J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Environmental Interactions

•  Chemical reaction between the material and its environment

•  Beneficial interactions: materials processing –  Carburization and nitriding hardens for wear resistance –  “Doping” adds electrically active species –  Interfacial compounds are used as diffusion barriers

•  Harmful interactions –  Oxidation

•  Materials “burn” slowly at high T –  Corrosion

•  Electrochemical reactions oxidize near room temperature

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Mechanism of Parabolic Oxidation

•  Diffusion through coherent oxide film –  Metal is ordinarily more mobile, diffuses to oxidize at free surface –  Growth is diffusion controlled –  Thickness increases roughly as mean diffusion distance (<x> = √2Dt)

•  Film diffusivity controls oxidation –  Oxidation is a high-T phenomenon (rate increases exponentially with T) –  Oxides with low D (high QD) are protective

•  Film forms, but cannot growth

metal

oxide 2e - O O 2 O = MO

M ++ δ

€

δ = k t

€

k = Aexp− QD

kT

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Mechanisms of Linear Oxidation

•  Linear oxidation is the addition of many parabolic steps –  Oxide does not fit perfectly on surface ⇒ mechanical strain –  Strain increases as film thickens –  At critical thickness, film ruptures, exposing fresh surface –  Process repeats

•  To suppress film rupture, suppress film growth –  Minimize diffusion through film

δ

t

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Engineering Oxidation Resistance: Alloying to Create Protective Films - Stainless Steel

•  The corrosion rate of Fe decreases with Cr –  Asymptotes at > 8% Cr (“stainless steel”)

•  Preferential incorporation of Cr into the oxide film –  Film is essentially Cr2O3 when Cr >8%.

•  Protective film no better than protective oxide –  Stainless steel liable to oxidation in presence of Cl (attacks Cr2O3) –  Stainless steel oxidizes at sufficiently high T

ln (k)

Cr (wt%)5 10 15 20

• Influence of Cr on the oxidation rate of Fe

€

k = Aexp −QD

kT

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Engineering Oxidation Resistance: Protective Coatings

•  Protect high temperature structures with oxidation-resistant coatings –  Ex: turbine blades in jet engines

•  Properties required of a protective coating –  Good oxidation resistance (Al, Cr) –  Resistance to spall (fracture of coating)

•  Matching coefficient of thermal expansion •  Intermediate bonding layer

•  Common choices: CoCrAlY, NiCrAlY –  Co, Ni, Cr/Al ratio control adjust thermal expansion –  Y improves adhesion at interface (often add additional “bonding layer”

protective coating

protected structure

bonding layer

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Environmental Interactions: Aqueous Corrosion

•  The primary source of degradation of structures –  Particularly steel structures (“rust”)

•  Corrosion is a low-temperature oxidation mechanism –  Normal oxidation is prevented by the natural oxide coating –  In corrosion, the protective coating does not automatically form –  In the reaction: M++ + O= = MO

•  The metal ions form at one location (the “anode”) •  The oxygen forms at another (the “cathode”) •  The two do not ordinarily develop a good protective coating

•  Corrosion is an electrochemical process –  Requires both electrical and chemical contact between

•  Anode, where electrons and metal ions are generated •  Cathode, where electrons are consumed, O= is generated

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Electrochemistry: The Galvanic cell completes the circuit

•  Two dissimilar metals (e.g., Zn, Cu) –  Connected electrically (e.g., by a wire) –  In contact with an electrolyte (e.g., ZnSO4|CuSO4) –  React according to potential (Δϕ = ϕCu - ϕZn)

•  Δϕ > 0 ⇒ Zn + 2Cu+ → Zn++ + Cu

•  Complete circuit permits dissolution –  Electrons swept from anode (Zn) to cathode (Cu) –  Ions (SO4

=) swept from cathode to anode

Half-cell potentials:

€

φZn = φZn0 +

RT2Fln[Zn++]

€

φCu = φCu0 +

RT2Fln[Cu++]

Zn

Cu Zn ++

Cu +

SO 4 =

SO 4 =

V

e - Zn ÷ Zn ++ + 2 Cu+ + e- ↔ Cu

Zn dissolves, Cu is plated

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

The Galvanic Series in Seawater

Increasingly anodic ‘ TinMagnesium NickelMagnesium alloys Brasses (Cu-Zn)Zinc CopperAluminum Bronzes (Cu-Sn)Al-Cu alloys Silver soldersMild steel Nickel (passive)Wrought iron Monel (70Ni-30Cu)Cast iron Titanium18Cr-8Ni stainless steel (non-passivated) 18Cr-8Ni stainless steel (passive)50Pb-50Sn solder GoldLead Increasing cathodic ’

•  The more anodic material in the couple is corroded

•  Note: alloys are (generally) cathodic to pure metals –  Free energy decreases on alloying

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Concentration Cell

•  Let a cell have Zn at both electrodes

•  If the Zn concentration is different –  A potential difference is developed –  The side with the lower Zn concentration has lower potential –  Lower Zn is the anode; is corroded

€

φZn = φZn0 +

RT2Fln[Zn++]

Zn ⇔ Zn++ + 2e-

€

Δφ =RT2Fln

Zn++[ ]1Zn++[ ]2

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

•  Anode reactions: –  Fe → Fe++ + 2e-

•  Cathode reactions: –  Normal cathode reaction:

2e- + 1/2O2 + H2O → 2(OH)-

–  Acidic solution: 2e- + 2H+ → H2

–  Strong potential: 2e- + H2O → 1/2H2 + (OH)-

•  Oxidation reaction: –  Fe++ + 2(OH)- → FeO + H2O –  Note FeO may not coat surface

