CorrosionRichard D. Sisson, Jr.

George F. Fuller ProfessorDirector of Manufacturing & Manufacturing Engineering

Mechanical Engineering DepartmentWorcester Polytechnic Institute

Worcester, MA 01609508-831-5335

Sisson@wpi.edu

CORROSION AND DEGRADATION

• Why does corrosion occur?

• What metals are most likely to corrode?

• How do temperature and environment affectcorrosion rate?

• How do we suppress corrosion?Keep it dry and cool!Keep it dry and cool!

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THE COST OF CORROSION• Corrosion:

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

ship, Sapona,off the coastof Bimini.

Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission.

• 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 Introduction to Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc., 1985.**Economic Report of the President (1998).

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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.

Fig. 17.6, Callister 6e. (Fig. 17.6 is courtesy LaQue Center for Corrosion Technology, Inc.)

Fig. 17.8, Callister 6e.(Fig. 17.8 from M.G.Fontana, CorrosionEngineering, 3rd ed.,McGraw-Hill BookCompany, 1986.)

8 FORMS OF CORROSION

Fig. 17.9, Callister 6e.

Uniform Corrosion& Corrosion Products

• Anode reactionFe Fe +2 + 2 e-

• Cathode ReactionO2 + 2 H2O + 4e- 4OH-

• Solubility Product– Fe(OH)2 Fe+2 + 2OH-– Ksp = [Fe+2] [OH]2 = some constant (T)

• Corrosion Product = RustRust = Fe(OH)2 etc.– Rust is much more complicated chemically!

CORROSION OF ZINC IN ACID• 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+ + 4 e − → 2H2O O2 + 2H2O + 4 e − → 4(OH)−

Zinc

oxidation reactionZn Zn 2+

2e -Acid solution

re duction reaction

H+H+

H2(gas)

H+

H+

H+

H+

H+

flow of e - in the metal

Adapted from Fig. 17.1, Callister 6e. (Fig. 17.1 is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)

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Galvanic Corrosion

• Two different metals electrochemically coupled– Anode– Cathode– Conductive path– Electrolyte!

CORROSION IN A GRAPEFRUIT or Potato!

CuZn

Zn2+

2e - oxidationreduction

AcidH+ H+

H+

H+

H+

H+

H+-+AnodeCathode

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2H+ + 2e − →H2 (gas )

O2 + 4H+ + 4 e − → 2H2O

STANDARD EMF SERIES

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• EMF series • Metal with smallerV corrodes.

• Ex: Cd-Ni cellmetalo

-

Ni

1.0 M Ni2+ solution

1.0 M Cd 2+ solution

+

Cd 25°C

mor

e an

odic

mor

e ca

thod

ic

AuCuPbSnNiCoCdFeCrZnAlMgNaK

+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

Data based on Table 17.1, Callister 6e.

GALVANIC SERIES• Ranks the reactivity of metals/alloys in seawaterm

ore

anod

ic

(act

ive)

mor

e ca

thod

ic

(iner

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

Based on Table 17.2, Callister 6e. (Source of Table 17.2 is M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)

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Crevice Corrosion

• Regions of restricted flow– Beneath bolt heads– Overlapping joints– Rivets

• Leads to pitting

Intergranular Corrosion

• Attack at or near grain boundaries• Typically associated with welding or heat

treating problems

Stress Corrosion Cracking

Corrosion protection

• Galvanic couples– Galvanized steel– Tin Cans?

• Impressed current• Paint• Vapor Phase Inhibitors

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

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

Metal oxideCONTROLLING CORROSION

• Reduce T (slows kinetics of oxidation and reduction)

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• 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. 17.13(a), Callister 6e. (Fig. 17.13(a) is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Co., 1986.)

Adapted from Fig. 17.14, Callister 6e. steel

zinczincZn 2+

2e - 2e -

e.g., zinc-coated nail

steel pipe

Mg anode

Cu wiree -

Earth

Mg2+

e.g., Mg Anode

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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.

Fig. 17.6, Callister 6e. (Fig. 17.6 is courtesy LaQue Center for Corrosion Technology, Inc.)

Fig. 17.8, Callister 6e.(Fig. 17.8 from M.G.Fontana, CorrosionEngineering, 3rd ed.,McGraw-Hill BookCompany, 1986.)

8 FORMS OF CORROSION

Fig. 17.9, Callister 6e.

• 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 ElectrodePotential are more likely to corrode relative toother metals.

• The Galvanic Series ranks the reactivity of metals inseawater.

• Increasing T speeds up oxidation/reduction reactions.• Corrosion may be controlled by:

-- using metals which forma protective oxide layer

-- reducing T

-- adding inhibitors-- painting--using cathodic protection.

SUMMARY

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