In luence o So D inks wi h Low pH on Di e en Ni-Ti
O hodon ic A chwi e Su ace Pa e ns
C. Abalos, A. Paul, A. Mendoza, E. Solano, C. Palazon, and F.J. Gil
(Submi ed No embe 5, 2011; in e ised o m May 16, 2012; published online July 20, 2012)
The aim o his s udy was o de e mine he influence o so d inks on he su ace o Ni-Ti a chwi es and
hei co osion beha io . A chwi es wi h di e en pa e ns (smoo h, sc a ch, dimple, and c ack) we e
selec ed and cha ac e ized by scanning elec on mic oscopy and lase con ocal mic oscopy. Imme sion es s
we e pe o med in a ificial sali a (pH 6.7) wi h a so d ink wi h a pH o 2.5 o 28 days. The esul s
showed an inc ease in he su ace de ec s and/o oughness o he dimple, c ack and sc a ch pa e ns wi h
he imme sion imes, and a dec ease in co osion esis ance. A ela ionship be ween he su ace pa e n and
he ex en o he co osion in Ni-Ti a chwi es wi h so d inks a low pH has been demons a ed. Pa e n
should be aken in o accoun in u u e s udies, and manu ac u ing p ocesses ha p oduce su ace de ec s
(especially c acks) should be a oided.
Keywo ds co osion, Ni-Ti, su ace
1. In oduc ion
The biocompa ibili y o nickel- i anium alloys (Ni-Ti) nea
equia omic composi ion is good in a ificial sali a, Ringe s
solu ion, and physiological saline solu ion (Re 1). Ne e he-
less, he co osion esis ance o Ni-Ti alloys dec eases in acid
sali a (Re 2-4) and chlo ine (Re 5) o fluo ide-con aining
solu ions (Re 5-15). Co osi e agen s de e io a e he passi e
film o he i anium oxide su ace (Re 7,12,14,15),
dec easing co osion esis ance o he alloy and hence i s
biocompa ibili y (Re 1-3).
In he las decade, many s udies ha e examined he ac o s
ha influence Ni-Ti alloys exposed o fluo ide. I is well known
ha he ou e i anium oxide laye is deg aded by hyd ogen
abso p ion (Re 14). Also, he e ec s o exposu e ime (Re 13,
14), fluo ide concen a ion (Re 6,7,12) and a mo e acidic pH
(Re 12,14), as well as hei ela ionships, ha e been s udied
(Re 13). A pH-dependen c i ical concen a ion has been
defined (Re 10,11) ha can deg ade he passi e film.
Ne e heless, in ela ion o ca bona ed d inks, only s udies on
he enamel coa ing ha a ec s adhe ence o b acke s can be
ound in he li e a u e. Such d inks a e a pa o he die o
young people ecei ing o hodon ic ea men , and can also
deg ade he o hodon ic a chwi e su ace.
When he passi e film is b oken, co osion akes place. As
a consequence, he elease o me allic ions in o he medium
(Re 3,6,10,15,16), su ace mo phological changes (Re 6,
13,14) including an inc ease in oughness (Re 3,6) and
ic ion coe ficien be ween he a chwi e and he b acke
(Re 17), a e p oduced. In hese e ec s, he wi e su ace plays
an impo an ole in co osion, al hough he published esul s
a e con o e sial. P e ious s udies (Re 3,16,18) poin o
su ace de ec s p oduced du ing he wi e manu ac u ing
p ocess as p e e en ial co osion a eas, while o he au ho s
ha e epo ed no such e idence (Re 7,19). Some in es i-
ga o s (Re 20) conside a chwi e oughness o be an indica o
o he endency owa d co osion, al hough o he s udies (Re
3,6,7,19) ha e ound no such ela ionship when including
esidual s esses and he deg ee o homogenei y as co osion
accele a o s (Re 7,19). This is o say ha di e en a chwi e
su aces ha e a be e o wo se esponse o co osi e fluo ide.
Thus, he a chwi e manu ac u e is a a iable in i sel (Re 6,
19). The main objec i e o his s udy was o de e mine
whe he so d inks wi h a low pH cause co osion o Ni-Ti
wi es.
The wi e su ace, su ace de ec s, and oughness a e ela ed
o he manu ac u ing p ocesses (Re 6) ha lead o di e en
su ace finishes (Re 21). A second objec i e o he p esen
s udy was o in es iga e he influence o he su ace (su ace
de ec s and oughness), as a esul o he manu ac u ing
p ocess, upon he co osion o Ni-Ti a chwi es. Di e en
comme cial b ands could p esen he same pa e n. In his
con ex , he influence o each pa e n upon so d ink co osion
was s udied by means o su ace analysis (scanning elec on
mic oscopy), oughness cha ac e iza ion (lase scanning con-
ocal mic oscopy), and co osion es s. The main hypo hesis is
ha di e en su ace pa e ns, de i ed om di e en manu ac-
u ing p ocesses, will espond di e en ly o co osion p oduced
by so d inks.
