Corrosion in Metallic Biomaterials

Question 1

Is biological corrosive environment constant?

Answer 1

No, oxygen levels and cell activity vary

Question 2

Where is a corrosion attack generally more common?

Answer 2

In multipart devices

Question 3

The overall effects of corrosion in biomaterials

Answer 3

Loss of material integrity and function

Corrosion products released and affect tissue

Question 4

Variables affecting corrosion rates

Answer 4

Composition of implant
Manufacturing variables
Handling 
Positioning of implant
Biological variables

Question 5

How does the composition of implant impact corrosion?

Answer 5

Variations within implant as well as implant to implant
surface microstructure
passivating oxide layer formation

Question 6

How do manufacturing variables affect corrosion?

Answer 6

Casting conditions
Metal purity
Amount of cold work
Heat treatments
Geometric variables

Question 7

How does handling affect corrosion?

Answer 7

During manufacture
sterilization
delivery
insertion

Question 8

Should you use a biomaterial just because of its corrosion resistance?

Answer 8

No, it can jeapardize mechanical and physical properties

Better for surface to be manipulated

Question 9

Chemistry of corrosion

Answer 9

Electrochemical reactions take place on metallic surface in aqueous electrolyte
Two reactions occur: anodic and cathodic

Question 10

Anodic reaction

Answer 10

M -> M+ + e-

ionization

Question 11

Cathodic reaction

Answer 11

Mn+ + n e- -> M (reduction)
More frequently:
Reduction of hydrogen to H2 gas
Reduction of dissolved oxygen

Question 12

Generic reactions involving materials

Answer 12

Ionization: formation of cation from base metal
Oxidation: Burning of a metal with gaseous O2
Hydroxylation: Metal with water under alkaline conditions to form passivating hydroxide
Solution reaction: Combination of metallic ions with other cations or anions

Question 13

Three regions of pourbaix diagram for chromium in water

Answer 13

Sum of equilibrium concentrations of all ions containing a particular metal element:
Corrosion: > 10-6M
Passivation: < 10-6M and passivation layer
Immunity: < 10-6 M and absence of passivation layer

Question 14

What are the dashed lines in a Pourbaix diagram?

Answer 14

Boundaries where water is stable (in between A and B)

B (top dashed line): above this, gaseous oxygen is released
A (bottom dashed line): below this, gaseous hydrogen is released

Question 15

Describe a “practical” Pourbaix diagram?

Answer 15

More closely simulated physiological conditions
The circles/ovals super imposed estimate various fluids
There is a reduction in the passivation area due to Cl- forming complexes with ions in solution

Question 16

How does the positioning of the implant impact the corrosion rates?

Answer 16

Local environment, mechanical stresses, relative motion between the various components with the host, small anatomical differences causing a large difference in pH and potential, lifetime of the implant, presence of oxygen

Question 17

How do do biological variables affect corrosion rates?

Answer 17

Presence of solution proteins or enzymes