Galvanic corrosion of dental implants
The most common form of corrosion occurring in titanium
implants is the galvanic corrosion or dissimilar corrosion.
Titanium has been chosen as the material of choice for
several trans-osseous and end-osseous implantations.
Long term studies and clinical observations have
established the fact that titanium is noble (due to the
presence of adherent oxide) and does not corrode in
human tissues however galvanic coupling of implant to
several other metallic restorations may induce one of the
several forms of corrosion. Thus coupling remains a great
concern for the metallic superstructures covering the
implant body.
Gold alloys have widely been used as a super structure
owing to their excellent biocompatibility, corrosion
resistance and mechanical properties. Owing to higher
cost of the precious metal alloys (noble alloys)used in
prosthodontics, it has led to the development of cost
effective semi-precious metallic alternatives(non-noble
alloys) [58-59] like gold-palladium, nickel-chromium,
cobalt-chromium, nickel-titanium and several other
titanium alloys.
Galvanic coupling occurs when two dissimilar metals are
placed in direct contact within the oral cavity (adjacent or
contralateral) or within the tissues. The complex corrosion
process occurring involves the electrochemical reactions
occurring at the dissimilar metals interface in presence of
electrolyte (saliva or oral fluids or body fluids) resulting in
the flow of electric current between them [37,60]. An invivo
(in the living tissue) electrochemical cell is formed
and galvanic current causes the corrosion of active metal
and the noble metal is protected. The current also passes
through the cellular junctions and tissues (desmosomes,
hemi-desmosomes and cellular attachments) thereby
activating the proprioceptors causing pain.
As already discussed in the section on hydrogen damage
above attention should be given to possible excessive
hydrogen uptake by titanium implant when it is galvanically
coupled to active metal superstructures. This situation is
of great concern in á titanium alloys when temperatures
exceed 80°C in aqueous electrolytes during implant
processing especially when hydrogen recombination
poisons, such as sulfides, arsenides and cyanides are
present.
Phenomena of galvanic corrosion
When two dissimilar metals with different electrode
potentials come in contact in the presence of a corrosive
electrolyte, a potential is generated. The net result is a
chemical reaction with oxidation occurring at the anode
and reduction occurring at the cathode, the electronic
exchange occurs through the contact and ionic exchange
occurs through the electrolyte.
The electrochemical cell reactions occurring at the
different electrodes, depending upon the pH and aeration
conditions as well the addition of oxidizers are [37]: