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In vitro corrosion behavior of four Ni−Cr dental alloys in lactic acid and sodium chloride solutions
In vitro corrosion behavior of four Ni-Cr dental alloys in lactic acid and sodium chloride solutions
J. Geis-Gerstorfer, H. Weber University of Tflbingen, West Germany
Geis-Gerstorfer J, Weber H. i n vitro corrosion behavior of four Ni-Cr dental alloys in lactic acid and sodium chloride solutions. Dent Mater 1987: 3: 289295. Abstract - This investigation was carried out to study the in vitro corrosion behavior of 4 nickelchromium alloys in different lactic acid and sodium chloride solutions. Electrochemical techniques like potentiodynamic potential curves and potential time curves were used to analyse the characteristics of these alloys. Substance loss was measured analytically (AAS) to determine kind and quantity of dealloying elements. The results reveal that Ni-Cr alloys currently used in dentistry show a broad spectrum of corrosion resistance (depending on their chemical composition), the best being equal to those of precious alloys, while the worst had up to a thousand times greater substance loss and showed high corrosion currents in the potential area of physiological interest. Ni-Cr alloys containing too small amounts of chromium and molybdenum yielded the worst results and showed a susceptibility to pitting corrosion.
Key words: in vitro corrosion behavior; potentiodynamic measurements; immersion tests; AAS; SEM-studies: potential time curves d. Geis-Gerstorfer, Zentrum far Zahn, Mund, und Kieferkrankheiten, Universit&t Tebingen, Osianderstrasse 2-8, D-7400 T0bingen, West Germany. Received June 4, 1986; accepted February 13, 1987.
The corrosion resistance of dental prostheses is of great importance because of possible biological reactions and because of a possible destruction of the restorations. Concerning metals, most of the allergic reactions are caused by nickel (1). Nickel-chromium alloys, containing 611-80 wt-% nickel, have been developed as alternatives to-more expensive gold alloys. If these base metal alloys are not corrosion resistant enough, products that result from reactions with the oral environment can initiate a sensitization dermatitis (2). To judge the extent of possible allergic sensitization, in vitro and in vivo corro-
sion studies have to discover kind and quantities of dissolved elements and have to characterize the passivity behavior. This study deals with the in vitro corrosion behavior.
Material and methods The corrosion resistance of 4 NickelChromium alloys was determined in different electrolytes by using electrochemical and substance loss measurements. The chemical compositions of these specimcns are given in Table 1. Three specimens per alloy were cast in a casting machine according to the
Table 1. Chemical composition of the alloys (main constituents). wt % Ni Cr Mo Fe Be Ga 19
A Wiron 88
B Microbond NP2
C Euro-Ceram
D Ultratek
61.2 24.7 9.9 1.9 -
66.9 12.5 7.2 5.3 7.1
72.5 23.1
80.1 11.4 2.5 0.07
Dental Materials 3:6, 1987
l.l -
1.7
-
-
techniques used routinely in dental laboratories. After grinding with No. 240 silicon carbide paper the specimens were ultrasonically cleaned for 5 min in aceton and then immersed in the test solution. The electrolytes employed were sodium chloride solutions of 0.02 M, 0.2 M, 1.0 M as well as 0.1 M lactic acid with 0.1 M sodium chloride. Fresh solutions were prepared before each measurement and then de-aerated with nitrogen (99.996%) for 2 h. Solution temperatures were controlled to 37~ + I~ The polarization data were obtained with specimens (10 x 5 • 1 ram) mounted in a specimen holder by squeezing them between platinum wires in order to avoid crevice corrosion (Fig. l). The potentiodynamic scans were initiated 0.25 V below E (Corr) and increased at a step rate of 0.1 mV/s to 1.0 V. A commercial computer controlled potentiostat (Model 351, Princeton Applied Research) and a standard corrosion cell with saturated calomel electrode (SCE) was used (Fig. l). Each test was repeated 3 times. A n immersion test with an aerated lactic
Geis-Gerstorfer & Weber
290
C
B
A
c
Corrosion
Potential
ECORR
o o
Wiron 88
I---i
-0.2 i .
.
.
.
.
.
Micro-Bond NP2
.
-O. 4
-~_~_ __--_ i .
B
.
'~-_-
- --
._ i ~ _ _
ILl
.
.
.
0.6
.
Ultratek Thermostat
A =WORKING
.
