midterm 380 lec 3

Corrosion

metal degradation by reactions with

the environment (carbon steel car corrodes)

Global cost

3.4% of GDP (2013 estimate, 2016

NACE study

Electroplating

deposition by reducing metal ions

Three questions to ask about corrosion

which reactions occur, are they favourable, how fast do they proceed

voltmeter in series

current doesn’t pass

measuring thermodynamic pus

Reference Cells

• Potentials are relative to the standard hydrogen electrode (SHE)

• Platinum provides an inert reaction surface

• Standard metal half-cell: pure metal in a 1 mol/L solution of its ions

• Values shown: 25 °C, standard conditions, zero curren

Standard EMF Series

• All entries are reduction potentials

• More positive values favour reduction

• Values refer to specified standards conditions

more at the bottom , more likely to oxides (sign flip)

delta v don’t get multiplied by stoich

nerst

• Equilibrium cell potential at the specified temperature and composition

• Use standard potentials at the same temperature

• Ion concentrations in mol/L (ideal, dilute solutions)

Metal-Ion Concentration Cells

•Lower metal-ion concentration gives a lower reduction potential (Nernst)

•The electrode in the dilute solution is the anode: it dissolves and its ion concentration rises

•This argument is for a metal-ion concentration cell; oxygen concentration cells use a different cathodic reaction

Corrosion Current and Metal Loss

• Cell potential predicts the favourable reaction direction

• Kinetics and mass transport determine the corrosion current

• A corroding surface has zero net external current, yet metal dissolves locally

Exchange Current Density

• A single metal in a solution of its own ions, at equilibrium

• Dissolution and deposition continue, at equal rates

• No net current and no net metal loss

• i0 (exchange current density): magnitude of each opposing

partial current density

i0 is a property of one redox couple at equilibrium

It is not a corrosion rate: corrosion couples two different

reactions

• Polarization:

electrode-potential shift caused by current flow

Overpotential

η = Vw − VE is positive

Oxide Growth Kinetics

•Parabolic: increasing diffusion distance slows growth through an adherent film

•Approximately linear: porous or spalling films repeatedly expose fresh

metal

•Logarithmic: very thin films (<100 nm), often near room temperature

Oxide Volume and Film Integrit/ Pilling–Bedworth Ratio

oxide volume /metal volume consumed

P–B < 1: insufficient oxide volume for full coverage (discontinuous oxide layer can never be protective)

P–B = 1: equal oxide and consumed-metal volumes

P–B > 1: excess oxide volume can generate compression and buckling

The ratio alone does not establish protectio

electrochemical rxn

mediated by electrons

only need a conductor

oxidation(anode) : loss electrons , reduction(cathode) : gain electrons (Leo ger)

half rxn happen at seperate electrically connected sites

require electronic and ionic path

2 possible run (potential must be positive)

Protective oxide

complete coverage without

excessive tensile or compressive stres

Geometric criterion Pilling–Bedworth Ratio

adhesion, transport and stress also govern protectio

Dry Corrosion: Oxidation in Gases

• Metal is oxidized and O2 is reduced at a solid–gas interface: still a redox process

• The oxide scale is both electrolyte (ion transport) and circuit (electron transport)

• A continuous, adherent scale slows transport and protects the meta

Cathodic Protection

• Supply electrons to the structure so it becomes the cathode: its own

oxidation is suppressed

• Sacrificial anode: a more active metal (Zn, Mg) is connected and

corrodes instead

• Impressed current: a DC rectifier drives electrons into the structure

IMAGE: RECTIFIER : APPLY VOLTAGHE TO A PIECE OF METAL DIRECTLY

Examples of cathodic protection

Corrosion prevention

Material selection Match alloy and cost to the service

environment

Environmental control Lower temperature, velocity or concentration;

remove dissolved O2

Inhibitors Adsorb on the surface to slow the anodic or cathodic reaction

Coatings Barrier (paint, polymer or metal layer) between metal and environment

Cathodic protection Make the structure the cathode: sacrificial anode or impressed curren

form of corrosion

•Uniform

•Galvanic

•Crevice

•Pitting

•Intergranular

•Selective leaching

•Erosion-corrosion

•Stress corrosion

Hydrogen embrittlement is a related degradation mechanism

Uniform Corrosion

Metal loss spread roughly evenly over the whole exposed surface

The most common form but the least dangerous: predictable, and measured as an average thickness loss (mm/yr)

