01 Corrosion

7/31/2019 01 Corrosion

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Environmental Effects on Materials p. 16.1

EMSE 201 Introduction to Materials Science & Engineering 2003 Mark R. De Guire rev. 04/14/03

Corrosion

is dissolution (or other breakdown via chemical

reaction) of a solid, in which a fluid supplies one ormore of the reactants

involves

thermodynamicstabilityof the solid in the fluidenvironment

kineticsof any breakdown reaction that mightoccur

Examples:

Electrochemical corrosion of metals

Oxidation or sulfidation of

Metals Non-oxide ceramics

Dissolution of refractories* by molten metals or glass

In contrast, erosion

is the physical removal of solid material by themechanicalaction of a flowing fluid

involves

fluid dynamics abrasion/wear resistance ofthe solid

In real situations, corrosion and erosion often occursimultaneously

* Refractories: structural materials used to contain and withstand high-temperature processes such as steelmaking and glass melting.


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THERMODYNAMIC CONSIDERATIONS (start)

Most metals are unstablein the presence of oxygen:

xM +y2 O2 MxOy Gox

Gox > 0 oxidation wont occur without energy input

Gox < 0 oxidation can occur spontaneously

Ellingham diagram Fig. 10.13, D.

R. Gaskell, Introduction toMetallurgical Thermodynamics, 2nd

ed. McGraw-Hill, 1981.

metal(M)

oxide(M O )x y

M(2y/x)+

O2

e

gas(w/O )

2

but rateof oxidation depends on diffusion of reactantsthrough oxide layer

Corrosion might just be a faster route

for approaching equilibrium (oxidation)


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THERMODYNAMIC CONSIDERATIONS (end)

Phase diagrams as an indicator of solid dissolution

Application: contain molten A in solid B

(e.g. A = metal or glass, B = refractory)

Goal: minimize how much B is dissolved in liquid

Which gives lowest CB,L?

A B

L

L

A B

T

T

L

A B

T

L

A B

T + L

+ L

+ L

+ L

CB,L

CB,L

CB,L

CB,L

CB

CB

CB

CB

Look for

Shallow eutectic T A-rich eutectic composn

High Tm,B


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AQUEOUS CORROSION (start)

Neutral metal atoms become cations in soln, e.g.:Zn(s) Zn2+(aq) + 2e (1)

oxidationoccurs at the anode

Electrons released in (1) reduce something else,

e.g. in an acidic solution:

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

reductionoccurs at the cathode

Net reaction:

Zn(s) + 2H+ Zn2+(aq) + H2(g) (3)

Other reactions are possible, depending on environment.

E.g., in aerated water:

2Fe(s) + O2(soln) + 2H2O 2Fe2+(aq) + 4OH

2Fe2+(aq) + 4OH 2Fe(OH)2

2Fe(OH)2 + 12 O2(soln) + H2O 2Fe(OH)3(s)rust


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AQUEOUS CORROSION (contd)

Note: an electrochemical reaction (e.g. reaction (3)) requires:

Oxidationof one species at an anode

Reductionof one species at a cathode

Transport ofionsthrough an electrolyte

Transport of electronsfrom anode to cathode

e.g. through an external circuit

Free energy considerations: If G < 0for rxn. (3) on p. 4

the energy released can drive an electrical load a battery

the reverse reaction can be made to happenwith electrical energy input

Plating

Consists of cations in soln being reducedto metalatoms & depositing on an electrically conductive surface

Is the reverse of reactions like (1) on p. 4:

Mn+(aq) + ne M(s) (4)


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THE STANDARD EMF SERIES (start)

When two species are present in an electrochemical couple,which gets oxidized and which gets reduced?

Consider a cell:

Metallic Fein an aqueous soln of 1.0 M Fe(II) salt

Metallic Znin an aqueous soln of 1.0 M Zn(II) salt

Separated by a slightly permeablemembrane

Electrodesconnected to a voltmeter

25C

1M [Zn ]2+

1M [Fe ]2+

ZnFe

e-e-

V

membrane

+

0.323 V

Fe2+(aq) reduces to Fe(s) Zn oxidizes to Zn2+(aq)

Fe plates on the Fe electrode Zn corrodes into the solution

Fe is cathodicw.r.t. Zn Zn is anodicw.r.t. Fe

voltmeter reads 0.323 V


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THE STANDARD EMF SERIES (end)

Net electrochemical reaction (from p. 6):Fe2+(aq) + Zn(s) Fe(s) + Zn2+

But replace Zn & Zn(II) soln with Cu & Cu(II) soln

Fe corrodes into solution as Fe2+(aq)

Cu2+ plates on the Cu electrode as Cu(s)

Voltmeter reads 0.780 V in opposite direction

Metals can be placed in a series indicating their relativetendencies to oxidize the standard emf series

Electrode reaction V, V

Increasing 13 Au3+ + e 13Au

+1.420

inertness Fe3+ + e Fe2+ +0.77112 Cu2+ + e

12

Cu+0.340

(arbitrary reference ) H+ + e 12 H2 0.000

12 Fe2+ + e

12

Fe0.440

Increasing 3 Cr3+ + e 3 Cr 0.744

reactivity12 Zn2+ + e

12

Zn0.763


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For alloys in seawater, the galvanic seriesprovides aqualitative listing (Table 17.2)


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APPLICATIONS OF THE STANDARD EMF SERIES (start)

Q.: What would be the voltage from a standard galvanic cellconsisting of Cr/Cr(III) and Fe/Fe(II)?

