CHAPTER 23:CORROSION AND DEGRADATIONCORROSION AND DEGRADATION

CORROSIONCorrosion:

The corrosion is the deterioration of the metals (oxidation) in different environments in presence of oxygendifferent environments in presence of oxygen.

Anodic reaction: M M+n + ne-

Cathodic reaction: 2H+ + 2e- H2 (acid environment) O2 + 2H2O + 4e- 4OH- (basic environment)

Example: CORROSION OF ZINC IN ACID• Two reactions are necessary:

-- oxidation reaction:-- reduction reaction:

Zn → Zn2+ + 2e−

2H+ + 2e− →H (gas )-- reduction reaction: 2H + 2e →H2(gas )

oxidation reactionH+

Zinc

Zn Zn2+

2e-Acid solution

H+H+H+

H+

flow of e- in the metal

reduction reaction

H+

H2(gas)H+

H+

in the metal

• Other reduction reactions:-- in an acid solution -- in a neutral or base solution

+ O2 + 4H+ + 4e− → 2H2O O2 + 2H2O + 4e− → 4(OH)−

STANDARD EMF SERIES• EMF series • Metal with smaller

V corrodes.• Ex: Cd-Ni cell

metalo

ic Au +1.420 Vmetal V

metalo

Ex: Cd Ni cell

- +

ca

tho

di

CuPbSn

+0.340- 0.126- 0.136

NiCd 25°C

mo

re c Ni

CoCd

- 0.250- 0.277- 0.403

DV = 0.153V

o

Ni

1 0 M 1 0 M

Cd 25°C

od

ic

CdFeCrZn

0.403- 0.440- 0.744- 0 763 1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

ore

an

o ZnAlMgNa

0.763- 1.662- 2.262- 2 714

5

m

NaK

- 2.714- 2.924

This is how we measure the potentials in the EMF Series:

• Two outcomes:--Metal sample mass --Metal sample mass

ne- H2(gas)

2e-

e- e-

ne-

e- e-

H+ +

2e-

Pla

tin

um

me

tal,

M

Mn+ ions

ne 2e

H+

H+ Mn+ ions

ne

Pla

tin

um

me

tal,

M H+

Pm

25°C 1M Mn+ sol’n 1M H+ sol’n

H

25°C 1M Mn+ sol’n 1M H+ sol’n

Pm

--Metal is the anode (-) --Metal is the cathode (+)

Vmetalo < 0 (relative to Pt) Vmetal

o > 0 (relative to Pt)

Standard Electrode Potential

EFFECT OF SOLUTION CONCENTRATION• Ex: Cd-Ni cell with

standard 1M solutions• Ex: Cd-Ni cell with

non-standard solutionso o RT X

- + VNi

o − VCdo = 0.153 VNi − VCd = VNi

o −VCdo −

RT

nFln

X

Y- +

n = #e-

per unitoxid/red

Ni

Y M X M

Cd TNi

1.0 M 1.0 M

Cd 25°C reaction(=2 here)F = Faraday'sY M

Ni2+ solution

X M

Cd2+ solution

1.0 M

Ni2+ solution

1.0 M

Cd2+ solutionFaraday sconstant=96,500C/mol.

• Reduce VNi - VCd byincreasing X--increasing X

--decreasing Y

Nernst equation is used to measure the potential difference:potential difference:

ionCF

RTVV ln0 +=

nFWhere:

V = New potential of half-cell, VV New potential of half cell, V

V0 = Standard potential, V

R = Real gas constant 8 3143 J/mol °KR = Real gas constant, 8.3143 J/mol K

T = Temperature, °K

n = Number of electrons transferred.

