corrossion and degradation

8/11/2019 Corrossion and Degradation

http://slidepdf.com/reader/full/corrossion-and-degradation 1/13

Chapter 17 - 1

ISSUES TO ADDRESS...• How does corrosion occur ?

• Which metals are most likely to corrode? • What environmental parameters affect

corrosion rate?

• How do we prevent or control corrosion?

Corrosion and Degradation ofMaterials



Chapter 17 - 2

• Deteriorative mechanisms are different for the three materialtypes:

1. Metals – actual material loss either by dissolution(corrosion) or by the formation of nonmetallic scale or film(oxidation).

2. Ceramics – resistant to deterioration which usually occursat elevated temperatures or in extreme environments,also called corrosion.

3. Polymers – may dissolve when exposed to a liquidsolvent, electromagnetic radiation (ultraviolet) and heat :degradation.



Chapter 17 - 3

• Corrosion :-- the destructive electrochemical attack of a material.-- Ex: Al Capone's

ship, Sapona,off the coastof Bimini.

• Cost: -- 4 to 5% of the Gross National Product (GNP)*

-- in the U.S. this amounts to just over $400 billion/yr**

* H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introductionto Corrosion Science and Engineering , 3rd ed., John Wiley and Sons, Inc.,1985.**Economic Report of the President (1998).

Photos courtesy L.M. Maestas, SandiaNational Labs. Used with permission.

THE COST OF CORROSION



Chapter 17 - 4

Corrosion is defined as the destructive and unintentional attackof a metal – electrochemical and begins at the surface.

Electrochemical – a chemical reaction in which there istransfer of electrons from one chemical species to another.

Metal atoms characteristically lose or give up electrons – oxidation reaction. The oxidation takes place is called anode.Oxidation sometimes called anodic reaction.

The electrons generated from the metal atom must betransferred to another chemical species – reduction reaction.The location at which reduction occurs is called the cathode.Reduction also called as cathodic reaction.



Chapter 17 - 5

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

Zn Zn 2 2e

2H 2e H2 (gas)

• Other reduction reactions in solutions with dissolved oxygen:-- acidic solution -- neutral or basic solution

O2 4H 4e 2H2O O2 2H2O 4e 4(OH)

Adapted from Fig. 17.1,Callister & Rethwisch 8e .(Fig. 17.1 is from M.G.Fontana, CorrosionEngineering , 3rd ed., McGraw-Hill Book Company, 1986.)

ELECTROCHEMICAL CORROSION

Zinc

Oxidation reaction Zn Zn2+

2e - Acidsolution

reduction reaction

H + H +

H2(gas)

H +

H +

H +

H +

H +

flow of e - in the metal

Ex: consider the corrosion of zinc in an acid solution



Chapter 17 - 6

STANDARD HYDROGEN ELECTRODE• Two outcomes:

0o

metal V (relative to Pt)

Standard Electrode Potential Adapted from Fig. 17.2,Callister & Rethwisch 8e .

-- Corrosion

-- Metal is the anode (-)

P l a t i n u m

m e

t a l , M

Mn+ions

ne - H2(gas )

25ºC1M Mn+ sol’n 1M H+ sol’n

2e -

e - e -

H + H +

-- Electrodeposition

-- Metal is the cathode (+)

Mn+ions

ne -

e - e -

25ºC1M Mn+ sol’n 1M H+ sol’n

P l a t i n u m

m e

t a l , M

H +

H + 2e -

0o

metal V (relative to Pt)

H2(gas)



Chapter 17 - 7

STANDARD EMF SERIES

metalo

• Metal with smaller

V corrodes.

• EMF series

AuCuPbSn

NiCoCdFeCrZn

AlMgNaK

+1.420 V+0.340- 0.126- 0.136

- 0.250- 0.277- 0.403- 0.440- 0.744- 0.763- 1.662- 2.363- 2.714- 2.924

metalV

metal

o

Data based on Table 17.1,Callister 8e .

m o r e a n o

d i c

m o r e

c a

t h o

d i c

D V =0.153V

o

Adapted from Fig. 17.2,Callister & Rethwisch 8e .

