articulo de corrosion dra liz

7/29/2019 Articulo de Corrosion Dra Liz

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Co r r o si o n beh a v i o r o f 2205 du p l e x st a i n l e ss st eelJ e f fr e y A . P l a t t , D D S , F A G D , a A n d r e s G u z m a n , D D S , b A r n a l d o Z u c c a r i , M D , D D S , bD a v i d W . T h o r n b u r g , P E , c B a r b a r a F . R h o d e s , B G S , a Y o s h i k i O s h i d a , P h D , aa n d B . K e i t h M o o r e , P h D aIndianapolis and LaPorte, Ind.The co r ros ion be hav io r o f 2205 d up lex s ta in less s tee l was comp ared w i th tha t o f A IS I t ype 316Ls ta in less s tee l . The 2205 s ta in less s tee l i s a po ten t ia l o r thodon t i c b racke t mate r ia l w i th l ow n icke lcon ten t (4 to 6 w t% ) , wh ereas the 316L s ta in less s tee l (n ickel co n ten t : 10 to 14 w t%) i s a cu r ren t l yused b rack e t ma te r ia l . Bo th s ta in less s tee ls were sub jec ted to e lec t rochem ica l and imm ers ion(c rev ice ) co r ros ion tes ts i n 37 C , 0 .9 w t % sod ium ch lo r ide so lu t i on . E lec t rochemica l tes t i ngind ica tes tha t 2205 has a longer pass iva t ion range than 31 6L . The co r ros ion ra te o f 2205 was0 .416 M PY (m i l li - inch per year) , whereas 316L exh ib i ted 0 .647 MPY . When 2205 w as coup led to316L w i th equa l su r face a rea ra ti o , the co r ros ion ra te o f 2205 reduce d to 0 .260 MPY, i nd ica t ingtha t 316L s ta in less s tee l behaved l i ke a sac r i f ic ia l anode . When 316L i s coup led w i th N iT i, TMA, o rs ta in less s tee l a rch w i re and w as sub jec ted to the immers ion co r ros ion tes t , i t was found th a t 316Lsu f fe red f rom c rev ice co r ros ion . On the o ther hand , 2205 s ta in less s tee l d id no t sho w any loca l i zedc rev ice co r ros ion , a l though the su r face o f 2205 was covered w i th co r ros ion p roduc ts , fo rme d whe ncoup led to N iTi and s ta in less s tee l w i res . Th is s tudy ind ica tes tha t cons ide r ing co r ros ion res i s tance ,2205 d up lex s ta in less s tee l is an improved a l te rna t i ve to 316L fo r o r tho don t i c b racke t fab r i ca t ionwhe n use d in conjunct ion wi th t i tan ium , i ts a l loys, or s ta in les s s teel arch wi res. (Am J OrthodDento fac Or thop 1997 ;112 :69-79 . )

Austenitic stainless steel (e.g., AISI type316L stainless steel) is the most commonly usedorthodontic bracket material. It typically has a com-position of 18 weight percent (wt%) chromium, 8wt% nickel (Ni), 2 to 3 wt% molybdenum, and a lowcarbon content . 1 Its mechanical properties , such asductility and wear resistance, make it attractive forthis application.The corrosion resistance and appearance ofstainless steel brackets are relatively good. However,this material is challenged by the hostile environ-ment in the mouth, as it is susceptible to localizedcorrosion in a low pH environment containing chlo-rine ions.Austenitic stainless steel exists as a face-centeredcubic crystalline structure, formed by heating thealloy above 912 C. 2 To maintain this structure whencooled, nickel is added to stabilize the austeniticphase. To minimize the risk of hypersensitivityreactions from nickel, the corrosion resistance of thea D e n t a l M a t e r i a l s, D e p a r t m e n t o f R e s t o r a t i v e D e n t i s t r y , In d i a n a U n i v e r -s i ty Scho ol o f Dent i s t ry .b p r o s t h o d o n t i c s , D e p a r t m e n t o f R e s t o r a t i v e D e n t i s t r y , I n d i a n a U n i v e r s i t ySchool o f Dent i s t ry .c T P O r t h o d o n t i c s , I n c .R e p r i n t r e q u e s t s t o : D r . Y o s h i k i O s h i d a , D e n t a l M a t e r i a l s L a b o r a t o r y ,I n d i a n a U n i v e r s it y S c h o o l o f D e n t i s t r y , 1 1 2 1 W . M i c h i g a n S t . , I n d i a n a p -olis, IN 46202-5186.C o p y r i g h t 1 9 9 7 b y t h e A m e r i c a n A s s o c i a t i o n o f O r t h o d o n t i s ts .0889-5406/97/$5.00 + 0 8/1/75384