Cathode Reactions in Fe Corrosion

V

Fe Fe

H2O

OH- OH-

Fe++

Fe++

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Galvanic Couples

•  Dissimilar metal contact –  Any two dissimilar conductors constitute a galvanic couple

•  Microstructural heterogeneities –  More stable grain or region (lowest free energy) is the cathode –  High free energy due to:

•  Mechanical deformation (defects) •  Chemical heterogeneity •  Phase or microstructural difference

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Oxygen Concentration Cells

•  Water immersion –  [O2] decreases with depth –  Cathode at surface –  Anode at depth ⇒  Corrosion below water line

•  Pitting corrosion –  O2 denuded at base of pit –  O2 replenished at surface –  Anode at pit base ⇒  Corrosion deepens pit

€

φ =ϕ º+ RTnFln

O2[ ]1/ 2

OH−[ ]2

2e- + 1/2O2 + H2O ↔ 2OH- Cathode:

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Crevice Corrosion

•  Oxygen concentration cells develop at crevices –  Rapid “crevice corrosion”

•  Attacks rivets, screw heads, etc .

€

φ =ϕ º+ RTnFln

O2[ ]1/ 2

OH−[ ]2

2e- + 1/2O2 + H2O ↔ 2OH- Cathode:

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Rate of Corrosion: Influence of Interface Polarization

•  Rate of corrosion proportional to current density (i)

•  Interface polarization –  Ions produced too rapidly to diffuse away –  Concentration build-up lowers Δφ –  Steady state when Δφ = ρi (Ohm’s Law for Acathode = Aanode)

•  ρ = electrical resistivity of conductor connecting anode to cathode

φ

log(i) i

Δφ=ρi

€

dmdt

= kiAI = iA

-

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Rate of Corrosion: Effect of Anode Area

•  Corrosion current determined by –  Polarization determined by current density, determines φ = φ(i) –  Actual current (I = iA) must be constant through circuit

•  As anode area shrinks relative to cathode –  Current density increases (corrosion rate increases) –  Corrosion potential increases –  Both change less strongly than area ratio

FeAnode electrolyte

Fe++

Fe++ OH-

OH-

cathode

I = iA

φ

log(I=iA)

Increasing AFe

Anode area increased Cathode area fixed

-

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Protection Against Corrosion

•  Break the circuit –  Break electrical contact between metals –  Break chemical contact with electrolyte

•  Provide an alternate circuit –  “cathodic protection”

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Corrosion Protection: Breaking the Circuit

•  Isolate anode from cathode –  Insert insulator at dissimilar metal surface –  Fe||Cu piping into a home –  Teflon “sleeves” for rivets on aircraft

•  Isolate metal from electrolyte by impermeable “paint” –  Can paint either or both, but should paint cathode –  Pin-hole break in anode risks catastrophic pitting corrosion –  “Passivation”: material paints itself with oxide, like Al, Cr, stainless

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Cathodic Protection

•  Make the material the cathode

•  Add a more anodic metal –  “Sacrificial anode” –  Must be in circuit

•  Impose a reverse voltage –  Reverse the sign of Δφ

FeMg

cathode

V

Fe

cathode

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Galvanizing

•  Protect metal by coating with an anodic material –  Coating protects metal like paint –  If coating is penetrated, cathodic protection kicks in –  Automotive protection: paint plus galvanizing (Zn||Fe) –  Aircraft protection: “alcladding” Al||Al alloy

•  Do not coat with cathodic layer

Zn

Fe

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Interfaces

•  The engineering importance of surfaces –  Wetting

•  Frying pans and car waxes •  Detergents •  Lubricants

–  Bonding •  Glues •  Solders

–  Catalysis •  Adsorption

–  Capillarity •  Tree sap and blood vessels

•  The thermodynamics of surfaces –  Surface tension –  Wetting criteria

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Interfaces and Wetting

•  Conditions of equilibrium - open system –  T, V (or A), {µ} fixed for interface

⇒  Ω(T,V,{µ}) = min = E - TS + ΣµkNk = - PV

•  Assign excess quantities to surface

•  ΩS(T,A,{µ}) = Ω - (Ωα + Ωβ )

transition shell dividing surface

€

ΩS =σA =min. (shape)

€

ΩS =σ =min. (state)

α

β

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Wetting: The Contact Angle

•  The “Young Equation” determines the “contact angle” –  Balance of forces at the periphery of a drop on a rigid surface

•  The wetting angle, θ ranges from 0 (wetting) to π (de-wetting)

€

cos(θ) =σ SV −σ SL

σ LV

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

Film Formation (Spreading)

•  Spreading: LV+SL interfaces have lower energy than SV –  Want for painting, coating, soldering, etc.

•  To promote spreading –  Raise σSV: e.g., clean the interface

•  Flux in soldering removes oxides from surface –  Lower σSL: e.g., include reactive species in L

•  Sn in solder forms intermetallic compounds with Cu, Ni or Au –  Lower σLV: e.g., add surfactant (species that adsorbs at LV interface)

•  Flux in solder coats surface, lowers σLV

S

L V

€

σ SL +σ LV ≤σ SV

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

De-Wetting

•  De-wetting: film of vapor preferred between S and L –  LV+SV interfaces have lower energy than SL –  Want for “non-stick” coatings (frying pans, car wax).

•  To promote de-wetting –  Lower σSV: e.g., add surfactants or low-σ coatings to solid

•  Teflon on frying pans –  Lower σLV: e.g., add surfactant (species that adsorbs at LV interface) –  Raise σSL: e.g., remove any possible surfactants or reactive species

€

σ SV +σ LV ≤σ SLS

L V