Corrosion and Creep of Dental Amalgam N II,s R GJcEKRD'ii and SVEIN ESPFvIK
NIOMQ Scandinavian Institutte of J)ental1 Ma trias, For sk-isnv ii. 1, Oslo 3, Norwzy The influence of stress on corrosion w1as c.ovestigated by a potentiostatic technzique. Ancalgrams with low cr ep, both with and wzithout y,-phasc, revealed smnall incrcascs in ciurrent with stress appli'cation while alloys witch hcigh creep exlhibited large increases in current. Rcsp-
ture of protective oxide due to imcroased stracn for high creepl alloys may exrplaic the pcc-
n1onenoun.
J Dent Res 57(1) :21-26 January 1978. Examination of amalgam restorations in extracted teeth revealed (a) corrosion of 2, (b) cracking of y/ at miargins.' Slight increases in corrosion potentials on application of stress in vitro have been decnonstrated.2 A correlation between creep rate and marginal breakdowvn has also been shown,3 but a direct cause and effec t relationship has not been established. The purpose of this investigaitioi x as to Received for publication October 4, 1976. Accepted for publication April 15, 1977. * Ivoclar A. G. Schaan, Lichtenstein.
Silamat®,
t Epofixti, Struers Scientific Instruments, Copenhagen, Denmark. $ Plastik 701cc, Kontakt Chemie, Rastatt, W.-Ger-
many.
FIG 1. Specinien uised in the studies.
ex alate the influence of stress oni corrosioi lsy
applying stress to the sample xxhen- potenitiostatic mciteasuaremients xxuere made, anid to cornelate the r-esuilts of the measuremnents with eeep rate and inir rostruicture.
MNaterials and Mlethods The five amialgams listed in Table I xsere used in the inv estigation. They xx ere ti iturated for 5 s in a mec hanical amialgamrator* and conidensed b)v hand in a stainless steel imiold prodlucing Specilens of a type designed for tensile tests. The tritoration time uisecd in thiese expeiliments iay cliffhr f romii those gixenLb the osart-ii facturer. The (lifTec-encies batx veen the alloys icav therefore be act entuated. At least 4 specimiiens wvere made froi eac h 1)1rand anid they xere stored foc I xes k at 37C(. Elcctrical leads were attta(bed and the ends of the spetciens xx cinebdded in epoxy resi'n Li I The srinfac es of the spec ecinins xvvre polished xvitih a 11111er c up and puniche f-lloxx\edlxd 1 insing( in distilled xw ater anid ethainol. At the interfacc betxxween the amialgam anad the resin, a plastic lIa(lurt. xxas applied to present leakage. 1The siri-
74
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21
GJERDET
22
&
J Dent Res January 1978
ESPEVIK
TABLE 1 INVESTIGATED AMALGAM ALLOYS Code
Batch No.
Hg/Alloy Ratio
74108
0.85:1.00
5 C 206
0.85:1.00
D
VS 31
0.86:1.00
H
Revalloy® S. S. White Ltd., Harrow, England
117234
1.10:1.00
R
Standalloy® F Degussa, Frankfurt, W. Germany
20041
1.00:1.00
S
Product Manufacturer
Caulk® Spherical Alloy The L. D. Caulk Co., Toronto, Canada Dispersalloy®
Johnson & Johnson Dental Products Co. New Jersey, USA Hi-Atomic®
G-C Dental Industrial Corp.
Tokyo, Japan
TABLE 2
TABLE 3
CREEP AT 37 C OF AMALGAMS INVESTIGATED (Two SAMPLES FOR EACH ALLOY, MEAN AND STANDARD DEVIATION)
CREEP OF ONE AMALGAM AS FUNCTION OF TEMPERATURE AND HEAT TREATMENT ON Two SAMPLES FOR EACH TEST CONDITION (MEAN AND STANDARD DEVIATION)
Code C
D H R S
Creep, %
3.10 1 0.03 0.28 + 0.04 0.66 +- 0.10 1.44 + 0.02 6.03 + 0.45
face area of the specimens exposed to the electrolyte was approximately 0.5 cm2 and the dimensions were 2 X 3 X 5 mm. The specimen was suspended in polyamid wires inside a double-walled beaker (Fig 2). One end of the specimen was fixed, while the other polyamid wire passed through a rubber diaphragm, across a pulley to a load which rested upon a laboratory jack. Lowering the jack subjected the sample to a stress of approximately 18 MPa. A stress of 1.6 MPa was continuously applied to ensure proper position of the specimen. The corrosion cell had a platinum counter electrode and a calomel reference electrode with a salt bridge. The electrodes and the specimen were connected to a potentiostat§ equipped with a scanning potentiometer.** Current § Wenking Model PCA 72 L®, G. Bank Elektronik, Gbttingen, W. Germany. ** Wenking Model SMP 72Rx,, G. Bank Elektronik, Gottingen, W. Germany.