stainless steel should be maximized to control thenickel ion release from the alloy.It has been reported that up to 21.5% of thepopula tion may exhibit allergic reaction to nickel onpatch testing? Case studies have indicated hyper-sensitivity reactions to nickel, stimulated by expo-sure to orthodont ic brackets. 4-8 It has also beenshown that t he quantity of nickel exposure is criticalif hypersensitivity symptoms are seen. 9The 2205 stainless steel, a duplex (or dual) phasestainless steel, is being investigated as a material fororthodontic bracket fabrication. Microstructure ofthe duplex stainless steels is a mixture of austeniticand delta-ferritic phases.TMThe delta-ferrite is hardand relatively less ductile. Austenite is softer andmore ductile. The combination of these phasesresults in steel harder than the single-phase aus-tenitic stainless steel (316L) and more ductile thansingle-phase ferritic stainless steel (430). The auste-nitic structure exhibits corrosion resistance becauseboth chromium and molybdenum are soluble in thematrix. Chromium adds to the overall resistancethrough a passivation process by forming a complexspinel-type passive film, (Fe,Ni)O(Fe,Cr)203 .11 Mo-lybdenum increases the ability of stainless steel toresist the localized corrosion including pitting andcrevice corrosion, particularly in environments con-taining chloride ion. 12The duplex stainless steels contain much less6 9


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7 0 Platt et aL American Journal of Orthodontics and Dentofacial O rthopedicsJuly 1997

1

t /i

~ 3 1 6 L$ . $ . "~ k 2 2 0 5 [S .S . 1

F ig . 1 . Samp le con f i gu ra t i on o f coup led tes t samp le .

, T M A w ir e"l' i N i w i reS S w i r e

//f

Jb lo c k , 3 1 6 L S S

~7~ 2 2 0 5 S S

F ig . 2 . Immers ion tes t samp le coup le fo r c rev i ce co r -ros ion .

Tab le I . C h e m i c a l c o m p o s i t i o n o f a d u a l - p h a s e 2 2 0 5 s t a i n le s ss t e e l a n d a s i n g l e - b a s e 3 1 6 L s t a i n l e s s s t e e l 132 2 05 (%) 3 1 6 L (%)l em en t

C 0 .0 3 m ax i m u m 0 .0 3 m ax i m u mNi 4-6 10-14Cr 31-23 16-18Mo 3-3 2-3M n 2 2S 0.03 0.03Fe balance balance

nickel than austenitic stainless steels, as seen inTable I. 13 If the corros ion resistance of the duplexstainless steel is equal to or better than the aus-tenitic stainless steel, the risk of nickel hypersensi-tivity should be reduced. The use of duplex stainlesssteels in oth er industries in highly corrosive environ-ments would suggest that the corrosion resistance ofthese materials is good.The purpose of this study is to compare thecorrosion beh avior o f 2205 stainless steel with that o f316L stainless steel through electrochemical polariza-tion studies and immersion crevice corrosion tests.M A T E R I A L S A N D M E T H O D SMater ia ls

All specimens of 316L and 2205 stainless steel used inthis study were supplied by TP Orthodontics, Inc. Speci-

mens (approximately 5 5 12 mm) were cast in a"Christmas tree" pat tern and separated with separatingdisks and air coolant. Mechanical polishing was per-formed with wet SiC papers up to grit No. 800 . Aftermechanical polishing, any samples with casting defectsevident on the surface were eliminated from the study.D e n s i t y ( g / c m 3 ) of each polished sample was calculatedand, if the calculated specific density was less than 99% ofthe averaged value, the sample was also eliminated fromthe study. The averaged value of density of 316L stainlesssteel was 7.949 -+ 0.076 g / c m 3 ; whereas that for 2205stainless steel was 7.953 -+ 0.068 g/cm3.Microst ruc tura l Observa t ion

After additional polishing with a suspension of 0.05~m alumina, one surface of each material was chemicallyetched with "aqua regia" at room temperature for 5


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American Journal of Orthodontics and Dentofacial Orthopedics Platt et aL 71Volume 112, N o . 1

2 2 0 5 i n N a C I ( 0 . 9 % ) N 2 s a t u r a t e d

0 .2

0 .0O>

. -0.213-

-0.4

r u n 1 r u n 2

E~or, (ru n I ) * ,

E , o = ( ru n 2 )= = =

~A

///~ c

10 5

F i g . 3 .

