16 Chapter Corrosion and Degradation of Materials
Intro
In metals actual material is loss either by dissolution/corrosion or by formation of
nonmetallic scale or film (oxidation)
Ceramic materials are relatively resistant to deterioration
o Only occurs at elevated temperatures or in rather extreme environments
For polymers mechanisms and consequences differ from those for metals and ceramics
(degradation)
o Polymers may dissolve when exposed to liquid solvent or they may absorb the
solvent and swell
o Electromagnetic radiation and heat may cause alterations in their molecular
structures
Corrosion = the destructive and unintentional attack of a metal; it is electrochemical and
ordinarily begins at the surface
Electrochemical Considerations
Oxidation = metal atoms characteristically lose or give up electrons also known as anodic
reaction
oM
Mn+ + neo M becomes a positively charged ion and in the process loses its n valence
electrons
o Fe
Fe2+ + 2e Anode = site at which oxidation takes place
Reduction = electrons generated from each metal atom that is oxidized must be
transferred to and become a part of another chemical species
o For example some metals undergo corrosion in acid solutions which have a high
concentration of hydrogen ions; H+ ions reduced as follows:
2H+ + 2eH2
o Redux reaction in an acid solution containing dissolved oxygen
O2 + 4H+ + 4e2H2O
o Redux reaction in a neutral or basic solution containing dissolved oxygen
O2 + 2H2O + 4e4(OH-)
o Redux of a multivalent metal ion to a lower valence state
Mn+ + eM(n-1)+
o Redux of a metal ion to its electrically neutral atom
Mn+ + neM
Cathode = location at which reduction occurs
Half-reactions = an individual oxidation or reduction reaction
A complete reaction = oxidation + reduction reaction
o Zn
Zn2+ + 2e- oxidation
o 2H+ + 2eH2 reduction
+
o Zn + 2H
Zn2+ + H2 complete reaction
Electrode Potentials
o Galvanic couple = two metals electrically connected in a liquid electrolyte
o Standard half-cell = an electrochemical cell consisting of a pure metal immersed
in a 1M aqueous solution of its ions, which is electrically coupled to the standard
hydrogen electode
Standard emf Series
o Emf series = a ranking of metallic elements according to their standard
electrochemical cell potentials
o Electrochemical cell potential
V0=V20 V10
Overall cell potential = reduction potential oxidation potential
Influence of Concentration and Temperature on Cell Potential
o Nernst equation electrochemical cell potential for two half-cells that are
electrically coupled and for which solution ion concentrations are other than 1 M
V=(V20 V10) RT/(nF)*ln([M1n+]/[M2n+]
F = Faradays constant; 96500
n = number of electrons
Simplified form at room temp: V=(V20 V10) (.0592/n)*log([M1n+]/
[M2n+]
Galvanic Series
o Galvanic series = a ranking of metals and alloys as to their relative
electrochemical reactivity in seawater
o Most metals and alloys are subject to oxidation or corrosion to one degree or
another meaning they are more stable in an ionic state than as metals
Corrosion Rates
Corrosion rate = rate of material removal as a consequence of the chemical action
CPR = KW/(At)
Expression relating corrosion rate and current density
o r = i/(nF)
Prediction of Corrosion Rates
Polarization
o Polarization = displacement of each electrode potential from its equilibrium value
o Overvoltage = magnitude of this displacement represented by
o Activation Polarization = condition wherein the reaction rate is controlled by the
one step in the series that occurs at the slowest rate
Activation energy barrier is associated with this slowest, rate limiting step
o Step Sequence:
Adsorption of H+ ions from the solution onto the zinc surface
Electron transfer from the zinc to form a hydrogen atom
H+ + eH
Combining of two hydrogen atoms to form a molecule of hydrogen
2H
H2
Coalescence of many hydrogen molecules to form a bubble
o For activation polarization
a = +/- log(i/i0)
i0 = exchange current density
and i0 are constants for the particular half cell
o equilibrium in a reaction occurs when rate of oxidation = rate of reduction
rred=roxid= i0/(nF)
o Concentration Polarization = exists when the reaction rate is limited by diffusion
in the solution
c = 2.3RT/(nF)*log(1- i/iL)
limiting diffusion current density
Passivity
passivity = loss of chemical reactivity, under particular environmental conditions, by
some active metals and alloys, often due to the formation of a protective film
o displayed by Cr, Fe, Ni, Ti, and many of their alloys
increase in passivity results in an increase the resistant in corrosion
intersection of reduction polarization curves and oxidation polarization curves gives the
value of the corrosion current density in the active and passive regions
Environmental Effects
variables in corrosion environment include fluid velocity, temperature, and composition
