Fretting Corrosion / erosion corrosion
The combination of corrosive fluid (saliva with several
enzymes and food particles) and high velocity in the oral
environment results in erosion-corrosion or fretting. It is
responsible for most of the metal release in tissue. Conjoint
action of chemical (enzymes and proteins) and mechanical
wear (mastication) during function further aggravates the
attack [36-37,55-56].In general during the passive
environments, the hard and tenacious TiO2 surface film
over the metal surface provides a superb barrier to
erosion-corrosion. For this reason titanium alloys can
withstand flowing water velocity as high as 30 m/s with
little or no metal loss. The ability of the oxide film to repair
itself when damaged and the intrinsic hardness of titanium
alloys both contribute to their excellent resistance to
erosion-corrosion.
Titanium alloys exhibit relatively high resistance to fluids
containing suspended solids. Critical velocities for
excessive metal removal depend upon the concentration,
shape, size, hardness of the suspended particles, fluid
impingement angle,[57] local turbulence, and titanium
alloy properties.
The typically low concentrations of organic material in oral
cavity is of little importance but continuous exposures to
local changes around the implant during function can lead
to finite removal of the metal as well as the cementing
material between the implant and superstructure there by
not only promoting erosion corrosion but crevice and
galvanic corrosion as well.
Intergranular corrosion/cracking
For intergranular cracking to occur reactive impurities may
segregate, or passivating elements may deplete at grain
boundaries. This results in grain boundaries as a
preferential region for corrosion owing to its high
susceptibility and so the grains might fall out of the surface
leading to material cracking [36-37]. This type of corrosion
is not usually observed in titanium oral implants.