104 10-3Log cur rent dens i ty mA/cm z

T a f e l s l o p e s o f p o l a r i z a t i o n c u r v e s o f F i g . 4 .2 2 0 5 i n N a C l ( 0 . 9 % ) N 2 s a t u r a t e d

1 0 - 2

2 . 0

1 . 8 - -

1 . 6 -1 . 4 -1 . 2 -

LUo 1 . 0 -_ > 0 . 8 -c 0 . 6 -0o . 0 . 4 -

0 . 2 -0 . 0 -

-0.2 - 0 . 4

r u n # 1 r u n # 2

I I I I I I I10-5 10-4 10-3 10-2 10-1 10 o 101 102

Log c u r ren t dens i t y m A /c m zF i g . 4 . P o l a r i z a t i o n c u r v e s o f u n c o u p l e d 2 2 0 5 s t a i n l e s s st e e l .


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72 Platt e t al. American Journal of Orthodontics and Dentofacial OrthopedicsJuly 1997

2205 type stainless teel

316L type stainless teel

Fig. 5. Optical microstructures.

se c o nd s . Th e m ic ro s t ru c tu re w a s p h o to g ra p h e d a t a ma g -nif ica t ion of 250X, wi th an opt ica l microscope .Corrosion Tests

Electrochemical corrosion tests--uncoupled." Two sam-ples of mechanica l ly pol ished 2205 sta in less s tee l and twoof 316L stain less s tee l were subjec ted to potent i odyna micpola r iza t io n tests in a 0.9 wt% NaC1 solu tion , n i t rogenpurged, a t 37 C, to s imula te the ora l environment . 14 Ascanning speed of 1 mV/second, over a potent ia l range of-400 to 1500 mV for 2205 and -700 to 1200 mV for 316Lsta in less s tee l was used . The resul t ing da ta curves wereplo t ted as corrosion potent ia l in V re fe renced to a SCE(sa tu ra t e d c a lo me l e l e c t ro d e ) v e r su s l o g c u r re n t d e n s i ty(mA/cm2).

Electrochemical corrosion tests--coupled: If two dis-s imi la r meta ls a re in contac t wi th each o ther , the m oren o b le m a te r i a l w il l b eh a v e in a c a th o d ic ( n o b l e ) ) 5 Wh e na 2205 or thodont ic bracke t i s combined wi th or thodont icarch wire , there i s the possib i l i ty of a ga lvanic (d issimi la r

meta l ) reac t ion when the couple i s exposed to the ora lenvironment . To form a corrosion couple , the rear sur-faces of 2205 and 316L sta in less s tee l samples were jo ine dby spot-weld ing a th in shee t o f 316L be twee n them. Thisspot-welded couple was embedded in to epoxy resin insuch a way tha t pol ished surfaces of each mater ia l wereexposed to the e lec t ro ly te , but the spot welded contac twas ent i re ly covered by resin (Fig . 1).An important fac tor in ga lvanic corrosion is e ffec t ofthe ra t io of the ca thodic and ano dic a reas. An u nfavorablearea ra t io consis ts of a la rge ca thode and a smal l anode .For a g iven current f low in the ga lvanic ce l l , a smal le ranode resul ts in a grea te r current densi ty and hence agrea te r corrosion ra te . a5 On the basis of the resul ts ofindiv idua l pola r iza t io n curves of uncoup led 2205 and316L, i t was found tha t 2205 mat er ia l behaves ca thodicwith respec t to 316L. To study the s urface a rea e ffect of a2205/316L couple , three d i f fe rent ra t ios of surface a reasbe tween 2205 and 316L sta in less s teels were tested : (1) 1 :1( i .e. , bo th mater ia ls had equa l exp osure a rea) , (2) 1 :0.5


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American Journal of Orthodontics and D entofacial O rthopedics Platt et aL 73Volume 112 , N o . 1

3 1 6 L i n 0 . 9 % N a C I s a t u r a t e d w i th N 2, 2

1.0 run 10.8 _ r0.60.4

>~ .2~ _ 0 . 0

-0.2 -0 . 4 , .

m-0.6

I I I ~ I I I10-5 10-4 10-3 10-2 10-1 10 0 101 10 2

Log current density mA/cm 2Fig. 6. Polarization curves of uncoupl ed 316L stainless steel.

2 2 0 5 1 3 16 L , N o M a s k ,i n N a C I ( 0 . 9 % ) N 2 s a t u r a t e d

LU(D>. m

"60 .

1.41.21.00.80.60.40.20.0

-0.2-0.4-0.6

= run 3 run 4 run 5 run 6

I I I I ~ I I I I I10 5 10 4 10 3 10 2 10 1 100 101 102 103

Log current density mA/cm zFig. 7. Polarization curves of coupled sampl e with equal surface area.


