Foundations of Materials Science and Engineering Third … · CHAPTER 13 Corrosion 1 ... Foundations of Materials Science and Engineering, 5th Edn. Smith and Hashemi

CHAPTER

13

Corrosion

1

Corrosion and corrosion protection:

Electrochemical corrosion of metals, Galvanic

cells, Types of corrosion, Oxidation of metals,

Corrosion control.

ISSUES TO ADDRESS...

• Why does corrosion occur?

• What metals are most likely to corrode?

• How do we suppress corrosion?

• What are the types of corrosion

• What is corrosion and how does it degrades the material?

• What is the effect of corrosion on ceramics? What is the effect of

corrosion on polymers?

Foundations of Materials Science and Engineering, 5th Edn. Smith and Hashemi

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Introduction

• Corrosion: Deterioration of a metal resulting

from chemical attack by its environment.

• Rate of corrosion depends upon temperature

and concentration of reactants and products.

• Metals have free electrons that setup

electrochemical cells within their structure.

• Metals have tendency to go back to low energy

state by corroding.

3



Al Capone's ship, Sapona, off the coast of Bimini.



Degradation of Polymer and Ceramics

• Engineering materials are subjected to numerous external

mechanical and environmental factors during their service.

Those factors include temperature, chemical attack,

mechanical vibration, applied mechanical loads, etc. Under

the influence of these factors the engineering materials loss

their potential to perform the intended task.

• Ceramics and polymers suffer corrosion by direct chemical

attack.

• Examples of Chemically Assisted Degradation

– Degradation of Rubber by Ozone

– Degradation of Poly(vinyl) Chloride (PVC) (formation of salt )

- Deterioration of acrylic paintings and pieces of art

- Decomposition of photographic films



Heat Exchanger for the Chemical Process made of 17 kms Zirconium



Oxidation – Reduction Reactions

• A metal (Eg – Zn) placed in HCl undergoes corrosion.

Zn + 2HCl ZnCl2 + H2

or

Zn + 2H+ Zn2+ + H2

also

Zn Zn 2+ + 2e- (Oxidation half cell reaction)

2H+ + 2e- H2 (Reduction half cell reaction)

• Oxidation reaction: Metals form ions at local anode.

• Reduction reaction: Metal is reduced in local charge at

local cathode.

• Oxidation and reduction takes place at same rate.

7



Standard electrode Half-Cell Potential of Metals

• Oxidation/Reduction half cell potentials are compared with standard hydrogen ion

half cell potential.

• Voltage of metal (Eg-Zn) is

directly measured against

hydrogen half cell electrode.

• Anodic to hydrogen More tendency to corrode

Examples:- Fe (-0.44), Na (-2.74)

• Cathodic to hydrogen Less tendency to corrode

Examples:- Au (1.498), Cu (0.33)

8



Table 13.1



Figure A galvanic cell is produced by two dissimilar metals. The more

“anodic” metal corrodes.



Macroscopic Galvanic Cells with 1M Electrolyte

• Two dissimilar metal electrodes immersed in solution of their own ions.

• Electrode that has more

negative oxidation potential

will be oxidized.

Zn Zn2+, Cu2+ Cu

Half cell reactions are

Zn Zn 2+ + 2e- E0 = -0.763 V

Cu Cu2+ + 2e- E0 = + 0.337 V

Or Cu2+ + 2e- Cu E0 = -0.337 V (negative sign)

Adding two reactions,

Zn + Cu2+ Zn2+ + Cu E0cell = [-0.763 -0.337 ] = -1.1V

Oxidized Reduced

11



GALVANIC SERIES

• Ranks the reactivity of metals/alloys in seawater

Based on Table 17.2, Callister

7e. (Source of Table 17.2 is

M.G. Fontana, Corrosion

Engineering, 3rd ed., McGraw-

Hill Book Company, 1986.)

