lecture 12 - degradation of metals and ceramics

what is the biological environment

aqueous environment containing various anions, cations, organic substances, and dissolved O2

what are the main anions in the biological environment

chloride, phosphate, bicarbonate

what are the main cations in the biological environment

sodium, calcium, potassium, magnesium

the biological environment is _____ to metals and some ceramics

corrosive

what does corrosion release

oxides

ceramics with what kinds of bonds are resistant to breakdown in the biological environment

strong ionic / partially covalent

material selection can/cannot be governed only by corrosion

cannot

corrosion happens only at the ____

surface

why are noble metals (gold, silver, platinum) rarely used for structural applications

because they have weak mechanical stability (malleable)

mechanical ___ must be taken into consideration in corrosion

stress

what is passivation

process of coating a metal to protect from further corrosion (creates an oxide on the surface)

what are some active metals used for passivation

• titanium

• chromium

• aluminum

corrosion is/is not a zero-order homogeneous process

is not (could be passivated or activated at any time)

is the rate of corrosion the same at all time?

no

what is the two-fold effect of corrosion

• loss of structural integrity (mechanically weaker)

• degradation products affect tissue

what is aqueous corrosion

electrochemical reactions taking place on a metallic surface in an aqueous electrolyte

what happens in an anodic reaction

electrons are produced (yields metallic ions)

what happens in a cathodic reaction

electrons are consumed

what is reduced in cathodic reactions

• hydrogen (2H+ 2e- → H2)

• oxygen in acidic solutions (O2 + 4H+ + 4e- → 2H2O)

• oxygen in basic and neutral solutions (O2 + 2H2O + 4e- → 4OH-)

both cathodic and anodic reactions are in _____

equilibrium

the _____ of cathodic and anodic reactions will stop corrosion

passivation

how does the standard electrode potential relate to metal reactivity

the more potential a metal has, the less reactive it will be

what is the driving force for corrosion

when equilibrium is displaced between the cathodic and anodic reaction

what factors can disrupt metallic equilibrium

• movement between implants and tissues

• ions forming complexes with protein

the ___ active metal will corrode

more

the ___ active metal will be protected

less

more active metals are ____

anodic

less active metals are ___

cathodic

what is an example of an anode

zinc

what is an example of a cathode

copper

if two electrodes are placed together, which direction will electrons flow in

towards the electrode with greater potential to make them equal (more reactive to less reactive)

what 2 things is corrosion resistance influenced by

• surface microstructural features

• environment

biological molecules _____ with the anodic/cathodic reactions in the environment

interfere

what 3 things does the stability of passivating the metal oxide layer depend on

• electrode potential (interacting proteins and cells)

• pH (inflammatory cells)

• oxygen availability (protein adsorption to surface could limit oxygen diffusion to certain regions)