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7 4 Platt e t al . Am erican Journal of Orthodontics an d Dentofacial O rthopedicsJuly 1997

2 2 0 5 /3 1 6 L , N o M a s k ,i n N a C I ( 0 . 9 % ) N 2 s a t u r a t e d

LUL~o9>. 1 . . ,t-

OO .

- 0 .2

-0 .3

-0 .4

-0 .5

-0 .61 0 5

1 . run 3 Lr un 4 run 5* r un 6

A ~& 0

0

" . . A "

% . %J t i i l l t j J i i = = l l I i i J p ~ l l l I I i =

1 0 -4 1 0 -3 1 0 -2

L o g c u r r e n t d e n s i t y m A / c m 2F i g . 8 . T a f e l s lo p e s o f p o l a r i z a t i o n c u r v e s o f F i g . 7 .

Table II . Corrosion rates

Maten I c o r r(vJl/cm2) Corrosion Figurera te (MPY ) no .2205 run i 0.8 0.370 5, 6

run 2 1.0 0.462 5, 6Ave rag e 0.416 +- 0.065316L run 1 1.5 0.693 7

run 2 1.3 0.601 7Ave rag e 0.647 --- 0.0652205/ 316L r un 3 0.45 0.208 10, 11

(1:1 ) run 4 0.55 0.254 10, 11run 5 0.50 0.231 10, 11run 6 0.75 0.346 10, 11

Average 0 . 2 6 0 0.0602205/316L run 1 1.5 0.693 12

(1:0.25) run 2 0.9 0.416 12run 3 1.0 0.462 12

Ave rag e 0.524 -+ 0.1482205/316L run 1 0.9 0.416 13

(1:0.5) run 2 1.2 0.554 13run 3 1.5 0.693 13

Av era ge 0.554 -- 0.139

( i .e ., ha lf o f 316L s ta in less s tee l was masked ) , and (3 )1 :0.25 ( i. e. , th re e q uar te r s o f 316L s ta in less s tee l wasm a s k e d ) , a s s e e n in F ig . 2 . Ma s k in g w a s a c c o m p l i s h e dw i th p l a s t i c t a p e . T h e c o r ro s io n p o te n t i a l w a s m o n i to re da n d re c o rd e d . T h e n th e c o u p le d s a m p le s w e re s u b je c t e dto s c a n n in g p o la r i z a tio n t e s t in g in th e s a m e m a n n e r a s th eu n c o u p le d s a m p le s .

E s t i m a t i o n o f c o r r o s io n r a te s : R e fe r r in g to F ig . 3 ,w h ic h i s e n la rg e d f ro m th e c a th o d ic a n d a n o d ic p o la r i z a -t io n p o r t io n s o f F ig . 4 , t h e c o r ro s io n c u r re n t , I . . . . i sr e l a t e d to th e s lo p e o f t h e p lo t t h ro u g h th e fo l lo w in gequa t ion16:

I . . . . = [3a[3C/{2"3([3A + [3C)} 8 E/ 8I (1 )w h e r e g E / g I i s s lope o f the po la r iza t ion res is tance p lo t ,13A, 13 is ano dic an d cath od ic Tafe l constan ts , an d Ico,~ isc o r ro s io n c u r re n t 0 x A ) .T h e c o r ro s io n c u r re n t , I . . . . c a n b e o b ta in e d g ra p h i -ca l ly as seen in F ig . 2 by f ind ing an in te rsec t ing p o in t o f [~aa n d [3 s lo pe s . T h e c o r ro s io n c u r re n t c a n b e r e l a t e d


7/11

American Journal of Orthodont ics and Dentofac ial Orthopedics P l at t e t al . 75Volume 112,No. 122051316L,314 M AS K,in Na Cl (0 .9%) Nasa tura ted

1 .41.2 --

1.0 --

0 . 8 - -

UJo 0 . 6 -og>._~ 0 . 4 -C

0 . 2 -0 ,0 - -

- 0 . 2 -

o r u n 1r u n 2r u n 3

z~

- 0 . 4 - o o [] o ~t,~o ~ ~ ; ~- 0 . 6 I I

1 0 - 5 1 0 - 4 1 0 - 3 1 0 - 2 1 0 - 1 1 0 0 1 0 1 1 0 2 1 0 3L o g c u r r e n t d e n s i t y m A / c m =

Fig. 9. Polarization curves of coup led sa mple wh en 3/4 of 316L sur face wa s masked.

