Study Guide 1 Flashcards
What are the different types of corrosion? (8)
Galvanic Corrosion Crevice Corrosion Pitting Corrosion Intergranular Corrosion Stress \& Galvanic Corrosion Stress Corrosion Fatigue Corrosion Fretting Corrosion
Galvanic Corrosion
Corrosion that mimics what occurs in a half cell.
2 metals: Zn & Cu are electrically coupled where electrons can transfer btw metals - like a wire.
Physiological fluid acts as a “salt bridge”.
*Note: If a stainless steal is coupled with another metal, it will undergo “anodic dissolution”.
Crevice Corrosion
Occurs in areas with a narrow, deep crack.
Usually btw screw & plate of bone fixation device.
Pitting Corrosion
Small flaw/disruption in passivation film.
Usually difficult to detect & leads to device failure.
Intergranular Corrosion
Grain boundaries are energetically active.
more active = anodic regions
(e.g. Occurs in stainless steel at the grain boundaries due to depletion of Cr; where Cr is required for passivating layer)
Stress & Galvanic Corrosion
Bending of a metal;
tensile side = anodic
compressive side = cathodic
Stress Corrosion Cracking
Occurs to a metal under tension in corrosive environment.
Cracks are often perpendicular to applied stress.
Fatigue Corrosion
Perturbing the passivating film around the implant via cyclic loading.
Fretting Corrosion
Motion disrupts the passivating layer.
Why does biological protein adsorption disrupt the formation of a passivating layer on a metal implant?
Biological proteins adsorb to the surface to:
a. Create a barrier that reduces O2 diffusion to the surface - which in turn disrupts the formation of the oxide passivation layer
b. Proteins can accept electrons from metals and the rxn is shifted towards the metal being dissolved.
* Note: Inflammatory cells can use a drop in pH to release strong oxidizing agents = increasing the passive layer.
Corrosion Control
Reduce the number of stress raisers in design.
Reduce galvanic corrosion by choosing metals which are close to each other in the galvanic series.
Choose cahtodic metals: Au, Ag, Pt; which are usually ductile & expensive
Choose metals that form passive layers. (e.g. alone/alloys)
Reduce intergranular corrosion by the heat treatment of stainless steel (why?)
Create passive layer prior implantation via “pre-treatment w/ nitric oxide”
Ceramic Degradation
metal = corrosion ceramic/polymer = degradation
Ceramics (FeO2) = passive layer around metals
Ceramics are usually more stable than metals, but require more energy to separate ionic bonds.
Ceramics = inert, resorbable, have controlled surface reactivity.
Ceramics undergo stress-induced degradation = brittle
Ceramic porosity = degradation
(e.g. pores create more SA for degradation & are also stress raisers = lead to more cracks)
*Need to a balance btw porosity and degradation.
Polymer Degradation
Degradation usually leads to discoloration, appearance of cracking (e.g. crazing), and change in mechanical properties.
This can occur via water, proteins, inflammatory cells, and mechanical stress.
2 Mechanisms: Swelling/Dissolution and Polymer Scission
Mechanisms of Polymer Degradation (2)
Swelling/Dissolution:
If polymer has hydrophilic domains, H2O disrupts 2ndary bonding btw polymer chains.
affects polymer crystallinity = affects mechanical & thermal properties
Polymer may dissolve completely due to solubility & number of covalent bonds btw polymer chains.
Polymer Scission:
Breaking of the primary bonds of polymer.
Decreases Mw = affect mechanical properties and Tg
Occurs via Hydrolysis & RedOx Rxns
Chain Scission via Hydrolysis
Typical degradation process in condensation polymers.
Depends on:
Reactivity of groups in the polymer backbone.
Interchain bonding.
Amount of media available to the polymer.
Number of cleavable groups and hydrophilic domains
Low/No crystallinity
Low Mw & low crosslink density polymers = hydrolysis
High SA to V ratio
Chain Scission via Oxidation
Free radicals (highly reactive species) break down covalent bonds via 3 processes:
- Initation: Loss of H+ from chain = highly reactive R-group
- Propogation: R-group interacts with O2 to form radical ROO- = accepts H+ from another part of chain
- Termination: Combining free radicals to create inert products
* Note: Oxidation usually caused by agents released by inflammatory cells. If a metal is complexed w/ a polymer, the polymer can break down & corrosion of the metal can lead to the oxidation of a polymer – Metal-catalyzed Oxidation.
Other forms of Polymer Degradation (2)
Environmental Stress Cracking: Subjected to tensile stress, and crack develops perpendicular to loading axis. (e.g. similar to stress corrosion cracking in metals; usually inflammatory cells are required for this type of cracking to occur.)
Enzyme-Catalyzed Degradation:
Lowers the energy required for hydrolysis/oxidation, enzymes break down natural/synthetic polymers. Usually occurs quickly in porous polymers.
Bulk Erosion & Surface Erosion
Bulk Erosion: change of shape/size of polymer w/in its core
Surface Erosion: change of shape/size of polymer upon its surface
*Note: Degradation = breaking of chemical bonds.
What’s the difference btw natural and synthetic polymers?
Synthetic: Degrade by hydrolysis/enzymes
Natural: Degrade by enzymes (depending on the material)
*Note: Hydrolysis Breakdown - Bulk/Surface Degradation.
Hydrolysis Breakdown (2)
Bulk Degradation: Rate of H2O into a polymer > the rate the polymer is converted to its water-soluble degradation products.
Surface Degradation:
The rate of H2O penetrates into the material
Biodegradable Ceramics
Typically degrade by erosison, but is influenced by:
Chemical susceptibility of material.
Amount of crystallinity.
Amount of H2O available.
SA to V ratio.
What are some tests use for measuring degradation?
In vivo testing
In vitro testing
What is a bioactive factor?
A factor that stimulates a cell.
Facilitated Degradation
Body provides neurtal pH & constant temp.
Ions in the blood initiate corrosion. (e.g. K+, Ca2+, Na+)
Inflammatory cells can attach to the biomaterial surface and release oxidizing agents (e.g. peroxides) and lower the pH in that area.
RedOx Rxns
LEO says GER
anode = oxidation = dissolved = increasingly active
(e.g. Zn)
cathode = reduction = expanded = increasingly inert
(e.g. Cu)
What is an electrochemical galvanic cell?
A battery.
Components:
Electrodes - Metal strips of Zn & Cu
Salt Bridge - KCl which provides ions to maintain neutral charge
*Note: Each metal is compared to a H+ electrode to rank oxidation behavior = STD Reduction Potentials. (e.g. metals with more (-) STD RP will always act as anode.)
Galvanic Series
Data of corrosive properties of metals in sea water.
Metals at the bottom of the list are more easily oxidized (creating electrons) than those at the top = anodic (e.g. Mn)
Nernst Equation
Depends on the temperature and concentration of ions.
E1: M1 –> M1+ + ne
E2: M2+ ne –> M2
where electrochemical potential for the entire cell is:
deltaE = (E2 - E1)
DE0 = (E20 – E10) – RT/nF ln ([M1n+]/[M2n+])
R - gas const in mol
T - temp in K
F - Faraday’s const in Coulombs
n - number of valence e transferred
