2012 etetcorrosion of metals final

7/29/2019 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

2012 etetcorrosion of metals final

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