d i re c t ly to th e c o r ro s io n r a t e th ro u g h th e fo l lo w in g e q u a -t ion ,corros ion ra te (MPY ) = 0 .13 X Ico~ x E .W. /d (2 )w h e r e M P Y i s mil l i - inches pe r yea r , E .W. is equ iva len tw e ig h t o f t h e c o r r o d in g s p e c ie s (g ) , w h ic h is e q u iv a le n t t ot h e a to m ic w e ig h t o f t h e c o r ro d in g e l e m e n t d iv id e d b y th ev a le n c e o f t h e e l e m e n t , d i s d e n s i ty o f t h e c o r ro d in ge le m e n t (g /c m 3) , a n d I . . . e q u a l s c o r ro s io n c u r re n t d e n s i ty(~A/cm2).Immersion crevice corrosion tests . T w e lv e b lo c k s o f3 1 6 L a n d 2 2 0 5 w e re p o l i s h e d o n a l l s ide s . A s s e e n in F ig .2 , t h re e ty p e s o f a rc h w i re s w e re s a n d w ic h e d b e tw e e np a i r s o f 3 1 6 L a n d 2 2 0 5 b lo c k s : t i t a n iu m -m o ly b d e n u m(T MA ) , s t a in l e s s s t e e l (S S ) , a n d t i t a n iu m -n ic k e l (N iT i ) .T w o a s s e m b l ie s w e re p re p a re d fo r e a c h s t a in le s s s t e el a n de a c h ty p e o f a rc h w i re . T h e a s s e m b l i e s w e re h e ld to g e th e rby p last ic c l ips and susp ende d in 0 .9 wt% NaC1 so lu t ion a t37 C fo r 5 weeks .

E a c h s p e c i m e n w a s e x a m i n e d f o r t h e p r e s e n c e o fv i s ib l e g e n e ra l c o r ro s io n a n d e x a m in e d u n d e r a s t e re o p -t i c a l m ic ro s c o p e fo r e v id e n c e o f c re v ic e c o r ro s io n , a

p ro c e s s th a t c a u s e s s e v e re lo c a l i z e d c o r ro s io n r e a d i lyv is ib le a t 40X magnif ica t ion .

T h i s t e s t w a s r e p e a te d tw ic e , r e s u l t in g in fo u r m e a -s u re m e n t s fo r e a c h c o m b in a t io n o f tw o b ra c k e t m a te r i a l s(2205 , 316L) and th ree a r ch wires (TM A, SS, NiTi) .

Corrosion product analysis . T o id e n t i fy th e c ry s t a ll i n es t ru c tu re s , c o r ro s io n p ro d u c t s w e re c o l l e c t e d a n d s u b -j e c t e d to t r a n s m is s io n e l e c t ro n d i f f r a ct io n (T E D ) , b e c a u s et h e a m o u n t o f c o l le c t ed c o r r o s i o n p r o d u c t w a s n o t l a r gee n o u g h to c o n d u c t x - ra y p o w d e r d i f f r a c t io n .T h e a c c e le ra t io n v o l t a g e w a s h e ld c o n s ta n t a t 1 00 kV .T h e d - s p a c in g s o f t h e c o r ro s io n p ro d u c t w e re c a l c u la t e df ro m th e d i a m e te r o f t h e d i f f r a c tio n r in g s. 14

R E S U L T SM i c r o s t r u c t u r a l O b s e r v a t i o n

O p t i c a l m i c r o s t r u c t u r e s c o n f i r m e d t h e m i x e ds t r u c t u r e o f t h e d u p l e x s t a i n l e s s s t ee l ( 2 2 0 5 ) a sc o m p a r e d w i t h t h e a u s t e n i ti c s i n g l e - p h a se 3 1 6 L , a ss e e n i n F i g . 5 . I n 2 2 0 5 , d e l t a - f e r r i t e ( m a r k e d p o o l s )i s p r e c i p i t a t e d i n th e p r i m a r y g a m m a - a u s t e n i t i c m a -


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7 6 PIatt et aL American Journal of Orthodontics and Dentofacial OrthopedicsJuly 1997

22051316L ,1 /2 M AS Kin Na C l (0 .9%) N2satura ted1 .41 .21 .00 .8

UJo 0 .6O9>-~ 0 .4

0 .20 .0

- 0 . 2- 0 . 4- 0 . 6

10 -6

0[]

o run 1o run 2

run 3

/xooO Zx O

O

I t I F I I I I I1 0 -5 1 0 - 4 10 -3 10 -2 10 -1 10 0 101 10 2 10 3

Log current density mA/cm zF ig . 1 0 . P o l a ri za t io n c u rv e s o f c o u p l e d s a m p l e w h e n h a lf o f 3 1 6 L s u r f a c e w a s m a s k e d .