Code
Condition
Creep, %
S S
23 C 37 C 37 C
1.09 ± 0.06 6.03 + 0.45
S
S
Heat treated 50 C
-
1.0* 18.9 (extrapolated from 3-hour test)
* From references 5 and 6.
and potential were continuously recorded on a strip chart recorder. The test solution used was an artificial saliva described by Fusayama and co-workers.4 The temperature of the solution was 37 C and it was stirred by a magnetic stirrer. After immersion of the specimen in the test solution, the open-cell potential was allowed to reach a stable value. A potential of -250 mV was then set up against the calomel electrode and the current to maintain this potential was recorded. When the current had decreased to a nearly constant value, the sample was loaded for 30 minutes, then unloaded. Polarization curves were recorded by scanning the potential from -250 mV in the positive direction with steps of 1 mV/s. At least 3 curves
Downloaded from jdr.sagepub.com at PENNSYLVANIA STATE UNIV on May 26, 2015 For personal use only. No other uses without permission.
Vol. 57 No. I
CORROSION & CREEP
were recorded from each specimen, alternately in the loaded and unloaded condition. One alloy was subjected to tests at 23 C, 37 C, and 50 C. In addition, one sample which had been heat treated at 70 C for 7 days was tested at 37 C. After tests had been performed, specimens were polished and examined in a scanning electron microscopet with a detector for back-scattered electrons. The y2 was determined on 4 randomly selected areas covering about 1/,50 of the cross section. Identification of the y, phase was done by an energy dispersive detector.$ Measurements of static creep were performed on cylindrical specimens which had been stored for 1 week at 37 C. The specimens were 4 mm in diameter and 7 mm long; the test temperature was 37 C 4 0.2 C. A compressive stress of 36 MPa was applied axially and the lengths were measured at 1 hour and 4 hours after loading and the differences in the readings expressed as percentages of the original lengths. t Jeol 50A®, Tokyo, Japan. t EDAX®, Prairie View, I1.
23
Results
The amalgams exhibited different degrees of increase in corrosion current when stress was applied (Fig 3). One-way analysis of variance was performed on the increases in current after 30 minutes of load application. Statistically significant differences between the allovs at the 95% confidence level were established except between alloys C and S, and H and D. The amalgams appeared to be re-passivated after the load had been removed. The creep rates of the amalgams are given in Table 2. A qualitative comparison between creep rates and increase in corrosion current showed that the largest increase in corrosion current was observed on high creep alloys. The increase in current due to stress application was reduced with reduction in temperature and as a function of heat treatment (Fig 4). In Table 3, creep rates as a function of corresponding temperatures and heat treatment were listed, indicating that alloy S exhibited high increase in corrosion current on stress application when the creep rate was high.
-
CALOMEL REFERENCE ELECTRODE
STIRRER
FIG 2. -Corrosion cell. Downloaded from jdr.sagepub.com at PENNSYLVANIA STATE UNIV on May 26, 2015 For personal use only. No other uses without permission.
GJERDET & ESPEVIK
24 6
r
I
'
1S_
6
5
I
E 0
4
LOADED
w 0
z
w
z 0 02
03
0 0
0 _
H
02
-
1
I
^~
==_~~~~~~~~~~~~~~
0
D
o I' 60 45 30 15 0 TIME, minutes I
1 u
I
6
9
12
I
15
FIG 4.-Effect of temperature and heattreatment on the corrosion current during loading of alloy S.
pAcm2
CURRENT
I
:3
TIME, minutes
FIG 3. Influence of loading on the current required to maintain -250mV across the specimen. The curves represent mean increases in current for the specimens. Standard deviations on the increases in current at 1, 15, and 30 minutes of load application are shown by vertical lines. The dotted lines represent the mean current for three specimens prior to loading.
if
Heat treated
L~~~~~~~~
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1
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.
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.
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j Dent Res January 1978
20
,
, UNLOADED
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LOADED
DO1i
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1
50 _
TI
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-24
FIG 5.-Example of polarization curves obtained from a specimen of alloy C in the loaded condition and unloaded condition.
00 _ I,
'50--II
-2!
aS I
,
,
Downloaded from jdr.sagepub.com at PENNSYLVANIA STATE UNIV on May 26, 2015 For personal use only. No other uses without permission.
