Lec15_16_Chemisorption_Corrosion.pdf

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Biomaterials Lecture

15

‐16

Surfaces: Chemisorption &

Corrosion

Unless otherwise noted, source Information for the following slides:

1) B. Ratner, A. Hoffman, F. Schoen, and J. Lemons: Biomaterials Science,2nd edition (San Diego: Elsevier Academic Press. 2004).

2) Butt, H‐J.; Graf, K.; Kappl Physics and Chemistry of Interfaces, 2nd Edition

(Wilet‐VCH: Weinheim 2006).

3) Modified from:Anne Mayes 3.051J/20.340J Materials for Biomedical

Application, MIT,

http://ocw.mit.edu,



Corrosion in Aqueous Environment



Electric Double Layer

• Electrical double layer: The structure of charge

accumulation and charge separation that always

occurs at the interface when an electrode is

immersed into an electrolyte solution.

• The excess charge on the electrode surface is

compensated by

an

accumulation

of

excess

ions

of the opposite charge in the solution.

• The amount of charge is a function of the

electrode potential. This structure behaves

essentially as a capacitor.

• There are

several

theoretical

models

that

describe

the structure of the double layer. The three most commonly used ones are the Helmholtz model, the Gouy‐Chapman model, and the Gouy‐

Chapman‐Stern model.

In other words, the double layer is formed in order to

neutralize the charged surface and, in turn, causes an

electrokinetic potential between the surface and any

point in the mass of the suspending liquid. This

voltage difference is on the order of millivolts and is

referred

to

as

the

surface

potential.



Adsorption phenomena



B. Ratner, A. Hoffman, F. Schoen, and J. Lemons: Biomaterials Science, 2nd edition (San Diego: Elsevier Academic Press. 2004). 433.

Anodic dissolution Cathodic protection

Galvanic corrosion



Less Reactive

More Reactive

• Commonly used surgical alloys contain highly reactive metals (i.e. with

high negative electrode potential) such as titanium, aluminum, chromium

• Because of high reactivity, these metals react with O2 to form an

impervious oxide layer firmly adherent to the metal surface, i.e.

passivation layer!!



• The most

common

type

of

fretting

is

caused

by

vibration.

The

protective

film on the metal surfaces is removed by the rubbing action and exposes

fresh, active metal to the corrosive action of the atmosphere.



B. Ratner, A. Hoffman, F. Schoen, and J. Lemons: Biomaterials Science, 2nd edition (San Diego: Elsevier Academic Press. 2004). 436.















Summary: Basic Principles of Corrosion

• In theory, corrosion resistance can be predicted from standard

electrode

potentials

(i.e.

explains

nobility

of

some

metals

and

considerable reactivity in others, not much use for prediction

in alloys)

• Irrespective of standard electrode potentials, the corrosion

resistance of

many

materials

is

determined

by

their

ability

to

become passivated by an oxide layer that protects the

underlying metal

•

Corrosion

processes

in

practice

are

influences

by

variation

in

surface microstructural features and in the environment that

disrupts the charge transfer equlibrium

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Lec15_16_Chemisorption_Corrosion.pdf