corton steel

Galvanic Corrosion

Two dissimilar metals electrically connected in

the same electrolyte

• The less noble (more active) metal becomes

the anode and corrodes faster

• Area effect: a small anode feeding a large

cathode corrodes very fast

Prevention

•Choose metals close together in the galvanic

series

•Avoid a small anode coupled to a large

cathode

•Insulate the joint to break the electronic

path

•Connect a third, more active metal as a

sacrificial anode

Crevice Corrosion

• Stagnant solution in a crevice cannot

exchange with the bulk

• O2 inside is consumed and not

replenished: O2 reduction moves

outside

• The O2-poor crevice becomes the

anode; acidification and Cl− build-up

accelerate attack

Control: weld instead of bolting, avoid

sharp corners, drain fully, use non-

absorbent gaskets

An oxygen concentration cell, not a

metal-ion cel

Pitting

• Local breakdown of the passive film

(often by Cl−) initiates a pit

• The tiny anodic pit is fed by the large

passive cathodic surface: very high local

current density

• Pits grow downward and can perforate a

wall with negligible overall mass loss

Intergranular

Corrosion

• Attack follows susceptible grain

boundaries

• Sensitized stainless steel: Cr23C6

precipitates at grain boundaries

(500–800 °C)

• The Cr-depleted zone beside

them loses its passivity

• Common in the heat-affected

zone of welds (weld decay

Selective Leaching and Erosion-Corrosion

Selective leaching

• One element of a solid-solution alloy is

preferentially dissolved

• Dezincification of brass: Zn dissolves, leaving

a porous, weak Cu-rich sponge

Erosion-corrosion

• Chemical attack plus mechanical wear by a

fast-flowing fluid, often carrying particles or

bubbles

• Severe when flow strips the passive film:

bends, elbows, pumps, impeller

Stress-Corrosion Cracking

•Tensile stress + a specific environment

(e.g., brass in ammonia, stainless steel

in chlorides)

•Cracks grow roughly perpendicular to

the stress, even in a mildly corrosive

medium

•Residual stress (welding, cold work) is

enough: no applied load is needed

Required combination: susceptible material,

tensile stress and environment

Remove any one of the three and cracking

stops: the basis for contro

Hydrogen

Embrittlement

•Atomic H diffuses into the lattice;

high-strength steels are most

susceptible

•Ductility and fracture toughness

drop sharply

•Sources: pickling, plating, H2S,

over-protective cathodic

protection

Passivity

• Passivity: a thin, adherent oxide film sharply lowers the dissolution rate

• Common in Cr, Fe, Ni, Ti, Al and their alloys

• Stainless steel (≥ 11 wt% Cr): protection comes from a Cr2O3-rich film

Combining Polarization Effects

•Start with the Nernst potential at bulk composition

•Add the activation and concentration overvoltages of that half-reaction

In the depletion term i is a positive magnitude, 0 ≤ i < iL; as i approaches iL, ηc grows without bound

concentration polarization

• Reactant supply limits the current

• Low current: diffusion keeps up

• High current: a depletion layer forms

• Here it limits the cathodic reaction (H+ or O2)

The curves meet before reactant supply limits the reduction current/The curves meet before the transport limit: mainly activation contr

Concentration polarization moves the intersection to a lower current

Less reactant supply gives a lower corrosion current

Corrosion Potential and Current

A common electrode potential couples metal

dissolution and H2 evolution

• At Vc, total anodic and cathodic currents are

equal and opposite

• The anodic dissolution current remains nonzero

Vc: corrosion potential

ic: corrosion current density

Zinc polarization

Overpotential, η = 0.000 V Equal dissolution and deposition rates. Net current = 0

Overpotential, η = +0.100 More oxidation current means faster zinc dissolution

Overpotential, η = +0.200 V More positive potential increases the oxidation rate

Tafel Relation

Activation Polarization

Interfacial reaction steps require activation

energy

• The slowest interfacial step limits the

activation-controlled rate

Hydrogen evolution on platinum

1. H+ reaches and adsorbs on Pt

2. Electrons reach the interface

3. Electron transfer forms adsorbed H

4. H atoms combine into H2

5. H2 molecules form bubbles

Activation overvoltage changes the relative forward and reverse rates

At equilibrium, the two rates are equal and the net current is zer