Vol. 57 No. I
CORROSION & CREEP
The chan,gc in crteep rate xn-ihlIheat tIeatment has been explained e lses hei e. I 1st conditiotos wNhich shosvecl loNv increase in corrosion current also demonstrated Ionv creep rates. Determ-lination of y.2 contenit shloxs ed that
specimIIe(lls mTlacle of alloy S, whichI is a high crecp allov, contained 3.3',- "/,-phase, whlile a heat-treated spectimnct of the same alloy conitained 4.2%. Specimnenis nmade of alloy II, which exhibit low reep conitained 3.2% Y2phase. The c hanges in corrosioni currents ( oulci
not be associated xsith different amoutnts of
7.-phase in tie anial-yas. ilxamliples of the 1i-0-
crostructure are shoxs-n in Figure 6. Although thetre vere conside able x atiatious in the re.sults, the potentiostatic curves indicate that thle breakdown potentials xvere more negative in the loaded conidition comparedx: .ith the unloadleid colditions (Fig5- ). Discussion
T1'he exceleiet ort rosioln resistatic e of clenita l. amalgarms has been showxn to be chic to a
protective oxide filin.7 When masticatory stresses are applied to a restoration the fili
25
n-iax lupture c1e to stirai. Dental amalglouims a1it Un11iciLIe 1)'eaiiIS(' tllhe'- 11MV e(Xhibit ctreep lates as hio11 as 5 If) I main-Iat a tensile stress of 19.6 MPa at 37 C' inidicating substantial per111anent deforimiationis at losv loads.8 Pmrcxious studclies haxe shoxxwn tlhat the corrosioim cuii rent p)aiik at approxiimatelyx-100 milV can be associated xx itlh the lmriakcloxsVi of patss itx of thle .phase. In thlis Sttluc the] inlcrease in corrosion cLrrent obserxed on the polarization curves also occurs at approximlately -100 lmY, indicating passix ity breakdoxs ii of the 'mQ )hase. rIlie chamixtes io c orrosoni c urclt with loath application acld the shiftini of the potentials for passixitv lreakdoxwn toxawacl iore neat(ratixe xvadtes x etc themefoie associatecd sith inereasecd 1)b eakd(cxsn of protect ixv filho otn the da
NIOMQ Scandinavian Institutte of J)ental1 Ma trias, For sk-isnv ii. 1, Oslo 3, Norwzy The influence of stress on corrosion w1as c.ovestigated by a potentiostatic technzique. Ancalgrams with low cr ep, both with and wzithout y,-phasc, revealed smnall incrcascs in ciurrent with stress appli'cation while alloys witch hcigh creep exlhibited large increases in current. Rcsp-
ture of protective oxide due to imcroased stracn for high creepl alloys may exrplaic the pcc-
n1onenoun.
J Dent Res 57(1) :21-26 January 1978. Examination of amalgam restorations in extracted teeth revealed (a) corrosion of 2, (b) cracking of y/ at miargins.' Slight increases in corrosion potentials on application of stress in vitro have been decnonstrated.2 A correlation between creep rate and marginal breakdowvn has also been shown,3 but a direct cause and effec t relationship has not been established. The purpose of this investigaitioi x as to Received for publication October 4, 1976. Accepted for publication April 15, 1977. * Ivoclar A. G. Schaan, Lichtenstein.
Silamat®,
t Epofixti, Struers Scientific Instruments, Copenhagen, Denmark. $ Plastik 701cc, Kontakt Chemie, Rastatt, W.-Ger-
many.
FIG 1. Specinien uised in the studies.
ex alate the influence of stress oni corrosioi lsy
applying stress to the sample xxhen- potenitiostatic mciteasuaremients xxuere made, anid to cornelate the r-esuilts of the measuremnents with eeep rate and inir rostruicture.
MNaterials and Mlethods The five amialgams listed in Table I xsere used in the inv estigation. They xx ere ti iturated for 5 s in a mec hanical amialgamrator* and conidensed b)v hand in a stainless steel imiold prodlucing Specilens of a type designed for tensile tests. The tritoration time uisecd in thiese expeiliments iay cliffhr f romii those gixenLb the osart-ii facturer. The (lifTec-encies batx veen the alloys icav therefore be act entuated. At least 4 specimiiens wvere made froi eac h 1)1rand anid they xere stored foc I xes k at 37C(. Elcctrical leads were attta(bed and the ends of the spetciens xx cinebdded in epoxy resi'n Li I The srinfac es of the spec ecinins xvvre polished xvitih a 11111er c up and puniche f-lloxx\edlxd 1 insing( in distilled xw ater anid ethainol. At the interfacc betxxween the amialgam anad the resin, a plastic lIa(lurt. xxas applied to present leakage. 1The siri-
74
Vol. 57 No. I Downloaded from jdr.sagepub.com at PENNSYLVANIA STATE UNIV on May 26, 2015 For personal use only. No other uses without permission.
