CHAPTER 16: CORROSION AND DEGRADATION

ISSUES TO ADDRESS...

• Why does corrosion occur?

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

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 2H2O 4e 4(OH)

Zinc

oxidation reactionZn Zn2+

2e-Acid solution

reduction reaction

H+H+

H2(gas)

H+

H+

H+

H+

H+

flow of e- in the metal

CORROSION OF ZINC IN ACID

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• Two outcomes:--Metal sample mass --Metal sample mass

Pla

tin

um

me

tal, M

Mn+ ions

ne- H2(gas)

25°C 1M Mn+ sol’n 1M H+ sol’n

2e-

e- e-

H+

H+

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

Vmetalo 0 (relative to Pt) Vmetal

o 0 (relative to Pt)

Standard Electrode Potential

STANDARD HYDROGEN (EMF) TEST

Mn+ ions

ne-

e- e-

25°C 1M Mn+ sol’n 1M H+ sol’n

Pla

tin

um

me

tal, M

H+ H+

2e-

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• EMF series • Metal with smaller V corrodes.• Ex: Cd-Ni cell

metalo

-

Ni

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

+

Cd 25°C

more

anodic

more

cath

odic 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- 2.262- 2.714- 2.924

metal Vmetalo

V = 0.153V

o

STANDARD EMF SERIES

CuZn

Zn2+

2e- oxidationreduction

Acid

H+ H+H+

H+

H+

H+

H+

-+AnodeCathode

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2H 2e H2(gas)

O2 4H 4e 2H2O

CORROSION IN A GRAPEFRUIT

- +

Ni

Y M

Ni2+ solution

X M

Cd2+ solution

Cd T

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• Ex: Cd-Ni cell with standard 1M solutions

• Ex: Cd-Ni cell with non-standard solutions

VNio VCd

o 0.153 VNi VCd VNi

o VCdo

RTnF

lnXY-

Ni

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

+

Cd 25°C

n = #e-

per unitoxid/redreaction(=2 here)F = Faraday'sconstant=96,500C/mol.

• Reduce VNi - VCd by --increasing X --decreasing Y

EFFECT OF SOLUTION CONCENTRATION

• Ranks the reactivity of metals/alloys in seawaterm

ore

anodic

(a

ctiv

e)

more

cath

odic

(i

nert

)

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

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GALVANIC SERIES

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Forms of

corrosion

• 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.

Rivet holes

attacked zones

g.b. prec.

FORMS OF CORROSION

• Stress & Saltwater... --causes cracks!

• Heat treatment: slows crack speed in salt water!

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

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“held at 160C for 1hr before testing”

increasing loadcrack

sp

eed

(m

/s)

“as-is”

10-10

10-8

Alloy 7178 tested in saturated aqueous NaCl solution at 23C

DETERIORATIVE

Uniform Corrosion: Rust!

Prevention:• Paint• Plate• Sacrificial anode

Galvanic Corrosion

Causes:

Dissimilar metalsElectrolyteCurrent Path

Described by Galvanic Series

Solutions:

Choose metals close in galvanic series

Have large anode/cathode ratios

Insulate dissimilar metals

Use “Cathodic protection”

Pitting and Creviced Corrosion

Prevention:

Weld – don’t rivet

Use non-absorbing gaskets

Polish surfaces

Add drains – avoid stagnant water

Adjust composition; e.g., add Mo to SS

Causes: concentration gradients in electrolyte cause some areas high in ion concentrations that accelerate oxidation

Intergranular Corrosion

Occurs in specific alloys – precipitation of corrosive specimens along grain boundaries and in particular environments

e.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

formation

Alloy with a material that has stronger carbide formation (e.g., Ti or Nb)

Erosion Corrosion

Causes: abrasive fluids impinging on surfaces

Commonly found in piping, propellers, turbine blades, valves and pumps

Solutions:

•Change design to minimize or eliminate fluid turbulence and impingement effects.

•Use other materials that resist erosion

•Remove particulates from fluids

Selective Leaching

• Occurs in alloys in which one element is preferentially removed – e.g., in Brass, Zinc is electrically active and is removed, leaving behind porous Copper

• Occurs in other metals, such as Al, Fe, Co, Cr

Solutions:

• Use protective coating to protect surfaces

• Use alternative materials

Stress Corrosion

Aka: stress corrosion cracking

Cracks grow along grain boundaries as a result of residual or applied stress or trapped gas or solid corrosion products

e.g., brasses are sensitive to ammonia

Stress levels may be very low

Solutions: Reduce stress levels

Heat treatment

Atmosphere control

Hydrogen Embrittlement

• Metals loose strength when Hydrogen is absorbed through surface, especially along grain boundaries and dislocations

• Often occurs as a result of decorative plating

• High strength steels particularly susceptible

• Can be removed by “baking” the alloy

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• Self-protecting metals! --Metal ions combine with O to form a thin, adhering oxide layer that slows corrosion.

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

Metal oxide

• 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

steel

zinczincZn2+

2e- 2e-

e.g., zinc-coated nail

steel pipe

Mg anode

Cu wiree-

Earth

Mg2+

e.g., Mg Anode

Corrosion prevention

Sacrificial Anode Applied Voltage

Surface coatings & Passivation

Some materials, such as Aluminum or Stainless Steel, form oxide barrier coatings that prevent oxidation at active surface – this is called “passivation”

Surface can be coated with protective layers: painted, anodized, plated (Caution!!! Cracks in plating or paint can lead to crevice corrosion!)

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• 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