increase in fluid velocity increases corrosion rate due to erosive effects
increase in temperature increases the corrosion rate
usually increasing the concentration of the corrosion material increases the rate of
corrosion
o however for the materials capable of passivation raising the corrosive content
could reduce the rate of corrosion because of active to passive transition
cold worked material is more susceptible to corrosion than annealing materials
Forms of Corrosion
eight forms of metallic corrosion
o uniform attack = form of electrochemical corrosion that occurs with equivalent
intensity over the entire exposure surface and often leaves behind a scale or
deposit
oxidation and reduction reactions occur randomly over the surface
rusting and tarnishing
most common form
o galvanic = occurs when two alloys or metals have different compositions are
electrically coupled while exposed to an electrolyte
the less noble or more reactive metal in particular environment will
experience corrosion; the more inert the more protected to corrosion
most common solution involved in corrosion is seawater
rate of galvanic attack depends on anode cathode ratio; smaller the anode
the greater the rate because it depends on density and area of current per
unit area
ways to reduce galvanic attack
if coupling of two dissimilar metals is necessary, choose two that
are close together in the galvanic series
avoid an unfavorable anode-to-cathode are ratio; use an anode area
as large as possible
o
o
o
o
o
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electrically insulate dissimilar metals from each other
electrically connect a third, anodic metal to the other two; this is a
form of cathodic protection
crevice = electrochemical corrosion may also occur as a consequence of
concentration differences of ions or dissolved gases in the electrolyte solution and
between two regions of the same metal piece; for such a concentration cell
corrosion occurs in the locale that has the lower concentration
prevented by using welded instead of bolted or riveted joints, using
nonabsorbing gaskets, removing accumulating deposits, and designing
containment vessels to avoid stagnant areas and ensure complete drainage
pitting = another form of very localized corrosion attack in which small pits or
holes form
attacks perpendicular to horizontal surface and goes undetected b/c of very
little material loss occurs until failure
oxidation occurs in pit reduction occurs at surface
intergranular = occurs along preferentially grain boundaries for some alloys and
in specific environments
macroscopic specimen disintegrates along grain boundaries
heating at temp between 500 and 800 degrees Celsius for long time period
susceptible to attack
weld decay; integranular is a big problem when welding stainless steels
protection occurs by: subjecting sensitized materials to high temp heat
treatment in which all chromium carbide particles dissolve; lowering the
carbon content below .03 wt% C so that carbide formation is minimal;
alloying the stainless steel with another metal such as niobium or titanium
which has a greater tendency to form carbides than chromium does so that
Cr remains in sold soln
selective leaching = found in solid solution alloys and occurs when one element
or constituent is preferentially removed as a consequence of corrosion processes
erosion-corrosion = arises from the combined action of chemical attack and
mechanical abrasion or wear as a consequence of fluid motion
harmful to passivate alloys that form a protective surface film b/c film
erodes and bare surface left behind
identified by surface grooves and contour waves
reduce it by changing design to eliminate fluid turbulence and
impingement effects; removing bubbles an particulates can help
stress corrosion results from the combined action of an applied tensile stress and a
corrosive environment both influences are needed
metals are ductile but failure behavior looks like failure of a brittle
material
causes: rapid temperature changes and uneven contraction
for two phase alloys each phase having different coefficients of
expansions can cause stress
for gaseous and solid corrosion products that are entrapped
internally can increase stresses
prevention: lower magnitude of stresses by reducing external load or
increasing cross-sectional area perpendicular to applied stress
Cathodic protection decreases stress corrosion
o Hydrogen Embrittlement = significant reduction in ductility and tensile strength
when atomic hydrogen penetrates into the material
Response to applied or residual tensile strength
Similar to stress corrosion in that brittle characterized failure occurs in
ductile materials when exposed to tensile stress and corrosive atmosphere
Cathodic protection increases hydrogen embrittlement
Increase in strength increases susceptibility to embrittlement
FCC alloys very resistant to embrittlement
