MSE Chap8 Corrosion
-
Upload
taye-zewdu -
Category
Documents
-
view
50 -
download
3
description
Transcript of MSE Chap8 Corrosion
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