Chapter 18: Corrosion and Degradation of Materials

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ISSUES TO ADDRESS...

• How does corrosion occur?

• Which metals are most likely to corrode?

• What environmental parameters affectcorrosion rate?

• How do we prevent or control corrosion?

Chapter 18:Corrosion and Degradation of Materials

• 01 Introduction• 02 Electrochemical Considerations• 03 Corrosion Rates• 04 Prediction of Corrosion Rates• 05 Passivity• 06 Environmental Effects• 07 Forms of Corrosion• 08 Corrosion Environments• 09 Corrosion Prevention• 10 Oxidation• 11 Swelling and Dissolution• 12 Bond Rupture• 13 Weathering

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01 Corrosion

• Corrosion is a natural process, which converts a refined metal to a more stable form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of materials (metals) by chemical reaction with their environment.

• Corrosion engineering is the field dedicated to controlling and stopping corrosion.

• Deteriorative mechanisms: – In metals, there is actual material loss either by dissolution

or by oxidation. – Ceramics, resistant to deterioration, but at elevated

temperatures or in rather extreme environments– Polymers, degradation

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• Corrosion:-- the destructive

electrochemical attack of a material.

-- Ex: Rusting of automobiles and other equipment

• Cost:-- 4 to 5% of the Gross

National Product (GNP)*-- in the U.S. this amounts

to just over $400 billion/yr**

THE COST OF CORROSION

© E

HSt

ock/

iSto

ckph

oto

Figun_18_p638a

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

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

02 ELECTROCHEMICAL CORROSION

Zinc

Oxidation reactionZn Zn2+

2e-Acid solution

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

Fig_18_02

An electrochemical cell consisting of iron and copper Electrodes

Iron corrodes(Oxidation)

Copper electrodeposits(Reduction)

Anode Cathode

Fig_18_03

An electrochemical cell consisting of iron and zincElectrodes

Zinc corrodes

Iron electrodeposits

Cathode Anode

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STANDARD HYDROGEN ELECTRODE• Two outcomes:

(relative to Pt)

Standard Electrode Potential

-- Corrosion

-- Metal is the anode (-)

Plat

inum

met

al, M Mn+

ions

ne- H2(gas)

25°C1M Mn+ sol’n 1M H+ sol’n

2e-

e-e-

H+H+

-- Electrodeposition

-- Metal is the cathode (+)

Mn+ions

ne-

e- e-

25°C1M Mn+ sol’n 1M H+ sol’n

Plat

inum

met

al, M

H+H+

2e-

(relative to Pt)

H2(gas)

Fig_18_04

The standard hydrogen reference half-cell

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STANDARD ElectoMotive Force (EMF) SERIES

metalo

• Metal with smallerV corrodes.

• EMF series

AuCuPbSnNiCoCdFeCrZnAlMgNaK

+1.420 V+0.340- 0.126- 0.136- 0.250- 0.277- 0.403- 0.440- 0.744- 0.763- 1.662- 2.363- 2.714- 2.924

metal Vmetalo

Data based on Table 18.1, Callister 9e.

mor

e an

odic

mor

e ca

thod

ic

∆V = 0.153V

o

-

1.0 M Ni2+ solution

1.0 M Cd2+ solution

+

25°C NiCd

Cdo

Nio

• Ex: Cd-Ni cellV < V ∴ Cd corrodes

Table_18_01

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CORROSION IN A GRAPEFRUIT

Zn2+

2e- oxidation reaction

AcidH+ H+

H+

H+

H+

H+

H+-+

Zn (anode)Cu (cathode)

reduction reactions

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EFFECT OF SOLUTION CONCENTRATION AND TEMPERATURE

• Ex: Cd-Ni cell withstandard 1 M solutions

-

Ni

1.0 M Ni2+ solution

1.0 M Cd2+ solution

+

Cd 25°C

• Ex: Cd-Ni cell withnon-standard solutions

n = #e-

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

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

- +

Ni

Y M Ni2+ solution

X M Cd2+ solution

Cd T

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

PlatinumGoldGraphiteTitaniumSilver316 Stainless Steel (passive)Nickel (passive)CopperNickel (active)TinLead316 Stainless Steel (active)Iron/SteelAluminum AlloysCadmiumZincMagnesium

mor

e an

odic

(act

ive)

mor

e ca

thod

ic(in

ert)

The Galvanic Series: The relative reactivities in seawater

15Table_18_02


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03 Corrosion rate• Corrosion penetration rate (CPR) or thickness

loss per unit time — as a function of specimen weight loss, density, area, and time of exposure– CPR = KW / ρAt (mm/yr)

• Expression relating corrosion rate and current density– r = I /nF (mole/m2.s)

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Fig_18_05

Electrochemical cell consisting of standard zinc and hydrogen electrodes that has been sort-circuited

04 Prediction of corrosion rate

Fig_18_06

Schematic representation of possible steps in the hydrogen reduction process, controlled by activation polarization.