8
~
Euro-Ceram
ELECTRODE
B : REFERENCE ELECTRODE
0
C = COUNTER ELECTRODE
Fig. l. Schematic representation of the corrosion cell ASTM Standard G 5.
acid - sodium chloride solution was examined to determine the substance loss (Fig. 2). Three specimens per alloy with a size of 3 0 x 2 0 x l mm were immersed together. At intervals of 2, 4, 7 and 14 days a small amount of the test solution was taken to be analysed by atom absorption spectrometry. After 14 days of immersion the specimens were used for SEM examination to study the corroded surfaces.
air
0 1
2
3 time [d]
4
5
Fig. 3. Corrosion potentials of the four alloys in de-aerated 0.1 M lactic acid +0.1 M sodium chloride over a period of 6 days.
Results The results of corrosion potential/time behavior of the 4 alloys in a de-aerated solution of 0.1 M lactic acid plus 0.1 M sodium chloride (pH = 2.3) over a period of 6 days is shown in Fig. 3. The corrosion potential of alloy A became increasingly more noble after immersion in the test solution and reached 0 V (SCE) after 6 days. The electrode potential of alloy B turned slowly to a more negative potential in the initial state and became stabilized at an equilibrium of about -0.25 V (SCE) after 2
0.1 M
lactic acid 0.1 M NaC[
2.3 pH 37oc T sample = 30x 2 0 x l mm size
Fig. 2. Schematic representation of the immersion test.
days. The molybdenum-free alloy C showed a marked trend to a more negative potential than alloys A and B, being - 0 . 8 V (SCE) or less, whereas alloy D became little more passive with time and remained at -0.65 V (SCE) Both alloys C and D produced a significant oszillation of electrode potential indicating intensive surface reactions. The alloys followed nearly the same sequence when recording potentiodynamic polarisation curves (Fig. 4). Comparing the rupture potentials alloy A had the highest value at 0.73 V (SCE) followed by alloy B with 0.25 V (SCE). Alloy C and D showed an active peak and onset of passivation began at - 0 . 2 8 V and - 0 . 2 2 V (alloy D). Alloy C revealed a beginning break down of the passivating film at - 0 . 1 5 V and alloy D at - 0 . 0 7 V (SCE) with the highest corrosion current. Potentiodynamic polarization measurements in different sodium chloride solutions (Figs. 5,6) signified a drastic decrease of rupture potential and a raised corrosion current of alloy B, C and D with increasing halide concentration as was expected. In comparison the rupture potential of alloy A (0.68 V) was little affected by the various sodium chloride concentrations whereas the magnitude of passive corrosion current rose half a decade. A reduction of passivity from 0.25 V to about - 0 . 3 V was observed for alloy B. Alloy C did not form a passive region even in the 0.02 M NaC1
Ni-Cr alloys & corrosion
291
Fig. 4. E-i diagram of the 4 alloys in de-aerated lactic acid/sodium chloride solution (sweep rate/). 1 mV/s).
0.1M lactic acid + 0.1 M NoCl 1.0
":>w 0.6 ~ o.2
W
-0.2 -0.6
0.0
0.2
0.4
0.6
0.8
1.0
i [mA/cm ]
0,02 M NaCI 0,2 M NaCI 1,0 M NaCI
-,,-,,1.0
i
i
i
i
,
i
i
i
-4,--,,-- 0,02 M NaCI 0,2 M NaCI -,,-,,-- 1,0 M NaCI i
ii
1,0
.4,
0.6
0.6
i
,
,
J
i
,
l
i
i
C
0.2 0.2 -0.2 >
-0.2
> -7
-6
-5
-4
IJ.J C) u3
LLJ
-0.6
-3 LU C.)
1.0
-7
-8
ILl
-6
D
-5
i
-/-,
i
-3
i
0.6 0.6 0.2
0.2
-0.2
-0.2
-0.6 -8
-0.6 -'7
-6
- 5
-4
log i [ A / c m 2]
-8
-7
-6
log i
Figs. 5 & 6. Potentiodynamic potential curves of alloys A, B, C, and D in deiaerated sodium chloride solutions. 19"
-5
-4
[ A/cm 2 ]
-3
Geis-Gerstorfer & Weber
292
Loss of Substance 16
[,g/c,,,~]
A
_Loss of Substance B
12
[llg/cm2]
~176176176 C
I .,~.~,~IN
8
2000 1
L
i I
6
D
0
0
0
~
~~176176i Fj,j?TIIp
I
"/"
2
400
400 200
2 3 4 56 8 10 20
time [d]
2 3 4 56 8 lO 20
time [d]
0
I
~ --'
2
200
~
-3 4 56 8 10
(]
20
/
//
/
~" >,.z
----
1
time [d]
c r'lY~ I
2
3 4 56 8 10
time [d]
Figs. 7 & 8. Analytically measured (AAS) substance loss-time behavior of alloys A, B, C, and D in aerated lactic acid-sodium chloride solution.