t r ix . In 316L, ca rb ide ( i .e . , Cr23C3) s p r e c i p i t a t e d i nt h e s i ng l e g a m m a - a u s t e n i t ic m a t r i x p h a s e .C o r r o s i o n P r o d u c t A n a l y s is

B e f o r e e s t i m a t i o n o f c o r ro s i o n r a t e , t h e t e r mE.W. (equ iva len t we igh t ) i n the eq ua t ion (2) must bek n o w n . T o d o so , t h e c o r r o s i o n p r o d u c t m u s t b ei d e n ti f ie d . T h e c o r r o s i o n p r o d u c t s c o l le c t e d f r o m b o t hs t ee l s w e r e r e d d i sh b r o w n a n d h a d t h e s a m e e l e c t r o ndi f f rac t ion pa t t e rns . A l l d i f f rac t ion li nes were iden t i f i edas be longing to F eO (O H) . 17 Thi s re su l t i s expec ted forFe -bas ed a l loys sub jec t ed to a re l a t ive ly mi ld aq ueou sc o r r o s i v e e n v i r o n m e n t . H e n c e t h e v a l u e o f E . W . =M /n = 55.85/2 = 27.93 (g) , an d dens i ty of i ron, d, =7.86 g/cm3.

Electrochemical corrosion test-- unc oup led. Figs .4 a n d 6 sh o w t h e p o l a r i z a t i o n c u r v e s o f 2 2 05 a n d3 1 6 L , r e sp e c t i v e l y . I t w a s f o u n d t h a t ( 1 ) c a t h o d i ca n d a n o d i c p o l a r i z a t i o n b e h a v i o r s o f b o t h s t e e l sa r e v e r y s i m i l a r to e a c h o t h e r u p t o t h e c u r r e n td e n s i t y o f 0 .1 m A / c m 2 ( a t a b o u t 4 0 0 m V ) , a n d ( 2 )

b e y o n d t h i s p o i n t , 3 1 6 L s t a i n l e s s s t e e l s h o w e de a r l i e r t r a n s p a s s i v a t i o n t h a n 2 2 0 5 s t a i n l e s s s t e e l ,a l t h o u g h a c l e a r p a s s i v a t i o n s t a g e w a s n o t i d e n t i -f i e d i n e i t h e r s t e e l . I n t h e t r a n s p a s s i v a t i o n r e g i m e ,t h e o x i d e ( p a s s i v e ) f i l m s t a r t s t o d i s so l v e , i n d i c a t -i n g th a t t h e m e t a l s u b s t r a t e i s n o l o n g e r p r o t e c t e df r o m t h e e n v i r o n m e n t b y t h e p a s s i v e fi lm . T o f in dt h e c o r r o s i o n c u r r e n t ( I c o r r ) , t h e c a t h o d i c t o a n -o d i c t r a n s i t i o n w a s r e p l o t t e d , a s s e e n i n F i g. 2 fo r2 2 05 s t e e l . T h e c a l c u l a t e d c o r r o s i o n r a t e s i n m i ll i-i n c h e s p e r y e a r ( M P Y ) a r e l i s t e d i n T a b l e I I . I tw a s f o u n d t h a t t h e c o r r o s i o n r a t e o f 2 2 05 w a s0 . 4 1 6 + _ 0 . 0 6 5 M P Y , w h i c h i s su p e r i o r t o 0 . 6 4 7 +_0 . 0 6 5 M P Y f o r 3 1 6 L .Electrochemical corrosion test-- cou pled . 1.E q u a l su r f a c e a r e a r a t i o o f 2 2 0 5 /3 1 6 L c o u p l e : F i g s.7 a n d 8 sh o w p o l a r i z a t i o n c u r v e s o f a 2 2 0 5 / 3 1 6 Lc o u p l e h a v i n g a n e q u a l s u r f a ce a r e a r a ti o . R e f e r r i n gt o F i g . 8 , i t w a s n o t i c e d t h a t t h e a n o d i c T a f e l s l o p e ,[31 , i s l a rge r t ha n the ca th odic s lope , [3c , sugg es t ingt h a t t h e c o r r o s i o n p r o c e s s i s c o n t r o l l e d b y t h e


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American Journal of Orthodontics and Dentofacial Orthopedics Pl at t et al. 77Volume 112, No. 1

Table III. Results of coupling corrosion testsMaterial Gener al corrosion

Block l~ ire Block 1 Block 2316L TiNi 1 1

1 12 22 2

316L TMA 1 10 02 20 0

316L SS 2 22 21 l1 1

2205 TiNi 1 10 0l 10 0

2205 TM A 0 00 00 00 0

2205 SS 1 00 00 1l 0

Crevice corrosionWire Block 1 Block 2

1 - - - -

1 - - - -

1 - - - -

1 + +1 -- --

1 -- --

1 + +2 + +2 + +1 - +1 +1 -- --

0 - - - -

l - - - -

0 - - - -

0 - - - -

000 - - - -

1 -- --

0 - - - -

1 -- --

1

For general corrosion:0: no corrosion observed.1: corrosion evident in isolated areas.2: corrosion evident in multiple areas.