05 Passivity• A number of metals and alloys

passivate, or lose their chemical reactivity, under some environmental circumstances. – involve the formation of a

thin protective oxide film – Stainless steels and

aluminum alloys• The active-to-passive behavior

may be explained by the alloy's S-shaped electrochemical potential-versus-log current density curve

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Fig_18_12


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06 Environmental Effects• The variables in the corrosion environment, which

include fluid velocity, temperature, and composition, can have a decided influence on the corrosion properties of the materials that are in contact with it.– increasing fluid velocity enhances the rate of

corrosion due to erosive effects, – Rates of reactions rise with increasing temp., conc.

of corrosive species (e.g., H+ ions in acids)

• Cold working or plastically deforming ductile metals is more susceptible to corrosion than the annealed state.

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• Uniform AttackOxidation & reductionreactions occur uniformly over surfaces.

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

• Stress corrosionCorrosion at crack tips when a tensile stress is present.

• GalvanicDissimilar metals arephysically joined in the presence of an electrolyte. Themore anodic metalcorrodes.

• Erosion-corrosionCombined chemical attack and mechanical wear (e.g., pipe elbows).

07 FORMS OF CORROSION

• Crevice Narrow and confined spaces.

Rivet holes

• IntergranularCorrosion alonggrain boundaries,often where precip.particles form.

attacked zones

g.b. prec.

• PittingDownward propagationof small pits and holes.

• Hydrogen embritlementa significant reduction in ductility that accompanies the penetration of atomic hydrogen into a metal/alloy

Fig_18_14

Fig_18_15

Immersed in seawater, crevice corrosion has occurred at the regions that were covered by washers.

Fig_18_16

Mechanism of crevice corrosion

The pitting of a 304 stainless steel plate by an acid chloride solution

Fig_18_18

Intergranular corrosion of stainless steel

Fig_18_19

Weld decay in a stainless steel

Impingement failure of an elbow that was part of steam condensate line

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Fi 18 21

Stress corrosion of a bent steel bar in sea water

Fig_18_22

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-- Use metals that passivate- These metals form a thin,

adhering oxide layer that slows corrosion.

• Lower the temperature (reduces rates of oxidation and reduction)

09 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 the corrosion environment -- e.g., Ni in basic solutions

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• Add inhibitors (substances added to solution that decrease its reactivity)-- 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.

CORROSION PREVENTION (ii)

Using a sacrificial anode

steel pipe

Mg anode

Cu wiree-

Earth

Mg2+

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

Adapted from Fig. 18.24, Callister & Rethwisch 9e. steel

zinczinc

Zn2+

2e- 2e-

e.g., zinc-coated nail

Galvanized Steel

e.g., Mg Anode

Fig_18_23

Fig_18_24


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10 Oxidation

• Oxidation of metallic materials by electrochemical action is also possible in dry, gaseous atmospheres

• An oxide film forms on the surface that may act as a barrier to further oxidation if the volumes of metal and oxide film are similar

• The kinetics of film formation may follow parabolic, linear, or logarithmic rate laws.

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Fig_18_25

Fig_18_26

Oxidation film growth curves

Table_18_03

Ao (AM)is molecular weight of the oxide (metal)ρo (ρM) is density of the oxide (metal)

= Vo/VM =(Ao/ ρo) / (AM/ ρM)

Corrosion of ceramics

• Ceramic materials, being compounds of metallic and nonmetallic elements, may be thought of as having already been corroded

• Ceramic materials, being inherently corrosion resistant, are frequently used at elevated temperatures and/or in extremely corrosive environments.

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11-13 Degradation of polymers• Deteriorate by noncorrosive processes. • Upon exposure to liquids, they may experience degradation

by swelling or dissolution.– solute molecules actually fit into the molecular structure.– occur when the polymer is completely soluble in the liquid.

• Scission, or the severance of molecular chain bonds, may be induced by radiation, chemical reactions, or heat. – Results in a reduction of molecular weight and a deterioration of the

physical and chemical properties of the polymer.

• Weathering: any resultant degradation caused by exposure to outdoor conditions

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• 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

Chapter 18: Corrosion and Degradation of Materials

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Transcript of Chapter 18: Corrosion and Degradation of Materials