A.:

12 Fe2+ + e

12 Fe V1 = 0.440 V

13 Cr e+

13 Cr3+ V2 = (0.744) V

12 Fe2+ + 13 Cr 12 Fe+ 13 Cr3+ V = 0.304 V

Note:

Balance es when writing half-cell reactions, butdo notmultiply V by a constant to match thereaction

V > 0 reaction occurs spontaneously

Q.: Which alloying elements in Co, Ni, or Fe alloys might beexpected to oxidize more readily than the host metal?

A.: (see Table 17.1)

Cr, Al, Y, Mg oxidation-resistant

superalloys


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APPLICATIONS OF THE STANDARD EMF SERIES (end)

Q.: How does a thin layer of Zn applied to plain carbonsteels (galvanized steel) provide corrosion protection?

A.:

acidic oroxygenated water

will oxidize iron

Fe

O2 Fe2+

zinc oxidizes& supplies electronsto keep Fe reduced

Fe

Zn2+2O

e

H+H+

Large ratio ofZn surface areaFe surface area Zn wont get depleted

Other examples of cathodic protection (Fig. 17.13)

underground

steel pipe

Mganode

eMg2+

earth(damp)

spontaneous oxidation of Mgsupplies electrons

to keep iron reduced

inertanode

e

DCpower

ioniccurrent

external power supplyconsumes energy

to provide electrons to iron


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CONCENTRATION EFFECTS THE NERNST EQUATION

If the solution concentrations in a galvanic couple are 1 M,corrosion tends to equalize the concentrations:

low [Fe ]2+high [Fe ]2+

FeFe

e-e- V

membrane

+ High-Fe side: Fe2+ + 2e Fe(cathodic plating)

Low-Fe side: Fe Fe2+ + 2e(anodiccorrosion)

in an ionic concentration cell,corrosion occurs evenbetween the same metal (!)

The Nernst equation:

V = (V2 V1) RTnF ln

[M1n+][M2n+]

R = 8.314 J mol-1

F = 9.648 104C mol-1n = # of es in half-cell reaction

Oxygen concentration cell:

high [O ]2

FeFe

e-e-

V

membrane

+

low [O ]2

Low-[O2] side:2Fe 2Fe2+ + 4e

anodic corrosionof aparticularly troublesome sort

High-[O2] side:O2 + 2H2O + 4e 4OH

no plating(if no metal salts areon the high-[O2] side of cell)


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EIGHT FORMS OF CORROSION (start)

1) Uniform attack tarnishing; widespread rust

2) Galvanic corrosionbetween dissimilar metals

Steel corrodes in electrochemical contactwith copper, brass

Mitigated by

large anode area

open circuit

third metal anodic to both

3) Crevice corrosion

4) Pitting oxygen concentration cell

Mechanism

Stagnant water in crevice or pit becomesdeoxygenated

Oxygen reduction at

large aerated surface oxidation of crevice metal

Mitigated by

welding (vs. rivets or bolts)

eliminating moisture (nonabsorbing gaskets)

removing accumulating deposits

design to ensure complete drainage


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EIGHT FORMS OF CORROSION (cont.) CreviceCorrosion

(Callister, Fig. 17.7)


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EIGHT FORMS OF CORROSION (end)

5) Intergranular corrosion Grain boundaries are areas of local high reactivity

Specific mechanisms involve segregation ofparticular elements into precipitates

6) Selective leaching

Dezincification of brass

Lead from glassware & ceramic glazes

7) Erosion-corrosion

Passive coating may be abraded away

Alleviated by reducing

fluid turbulence

bubbles & particulates

8) Stress corrosion

Local stresses (residual or external) increasecorrosion

High strain energy

High dislocation density

Alleviated by reducing stress:

Larger area Lower loads Annealing


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OXIDATION

xM xM(2y/x)+ + (2y)ey2 O2 + (2y)e

yO2

xM(2y/x)+ + yO2 MxOy

xM(s) +y2 O2(g) MxOy(s)

Will the oxide layer be

porous? or rotective?

Pilling-Bedworth ratio(PBR)

PBR oxide volume producedmetal volume consumed =

MWoxidedmetalxMWmetaldoxide

PBR < 1 porous oxide

1 < PBR < 2 protective oxide

2 < PBR oxide is heavily compressed spalling


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OXIDATION KINETICS

A porous oxide allows penetration of molecular oxygen

metaloxide

O

O

2

2

z Rate of film formation is constant:

dzdt = c1

Linear growth:

z = c1t + c2

A protective film requires diffusion through oxide layer forfurther oxidation:

metaloxide

M(2y/x)+

(2y)e

metaloxide

(2y)e

yO2

D >> DM,oxide O,oxide

D >> DM,oxideO,oxide

z z Recall Ficks first law:

JOorM = DOorMcOorM

z

dzdt J dzdt 1z

z2 = c3t + c4

parabolic growth

Unlike linear growth, as z increases, growth slows:

orm

t

linear

parabolicz

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