For crevice corrosion:-: no crevice corrosion.+: crevice corrosion present.

a n o d ic r e a c t i o n . A p r e c ip i t a t e d f i lm i s f o r m e d o na n o d ic a r e a s b e c a u se o f t h e a n o d e c o n t r o l t y p epola r iza t ion . 18

A s s e e n i n T a b le I I , f o r t h e c o u p le d d a t a w i the q u a l a r e a s , t h e c a l c u l a t e d v alu e s o f c o r r o s io n r a t ef o r r u n s 3 t h r o u g h 6 a g r e e d w e ll . T h e c o r r o s io n r a t eof 0 .260 __ 0 .060 MPY for 2205 in the 2205/316Lcouple was much le ss than 0 .416 _+ 0 .065 MPY foruncoupled 2205. I t i s sugges ted tha t the le ss noble316L s ta in less s tee l se rve a s a sac r i f ic ia l anode topro tec t the 2205 s ta in less s tee l .2 . U n e q u a l su r f a c e a r e a r a t i o o f 2 2 0 5 /31 6 L c o u -ple : F ig . 9 shows pola r iza t ion curves of 2205/316Lcouples when the sur face a rea ra t io i s 1 :0 .25;whereas Fig. 10 represents polar ization curves of cou-ples when the surface area ra tio is 1:0.5. The results ofcalculated corro sion ra tes are a lso listed in Tab le I I . I twas fou nd th at th ere is no signif icant difference incor ros ion ra te s for both unequa l sur face a rea ra t ios(i.e. , 0.524 _+ 0.148 MPY for surface area ratio of1:0.25 and 0.554 +_ 0.139 M PY for 1:0.5). Th is findingdoes not agree wi th the genera l ly accepted concept of

Fig. 11. General v iew of crevice corrosion tested sam-ples.

the surface area effect .15 On e of the possible rea sonsfor this might be the fact that the surface of the 316Lstainless steel a t the edge of the masking tape wasseverely a ttacked by localized crevice corrosion. Con-sequently , a ma jor i ty of cor ros ion cur ren t was concen-t ra ted on these c rev ice cor ros ion s i te s r a the r thandis t r ibu ted ov er the un covered sur face .Immersion Crevice Corrosion Test

Fig. 11 shows typical general views of crevicecor ros ion te s t couples wi th wires . Af te r d issembl ing


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78 Platt et aL American Journal of Orthodontics and Dentofacial OrthopedicsJuly 1997

ar e a c ove r e d wi th wir e

Fig. 12. Crevice corrosion on 316L stainless steel, asmarked on Fig. 11.

the test couples, the extent of general corrosion wasevaluated and scored in three ranks, as indicated inTable III. The 2205 stainless steel exhibited bettercorrosion resistance in the immersion corrosiontests. After sampling the corrosion products, bothsurfaces and wires were cleaned with alcohol anddistilled water. I t was found that the surface of 2205was contaminated with corrosion products formedfrom the wire materials, with little evidence ofcorrosion of the 2205. Then the surfaces wereexamined for crevice corrosion. Fig. 12 was takenfrom the area marked with an arrow mark on Fig.11. Fig. 12 shows crevice corrosion of 316L on areaswhere the arch wire was in contact. Intense localizedcorrosion frequent ly occurs within crevices andother shielded areas on metal surfaces exposed tocorrosives. This type of attack is usually associatedwith small volumes of stagnant solution caused by

holes, gasket surfaces, lap joints, surface deposits,and crevices unde r bol t and rivet hea dsJ 5 It isbelieved that crevice corrosion results from differ-ences in metal ion or oxygen concentration betweenthe crevice and its surroundings. Under the depositarea, oxygen depletion takes place. After oxygen isdepleted, no fur ther oxygen reduc tion occurs, al-though the dissolution of metal continues. Thistends to produce an excess of positive charge in thesolution that is balanced by the migration of chlo-ride ions into the crevice. This results in an in-creased concentration of metal chloride with a pHvalue as low as 3 to 4 within the crevice. As thecorrosion within the crevice increases, the rate ofoxygen reduction on adjacent surfaces increases.This cathodically protects the external surfaces.Thus dur ing crevice corrosion, the a ttack is localizedwithin shielded areas, while the remaining surfacesuffers little or no damage. 15

The right column of Table III shows the resultsof crevice corrosion with " - " and " + " notations.None of the 2205 samples exhibit crevice corrosion,whereas the 316L specimens did, as seen in Fig. 12.DISCUSSION

The use o f 2205 duplex stainless steel in place of316L appears to result in lower corrosion in asimulated oral environment. However, a corrosioncouple alloy may dramatically decrease this benefit.In particular, NiTi in contact with 2205 produces areduction in corrosion resistance.

