Scimatp Lecture 14 Corrosion

7/27/2019 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)



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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



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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

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Scimatp Lecture 14 Corrosion