C. Abalos, Depa men o Den al Ma e ials, School o Den is y,
Uni e si y o Se ille, Se ille, Spain; A. Paul, Depa men o
Mechanical and Ma e ials Enginee ing, School o Enginee ing,
Uni e si y o Se ille, Se ille, Spain; A. Mendoza, Depa men o
Pedia ic Den is y, School o Den is y, Uni e si y o Se ille, Se ille,
Spain; E. Solano and C. Palazon, Depa men o O hodon ics, School
o Den is y, Uni e si y o Se ille, Se ille, Spain; and F.J. Gil,
Depa men o Ma e ials Science and Me allu gical Enginee ing,
Technical Uni e si y o Ca alonia, Diagonal 647, 08028 Ba celona,
Spain. Con ac e-mail: ancesc.xa ie [email p o ec ed].
JMEPEG (2013) 22:759–766 ASM In e na ional
DOI: 10.1007/s11665-012-0311-3 1059-9495/$19.00
Jou nal o Ma e ials Enginee ing and Pe o mance Volume 22(3) Ma ch 2013—759
2. Ma e ials and Me hods
2.1 P elimina y S udy
The su aces o 16 Ni-Ti a chwi es (0.016 90.022 inches)
om nine comme cial b ands we e s udied in he as- ecei ed
condi ion (Re 22)(Table 1). The su ace mo phology and
composi ion we e e alua ed by scanning elec on mic oscopy
(SEM) (Philips XL-30 Philips, Eindho en, Ne he lands) and
ene gy dispe si e spec ome y (EDS) (Edax In e na ional,
Mahwah, USA), while oughness was measu ed using a lase
scanning con ocal mic oscope sys em (LSCM) (Leica TCS-
SP2, We zla , Ge many).
The a chwi es we e classified acco ding o hei su ace
pa e ns using 18 a iables. Fi e a iables we e ound o be
co ela ed o he pa e n: dimples, sc a ches, c acks, po es, and
smoo h/no de ec . As a esul , he Ni-Ti a chwi es we e classified
acco ding o hese fi e pa e ns (Table 1) depending on he
p edominan su ace de ec s. The di e en su ace de ec s a e
de e mined in he ollowing way: As- ecei ed ma e ial can be
obse ed in Fig. 1a. Po e is a deep, usually ounded de ec .
Mic opo e is a po e wi h a diame e o less han one mic ome e .
Dimple is a small hole, an ellip ical o ounded dep ession on he
su ace simila o he dimples in a gol ball (Fig. 1c). C ack is a
long, deep fissu e wi h i egula edges and a iable wid h
(Fig. 1e). Las ly, sc a ch is a long fissu e wi h smoo h edges
and cons an wid h, in which he bo om is clea ly isible
(Fig. 1g).
2.2 Ma e ial and Imme sion Tes
Eigh Ni-Ti wi es wi h nea equia omic composi ions made
by di e en manu ac u e s we e used in his s udy. Each su ace
pa e n (dimple, sc a ch, c ack, po e, and smoo h/no de ec )
co esponded o wo di e en comme cial b ands (Table 1).
The po ous su ace pa e n was elimina ed om his s udy
because he e was only one manu ac u e . Fo he es s, use was
made o fi e samples o di e en a chwi es om he same
ba ch (n= 40). The samples we e 20 mm segmen s ex ac ed
om he s aigh pa o he a ch. P io o imme sion
es ing, each specimen was cleaned and deg eased o 5 min
in ace one.
Samples we e placed in a p opylene ube (15 mL) wi h
2 mL o a ificial sali a modified by Fusayama (sali a com-
posi ion: NaCl (0.4 g/L), KCl (0.4 g/L), CaCl
2
Æ2H
2
O (0.906 g/L),
NaH
2
PO
4
ÆH
2
O (0.690 g/L), Na
2
SÆ9H
2
O (0.005 g/L), KSCN
(0.3 g/L), and u ea (1 g/L) (Re 2,3,7,8)) a pH 6.7 (mic opH
2001, C ison Ins umen S.A., Ba celona, Spain) o 28 days a
37 C. Du ing his ime, he samples we e placed in 15 mL
p opylene ubes wi h 2 mL o so d ink wi h a pH o 2.5 e e y
3 days (10 days in o al). The imme sion sequence was: 1 min
in so d ink ollowed by insing o a ificial sali a o 3 min
(5 imes), acco ding o he Van Eygen p o ocol (Re 23). The
o al imme sion ime was 50 min in so d ink co esponding o
he consump ion o one can o so d ink pe day o 28 days
(Re 24). Du ing his p ocess, a e each imme sion in so
d ink, he specimens we e placed in esh a ificial sali a used
in his expe imen .