-

1.0 MNi2+ solution

1.0 MCd 2 + solution

+

25ºC NiCd

Cdo

Nio

• Ex: Cd-Ni cellV < V Cd corrodes



Chapter 17 - 8

EFFECT OF SOLUTION CONCENTRATION ANDTEMPERATURE

• Ex: Cd -Ni cell with

standard 1 M solutionsV

Nio V

Cdo 0.153 V

-

Ni

1.0 MNi2+ solution

1.0 MCd 2 + solution

+

Cd 25ºC

• Ex: Cd -Ni cell with

non-standard solutions

Y

X ln

nF

RT V V V V

o

Cd

o

NiCdNi

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

• Reduce V Ni - V Cd by-- increasing X -- decreasing Y -- increasing T

- +

Ni

Y MNi2+ solution

X MCd 2 + solution

Cd T



Chapter 17 - 9

GALVANIC SERIES• Ranking of the reactivity of metals/alloys in seawater

Based on Table 17.2, Callister &Rethwisch 8e . (Source of Table17.2 is M.G. Fontana, CorrosionEngineering , 3rd ed., McGraw-Hill Book Company, 1986.)

PlatinumGoldGraphiteTitaniumSilver

316 Stainless Steel (passive)Nickel (passive)CopperNickel (active)TinLead

316 Stainless Steel (active)Iron/Steel

Aluminum AlloysCadmiumZincMagnesium

m o r e a n o

d i c

( a c

t i v e )

m o r e c a

t h o

d i c

( i n e r t )



Chapter 17 - 10

• Uniform Attack Oxidation & reductionreactions occur uniformlyover surfaces.

• Selective Leaching Preferred corrosion ofone element/constituent[e.g., Zn from brass (Cu-Zn)].

• Stress corrosion Corrosion at crack tipswhen a tensile stressis present.

• Galvanic Dissimilar metals arephysically joined in thepresence of anelectrolyte. Themore anodic metalcorrodes.

• Erosion-corrosion Combined chemical attack andmechanical wear (e.g., pipeelbows).

FORMS OF CORROSION

Forms of

corrosion

• Crevice Narrow andconfined spaces.

Fig. 17.15, Callister & Rethwisch 8e . (Fig. 17.15is courtesy LaQue Center for Corrosion

Technology, Inc.)

Rivet holes

• Intergranular Corrosion alonggrain boundaries,

often where precip.particles form.

Fig. 17.18, Callister &Rethwisch 8e .

attackedzones

g.b.prec.

• Pitting Downward propagationof small pits and holes.

Fig. 17.17, Callister &Rethwisch 8e . (Fig. 17.17from M.G. Fontana,Corrosion Engineering ,3rd ed., McGraw-Hill BookCompany, 1986.)



Chapter 17 - 11

-- Use metals that passivate

- These metals form a thin,adhering oxide layer thatslows corrosion.

• Lower the temperature (reduces rates of oxidation andreduction)

CORROSION PREVENTION (i)

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

Metal oxide

• Apply physical barriers -- e.g., films and coatings

• Materials Selection -- Use metals that are relatively unreactive in thecorrosion environment -- e.g., Ni in basic solutions



Chapter 17 - 12

• Add inhibitors (substances added to solution that decreaseits reactivity)-- Slow oxidation/reduction reactions by removing reactants

(e.g., remove O 2 gas by reacting it w/an inhibitor).-- Slow oxidation reaction by attaching species to

the surface.

CORROSION PREVENTION (ii)

Adaptedfrom Fig.17.22(a),Callister &Rethwisch8e .

Using a sacrificial anode

steelpipe

Mganode

Cu wire e -

Earth

Mg2+

• Cathodic (or sacrificial) protection -- Attach a more anodic material to the one to be protected.

Adaptedfrom Fig.17.23,Callister &Rethwisch8e . steel

zinczincZn 2+

2e - 2e -

e.g., zinc-coated nail

Galvanized Steel

e.g., Mg Anode



Chapter 17 - 13

• Metallic corrosion involves electrochemical reactions

-- electrons are given up by metals in an oxidation reaction-- these electrons are consumed in a reduction reaction• Metals and alloys are ranked according to their

corrosiveness in standard emf and galvanic series .• Temperature and solution composition affect corrosion

rates.• Forms of corrosion are classified according to mechanism• Corrosion may be prevented or controlled by:

-- materials selection

-- reducing the temperature-- applying physical barriers-- adding inhibitors-- cathodic protection

SUMMARY

corrossion and degradation

Documents