Platinum

Gold

Graphite

Titanium

Silver

316 Stainless Steel

Nickel (passive)

Copper

Nickel (active)

Tin

Lead

316 Stainless Steel

Iron/Steel

Aluminum Alloys

Cadmium

Zinc

Magnesium

mo

re a

no

dic

(a

ctive)

more

cath

odic

(inert

)



Figure A steel bolt in a brass plate creates a galvanic cell

Brass is an alloy of

copper and zinc



Types of Corrosion

• Uniform or general attack corrosion: Reaction proceeds

uniformly on the entire surface.

Controlled by protective coatings, inhibitors and

cathodic protection.

• Galvanic or two metal corrosion: Electrochemical

reaction leads to corrosion of metal.

Zinc coatings on steel protects steel as zinc is

anodic to steel and corrodes.

Large cathode area to small anode area should be

avoided.

14



Grain – Grain boundary Electrochemical cells

• Grain boundaries are more chemically active (anodic)

and hence get corroded by electrochemical attack.

• Grain boundaries are at higher energy due to impurities

that migrate to grain boundaries.

Cartridge Brass

Grain

Boundary

Grain boundary

(anode)

15



Pitting Corrosion

• Pitting: Localized corrosive attacks that produces holes or

pits in a metal.

• Results in sudden unexpected failure as pits go undetected

(covered by corrosion products).

• Pitting requires an initiation

period and grows in

direction of gravity.

• Pits initiate at structural

and compositional

heterogeneities.

Pitting of stainless steel 16



Growth of Pit

• Growth of pit involves dissolution of metal in pit

maintaining high acidity at the bottom.

• Anodic reaction at the

bottom and cathodic

reaction at the metal

surface.

• At bottom, metal chloride + water Metal hydroxide +

free acid.

• Some metals (stainless steel) have better resistance than

others (titanium).

17



Crevice Corrosion

• Localized electrochemical corrosion in crevices and under shielded surfaces where stagnant solutions can exist.

• Occurs under valve gaskets, rivets and bolts in alloy systems like steel, titanium and copper alloys.

• Anode: M M+ + e-

• Cathode:O2 + 2H2O + 4e- 4OH-

• As the solution is

stagnant, oxygen is used up

and not replaced.

• Chloride ions migrate to

crevice to balance positive charge and form metal hydroxide and free acid that causes corrosion.

18



Intergranular Corrosion

• Localized corrosion at and/or adjacent to highly reactive

grain boundaries resulting in disintegration.

• When stainless steels are heated to or cooled through

temperature range (500-8000C) chromium carbide

precipitate along grain boundaries.

• When exposed to corrosive environment, the region next

to grain boundaries become anodic and corrode.

19



Stress Corrosion

• Stress corrosion cracking (SCC): Cracking caused by combined effect of tensile stress and corrosive environment.

• Only certain combination

of alloy and environment

causes SCC.

• Crack initiates at pit or

other discontinuity.

• Crack propagates perpendicular

to stress

• Crack growth stops if either stress or corrosive environment is removed.

20



Stress Corrosion

A hanger with large Stress

Corrosion Cracks

stress corrosion crack

http://www.google.com.om/imgres?imgurl=http://www.climbcaymanbrac.com/wp-content/uploads/2011/05/OoAfrica2c.jpg&imgrefurl=http://www.climbcaymanbrac.com/%3Fpage_id%3D28&usg=__BS1kfqw2t53i3J980wtgL2cmMLA=&h=396&w=321&sz=51&hl=en&start=6&zoom=1&tbnid=zQ1XHpL1ilmDvM:&tbnh=124&tbnw=101&ei=pwetT86nJcWsrAe17fimDA&prev=/search%3Fq%3Dstress%2Bcorrosion%26hl%3Den%26rlz%3D1R2GGLL_en%26tbm%3Disch&itbs=1

http://eddycurrent.net/gallery/var/albums/Tube-Defects/stress_corrosion_crack.gif?m=1297144357



Fretting Corrosion

• Fretting corrosion: Occurs at interface between materials

under load subjected to vibration and slip.

Metal fragments get oxidized and act as abrasives

between the rubbing surfaces.