2.3 Su ace Cha ac e iza ion
The su ace opog aphy o he Ni-Ti a chwi es was exam-
ined be o e and a e he imme sion es s. SEM mic og aphs
and we e aken o ep esen a i e deg aded a eas o he su ace.
The images we e used o measu e he su ace occupied by each
de ec (dimples, sc a ches, c acks, and po es) in h ee di e en
a eas o 10,000 lm
2
. The h ee measu emen s we e hen
a e aged. Thus, fi e samples pe b and o wi e (n= 40) and
h ee a eas pe specimen we e analyzed.
The su ace oughness o he Ni-Ti a chwi es we e analyzed
be o e and a e he imme sion es s by means o LSCM. The
mic oscope was p e iously calib a ed using a Mi u oyo
p ecision Re e ence Specimen (Code no. 178-601) wi h Ra
= 3.1 lm— he e o coe ficien being below 0.3%. The images
we e p ocessed using Leica Con ocal so wa e (Leica Mic o-
sys ems GMBH, We zla , Ge many). Mic og aphs co espond-
ing o 750 linea mic ons we e aken. Fi e a eas o 10,000 lm
2
we e andomly selec ed om each opog aphic image and he
oughness was measu ed. Ra, Rms, Rp, and R alues ob ained
Table 1 A chwi es used in he p elimina y and expe imen al s udies
Code A chwi e Company Ba ch Composi ion(b) Pa e n
NC(a) Ni inol Classic(a) 3M-Uni ek V5304 53.0/47.0 Smoo h
RS(a) Reflex Supe elas ic(a) TPO 1567027 52.9/47.1 Smoo h
AL(a) Align XF(a) O mco 030880842 53.4/46.6 Dimple
OR(a) O honol(a) RMO A07005 52.9/47.1 Dimple
NCu Ni iCu 35 O mco 07E278E 47.2/46.5/6.3(c) Dimple
TS(a) Ti anol Supe elas ic(a) Fo es aden 28636946 53.9/46.1 C ack
M(a) Memo ia(a) Leone 07132601 54.0/46.0 C ack
TM(a) The momemo ia(a) Leone 08022001 53.9/46.1 Sc a ch
G4(a) G4(a) G&H 142669 53.3/46.7 Sc a ch
T Tensic Den au um 54301 56.5/43.4 Sc a ch
NH Ni inol HA 3 M-Uni ek AP1FY 53.0/47.0 Sc a ch
RL Rama i an Li e Den au um 60648 54.5/45.5 Sc a ch
M5 M5 G&H 146755 53.3/46.6 Sc a ch
TA The maloy RMO A07304 53.4/46.6 Sc a ch
RH Reflex Hea -Ac i a ed TPO 1297060 53.2/46.7 Sc a ch
NS Neo Sen alloy GAC H326 53.1/46.8 Po ous
(a)A chwi es used in he expe imen al s udy. (b)%Ni/%Ti (w .%). (c)%Ni/%Ti/%Cu (w .%)
760—Volume 22(3) Ma ch 2013 Jou nal o Ma e ials Enginee ing and Pe o mance
in each a ea we e a e aged. Ra and Rms ep esen he
a i hme ical mean o he absolu e alues and he oo -mean-
squa e alue o he scanned su ace p ofile, espec i ely. Rp
and R in u n ep esen he highes and lowes alues o he
scanned p ofile. Thus, o oughness s udies, fi e a eas om
fi e samples o each a chwi e we e analyzed (n= 40).
2.4 Co osion Tes s
A chwi es we e sec ioned o p oduce co osion es samples.
A 25-mm leng h o each a chwi e wascu wi h s e ile o hodon ic
plie s and was isola ed wi h wax a he poin o in e phase
be ween he es ing solu ion and he ai . The co osion es s we e
Fig. 1 Rep esen a i e SEM images o Ni-Ti wi es. As- ecei ed: (a) smoo h pa e n, (c) dimple pa e n, (e) c ack pa e n, and (g) sc a ch
pa e n. So d ink- ea ed: (b) smoo h pa e n, (d) dimple pa e n, ( ) c ack pa e n, and (h) sc a ch pa e n
Jou nal o Ma e ials Enginee ing and Pe o mance Volume 22(3) Ma ch 2013—761
pe o med in acco dance wi h he ISO-s anda d (Re 25) o
co osion es ing.
