commercial archwires of stainless steel ( ss ) , nickel - titanium ( niti ) , titanium molybdenum alloy ( tma ) , and low - friction tma ( l - tma ) were used ( 0.017  0.025 ormco , glendora , california , usa ) .
scanning electron microscope ( hitachi , su6600 , japan ) was used to observe the surface morphology of the archwires .
elemental composition of the archwires was assessed using energy dispersive spectroscope ( eds ) ( horiba - emax ) .
atomic force microscope ( afm ) ( xe-100 park , korea ) was used to evaluate the three - dimensional surface ra of the archwires . a potentiostat ( gill ac , acm instruments , england ) was used to perform the linear polarization test , a fast and nearly nondestructive electrochemical technique .
a saturated calomel electrode and platinum were used as the reference electrode and counter electrode , respectively .
modified fusayama artificial saliva ( nacl [ 400 mg / l ] , kcl [ 400/mg / l ] , cacl2 h2o [ 795
mg / l ] , na2h2 po4 h2o [ 690mg / l ] , kscn [ 300
mg / l ] , na2 s.9h2o [ 5 mg / l ] , and urea [ 1000 mg / l ] ) with a ph of 6.5 at 37c was used as the corrosion test electrolyte . to evaluate the effect of fluoride concentration on these archwires , 0.5% of naf ( simulating the fluoride concentration contained in commercial fluoridated toothpastes )
eight groups were taken , and the sample size was 10 for each of these groups .
the experimental groups are mentioned below : 
 group 1 : ss in artificial salivagroup 2 : niti in artificial salivagroup 3 : tma in artificial salivagroup 4 : l - tma in artificial salivagroup 5 : ss in artificial saliva with 0.5% nafgroup 6 : niti in artificial saliva with 0.5% nafgroup 7 : tma in artificial saliva with 0.5% nafgroup 8 : l - tma in artificial saliva with 0.5% naf . 
 
group 1 : ss in artificial saliva group 2 : niti in artificial saliva group 3 : tma in artificial saliva group 4 : l - tma in artificial saliva group 5 : ss in artificial saliva with 0.5% naf group 6 : niti in artificial saliva with 0.5% naf group 7 : tma in artificial saliva with 0.5% naf group 8 : l - tma in artificial saliva with 0.5% naf .
the linear polarization tests were carried out after dipping the archwire samples into the test electrolyte for 2 h. the linear polarization values were measured from 10 mv to + 10 mv with a scan rate of 0.1 mv / s . from the present test , the polarization resistance ( rp [  cm ] ) was obtained which is inversely proportional to the corrosion rate and directly proportional to corrosion resistance .
eight groups were taken , and the sample size was 10 for each of these groups .
the experimental groups are mentioned below : 
 group 1 : ss in artificial salivagroup 2 : niti in artificial salivagroup 3 : tma in artificial salivagroup 4 : l - tma in artificial salivagroup 5 : ss in artificial saliva with 0.5% nafgroup 6 : niti in artificial saliva with 0.5% nafgroup 7 : tma in artificial saliva with 0.5% nafgroup 8 : l - tma in artificial saliva with 0.5% naf . 
 
group 1 : ss in artificial saliva group 2 : niti in artificial saliva group 3 : tma in artificial saliva group 4 : l - tma in artificial saliva group 5 : ss in artificial saliva with 0.5% naf group 6 : niti in artificial saliva with 0.5% naf group 7 : tma in artificial saliva with 0.5% naf group 8 : l - tma in artificial saliva with 0.5% naf .
the linear polarization tests were carried out after dipping the archwire samples into the test electrolyte for 2 h. the linear polarization values were measured from 10 mv to + 10 mv with a scan rate of 0.1 mv / s . from the present test , the polarization resistance ( rp [  cm ] ) was obtained which is inversely proportional to the corrosion rate and directly proportional to corrosion resistance .
figure 2 and table 1 show the afm observations and the corresponding surface ( ra , nm ) .
statistical analysis using one - way analysis of variance ( anova ) showed a significant influence on the ra value ( p < 0.003 ) .
ss ( fe - 73.2% , cr - 18.8% , ni - 8% ) , niti ( ni - 55.1% , ti - 44.9% ) , tma ( ti - 78% , mo - 11.5% , zr - 6% , sn - 4.5% ) , and l - tma ( ti - 78% , mo - 10.5% , zr - 5.5% , sn - 4.5% , n - 1.5% ) .
the ranking of mean rp value was l - tma > tma > niti > ss . table 3 reveals that rp value of all archwires decreased drastically in artificial saliva with 0.5% naf .
the collected data were subjected to statistical analysis using one - way anova and was found that there was a statistically significant influence on the rp value ( p < 0.001 ) . scanning electron microscopic images .
niti : nickel titanium , tma : titanium molybdenum , l - tma : low - friction titanium molybdenum alloy atomic force microscopic images .
niti : nickel titanium , tma : titanium molybdenum , l - tma : low - friction titanium molybdenum alloy surface roughness ( ra , nm ) for different wires polarization resistances (  cm ) for various groups in artificial saliva without sodium fluoride polarization resistances (  cm ) for various groups in artificial saliva with sodium fluoride
the ss and titanium alloys used in orthodontic appliances rely on the formation of a passive surface oxide film to resist corrosion .
the protective layer is not infallible ; it is susceptible to both mechanical and chemical disruption .
potentiodynamic polarization experiments and scanning electron microscopic observations of archwires composed of ss , niti , and tma exposed to electrochemical corrosion in artificial saliva have shown evidence of pitting corrosion formed on the wire surface .
