Download - Scimatp Lecture 14 Corrosion
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CORROSION reduction in performance of component during
lifetime
the deterioration of materials by chemical
interaction with their environment.Also refers to degradation of plastics, concrete
and wood, butgenerally refers to metals.
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CORROSION
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With focus on upgrading the quality and
safety of national roads, DPWH has been
provided with budget of P29.7-billion for
the rehabilitation and maintenance of
national roads and bridges. The amount is11.8 percent higher than this years P26.5
billion.
Message
of
His Excellency Benigno S. Aquino III
President of the Philippines
to
The Fifteenth Congress of the Philippines
On The National Budget for 2012
[July 26, 2011]
CORROSION
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Why do metals corrode? They are more stable in an ionic state than as
metals
Essentially, all metals occur in nature as
compounds oxides, carbides, carbonates,
silicates, sulfides, and sulfates
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Chemical Corrosion Direct dissolution
A material dissolves in a corrosive liquid
Liquid metal attack
Selective leaching
Selective dissolution of one particular element
in an alloy
Dezincification
Graphitic Corrosion
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Dezincification
The zinc component of the metal is leached
out to leave copper. Deep pitting is initiated
and the pipe becomes porous prior to
producing an actual leak
Graphitic Corrosion
Occurs when iron (in gray cast iron) is
selectively dissolved in water or soil leaving
behind interconnected graphitic flakes and a
corrosion product
Chemical Corrosion
http://t2.gstatic.com/images?q=tbn:YFRGRV0xQhQ4
NM:http://www.corrview.com/images/corr_07d.jpg
http://www.andersonmaterials.com/images/metal3.jpg
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Electrochemical Corrosion Occurs when metal atoms lose electrons and become
ions
Most common form of attack of metals
Occurs most frequently in an aqueous medium
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OXIDATION (Anodic Reaction)
Basic principles:
Metals in aqueous solutions tend to form ions
Electrochemical Corrosion
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H+
H+H2
REDUCTION (Cathodic Reaction)
Electrochemical Corrosion
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Electrochemical Corrosion
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What happens to the metal ions?
They can simply remain in solution They can react with the electrolyte to form a
solid product This may take the form of a film, which may
protect theunderlying metalPASSIVATION
Electrochemical Corrosion
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Galvanic Corrosion Occurs when dissimilar metals are in contact
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GALVANIC CELL
Cu
1.0 M CuSO4
Zn
1.0 M ZnSO4
cathode half-cell
Cu+2 + 2e- Cu
anode half-cell
Zn Zn+2 + 2e-
-+ 1.1 volts
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Au
Cu
Pb
Sn
NiCo
Cd
Fe
Cr
ZnAl
Mg
Na
K
+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
Vmetal
o
Data based on Table
17.1, Callister 6e.
STANDARD EMF SERIES
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Ranks the reactivity of metals/alloys in seawater
PlatinumGold
Graphite
Titanium
Silver
316 Stainless Steel
Nickel (passive)Copper
Nickel (active)
Tin
Lead
316 Stainless Steel
Iron/SteelAluminum Alloys
Cadmium
Zinc
Magnesium
Based on Table 17.2, Callister
6e. (Source of Table 17.2 isM.G. Fontana, Corrosion
Engineering, 3rd ed.,
McGraw-Hill Book Company,
1986.)
GALVANIC SERIES
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Corrosion typesUniform corrosion Oxidation and reduction
reactions occur
randomly across whole
surface
Localised corrosion Oxidation reaction
concentrated at one area
of surface
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Crevice Corrosion Occurs in water when oxygen content varies across
component
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Crevice Corrosion
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Pitting Corrosion Similar to crevice corrosion
Arises from local breakdown in protective oxidefilm
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Protecting Against Corrosion Isolate metal surface from electrolyte
Passivation
Paint
Inhibit anodic reaction Cathodic protection or Sacrificial protection
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Protecting Against Corrosion Inhibit cathodic reaction
Minimize dissolved oxygen
Reduce conductivity of electrolyte
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Polymer Degradation
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www.nace.org/nace/content/publicaffairs/images_cocorr/ccsupp.pdf
http://www.cmse.ed.ac.uk/MSE3/Topics/MSE2-
06/Lecture%20corrosion.pdf
Askeland, Donald (2010) Materials Science and
Engineering
http://academic.pgcc.edu/~ssinex/E_cells.ppt http://en.wikipedia.org/wiki/Silver_Bridge
http://www.cmse.ed.ac.uk/MSE3/Topics/MSE2-06/Lecture%20corrosion.pdfhttp://www.cmse.ed.ac.uk/MSE3/Topics/MSE2-06/Lecture%20corrosion.pdfhttp://academic.pgcc.edu/~ssinex/E_cells.ppthttp://academic.pgcc.edu/~ssinex/E_cells.ppthttp://www.cmse.ed.ac.uk/MSE3/Topics/MSE2-06/Lecture%20corrosion.pdfhttp://www.cmse.ed.ac.uk/MSE3/Topics/MSE2-06/Lecture%20corrosion.pdfhttp://www.cmse.ed.ac.uk/MSE3/Topics/MSE2-06/Lecture%20corrosion.pdf