The es s we e ca ied ou wi h a Vol alab PGZ 301
po en ios a (Radiome e , Copenhagen, Denma k) con olled by
Vol amas e 4 so wa e (Radiome e Analy ical, Villeu banne
Cedex, F ance). The es ing solu ion was a ificial sali a and
a ificial sali a wi h so d ink kep a a con olled cons an
empe a u e o 37 C. The elec ical se -up used o measu e he
elec ochemical pa ame e s and sample geome y is ep esen ed
in Fig. 2. The e e ence elec ode was an Ag/AgCl/KCl
elec ode (E= 0.222 V). The auxilia y elec ode used was a
pla inum elec ode wi h a su ace o 240 mm
2
(Radiome e
Analy ical, Villeu banne, F ance).
The open ci cui po en ial was moni o ed o 3 h in o de o
allow le eling o o he alue be o e he pola iza ion esis ance
es . The Cyclic Vol amme y assay was pe o med by scanning
he po en ial o he alloy o he sample a 0.25 mV/s wi h he
minimum cu en se a 1 A and he maximum a +1 A, wi h a
minimum ange se a 100 lA be ween 300 and +2000 mV
a ound he OCP alue. Reco dings o he a ia ion in gal anic
cu en densi y, po en ial, e c., we e ob ained, and he Ta el
slopes we e de e mined om he E ans diag ams. In o de o
de e mine hese diag ams, i is e y impo an o eco d he
pola iza ion cu es in a pseudo-s a iona y manne . In his case,
he 250 min o imme sion o he specimens su ficed.
Open ci cui po en ial (E
ocp
), co osion po en ials (E
co
),
and co osion cu en s (i
co
) we e eco ded o he di e en
samples es ed. These pa ame e s a e defined as:
•Open ci cui po en ial (E
ocp
): Po en ial o an elec ode
measu ed wi h espec o a e e ence elec ode o ano he
elec ode when no cu en flows o o om he ma e ial.
•Co osion po en ial (E
co
): Po en ial calcula ed a he
in e sec ion whe e he o al oxida ion a e is equal o he
o al educ ion a e.
•Co osion cu en densi y (i
co
): Cu en di ided by he
su ace o he elec ode. This is he size o he anodic
componen o he cu en which flows a he co osion
po en ial E
co
. Since by defini ion he esul ing cu en is
equal o ze o a ha po en ial, he ca hodic componen is
o equal size, bu o opposi e sign. The measu ed esul -
ing cu en being ze o a he co osion po en ial, he co -
osion cu en densi y i
co
can only be ob ained by
indi ec me hods, e.g., om he Ta el equa ion. Ta el
cons an s aand ba e he Ta el p opo ionali y cons an s
o anodic (oxida ion) and ca hodic ( educ ion) eac ions
o a me al.
2.5 S a is ical Analysis
Mean alues and s anda d de ia ions o each o he eigh
b ands and he ou su ace pa e ns we e compu ed om he
da a ob ained du ing su ace cha ac e iza ion, o he
as- ecei ed a chwi es as well as a e imme sion es ing. Since
he a chwi es used had di e en ini ial alues, in o de o
ensu e alid compa ison o he s a is ical esul s i was deemed
app op ia e o use he di e ence be ween he as- ecei ed and
pos -imme sion es alues eco ded o each wi e es ed. Thus,
he final da a e e o he inc eases o dec eases in he a ea
occupied by he su ace de ec s o su ace oughness. Posi i e
alues imply su ace deg ada ion, while ze o o nega i e alues
indica e he absence o co osion. S a is ical analysis was
ca ied ou using analysis o a iance (ANOVA) and he pos
hoc Bon e oni es , o he compa ison o mean alues.
S a is ical significance was accep ed o a£0.05. The da a
we e analyzed using he SPSS e sion 14.0 s a is ical package
(SPSS Inc., Chicago, IL, USA).
3. Resul s
Table 2shows he esul s o he su ace de ec s in he
as- ecei ed a chwi es. Quan ifica ion was done wi h he su ace
alues o each de ec and he o al su ace de ec s o each
a chwi e. The inc emen al alue also e e ed o each su ace
pa e n included o he a chwi es a e he imme sion es s in
sali a wi h so d ink. The e we e significan di e ences in he
dimple, c ack, and sc a ch pa e ns. Figu e 1shows ep esen-
a i e SEM images o he changes in he su ace pa e ns a e
he co osion es s. In he sc a ch pa e n, pi ing co osion
simila o co osion caused by fluo ide was obse ed (Fig. 1h).
In he dimple and c ack pa e ns he e was no pi ing, bu an
inc ease in he su ace occupied by de ec s was seen. The
numbe o dimples (Fig. 1d) and c acks (Fig. 1 ) inc eased,
espec i ely.
The su ace oughness da a (LSCM) o he a chwi es in he
as- ecei ed and pos -imme sion condi ions a e exp essed in
Table 3. The e was a significan di e ence o he c acked
pa e n, whe e he e was a 34% inc ease in su ace oughness.
