass (Cu-Zn)).

Stress corrosionStress & corrosionwork togetherat crack tips.

GalvanicDissimilar metals arephysically joined. The

more anodic onecorrodes.(see Table17.2) Zn & Mgvery anodic.

Erosion-corrosionBreak down of passivatinglayer by erosion (pipeelbows).

FORMS OF CORROSION

Formsof

corrosion

Crevice Between twopieces of the same metal.

Fig. 17.15, Callister 7e. (Fi g. 17.15 iscourtesy LaQue Center for CorrosionTechnology, Inc.)

Rivet holes

IntergranularCorrosion alonggrain boundaries,often where specialphases exist.

Fig. 17.18, Callister 7e.

attackedzones

g.b.prec.

PittingDownward propagationof small pits & holes.

Fig. 17.17, Callister 7e.(Fig. 17.17 from M.G.Fontana, CorrosionEngineering, 3rd ed.,McGraw-Hill BookCompany, 1986.)

Chapter 17 -

Forms of Corrosion

Uniform corrosion of a single metal

usually an electrochemical reaction atgranular level

relatively slow and predictable

rusting of exposed steel, tarnishedsilver

easily corrected with coatings andregular maintenance

Chapter 17 -

Forms of Corrosion

Galvanic Corrosion

2 dissimilar metals, electrolyte,electrical connection and oxygen

Pitting Corrosion

Localized corrosion forming holes orindentations

Difficult to initially detect


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Chapter 17 -

Forms of Corrosion

Crevice Corrosion

narrow crevice filled with ionizedsolution

Oxygen-rich on the outside, oxygen-poor on the inside

metals oxidize with salt anions FeCl2and pH rises in cathodic zone

H+ may destroy passivity

Chapter 17 -

Forms of Corrosion

Intergranular Corrosion

corrosion along grain boundaries atmicroscopic level

stainless steels and heat treated high-strength steels

carbides precipitate along grainboundaries leaving these areas with noalloyed Chromium

Welds can have this same depletioneffect

Chapter 17 -

Forms of Corrosion

Cavitation and Erosion in Pipe

particulate matter

turbulent flow

abrades away the corrosion product

abrasion of zinc coatings

Chapter 17 -

Forms of Corrosion

Stress Corrosion Cracking

tensile stress and corrosiveenvironments

cracks are initiated at corrosion areas

tensile stresses propagate the crack

corrosion further deteriorate crack

etc.....

Chapter 17 -

Reinforcement Corrosion

Chapter 17 -

Corrosion of Metals in ConcreteReinforcing Steel & Prestressing

Steel

Concrete is Normally Highly Alkaline

Protects Steel from Rusting if Properly

Embedded

If Corrosion Occurs, the Reaction Products

are Greater in Volume Than the

Original Steel

Corrosion Initiation and Rate Depends On

Amount of Concrete Cover, Quality of Concrete

Details of Construction, & Exposure to Chlorides


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Chapter 17 - Chapter 17 - 44

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!).

CONTROLLING CORROSION

Metal (e.g., Al,stainless steel)

Metal oxide

Adapted from Fig. 17.22(a),Callister 7e. (Fi g. 17.22(a) isfrom M.G. Fontana, CorrosionEngineering, 3rd ed., McGraw-HillBook Co., 1986.)

steelpipe

Mganode

Cu wiree-

Earth

Mg2+

e.g., Mg Anode

Cathodic (or sacrificial) protection-- Attach a more anodic material to the one to be protected.

Adaptedfrom Fig.17.23,Callister

7e. steel

zinczinc

Zn2+

2e- 2e-

e.g., zinc-coated nail

Chapter 17 -

Corrosion Prevention

Coatings

Barrier films

Inhibitive Pigments

Sacrificial treatments

Paint

Active CathodicProtection

Chapter 17 -

Activation polarization



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Chapter 17 -

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