as for ss archwires , it is well known that chromium element in the ss alloy can form a thin and adherent cr2o3-based protective film which provides the corrosion resistance of a substrate alloy .
a minimum chromium content of around 11% is required to form a protective passive film on the ss wire . in case of niti archwires ,
the tio2-based ( also small traces of nio ) passive film can provide a good measure of niti alloy biocompatibility .
tma wires form a passive film of tio2 and traces of mo3 , zro2 , sno , and l - tma form the same protective layer as tma wire together with the formation of traces of no due to ion bombardment of nitrogen .
edie et al . in their study stated that the corrosion potential of ss and niti is not different .
however , the present study [ table 3 ] suggests that more surface pitting and corrosion occurred in ss than in niti , tma , and l - tma .
these results were almost similar to the results reported by suarez et al . and kim and johnson .
they concluded that the titanium wires appear to be the most inert wire of those tested and are unlikely to release metal ions when used intraorally .
however , in this study , l - tma was found to have a slightly higher corrosion resistance than tma wires .
the improvement in corrosion resistance of l - tma was believed due to the presence of no in the outermost surface of the alloys .
it was found that tio2-based passive film formed on titanium metal has better corrosion resistance in acidic artificial saliva than the cr2o3-based passive film on ss .
this result was also consistent with a previous study which has also concluded that tio2 passive film is more corrosion resistant than cr2o3 .
the better corrosion resistance of tma wires than niti might be due to the increased titanium content in them ( 78% in tma and 45% in niti ) .
this is clearly seen in the present study in which the composition of the archwire is determined by energy dispersive spectroscope ( eds ) .
a previous study had suggested that surface ra of orthodontic archwires ought to be taken as an important indicator of the trend toward archwire corrosion resistance . in our study
[ table 1 ] , tma showed the highest surface ra value and ss the least surface ra . from the potentiostatic tests , it was confirmed that tma and l - tma exhibited the highest corrosion resistance .
contrary to this , earlier studies were reported but our study was consistent with the study reported by danto et al . from this
, it can be concluded that chemical composition of the wire is the primary factor influencing the corrosion resistance and surface ra is only secondary . from [ table 3 ] , it is very clear that the corrosion resistance of all wires had reduced drastically in the presence of 0.5% naf ( although l - tma showed the highest rp value ) .
the cr2o3 passive film of ss reacts with naf and the following reaction takes place . cr2o3 + 2naf  crf2 + na2o + cro2 
  in case of niti and tma wires , tio2 reacts with naf to form titanium - fluoride complex compound .
tio2 + naf  na2 tif6 however , tma and l - tma archwires showed a better corrosion resistance than niti .
this may be due to the presence of some other oxides in the surfaces passive film ( moo3 , zro , and sno ) .
this was followed by niti , l - tma , and ss in decreasing order . from the potentiostatic study
, it was found that tma and l - tma had the highest corrosion resistance .
this indicates that the chemical composition of the wire is the primary influential factor to have high corrosion resistance and surface ra is only secondary . in the present study
, the rp values were measured after exposing the archwires to artificial saliva and artificial saliva - containing sodium fluoride ( 0.5% ) for 2 h. this simulated the in vivo corrosion resistance of the archwires in the oral cavity .
it also simulated the in vivo corrosion resistance of archwires when residual fluorinated toothpastes interacted with the archwires 2 h after brushing .
it was seen that the in vivo corrosion resistance of the archwires in the oral environment might decrease if the highly fluorinated toothpastes were used and/or the exposure time of archwires to residual fluorinated toothpastes was prolonged .
therefore , complete removal of residual high - fluorinated toothpastes from the crevice between archwire and bracket during tooth brushing is required .
furthermore , the repair of the protectiveness of the surface oxide film on the archwires might occur after full removal of residual fluorinated toothpastes by mechanical tooth brushing . however , the effect of a lesser concentration of fluoride and variation in the exposure time to this particular concentration of fluoride on the archwires is still to be investigated .
 objective:(1 ) to evaluate the corrosion resistance of four different orthodontic archwires and to determine the effect of 0.5% naf ( simulating high fluoride - containing toothpaste of about 2250 ppm ) on corrosion resistance of these archwires . ( 2 ) to assess whether surface roughness ( ra ) is the primary factor influencing the corrosion resistance of these archwires.materials and methods : four different archwires ( stainless steel [ ss ] , nickel - titanium [ niti ] , titanium molybdenum alloy [ tma ] , and ion - implanted tma ) were considered for this study . 
 surface characteristics were analyzed using scanning electron microscopy , atomic force microscopy ( afm ) , and energy dispersive spectroscopy . 
 linear polarization test , a fast electrochemical technique , was used to evaluate the corrosion resistance , in terms of polarization resistance of four different archwires in artificial saliva with naf concentrations of 0% and 0.5% . 
 statistical analysis was performed by one - way analysis of variance.results:the potentiostatic study reveals that the corrosion resistance of low - friction tma ( l - tma ) > tma > niti > ss . 
 afm analysis showed the surface ra of tma > niti > l - tma > ss . 
 this indicates that the chemical composition of the wire is the primary influential factor to have high corrosion resistance and surface ra is only secondary . 
 the corrosion resistance of all wires had reduced significantly in 0.5% acidic fluoride - containing artificial saliva due to formation of fluoride complex compound.conclusion:the presence of 0.5% naf in artificial saliva was detrimental to the corrosion resistance of the orthodontic archwires . 
 therefore , complete removal of residual high - fluorinated toothpastes from the crevice between archwire and bracket during tooth brushing is mandatory . 
Materials and Methods
Experimental groups
Results
Discussion
Conclusion
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