Fig. 2 Schema ic ep esen a ion o o hodon ic a chwi es (a) and he elec ical se -up used o measu ing he elec ochemical pa ame e s (b)
762—Volume 22(3) Ma ch 2013 Jou nal o Ma e ials Enginee ing and Pe o mance
In he c ack pa e n, Ra changed om 0.35 lm in he
as- ecei ed a chwi es o 0.47 lm in he pos -imme sion
samples. Figu e 3p o ides ep esen a i e LSCM images o
he ou pa e ns s udied.
The esul s ob ained o E
co
,i
co
, and E
ocp
du ing co osion
es ing a e p esen ed in Table 4.
The a chwi es es ed in a ificial sali a clea ly showed
g ea e esis ance o co osion han hose es ed in a ificial
sali a wi h so d ink. Fo he smoo h a chwi es he di e ence
in E
co
due o imme sion in so d ink was abou 100 mV.
Howe e , o he su ace wi h dimples he alue was abou
150 mV, and o he c ack and sc a ch su aces i was a ound
200 mV. The de ec s wi h mo e in e nal ene gy (c acks and
sc a ches) a e mo e sensi i e o co osion.
The a chwi es wi h sc a ches and c acks showed he g ea es
endency owa ds co osion, wi h a e age alues o 650 mV
in a ificial sali a and 850 mV wi h so d ink. The a chwi es
wi h a smoo h su ace we e ound o be less esis an o
co osion, wi h an E
co
o 515 and 600 mV, in a ificial
sali a and wi h so d ink, espec i ely.
The minimum cu en densi y co esponded o he smoo h
pa e n (3.85 o 3.0 lA/cm
2
), while he maximum alues
we e obse ed in he sc a ch pa e ns (0.59 o 0.22 lA/cm
2
)
in a ificial sali a. The p esence o so d ink p oduced a
dec ease in bo h ype o pa e ns (be ween 0.3 and 1 lA/cm
2
).
The open cu en po en ial alues showed g ea a iabili y
be ween he measu emen s ob ained. Howe e , he alues o
he smoo h and dimple pa e ns we e highe han hose o he
c ack and sc a ch pa e ns.
4. Discussion
In ela ion o he me hodological design, he equi alen o
one so d ink was used as a e age daily consump ion.
Ne e heless, consump ion has been es ima ed o be 12 cans
pe week pe pe son (565 mL/day) (Re 26). In Ame ican
eenage s be ween he ages o 12 and 19 yea s, consump ion
can be as high as 600-800 mL/day (Re 27). Thus, he a e age
consump ion used in his s udy can be ega ded as egula .
In ela ion o he imme sion es , no p e ious s udies
in ol ing he exposu e o o hodon ic wi es o ca bona ed
d inks ha e been ound. Due o he lack o da a, in i o enamel
exposu e p o ocols we e used (Re 23,24). Acco dingly, he
co osion esul s ob ained a e no due o excessi e exposu e o
he media. Fu he mo e, i is suspec ed ha du ing o hodon ic
ea men , he con ac ime could be e en highe han o
Table 2 Su ace a ea o de ec s and a ia ion (DDe , ob ained by sub ac ing a e age o al de ec s) be o e and a e
imme sion es ing
Ni-Ti a chwi es
Manu ac u ing de ec s (in 10,000 lm
2
)
Pa e n
DDe -sali a +
so d ink
C acks Po es lPo es Sc a ches(a) Dimples To al Mean SD
Reflex SE 0 0 0.8 2.9 0 3.7 Smoo h 6.7 16.9
Ni inol Classic 0 174 1.9 0 25 200.1
O honol 0 228 1.5 0 6215 6444.5 Dimple 252(b) 68.2
Align XF 73.7 258 3,3 0 1302 1637
Memo ia 577.5 5.1 1.7 12.8 176 773.1 C ack 768(b) 72.7
Ti anol SE 264.5 35.6 1.3 0 0 301.4
G4 37.5 94.95 5.7 11.2 0 149.3 Sc a ch 1306(b) 46.7
The momemo ia 7.5 22.8 0.66 7.5 84.78 123.2
Dimple, c ack, and sc a ch pa e ns show s a is ically significan a ia ions
(a)lm pe 100 lm in linea sc a ches. (b)Mean significan di e ence (p<0.05) by ANOVA pos hoc Bon e oni es
Table 3 Su ace oughness da a and a ia ion be o e and a e imme sion es ing
Ni-Ti a chwi es
Su ace oughness as- ecei ed (10,000 lm
2
) Sali a + so d ink
Ra Rms R Rp Pa e n Ra Rms DRa DRms
Reflex SE 0.22 0.31 2.31 1.20 Smoo h 0.32 (0.13)(a) 0.45 (0.18) 0.03 (0.01) 0.04 (0.01)
Ni inol Classic 0.46 0.61 3.46 2.76
O honol 1.32 1.71 8.45 4.53 Dimple 1.01 (0.35) 1.35 (0.47) 0.01 (0.01) 0.01 (0.01)
Align XF 0.70 0.95 7.60 4.09
Memo ia 0.44 0.60 4.30 2.23 C ack 0.35 (0.13) 0.48 (0.17) 0.12(b) (0.04) 0.14(b) (0.04))
Ti anol SE 0.25 0.37 3.33 1.18
G4 0.41 0.53 3.75 2.9 Sc a ch 0.42 (0.11) 0.54 (0.14) 0.01 (0.01) 0.03 (0.01)
The momemo ia 0.43 0.57 3.93 2.74
C ack pa e n shows s a is ically significan a ia ions
(a)S anda d de ia ions a e gi en in pa en heses. (b)Mean significan di e ence (p<0.05) by ANOVA pos hoc Bon e oni es
Jou nal o Ma e ials Enginee ing and Pe o mance Volume 22(3) Ma ch 2013—763

enamel, because he b acke -a chwi e se complica es he sel -
cleaning mechanisms and he sali a insing e ec .
