Corrosion of Metallic Biomaterials Part 2 Flashcards
Forms of corrosion
Uniform Galvanic (Bimetallic) Intergranular Crevice Pitting Erosion and Fretting Stress
Uniform Corrosion
Most common form
Corrosion and Passivation regions of pourbaix diagram
Occurs in most metals where there’s no equilibrium in constituent ions concentrations
Electron current flow from anodic to cathodic region
Galvanic (Bimetallic Corrosion)
Happens when two metals are in physical contact, and immersed within ionic conducting fluid
Conditions to be met:
- Differences in electrochemical potential of metals
- Contact between them
- Presence of electrolyte
Occurrence of galvanic corrosion
Tendency to naturally occur because screws and plates made by different process
Mixing materials, other metallic foreign bodies, different heat treatments, and improper heat treatments can all cause it
Intergranular corrosion
Most common in cast metal alloys - more grains with impurities at boundaries
Different chemistry from grain boundary and grains, leading to different electrochemical potential
Cathodic regions - inclusions at grain boundary - these corrode grain surface
How to reduce integranular corrosion
Proper heat treatment
Crevice Corrosion
Requires narrow deep crack
Oxygen depletion of crevice, anodic metal corrosion along crevice faces, and cathodic protective conditions around crevices mouth
Static solution conditions favour it
matrix acts as cathode crack as anode
Examples of crevice corrosion
Easily seen in contact areas of multipart devices
Stress concentrations can develop due to reduction in cross section of plate at hole
May change mechanical properties if subjected to cyclic loading
Pitting corrosion
Special case of crevice corrosion where autocatalysis plays most important role
Scratches, handling damage, and inclusions may initiate it by removing oxide layer
Appear as freckles
May initiate crack propagation and result in large localized damage by acting as anode, while remaining surface acts as cathode
Found in older SS and Co-Cr joint components but rare in Ti alloys
Erosion and fretting corrosion
Like pitting but pits are elongated in direction of flux
May physically erode passivated layer in extreme cases
Fretting may occur if implant host interface is loose of fixation is poor.
Fretting difficult to distinguish from wear
Stress Corrosion
Tensile stress increases chemical activity of metals
One side of metal in tension, other in compression
Convex side becomes aniodic, with respect to concave side
Corrosion attacks convex surface
Four ways proteins in cells can influence rate of corrosion by interfering with anodic and cathodic reactions
- Biological molecule consumes product of aniodic of cathodic ractions
- Proteins and cells interact with charges formed at interface and affect electrode potential
- Protein adsorption affects diffusion of oxygen to certain regions of surface, leading to breakdown of passive layer
- Cathodic reaction forms hydrogen, which builds up and inhibits cathodic reaction
Magnesium and its alloys
Density 4.5 times lower than SS
May be applied as temporary orthopaedic implants since they corrode
Low corrosion resistance in electrolytic aqueous environment
Usually have enough mech integrity for 12-18 weeks in vivo, enough for bone healing
Corrosion of Magnesium alloys
Corrodes too rapidly and Cl in environment rapidly makes it lose mech integrity
Its corrosion also releases hydrogen gas at a rate too high
Alloying and protective coatings needed if we want to use Mg alloys in biomaterials