Occurs in tight-fitting surfaces (between shafts and

bearings or sleeves)

22

chain fretting

corrosion due to

lack of lubrication

Fretting Corrosion of a

Fence Post and Wires

Frettin Corrosion Pay

close attention to sharp

instrument edges and

tips

http://www.google.com.om/imgres?imgurl=http://www.iahcsmm.org/Recertification/LessonPlans/images/CIS211pics/exampleOfFrettingCorrosion.jpg&imgrefurl=http://www.iahcsmm.org/Recertification/LessonPlans/CIS_lessonPlans/CIS_Lessons/CIS_211.html&usg=__UGtzMoflDzg4j7TN5LZ2DXA4zbg=&h=333&w=300&sz=11&hl=en&start=2&zoom=1&tbnid=nJCB8rbiyPBicM:&tbnh=119&tbnw=107&ei=Of2sT8rGCYrXrQeV79yeDA&prev=/search%3Fq%3Dfretting%2Bcorrosion%26hl%3Den%26sa%3DX%26rlz%3D1W1GGLL_en%26tbm%3Disch%26prmd%3Dimvns&itbs=1

http://www.google.com.om/imgres?imgurl=http://www.vibrationconsulting.com/UserFiles/Image/chain.PNG&imgrefurl=http://www.vibrationconsulting.com/pgwitnessedshoptesting.aspx&usg=__tmfr6pwqupXBv1RZFX8BsBsLPp8=&h=265&w=419&sz=288&hl=en&start=17&zoom=1&tbnid=ZBNMXBHGH1An2M:&tbnh=79&tbnw=125&ei=Of2sT8rGCYrXrQeV79yeDA&prev=/search%3Fq%3Dfretting%2Bcorrosion%26hl%3Den%26sa%3DX%26rlz%3D1W1GGLL_en%26tbm%3Disch%26prmd%3Dimvns&itbs=1

http://www.google.com.om/imgres?imgurl=http://corrosion.ksc.nasa.gov/images/fret4.jpg&imgrefurl=http://corrosion.ksc.nasa.gov/fretcor.htm&usg=__LdmNPPqXSy47RgBB8mCDlqOi7Ek=&h=268&w=356&sz=20&hl=en&start=1&zoom=1&tbnid=VqPQa5EM33nJvM:&tbnh=91&tbnw=121&ei=Of2sT8rGCYrXrQeV79yeDA&prev=/search%3Fq%3Dfretting%2Bcorrosion%26hl%3Den%26sa%3DX%26rlz%3D1W1GGLL_en%26tbm%3Disch%26prmd%3Dimvns&itbs=1



Selective Leaching

• Selective leaching (dealloying): Selective removal of one

element of alloy by corrosion.

Example: Dezincification Selective removal

of zinc from copper on brasses.

Weakens the alloy as single metal might not have

same strength as the alloy.

Other examples are the

loss of nickel, tin and chromium from copper

alloys

Loss of iron from cast iron

Loss of nickel from alloy steels



Selective Leaching

Dezincification of Pipe Fittings

Corrosion is an inevitable phenomena.

The spotted effect on this propeller is the

result of selective leaching.

One of the less noble elements in the casting

is being leached out of the bronze, leaving

pockets of copper behind.

This propeller will eventually corrode away

if the anodes are not corrected. The

propeller needs replacing. .



Erosion Corrosion

• Erosion corrosion: Acceleration in rate of corrosion due

to relative motion between corrosive fluid and surface.

• Pits, grooves, valleys appear on surface in direction of

flow.

• Corrosion is due to abrasive action and removal of

protective film.

25



Cavitation Damage

• Cavitation damage: Caused by collapse of air bubbles or

vapor filled cavities in a liquid near metal surface.

• Rapidly collapsing air bubbles produce very high pressure

(60,000 PSI) and damage the surface.

• Occurs at metal surface when high velocity flow and

pressure are present.

Cavitation of a nickel alloy pump

impeller blade exposed to a

hydrochloric acid medium.