The esul s e e ed o he main objec i e o his s udy show
so d ink a low pH o exe co osi e ac ion on he su ace o
Ni-Ti o hodon ic a chwi es, al hough in di e en ways
depending on he su ace pa e n. In p e ious esea ch wi h
o he co osi e media (Re 16,19), i was shown ha he
manu ac u e o Ni-Ti wi es exe s a s a is ically significan
influence upon co osion esis ance, since he as- ecei ed
comme cial Ni-Ti a chwi es had di e en su ace mo phologies
(Re 6,7,16). The esul s ob ained ag ee wi h his influence o
he manu ac u e upon co osion esis ance. Ne e heless,
p e ious s udies (Re 5-7,10,12,13,16,19) ela e co osion
o he b ands o a chwi es s udied, using only one o ou
a chwi es om di e en manu ac u e s. This s udy may ha e
some ad an ages, since a gi en pa e n indica es a gi en su ace
finish, which allows us o g oup se e al b ands o manu ac-
u e s wi hin he same pa e n. The su ace finish is ela ed o
he manu ac u ing p ocess which, in u n, is ela ed o co osion
esis ance (Re 6,22,28). The e o e, he su ace pa e n should
be aken in o accoun bo h in p esen (Re 29) and in u u e
co osion esea ch.
Fig. 3 Rep esen a i e lase scanning con ocal mic oscope sys em (LSCM) images o Ni-Ti wi es. As- ecei ed: (a) smoo h pa e n, (c) dimple
pa e n, (e) c ack pa e n, and (g) sc a ch pa e n. So d ink- ea ed: (b) smoo h pa e n, (d) dimple pa e n, ( ) c ack pa e n, and (h) sc a ch
pa e n
Table 4 Mean alues ob ained om po en ios a ic pola iza ion plo o di e en Ni-Ti a chwi es s udied in a ificial sali a
solu ion
Ni-Ti a chwi es
A ificial sali a A ificial sali a + So d ink
i
co
,lA/cm
2
E
co
,mV E
ocp
,mV i
co
,lA/cm
2
E
co
,mV E
ocp
,mV
Reflex SE Smoo h 3.85 520 +555 2.87 600 +545
Ni inol Classic Smoo h 3.00 515 +568 2.84 601 +524
O honol Dimple 1.67 565 +450 1.38 691 +479
Align XF Dimple 0.88 579 +520 0.79 674 +488
Memo ia C ack 1.36 624 +401 1.08 796 +409
Ti anol SE C ack 0.67 657 +321 0.11 809 +405
Tensic Sc a ch 0.22 708 +320 +0.03 894 +307
Ni inol HA Sc a ch 0.59 606 +290 +0.24 887 +301
764—Volume 22(3) Ma ch 2013 Jou nal o Ma e ials Enginee ing and Pe o mance
Ra ing o he su ace pa e ns in e ms o co osion
esis ance om low o high is as ollows: c ack, sc a ch,
dimple, and smoo h. In he c ack and sc a ch pa e ns, hese
de ec s a e esponsible o he inc ease in o al su ace de ec s,
p oducing an inc ease in he discon inui ies in he a chwi e
su ace (Fig. 1 -h). In he dimple pa e n, he e is an inc ease in
he numbe o de ec s, p oducing a mo e i egula su ace, bu
wi hou inc easing he su ace discon inui ies (Fig. 1d). The
smoo h pa e n shows no signs o co osion (Fig. 1b). The e is
some deba e in he case o fluo ide co osion ( he mos widely
s udied ype o co osion) be ween hose au ho s (Re 3,16,18)
who conside su ace de ec s p oduced du ing he manu ac u -
ing p ocess o be he cause o co osion and hose who do no
(Re 7,19). Upon e iewing he esul s, i is seen ha su ace
de ec s can play an impo an ole in a chwi es co osion by
ca bona ed d inks. Fu he s udies a e needed o confi m his
hypo hesis (in ol ing ion leaching, pola iza ion cu es, e c.).