A Wiron 88
D
B
C
Micro-Bond NP2
EuPO Cenam
UltraLek
Co
Ni
\
/
C r ~ o Ni
~'--~\~
Cr
~Cr
Mn
Figs. 9 & 10. Percentage distribution of the analysed elements of alloys A, B, C, and D after 7 days immersion.
Fig. 11. SEM micrograph of Wiron 88.
20
Ni-Cr alloys & corrosion
293
Figs. 12 & 13. SEM micrographs of Microbond NP2.
solution. The aggressive chloride attack exhibited a definite increase of corrosion rate for alloy D. Using an immersion test with an aerated lactic acidsodium chloride solution (Fig. 2), substance loss was measured analytically by AAS (3). The substance loss-time behavior is illustrated in Figs. 7 and 8. Alloy A had the lowest substance loss and alloy B showed slightly higher values. Both were under 16 btg/cm2 in 14 days. The predominant elements dissolved wei-e nickel and molybdenum, followed by small amounts of chromium and iron. A large difference in substance loss was observed between the alloys A/B and C/D with up to a
thousand times greater values. From large grains and some Cr and Mo enboth alloys, C and D, nickel was the main dissolved element followed by chromium. No molybdenuym could be found in the electrolyte of alloy D. Conspicuously great quantities of the controversial beryllium were analysed. A percentual distribution of the elements analysed after 7 days of immersion is plotted in Figs. 9 and t0. Alloy A was most corrosion resistant and showed only few pits in the SEM examination (Fig. 11). Alloy B, consisting of an almost single phase structure, with richcd small zones, showed some even attack over the total surface (Figs. 12,
13). Around the small zones the original unattacked surface remained, probably an effect of galvanic coupling. The molybdenum free alloy C was very succcptible to pitting corrosion. Figs. 14 and 15 illustrate the two-phase microstructure of alloy C. After a short time a strong etching was already visible. The beryllium containing Alloy D produces a substantial quantity of NiBe eutectic working as an anode which was preferentially dissolved. Figs 16 and 17 show an attack reaching deep into the structure causing dissolution of the Ni-Bc eutectic but leaving an alumina rich film with a thickness of 0.5 to 1.0 gm.
294
Geis-Gerstorfer & Weber
Figs. 14 & 15. SEM micrographs of Euro-Ceram.
Discussion The possibilities of becoming sensitized to metals are diverse. Taking into account all the quantities of oral metal ingestion, (e.g. the daily food consumption contains 300 to 600 btg nickel [1,4]) the quantities of nickel dissolution from dental nickel alloys have to be discussed in comparison. In this investigation, the relatively corrosion resistant alloys A and B showed a loss of nickel of about 3 btg/cm-"a day, whereas
alloys C and D showed a nickel loss of about 500-1000 gg/cm z a day in the electrolyte used (0.1 M lactic acid/sodium chloride). With regard to the nickel content in food and drinking water, alloys A and B can be considered to be not critical but it is doubtful wheather the substance loss of alloys C and D is harmless. In principle it seems that Ni-Cr alloys in the oral environmerit only cause soft tissue inflammation and sensitization dermatitis if pa-
tients have already been sensitized before or if the substance loss of the used alloy is very high. From this point of view alloys A and B should be preferred to alloys C/D. In the future, systematic investigations should be taken to find an ideal artificial saliva, and the influence of alloying elements on corrosion resistance. Furthermore, the results obtained from studies like this one should be correlated with clinical data.
N i - C r alloys & c o r r o s i o n
295
Acknowledgement - This study was supported by DFG:SFB 175 "Implantologie": Projektbireich.
References 1. Blanco Dalman L. The nickel problem. J Prosthet Dent 1982: 48: 99. 2. Fisher A. Contact dermatitis. Philadelphia: Lea & Febiger, 1973. 3. Geis-Gerstorfer J, Sauer KH, Weber H. In vitro Korrosionsuntersuchungen zum Massenverlust von Nichtedelmetallegierungen. 4. Louria DB. The human toxicity of certain trace elements. A n n Int M e d 1972: 76: 307.