• Uniform Attack Oxidation & reduction

occur uniformly over

surface. • Selective Leaching Preferred corrosion of

one element/constituent

(e.g., Zn from brass (Cu-Zn)).

• Stress corrosion Stress & corrosion

work together

at crack tips.

• Galvanic Dissimilar metals are

physically joined. The

more anodic one

corrodes.(see Table

17.2) Zn & Mg

very anodic.

• Erosion-corrosion Break down of passivating

layer by erosion (pipe

elbows).

FORMS OF CORROSION

Forms of

corrosion

• Crevice Between two

pieces of the same metal.

Fig. 17.15, Callister 7e. (Fig. 17.15 is

courtesy LaQue Center for Corrosion

Technology, Inc.)

Rivet holes

• Intergranular Corrosion along

grain boundaries,

often where special

phases exist.

Fig. 17.18, Callister 7e.

attacked

zones

g.b.

prec.

• Pitting Downward propagation

of small pits & holes. Fig. 17.17, Callister 7e.

(Fig. 17.17 from M.G.

Fontana, Corrosion

Engineering, 3rd ed.,

McGraw-Hill Book

Company, 1986.)



Corrosion Control – Material Selection

• Metallic Metals:

Use proper metal for particular environment.

For reducing conditions, use nickel and copper

alloys.

For oxidizing conditions, use chromium based

alloys.

• Nonmetallic Metals:

Limit use of polymers in presence of strong

inorganic acids.

Ceramics have better corrosion resistance but are

brittle.

28



Coatings

• Metallic Coatings: Used to protect metal by separating

from corrosive environment and serving as anode.

Coating applied through electroplating or roll

bonding.

might have several layers.

• Inorganic coatings: Coating steel with ceramic.

Steel is coated with porcelain and lined with glass.

• Organic coatings: Organic polymers (paints and

varnishes) are used for coatings.

Serve as barrier but should be applied carefully.

29



Figure (a) Galvanized steel consists of a zinc coating on a steel substrate. Since zinc is anodic

to iron, a break in the coating does not lead to corrosion of the substrate. (b) In contrast, a

more noble coating such as “tin plate” is protective only as long as the coating is free of breaks.

At a break, the anodic substrate is preferentially attacked.

Zinc coated steel roofing

Zinc coated wire

Tin coated steel bolts

A steel grinder coated with

zinc



Design

• General design rules:

Provide allowance for corrosion in thickness.

Weld rather than rivet to avoid crevice corrosion.

Avoid dissimilar metals that can cause galvanic corrosion.

Avoid excessive stress and stress concentration.

Avoid sharp bends in pipes to prevent erosion corrosion.

Design tanks and containers for early draining.

design so that parts can be easily replaced.

Design heating systems so that hot spots do not occur.

31



Alteration Environment

• Lower the temperature Reduces reaction rate.

• Decrease velocity of fluids Reduces erosion

corrosion.

• Removing oxygen from liquids reduces

corrosion.

• Reducing ion concentration decreases corrosion

rate.

• Adding inhibitors inhibitors are retarding

catalysts and hence reduce corrosion.

32



• Self-protecting metals! -- Metal ions combine with O

to form a thin, adhering oxide layer that slows corrosion.

• 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!).

CONTROLLING CORROSION

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

Metal oxide

Adapted from Fig. 17.22(a),

Callister 7e. (Fig. 17.22(a) is

from M.G. Fontana, Corrosion

Engineering, 3rd ed., McGraw-Hill

Book Co., 1986.)

steel pipe

Mg anode

Cu wire e -

Earth

Mg 2+

e.g., Mg Anode

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

Adapted

from Fig.

17.23,

Callister

7e. steel

zinc zinc

Zn 2+

2e - 2e -

e.g., zinc-coated nail



• Corrosion occurs due to: -- the natural tendency of metals to give up electrons.

-- electrons are given up by an oxidation reaction.

-- these electrons then used in 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

Documents

Foundations of Materials Science and Engineering Third … · CHAPTER 13 Corrosion 1 ... Foundations of Materials Science and Engineering, 5th Edn. Smith and Hashemi