21
GJERDET
22
&
J Dent Res January 1978
ESPEVIK
TABLE 1 INVESTIGATED AMALGAM ALLOYS Code
Batch No.
Hg/Alloy Ratio
74108
0.85:1.00
5 C 206
0.85:1.00
D
VS 31
0.86:1.00
H
Revalloy® S. S. White Ltd., Harrow, England
117234
1.10:1.00
R
Standalloy® F Degussa, Frankfurt, W. Germany
20041
1.00:1.00
S
Product Manufacturer
Caulk® Spherical Alloy The L. D. Caulk Co., Toronto, Canada Dispersalloy®
Johnson & Johnson Dental Products Co. New Jersey, USA Hi-Atomic®
G-C Dental Industrial Corp.
Tokyo, Japan
TABLE 2
TABLE 3
CREEP AT 37 C OF AMALGAMS INVESTIGATED (Two SAMPLES FOR EACH ALLOY, MEAN AND STANDARD DEVIATION)
CREEP OF ONE AMALGAM AS FUNCTION OF TEMPERATURE AND HEAT TREATMENT ON Two SAMPLES FOR EACH TEST CONDITION (MEAN AND STANDARD DEVIATION)
Code C
D H R S
Creep, %
3.10 1 0.03 0.28 + 0.04 0.66 +- 0.10 1.44 + 0.02 6.03 + 0.45
face area of the specimens exposed to the electrolyte was approximately 0.5 cm2 and the dimensions were 2 X 3 X 5 mm. The specimen was suspended in polyamid wires inside a double-walled beaker (Fig 2). One end of the specimen was fixed, while the other polyamid wire passed through a rubber diaphragm, across a pulley to a load which rested upon a laboratory jack. Lowering the jack subjected the sample to a stress of approximately 18 MPa. A stress of 1.6 MPa was continuously applied to ensure proper position of the specimen. The corrosion cell had a platinum counter electrode and a calomel reference electrode with a salt bridge. The electrodes and the specimen were connected to a potentiostat§ equipped with a scanning potentiometer.** Current § Wenking Model PCA 72 L®, G. Bank Elektronik, Gbttingen, W. Germany. ** Wenking Model SMP 72Rx,, G. Bank Elektronik, Gottingen, W. Germany.
Code
Condition
Creep, %
S S
23 C 37 C 37 C
1.09 ± 0.06 6.03 + 0.45
S
S
Heat treated 50 C
-
1.0* 18.9 (extrapolated from 3-hour test)
* From references 5 and 6.
and potential were continuously recorded on a strip chart recorder. The test solution used was an artificial saliva described by Fusayama and co-workers.4 The temperature of the solution was 37 C and it was stirred by a magnetic stirrer. After immersion of the specimen in the test solution, the open-cell potential was allowed to reach a stable value. A potential of -250 mV was then set up against the calomel electrode and the current to maintain this potential was recorded. When the current had decreased to a nearly constant value, the sample was loaded for 30 minutes, then unloaded. Polarization curves were recorded by scanning the potential from -250 mV in the positive direction with steps of 1 mV/s. At least 3 curves
Downloaded from jdr.sagepub.com at PENNSYLVANIA STATE UNIV on May 26, 2015 For personal use only. No other uses without permission.
Vol. 57 No. I
CORROSION & CREEP
were recorded from each specimen, alternately in the loaded and unloaded condition. One alloy was subjected to tests at 23 C, 37 C, and 50 C. In addition, one sample which had been heat treated at 70 C for 7 days was tested at 37 C. After tests had been performed, specimens were polished and examined in a scanning electron microscopet with a detector for back-scattered electrons. The y2 was determined on 4 randomly selected areas covering about 1/,50 of the cross section. Identification of the y, phase was done by an energy dispersive detector.$ Measurements of static creep were performed on cylindrical specimens which had been stored for 1 week at 37 C. The specimens were 4 mm in diameter and 7 mm long; the test temperature was 37 C 4 0.2 C. A compressive stress of 36 MPa was applied axially and the lengths were measured at 1 hour and 4 hours after loading and the differences in the readings expressed as percentages of the original lengths. t Jeol 50A®, Tokyo, Japan. t EDAX®, Prairie View, I1.