F = Faraday constant, 96,500 C/mol or A· s/mol

Cion = Molar concentration of ions

If we need to measure the amount of mass lost by corrosion we use Faraday equation:corrosion we use Faraday equation:

iAtMItMw ==

nFnFw ==

Where:

l t i htw = lost weight, g

M = Atomic mass of the metal, g/mol

t = time, sec

I = current density, A/cm2

n = number of electrons transferred

F = Faraday constant, 96,500 C/mol or A.s/moly , ,

A = area, cm2

Example: Corrosion in a grapefruit

Cu-+AnodeCathode

CuZn

Zn2+H+ H+

2e- oxidationreduction

H+

Acid

H+ +H+

HH+

2H+ + 2e− →H2(gas )

O 4H+ 4e− 2H O

H+ H+

O2 + 4H+ + 4e → 2H2Od

ic PlatinumGoldG hit

GALVANIC SERIES

Ranks the reactivity f t l / ll i c

ath

on

ert

) GraphiteTitaniumSilverof metals/alloys in

seawater

mo

re c (in

316 Stainless SteelNickel (passive)C

c

m CopperNickel (active)Tin

an

od

iciv

e)

TinLead316 Stainless SteelI /St l

mo

re a

(ac

ti Iron/SteelAluminum AlloysCadmium

m

CadmiumZincMagnesium

FORMS OF CORROSION

• Uniform AttackOxidation & reductionOxidation & reductionoccur uniformly oversurface.

• Crevice Between twoi f th t lpieces of the same metal.

Rivet holes

FORMS OF CORROSION (cont.)

• Stress corrosionStress & corrosionwork togetherat crack tips.

• GalvanicDissimilar metals arephysically joined Thephysically joined. Themore anodic onecorrodes.(see Table17.2) Zn & Mgvery anodic.

FORMS OF CORROSION (cont.)

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

Pitti• PittingDownward propagationof small pits & holes.p

FORMS OF CORROSION (cont.)

• Erosion-corrosionBreak down of passivatingBreak down of passivatinglayer by erosion (pipeelbows).

g.b. prec. • Intergranular

attacked

gCorrosion alonggrain boundaries,often where special

h i tzones phases exist.

CONTROLLING CORROSION• Self-protecting metals!

--Metal ions combine with Ot f thi dh i id l th t l ito form a thin, adhering oxide layer that slows corrosion.

Metal (e.g., Al, Metal oxide

stainless steel)

R d T• 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 toy g p

the surface (e.g., paint it!).

CONTROLLING CORROSION (cont.)

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

Z 2+e.g., zinc-coated nail e.g., Mg Anode

zinczincZn 2+

steel Mg

Cu wiree-Mg 2+

steel2e - 2e -

s eepipe

Mg anode

Earth

Mg 2

Rate of oxidation

Rate of oxidation and the tendency of the film to protect the metal from further oxidation 0ρAthe metal from further oxidation are related to the relative volumes of the oxide and metal. 0

0

ρρ

M

M

A

AratioBP =−

Where:

P-R ratio = Pilling-Bedworth ratiog

A0 = is the molecular (or formula) weight of the oxide

AM = is the atomic weight of the metalAM = is the atomic weight of the metal

ρ0 = oxide density

t l d itρ0 = metal density

Oxidation rate (kinetics)Where:

ω = weight gain per unit areaω weight gain per unit area

t = time

d th t t κL, κp and κe are the constant linear, parabolic and logarithmic respectively.

Linear: ω = κLt

p y

C and A are constant.

L

Parabolic ω2 = κpt + C

i h i l (C A)Logarithmic ω = κelog (C·t + A)

Example: Artificial total hip replacement

X-rays of a normal hip joint Schematic diagram and x-ray of an X-rays of a normal hip joint and fractured hip joint

g yartificial total hip replacement

Artificial total hip joint jreplacement designs

Artificial total hip replacement (cont.)

Three types of biomaterials are used for hip implants:

• Austenitic stainless steel - 316L with low sulfur content (< 0.002 wt%) and extremely low carbon

• Cobalt-nickel-chromium-molybdenum – MP35N with a composition 35 wt% Co, 35 wt% Ni, 20 wt% Cr, and 10 wt% Mo.

• Titanium – Ti-6Al-4V with a composition 90 wt% Ti, 6 wt% Al and 4 wt% V

Artificial Total Hip replacement

SUMMARY• Corrosion occurs due to:

--the natural tendency of metals to give up electrons.--electrons are given up by an oxidation reaction.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 toPotential are more likely to corrode relative toother metals.

• The Galvanic Series ranks the reactivity of metals intseawater.

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

-- using metals which forma protective oxide layer

-- reducing T

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