Reduce likelihood of occurrence by reducing tensile strength via heat
treatment, removal of source of hydrogen, baking alloy to eliminate any
dissolved hydrogen, substitution of a more embrittlement-resistant alloy
Corrosion Environments
Environments include atmosphere, aqueous solution, soils, acids, bases, inorganic
solvents, molten salt, liquid metals, and the human body
Atmospheric corrosion accounts for the greatest losses
o NaCl makes marine atmosphere highly atmosphere
Corrosion Prevention
Most common prevention method is to identify the corrosion environment and base the
material off of that
Changing the environment could influence corrosion
o Decreasing the fluid temperature and/or velocity can decrease the corrosion rate
Changing the concentration of the material can have a positive effect; like passivation in
a metal
Inhibitors = substances that, when added in relatively low concentrations to the
environment, decrease its corrosiveness
o Depends on alloy and corrosive environment
o Used in closed systems
Prevention include exclusion of air because dissolved oxygen may enhance corrosivity
Physical barriers to corrosion are applied on the surfaces in form of films and coatings
Cathodic Protection
o Can be used for all 8 types of corrosion
o Supplies electrons from an external source to the metal to be protected and
making it a cathode
o One technique of cathodic protection is when a sacrificial anode (another metal
that is more reactive in the environment) goes through oxidation and is connected
to the metal electrically
Usually the anode is magnesium and zinc
o Galvanizing = a layer of the zinc is applied to the surface of steel by hot dipping
Process is slow because of the large anode to cathode ratio of surface area
o Another method of cathodic protection involves the source of electrons being an
impressed current from an external dc power source
Negative terminal attached to metal that needs to be protected ad other to
an inert anode
Oxidation
M
Mn+ + ne Scaling = tarnishing = dry corrosion = oxide layer or scale forms on the surface of the
metal
aM + (b/2)O
MaOb
Scale Types
o Rate of oxidation and the tendency of the film to protect the metal from further
oxidation are related to the relative volumes of the oxide and metal
o Pilling-Bedworth ratio; P-B ratio = AOM/(aAMO)
a=1 if divalent otherwise not divalent; a=coefficient of metal species for
the overall oxidation reaction
o if the ratio is greater than 2-3 then the oxide coating may crack and flake off;
protected if between 1 to 2
Kinetics
o Primary concern relative to metal oxidation is the rate at which the reaction
progresses
o Rate of reaction determined by measuring the weight gain per unit area asa
function of time
Weight gain, W2= K1t+K2 where K are the time independent constants at
a given temp
For porous or flaking off metals (P-B<1 or >2) oxidation rate is linear
W=K3t
For thin oxide layers
W=K4log(K5t+K6)
Corrosion of Ceramic Materials
Immune to corrosion unless involved with chemical dissolution
Degradation of Polymers
For polymers the environment can cause deterioration to it but it is called degradation
instead of corrosion
Physiochemical instead of electrochemical
Deteriorate by swelling, dissolution, covalent bond rupture
Swelling and Dissolution
Swelling = liquid or solute diffuses into and is absorbed within the polymer; the small
solute molecules fit into and occupy positions among the polymer molecules
o Macromolecules are forced apart in such a way that the specimen expands or
swells
o Increase in separation leads to weaker secondary bonds leading to softer and more
ductile material
Dissolution = occurs when the polymer is completely soluble; continuation of swelling
Bond Rupture
Polymers may also experience degradation by a process termed scission
o The severance or rupture of molecular chain bonds
o Causes a point of separation at scission point and a reduction MW
Radiation Effects
o Certain types of radiation possess sufficient energy to penetrate a polymer
specimen and interact with the constituent atoms or their electrons
X-rays, electron beams, beta and gamma rays, UV
o Ionization = the radiation removes on orbital electron from a specific atom,
converting that atom into a positively charged ions
o One of the covalent bonds associated with the specific atom is broken, and there
is a rearrangement of atoms or groups of atoms at that point
o Stabilizers may be added to protect from radiation
o Irradiation can cause cross linking
Chemical Reaction Effects
o Oxygen, ozone and other substances can cause or accelerate chain scission as a
result of chem reaction
o Most common in vulcanized rubbers
o Chain severed at double bond
Thermal Effects
o Thermal degradation corresponds to the scission of molecular chains at elevated
temperatures
o Higher the bond energy higher the thermal stability
Weathering
Degradation because of exposure to outdoor conditions