23
Results
The amalgams exhibited different degrees of increase in corrosion current when stress was applied (Fig 3). One-way analysis of variance was performed on the increases in current after 30 minutes of load application. Statistically significant differences between the allovs at the 95% confidence level were established except between alloys C and S, and H and D. The amalgams appeared to be re-passivated after the load had been removed. The creep rates of the amalgams are given in Table 2. A qualitative comparison between creep rates and increase in corrosion current showed that the largest increase in corrosion current was observed on high creep alloys. The increase in current due to stress application was reduced with reduction in temperature and as a function of heat treatment (Fig 4). In Table 3, creep rates as a function of corresponding temperatures and heat treatment were listed, indicating that alloy S exhibited high increase in corrosion current on stress application when the creep rate was high.
-
CALOMEL REFERENCE ELECTRODE
STIRRER
FIG 2. -Corrosion cell. Downloaded from jdr.sagepub.com at PENNSYLVANIA STATE UNIV on May 26, 2015 For personal use only. No other uses without permission.
GJERDET & ESPEVIK
24 6
r
I
'
1S_
6
5
I
E 0
4
LOADED
w 0
z
w
z 0 02
03
0 0
0 _
H
02
-
1
I
^~
==_~~~~~~~~~~~~~~
0
D
o I' 60 45 30 15 0 TIME, minutes I
1 u
I
6
9
12
I
15
FIG 4.-Effect of temperature and heattreatment on the corrosion current during loading of alloy S.
pAcm2
CURRENT
I
:3
TIME, minutes
FIG 3. Influence of loading on the current required to maintain -250mV across the specimen. The curves represent mean increases in current for the specimens. Standard deviations on the increases in current at 1, 15, and 30 minutes of load application are shown by vertical lines. The dotted lines represent the mean current for three specimens prior to loading.
if
Heat treated
L~~~~~~~~
z
0
1
50 C
/
I
I
LJ
.
.
.
r23 C 37^C
/ /
z
j Dent Res January 1978
20
,
, UNLOADED
5
,0
0 cc
>
LOADED
DO1i
1(
1
50 _
TI
il
LL
0
-24
FIG 5.-Example of polarization curves obtained from a specimen of alloy C in the loaded condition and unloaded condition.
00 _ I,
'50--II
-2!
aS I
,
,
Downloaded from jdr.sagepub.com at PENNSYLVANIA STATE UNIV on May 26, 2015 For personal use only. No other uses without permission.
Vol. 57 No. I
CORROSION & CREEP
The chan,gc in crteep rate xn-ihlIheat tIeatment has been explained e lses hei e. I 1st conditiotos wNhich shosvecl loNv increase in corrosion current also demonstrated Ionv creep rates. Determ-lination of y.2 contenit shloxs ed that
specimIIe(lls mTlacle of alloy S, whichI is a high crecp allov, contained 3.3',- "/,-phase, whlile a heat-treated spectimnct of the same alloy conitained 4.2%. Specimnenis nmade of alloy II, which exhibit low reep conitained 3.2% Y2phase. The c hanges in corrosioni currents ( oulci
not be associated xsith different amoutnts of
7.-phase in tie anial-yas. ilxamliples of the 1i-0-
crostructure are shoxs-n in Figure 6. Although thetre vere conside able x atiatious in the re.sults, the potentiostatic curves indicate that thle breakdown potentials xvere more negative in the loaded conidition comparedx: .ith the unloadleid colditions (Fig5- ). Discussion
T1'he exceleiet ort rosioln resistatic e of clenita l. amalgarms has been showxn to be chic to a
protective oxide filin.7 When masticatory stresses are applied to a restoration the fili
25
n-iax lupture c1e to stirai. Dental amalglouims a1it Un11iciLIe 1)'eaiiIS(' tllhe'- 11MV e(Xhibit ctreep lates as hio11 as 5 If) I main-Iat a tensile stress of 19.6 MPa at 37 C' inidicating substantial per111anent deforimiationis at losv loads.8 Pmrcxious studclies haxe shoxxwn tlhat the corrosioim cuii rent p)aiik at approxiimatelyx-100 milV can be associated xx itlh the lmriakcloxsVi of patss itx of thle .phase. In thlis Sttluc the] inlcrease in corrosion cLrrent obserxed on the polarization curves also occurs at approximlately -100 lmY, indicating passix ity breakdoxs ii of the 'mQ )hase. rIlie chamixtes io c orrosoni c urclt with loath application acld the shiftini of the potentials for passixitv lreakdoxwn toxawacl iore neat(ratixe xvadtes x etc themefoie associatecd sith inereasecd 1)b eakd(cxsn of protect ixv filho otn the da
