CONTENTS

1. Introduction

2. Corrosion mechanism

3. Corrosion rates

4. Forms of corrosion

5. Controlling corrosion

Material Science and EngineeringSchool of Chemical and Bio Engineering

LECTURE 11

CorrosionDec 31, 2013

CORROSION

Why does corrosion occur?

What metals are most likely to corrode?

How do temperature and environment affectcorrosion rate?

How do we suppress corrosion?

Corrosion

INTRODUCTION Classification of materials

Metals Ceramics Polymers Composites

Deterioration of materials

Corrosion Leaching Degradation

Year Billion (109) USD

1976 70

1982 126

2002 276

Table 1. Estimated direct annual costs of corrosion in USA.

ECONOMIC IMPACT

Indirect costs

Direct costs

is a destructive and unintentional attack of a metal;

it is electrochemical and ordinarily begins at the surface.

DEFINITION

W.D. Callister, Materials Science and Engineering, An Introduction, John Wiley & Sons, New York etc., 1991, p.563.

Figure 1. Golden Gate Bridge, San Francisco.http://en.wikipedia.org/wiki/Golden_Gate_Bridge

GOLDEN GATE, San Francisco

Due to corrosion original lead paint was replaced by zinc-silicate and acrylic topcoat.

It took 30 years to complete the project.

Currently 38 painters and 17 ironworkers continually work on protection andreplacement of corroded parts.

Corrosion

Fe2O3 + 3CO → 2Fe + 3CO2

ORE(metal oxides in minerals)

EXTRACTION METALS

CORROSION

2Al2O3 + 3C → 4Al + 3CO2

Corrosion is an elctrochemical process thus,

it is a chemical reaction in which there is

transfer of electrons from one chemical

species to another.

Corrosion environment

AQUAEOUS

GASEOUS

AQUEOUS ELECTROLYTE

water in which ions are dissolved.

CORROSION MECHANISM

Figure 3. A simple electrochemical cell.

Corrosion

CORROSION MECHANISM

4Fe + 6H2O + 3O2 4Fe(OH)3

gives ferric hydroxide

2Fe(OH)3 Fe2O3 3H2Ogives iron oxide (rust) and water

Basic “rusting” or corrosion requirements

1. The metal is oxidized at the anode of an electrolytic cell2. Some ions are reduced at the cathode3. There is a potential or voltage difference between the anode and cathode4. An electrolyte (fluid) must be present5. The electrical path must be completed

Corrosion of steel under a drop of water

H2O + ½O2 + 2eˉ → 2OH ˉ

Anode Fe → Fe2+ + 2eˉ

Cathode Fe2+ + 2OHˉ → Fe(OH)2

CuZn

Zn2+

2e- oxidationreduction

Acid

H+H+

H+

H+

H+

H+

H+

-+AnodeCathode

2H 2e H2(gas )

O2 4H 4e 2H2O

CORROSION IN A GRAPEFRUIT

Corrosion

Corrosion

CORROSION MECHANISM

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)

CORROSION OF ZINC IN ACID

Electrode Potentials

Corrosion

Cu2+ + Fe Fe2+ + Cu

Corrosion

2H 2e H2(gas )

The standard hydrogen electrode consists ofan inert platinum electrode in a 1 Msolution of H+ ions, saturated with hydrogengas that is bubbled through the solution at apressure of 1 atm and a temperature of 25 oC.

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-

Corrosion

Corrosion

• EMF series • Metal with smallerV corrodes.

• Ex: Cd-Ni cellmetalo

-

Ni

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

+

Cd 25°C

mo

re a

no

dic

mo

re c

ath

od

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

DV = 0.153V

o

Data based on Table 17.1, Callister 6e.

STANDARD EMF SERIES

Corrosion

- +

Ni

Y M

Ni2+ solution

X M

Cd2+ solution

Cd T

• Ex: Cd-Ni cell withstandard 1M solutions

• Ex: Cd-Ni cell withnon-standard solutions

VNio VCd

o 0.153 VNi VCd VNi

o VCdo

RT

nFln

X

Y-

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 AND TEMPERATURE

Corrosion

Nernst Equation

THE GALVANIC SERIES

Corrosion

A more realistic and practical ranking that represents the relative reactivities of a number of metals and commercial alloys in seawater.

Corrosion

CORROSION RATES

CPR: The rate of material removal as a consequence of the chemical action may be expressed as the corrosion penetration rate (CPR),or the thickness loss of material per unit of time.

where W is the weight loss after exposure time t; ρ and A represent the density and exposed specimen area, respectively, and K is a constant 87.6 mm/yr or 534 mpy. W, ρ ,A, and t are specified in units of milligrams, grams per cubic centimeter, square centimeters, and hours.

CPR< 0.50 mm/yr -------Acceptable

Rate: Expression relating corrosion rate and current density

where, again,n is the number of electrons associated with the ionization of each metal atom, and f is 96,500 C/mol.

Self-help Problems:

EXAMPLE

This problem asks to calculate the CPR in both mpy and mm/yr for a thick steelsheet of area 100 in.2 which experiences a weight loss of 485 g after one year.Employment of CPR Equation leads to

= 0.952 mm/yr

Also

CPR =

= 37.4 mpy

CPR = KW

A t=

(87.6)(485 g) 10 3 mg / g 7.9 g / cm 3 100 in.2 (2.54 cm / in.)2 (24 h / day)(365 day / yr)(1 yr)

)1)(/365)(/24(.100/9.7

/10)485)(534( = CPR

23

3

yryrdaydayhincmg

gmgg

Corrosion

A thick steel sheet of area 100 in2 is exposed to air near the ocean. After a one-year period it was found to experience a weight loss of 485 g due to corrosion. To what rate of corrosion, in both mpy and mm/yr, does this correspond?

Material properties Metallurgical factors Passivity Environment

Metallurgical factors Chemical segregation Presence of multiple phases Inclusions Cold Work Non-uniform stresses

FACTORS AFFECTING CORROSION

Passivity Example with steel in

nitric acid…dilute solutions will cause rapid attack, strong solutions have little visible effect.

Surface film can be formed

Some types of steel may do this with rust

Aluminum does this

Corrosion

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. 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. 17.9, Callister 6e.

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

FORMS OF CORROSION

Corrosion

Self-protecting metals!

Metal ions combine with O2 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 tothe surface (e.g., paint it!).

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

CONTROLLING CORROSION

Corrosion

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

Corrosion