Du ing manu ac u e o he a chwi es, hose p ocesses ha lead
o he o ma ion o su ace de ec s, especially c acks, should be
a oided.
C ack and sc a ch pa e ns p esen ed g ea e co osion
beha io due o he high su ace ene gy p oduced by plas ic
de o ma ion, especially in he ip, esul ing in high s ess
co osion. The modynamically, he zones wi h mo e ene gy a e
mo e suscep ible o elec ochemical a ack. C acks also can lead
o c e ice co osion. Howe e , o he su ace pa e ns p esen ed
g ea e co osion esis ance, as is he case o smoo h and dimple
pa e ns. F om hese esul s i can be obse ed ha he ole o
oughness in co osion beha io is no significan in compa -
ison wi h he de ec s.
In he fluo ide co osion mechanism, which has been
ex ensi ely s udied in he las decade, he fluo ide ions can
de e io a e he passi e film o he i anium oxide su ace (Re 7,
12,14,15) by hyd ogen abso p ion (Re 14), dec easing he
co osion esis ance o he alloy and hence i s biocompa ibili y
(Re 1-3). In his case, he ca bona ed d ink has a low pH (2.5)
and con ains acids. The o al acid le el (known as i a able
acid), a he han he pH, can be an impo an ac o in co osion
because i de e mines he ac ual hyd ogen ion a ailabili y
(Re 30) o in e ac ion wi h he passi e film, in a way simila o
fluo ide co osion (Re 6,10,11,13). Mos so d inks con ain
one o mo e ood acidulan s; phospho ic and ci ic acid a e
common bu malic, a a ic, and o he o ganic acids also may be
p esen (Re 31). These acids a e esponsible o he cha ac e -
is ics o Ni-Ti a chwi es co osion p oduced by so d inks.
The co osi e e ec s o so d inks a e mainly eflec ed by
an inc ease in he p e-exis ing as- ecei ed su ace de ec s
(Table 2). The p esen s udy eco ded he gene alized o ma ion
o small c acks, simila o he pi s p oduced du ing fluo ine
co osion (Re 5,13,15), bu only on he a chwi es wi h a
sc a ch pa e n (Fig. 1h). The c ack pa e n yielded g ea e
inc eases in su ace de ec con en om he p e-exis ing c acks
(Fig. 1 ). C acks filled wi h impu i ies in he as- ecei ed
a chwi es g ow and become mo e p onounced. The as- ecei ed
impu i ies ha e been p e iously desc ibed and ela ed o
a chwi e de e io a ion by o he au ho s (Re 32). In he absence
o u he s udies, he mos p onounced so d ink e ec upon
he Ni-Ti alloy is seen o be i s abili y o p oduce o enla ge
c acks which gene a e su ace discon inui ies and ha e de i-
men al e ec s on he passi e film (Re 15). C acks ac as
c e ices ha induce co osion h ough he o ma ion o a
di e en ial ae a ion cell. In he a chwi es wi h a dimple pa e n
(Fig. 1c, d), and al hough an inc ease in numbe is obse ed,
he e is no su ace discon inui y (c e ices) and, coupled wi h
he smoo h pa e n, his is he mos co osion esis an pa e n.
In u u e esea ch i would be in e es ing o measu e he e ec
o ion leaching upon he a e age diame e and mass o he
a chwi es in o de o be e unde s and he co osi eness and
e ec s o so d ink on he passi e laye . On compa ing hese
findings wi h hose ound du ing fluo ide co osion, se e al
ele an di e ences a e obse ed. The mos equen conse-
quences o fluo ide imme sion a e a gene al (Re 5,13-15)o
localized a ack (Re 10,15) in he o m o pi s. Ano he
cha ac e is ic e ec is he change in colo : da k s ains (Re 13),
whi e spo s (Re 9,13-15), o da k spo s ha disappea a e he
imme sion es s in as- ecei ed a chwi es (Re 12-14,32,33).
Such changes in colo desc ibed du ing co osion in fluo ide
media we e no ound in he p esen s udy. A hypo hesis o
p o e is ha so d inks, unlike fluo ide, use p e-exis ing
de ec s as nuclea ion si es o co osion-c acks being he mos
impo an de ec s because o he exis ence o a c e ice.
Rega ding he inc ease in oughness a e imme sion in so
d ink, co ela ions wi h he c ack pa e n we e ound (Table 3).
