2012 etetCorrosion of Metals Final
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Transcript of 2012 etetCorrosion of Metals Final
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Corrosion of Metals
http://www.keytechinc.com/blog/wp-content/uploads/2010/11/Rusty-Bolts.jpghttp://www.keytechinc.com/blog/wp-content/uploads/2010/11/Rusty-Boat.jpg -
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Lecture Outline
What is corrosion? Types of corrosion Mechanism of corrosion Types of electrochemical
corrosion Effect of corrosion Prevention of corrosion
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What is corrosion? Corrosion is the destruction of a metal or an
alloy by chemical or electrochemical reaction
with its surrounding environment or medium.
Bodyenvironment tissue fluids contains water,
salt, dissolved oxygen, bacteria, proteins and
various ions such as chloride and hydroxide.
This is aggressive environment for metal
implants
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Types of corrosion
1.Chemical corrosion (dry corrosion):Occurs in the absence of fluids or water
2.Electrochemical corrosion:It is a combination of chemical reactionand a flow of electric current
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Mechanism of Electrochemical corrosion
1. Two dissimilar metals with different
electrode potentials
2. Electrolyte
3. Chemical reaction
http://en.wikipedia.org/wiki/Electrode_potentialhttp://en.wikipedia.org/wiki/Electrode_potential -
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At the anode: Oxidation reactiontakes place (giving up metal ions)
At the cathode: Reduction reactiontakes place (gaining metal ions)
i.e. Anode dissolves (corrodes) while the cathode remains intact.
At the anode:metal oxidized and the electrons areliberated from the metal to form +ve metal ions.
The liberated electrons dissolve into the electrolyte,
and deposition is formed on the cathodic metal
Mechanism of corrosion
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Types of electrochemical corrosion
1. Galvanic corrosion
When two dissimilar metals are connected electrically
through an electrolyte one metal will become anodic with
reference to the other and the potential difference will create
an electric current causing the more anodic metal to corrode
Occur at the surface of one metal
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2. Stress corrosion
When an implant is subjected to stress,the corrosion process could be accelerated due to
the mechanical energy. Under tensile stresses
corrosion can form minute crack which increases
in size as a result of stress concentration resultingin crack growth and failure of implant.
Stress-Corrosion Cracking
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3.Fatigue corrosion
If low mechanical stress(below the proportional limit of
the implant material) is repeated for long period of time
then fatigue stress corrosion takes place as in the femoral
stem of the hip joint and hip nails made of stainless steel.
4. Pitting corrosion
Corrosion in which pits form on the metal surface.The passivity is locally broken by the chloride.
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What produce Corrosion?
1. Irregularities of metal surface.
2. Difference in compositionof the metal surface
(alloying; combination 2 or more metals).
3. Stress induced during manufacturing of
implants.
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Protection from corrosion:Passivation:
Passivation is the formation of stable surface oxidelayer that prevents further oxidation of the metal.
Passivation corresponds to the transformation of
an active surface which is corroding to an inactive
surface, by formation of a passivation layer.
Chromium?????
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Effect of corrosion:
Corrosion products (ions) released in the body may lead to:
1. local pain and swelling in the region of the
implant, with no evidence of infection.
2. Metal ions may migrate to other organs of
the body causing inflammation.
3. cracking or flaking of the implant may lead
to mechanical failure of implant. Most of
these failures are due to fatigue.
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TO prevent corrosion:
Use appropriate metals
Avoid implantation of dissimilar metals
Use highly polished metal implant
A metal that does not corrode in one part
of the body may corrode somewhere else
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Home Work:
1. Describe the mechanism of electrochemical
corrosion.
2. List the effect of corrosion on the body and
on the implant
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Biomaterials Science. An introduction to materials
in medicine. Ratner B,D, Hoffman A, Schoen
F, Lemons J, E . First Edition,.1996
Biomaterials :An introduction Park j, Lakes R.S.
Third Edition, 2007
References