The results of the uncoupled electrochemicalcorrosion tests are consistent with previous findings.Cigada performed electrochemical corrosion testson several stainless steel wires containing Cr, Ni,and Mo in physiologic solutions at 40 C. 19 Duplexstainless Steel exhibited a passivation range from-200 to 650 mV, results very similar to those ob-tained in the current study.

As seen clearly in Fig. 7, the 316L stainless steeldid not show a clear passivation under the currenttesting conditions and it is obvious that 2205 stain-less steel exhibits much better corrosion resistanceagainst 37 C, 0.9 wt% NaCI aqueous solution.If two dissimilar materials are in electrical con-tact, the materials should behave differently thanwhen they were exposed to corrosion media sepa-rately. It is also generally believed that the magni-tude of this galvanic corrosion depends on the ratioof exposed surface area of the two materials. Hence,tests in this study were designed to simulate theexposed surface areas in contact when an arch wirelies in the slot of an orthodontic bracket.


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American Journal of Orthodontics and Dentofacial Orthopedics Plat t e t a l . 79Volume 1 1 2 , N o . 1C O N C L U S I O N S

O u r f i n d i n g s i n d i c a t e t h a t ( 1 ) 2 2 0 5 s t a i n l e s s s t e e le x h i b i t s b e t t e r c o r r o s i o n r e s i s t a n c e t h a n 3 1 6 L s t a i n l e s ss t e e l ; ( 2 ) w h e n 2 2 0 5 i s c o u p l e d w i t h a l e s s c o r r o s i o nr e s i s t a n t m a t e r i a l ( 3 1 6 L s t a i n l e s s s t e e l ) , 2 2 0 5 s t a i n l e s ss t e e l s h o w s d e c r e a s e s c o r r o s i o n ; a n d ( 3 ) t h i s t r e n d i sr e d u c e d w h e n t h e e x p o s e d s u r f a c e a r e a s h a v e a a n o d i c /c a t h o d i c r a t i o le s s t h a n 1 a n d t h a t 2 2 0 5 , u n l i k e 3 1 6 L , i s n o ts u b j e c t t o c r e v i c e c o r r o s i o n .

F r o m t h e s t a n d p o i n t o f c o r r o s i o n r e s i s t a n c e , t h e u s eo f 2 2 0 5 a s a n o r t h o d o n t i c b r a c k e t m a t e r i a l s e e m s t o b ej u s t i fi e d w h e n t h e a r c h w i r e m a t e r i a l i s s ta i n l e s s s t e e l o rt it a n i u m . U s e o f t h is a l l o y c o u l d d e c r e a s e t h e a m o u n t o fc o r r o s i o n p r o d u c t s t o w h i c h a p a t i e n t w o u l d b e e x p o s e dt h a t c o u l d m i n i m i z e n i c k e l a l l e r g y p r o b l e m s p o t e n t i a l l ya s s o c i a te d w i t h o r t h o d o n t i c t r e a t m e n t .

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A A O1 9 9 81 9 9 92000

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M E E T I N G C A L E N D A R- - D a l la s , T e x a s , M a y 1 6 t o 2 0 , D a l la s C o n v e n t i o n C e n t e r- - S a n D i e g o , C a li f. , M a y 1 5 t o 1 9 , S a n D i e g o C o n v e n t i o n C e n t e r- - C h i c a g o , II1 ., A p r il 2 9 t o M a y 3 , M c C o r m i c k P l a c e C o n v e n t i o n C e n t e r

( 5 th I O C a n d 2 n d M e e t i n g o f W F O )- - T o r o n t o, O n t a r io , C a n a d a , M a y 5 t o 9 , T o r o n t o C o n v e n t i o n C e n t e r- - B a l t i m o r e , M d . , A p r i l 2 0 t o 2 4 , B a l t i m o r e C o n v e n t i o n C e n t e r- - H a w a i i a n I s l a n d s, M a y 2 t o 9 , H a w a ii C o n v e n t i o n C e n t e r- - O r l a n d o , F l a . , M a y 1 t o 5 , O r l a n d o C o n v e n t i o n C e n t e r

articulo de corrosion dra liz

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