The inc ease in oughness is ela ed o co osion (Re 3,6,7),
al hough no he ini ial oughness (Re 3,7,19). The esul s
ob ained a e in ag eemen wi h he findings e e ed o he
dimple pa e n, wi h high ini ial oughness, which does no
significan ly a ec pos -imme sion oughness. Ne e heless,
his is in disc epancy wi h o he in es iga o s (Re 20) who
poin o ini ial oughness as a ac o ela ed o co osion. On he
o he hand, he inc ease in oughness is no only ela ed o he
inc ease in he numbe o de ec s. In he dimple and sc a ch
pa e ns he numbe o de ec s inc ease bu no so oughness.
The explana ion o his is ha he su ace occupied by he
de ec s in some a chwi es ep esen s abou 10% o he o al
su ace. Thus, oughness depends on he numbe o de ec s bu
also on he emaining su ace. Taking in o accoun ha
imme sion in fluo ide solu ion p oduces a gene alized inc ease
in oughness ha is dependen upon concen a ion and pH
(Re 5,6), in he as- ecei ed a chwi es wi h ew su ace de ec s
he inc ease in oughness essen ially depends on he p ocesses
aking place on he su ace ee o de ec s. In exposu e o so
d ink, an inc ease in oughness in he c ack pa e n is obse ed
whe e he numbe o p e-exis ing de ec s is significan . This
inc ease can influence pos -imme sion oughness. I would be
in e es ing o s udy he changes in oughness p oduced by
fluo a ed solu ions and o compa e hem wi h hose p oduced
by ca bona ed d inks, in he same way ha he di e ence has
been s udied be ween he ac ions o chlo ide-con aining
solu ions on Ni-Ti a chwi es and hose exposed o fluo ide
media (Re 5). The oughe su ace condi ion in fluo ide can be
asc ibed o he gene al co osion o Ni-Ti alloy in fluo ide
solu ions (Re 5). As he acid media in so d ink and fluo ide
a e di e en , he e should be di e ences be ween hem.
I may be no ed ha he po en ials in an open ci cui o he
all a chwi es adop posi i e alues, and consequen ly he Ni-Ti
a chwi es s udied emain in hei immune ange. The highe he
cu en densi y a a gi en po en ial, he mo e p one he ma e ial
is o co osion. Smoo h and dimple pa e ns had he mos
passi e (2.87, 2.84, 1.38, and 0.79 lA/cm
2
) and sc a ch
he mos ac i e (+0.03, +0.24 lA/cm
2)
c i ical cu en densi y
alues in a ificial sali a wi h fluo ide solu ion.
The di e ences in cu en densi y a a gi en po en ial
be ween smoo h and dimple pa e ns a e due o he in e nal
s esses p oduced in he su ace o he Ni-Ti by he machining
p ocess, which a o he co osion p ocesses.
Jou nal o Ma e ials Enginee ing and Pe o mance Volume 22(3) Ma ch 2013—765
Obse ed su ace changes canno be di ec ly ela ed o
changes in he mechanical p ope ies o Ni-Ti a chwi es (Re
13). Nei he su ace de ec s no oughness can be conside ed he
only ac o s in ol ed in co osion; su ace esidual s ess, he
amoun o esidue and he deg ee o homogenei y o mic os uc-
u es (Re 5,7,19), among o he ac o s, mus also be aken in o
accoun . The o hodon is should ins uc he pa ien on ca bon-
a ed d ink in ake du ing ea men , sugges ing he use o a s aw
o minimize he co osi e e ec on Ni-Ti a chwi es.
5. Conclusions
In conclusion, so d inks wi h low pH alues exe co osi e
ac ion on he su ace o Ni-Ti o hodon ic a chwi es, al hough in
di e en ways depending on he su ace pa e n. A chwi es wi h
c acked o sc a ched su ace pa e ns a e he leas co osion
esis an . C acks as p e-exis ing manu ac u ing de ec s show he
g ea es su ace de e io a ion, due o he in e nal ene gy s o ed.
Howe e , he ini ial oughness o a chwi es is no ela ed o he
inc ease in oughness when hey a e exposed o so d ink. The
c acked su ace pa e n mo eo e shows he g ea es inc ease in
oughness; he e o e, c acks should be a oided in he a chwi e
su ace pa e n. The absence o s anda d su ace ea men
p o ocols o Ni-Ti esul s in a iabili y in he su ace finish,
which in u n esponds di e en ly o co osion. Such p o ocols
should be help ul in secu ing be e quali y su aces.
Acknowledgmen s
The au ho s hank D . Juan Luis Ribas (LSCM expe ) and he
Cen e o Technology and Inno a ion Resea ch, Uni e si y o
Se ille (CITIUS), o hei collabo a ion in his s udy.
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766—Volume 22(3) Ma ch 2013 Jou nal o Ma e ials Enginee ing and Pe o mance