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8/8/2019 c7-Corrosion & Degradation of Materials
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CORROSION & DEGRADATIONCORROSION & DEGRADATION
OF METALSOF METALS
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CORROSIONCORROSION
Figure 1 is a view of the failure origin showing severe areas ofsurface corrosion. The corrosive environment is the typical
salt/water mixture resulting from the deicing of roads during
the winter.
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DEFINITION OF CORROSION
Slow and progressive destruction of a
metal brought about by the action of
an external agent and various causes
of a mechanical nature.
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Corrosive EnvironmentspH
(Remember! pH 0 to 7: acidic, pH 7 to 14: alkaline)
Lower pH
Higher the amount of free acid present
more corrosive the solution
Aggressive species
Aggressive species are corrosive materials that can penetrate
and brake down protective surface films or modify the shape of
the cathodic polarization curve.
Ex: chloride ion, sulphur ion
Oxidizing species
Oxidizing species exhibit mild to severe effects on corrosiveness.Oxidizing species depolarize corrosion reactions.
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To forms of metal deterioration can
be stated:
DIRECT OXIDATION:
Where the metals atoms combine with theaggressive substance:
2Fe +O2 ------2FeO.
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ELECTROCHEMICAL CORROSIONELECTROCHEMICAL CORROSION
For metals, corrosion process is an electrochemicalprocess where metal will loose electron(s)
M Mn+ + ne-
Fe Fe2+ + 2e- or Al Al3+ + 3e-
The side which oxidation occur
Electron(s) generated from the metal (duringoxidation) must be transferred to form a chemicalspecies.
Process of gaining electron(s)
2H+
+ 2e-
H2 Reduction occurs at
oxidationoxidation
anodeanode
reductionreduction
cathodecathode
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Oxidation:Zn Zn2+ + 2e-
Reduction2H+ + 2e- H2 (gas)
Acid solutionAcid solution
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Standard EMF Series
AuCu
Pb
Sn
Ni
Co
Cd
Fe
Cr
Zn
Al
Mg
Na
K
more cathodic
more anodic
EMF series is gathered bycoupling a standard half cellof a metal to standardhydrogen electrode
EMF series give corrosiontendencies of metals
Those on top >difficult to oxidise
Those on bottom ....>easy to be oxidised
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Uniform AttackOxidation & reduction
occur uniformly over
surface. Selective Leaching
Preferred corrosion ofone element/constituent
(e.g., Zn from brass (Cu-Zn)).
Stress corrosion
Stress & corrosionwork together
at crack tips.
GalvanicDissimilar metals arephysically joined. The
more anodic one
corrodes.
Erosion-corrosionBreak down of passivating
layer by erosion (pipe
elbows).
FORMS OF CORROSION
Formsof
corrosion
Crevice Between twopieces of the same metal.
Fig. 17.15, Callister 7e. (Fig. 17.15 iscourtesy LaQue Center for Corrosion
Technology, Inc.)
Rivet holes
IntergranularCorrosion along
grain boundaries,
often where specialphases exist.
Fig. 17.18, Callister 7e.
attackedzones
g.b.
prec.
Pitting
Downward propagationof small pits & holes.
Fig. 17.17, Callister 7e.(Fig. 17.17 from M.G.Fontana, CorrosionEngineering, 3rd ed.,McGraw-Hill BookCompany, 1986.)
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Erosion-corrosion
Erosion corrosion: acceleration in rate
of corrosion due to relative motionbetween corrosive fluid & surface
Found mostly in piping; especially atbends, elbows or abrupt change in
pipe diameter where fluids becometurbulence or fluid change directionsuddenly
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Pitting corrosion
Localized corrosion attack
Small pits/holes form
Oxidation occurs within the pitsitself
A pit must form from detects likescratches or slight variation incombination
Undetected and suddenunexpected failure
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Crevice corrosion
Corrosion occurs because of different
in concentration electrolyte aroundmetal parts
Occurs under valve, washers,
insulation material, fastener heads,surface deposits, coatings, threads, lap
joints & clamps
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Galvanic corrosion
Two metals or alloys havingdifferent compositions areelectrically coupled while
exposed to an electrolyte
Electrochemical reactionleads to corrosion of metal
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GALVANIC CORROSIONGALVANIC CORROSION
2 dissimilar metal electrodesimmersed in solution of their ownions.
Electrode that has morenegative oxidation potential willbe oxidized.
Zn Zn2+
Cu2+ Cu
oxidized
reduced
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GALVANIC SERIESGALVANIC SERIESranks of the reactivity of metals/alloys in seawater
Galvanic seriesgives the cathodic,
anodic relationshipbetween metalsE.g in seawater, Zinc
is more active thanaluminum
Series is determinedexperimentally forevery corrosiveenvironment
PlatinumGoldGraphiteTitaniumSilver
316 Stainless SteelNickel (passive)CopperNickel (active)Tin
Lead316 Stainless SteelIron / SteelAluminum alloysCadmiumZinc
Magnesium
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Intergranular corrosion
Corrosion occurs along grain
boundaries for alloys in specificconditions
Specimen disintegrates along the
grain boundaries and result in failure
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Corrosion Prevention MaterialsSelections & Design
Choose a right metal once the workingenvironment has been detected. Use less
active metal (galvanic series and EMF) Avoid dissimilar metals that can cause
galvanic corrosion
Provide allowance for corrosion of thickness
Allow complete drainage Make sure parts can be replaced
Weld rather than rivet
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Self-protecting metals!-- Metal ions combine with O
to form a thin, adhering oxide layer that slows corrosion.
C
hange the environment e.g. lowering the fluidtemperature, pressure, concentration and velocity- 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 to
the surface (e.g., paint it!).
Corrosion Prevention Environment
Metal (e.g., Al,stainless steel)
Metal oxide
Cathodic (or sacrificial) protection
-- Attach a more anodic material to the one to be protected.
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Corrosion Prevention Anticorrosion TreatmentsAnticorrosion Treatments
METAL coatings:
via ELECTROLYSIS: Protective films of metal using the coating asthe anode and the part being coated as the cathode,employing sulphates or cyanides as an electrolyte. The substance
is broken down by applying an electrical voltage over acontrolled period at a controlled current, giving us the desired
layer. Steel parts should be covered with 3-4 alternating layers ofCu, Ni, Cr.
NICKEL PLATING.
CHROME PLATING.
COPPER PLATING.
via IMMERSION IN MOLTEN METAL: Immersing the piece in a bathof molten protective metal.
GALVANISING. Pipes/tubes.
TINNING. Tinplate.
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via METALLISING: Using a legalising spray gun to coat the
piece in molten metal.
Zn,P, Al, Stainless steel.
via CASE HARDENING: Methods in which the piece to becoated is covered in powdered Zn and Naphthalene / Cr,
alumina / Aluminium / Si , heated up to set temperature for aspecific time to create a coating on the piece.
SHERARDIZING.
CHROMIZING.
CALORIZING.
SILICATION.
via PLATING: Superimposing plates of metal on one or bothsides of the metal to be protected..
Cu, brass, Ni, Cupro-nickel, Stainless steel.
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NON METALLIC:
inorganic:
Enamel. Via thermo chemical treatment.
PHOSPHATING (immersing the steel pieces in an aqueoussolution of metal phosphate at 100C).
ANODISING (forcing aluminium to oxidise electrolytic allyin order to strengthen the layer of oxide, giving it greater
protection), BLUING (treatment applied to steels toprotect them against oxidation, producing a darkprotective layer of magnetite. Heated up to 200C andthen tempered in oil).
organic:
Paints.
CATHODIC PROTECTION:
Steel pipes using Mg or Zn. Ships propellers which are made of Cu
using
electrodes made of Zn & Mg.
Recommended