Figs. 16 & 17. SEM micrographs of Ultratek.
J. Geis-Gerstorfer, H. Weber University of Tflbingen, West Germany
Geis-Gerstorfer J, Weber H. i n vitro corrosion behavior of four Ni-Cr dental alloys in lactic acid and sodium chloride solutions. Dent Mater 1987: 3: 289295. Abstract - This investigation was carried out to study the in vitro corrosion behavior of 4 nickelchromium alloys in different lactic acid and sodium chloride solutions. Electrochemical techniques like potentiodynamic potential curves and potential time curves were used to analyse the characteristics of these alloys. Substance loss was measured analytically (AAS) to determine kind and quantity of dealloying elements. The results reveal that Ni-Cr alloys currently used in dentistry show a broad spectrum of corrosion resistance (depending on their chemical composition), the best being equal to those of precious alloys, while the worst had up to a thousand times greater substance loss and showed high corrosion currents in the potential area of physiological interest. Ni-Cr alloys containing too small amounts of chromium and molybdenum yielded the worst results and showed a susceptibility to pitting corrosion.
Key words: in vitro corrosion behavior; potentiodynamic measurements; immersion tests; AAS; SEM-studies: potential time curves d. Geis-Gerstorfer, Zentrum far Zahn, Mund, und Kieferkrankheiten, Universit&t Tebingen, Osianderstrasse 2-8, D-7400 T0bingen, West Germany. Received June 4, 1986; accepted February 13, 1987.
The corrosion resistance of dental prostheses is of great importance because of possible biological reactions and because of a possible destruction of the restorations. Concerning metals, most of the allergic reactions are caused by nickel (1). Nickel-chromium alloys, containing 611-80 wt-% nickel, have been developed as alternatives to-more expensive gold alloys. If these base metal alloys are not corrosion resistant enough, products that result from reactions with the oral environment can initiate a sensitization dermatitis (2). To judge the extent of possible allergic sensitization, in vitro and in vivo corro-
sion studies have to discover kind and quantities of dissolved elements and have to characterize the passivity behavior. This study deals with the in vitro corrosion behavior.
Material and methods The corrosion resistance of 4 NickelChromium alloys was determined in different electrolytes by using electrochemical and substance loss measurements. The chemical compositions of these specimcns are given in Table 1. Three specimens per alloy were cast in a casting machine according to the
Table 1. Chemical composition of the alloys (main constituents). wt % Ni Cr Mo Fe Be Ga 19
A Wiron 88
B Microbond NP2
C Euro-Ceram
D Ultratek
61.2 24.7 9.9 1.9 -
66.9 12.5 7.2 5.3 7.1
72.5 23.1
80.1 11.4 2.5 0.07
Dental Materials 3:6, 1987
l.l -
1.7
-
-
techniques used routinely in dental laboratories. After grinding with No. 240 silicon carbide paper the specimens were ultrasonically cleaned for 5 min in aceton and then immersed in the test solution. The electrolytes employed were sodium chloride solutions of 0.02 M, 0.2 M, 1.0 M as well as 0.1 M lactic acid with 0.1 M sodium chloride. Fresh solutions were prepared before each measurement and then de-aerated with nitrogen (99.996%) for 2 h. Solution temperatures were controlled to 37~ + I~ The polarization data were obtained with specimens (10 x 5 • 1 ram) mounted in a specimen holder by squeezing them between platinum wires in order to avoid crevice corrosion (Fig. l). The potentiodynamic scans were initiated 0.25 V below E (Corr) and increased at a step rate of 0.1 mV/s to 1.0 V. A commercial computer controlled potentiostat (Model 351, Princeton Applied Research) and a standard corrosion cell with saturated calomel electrode (SCE) was used (Fig. l). Each test was repeated 3 times. A n immersion test with an aerated lactic
Geis-Gerstorfer & Weber
290
C
B
A
c
Corrosion
Potential
ECORR
o o
Wiron 88
I---i
-0.2 i .
.
.
.
.
.
Micro-Bond NP2
.
-O. 4
-~_~_ __--_ i .
B
.
'~-_-
- --
._ i ~ _ _
ILl
.
.
.
0.6
.
Ultratek Thermostat
A =WORKING
.
8
~
Euro-Ceram
ELECTRODE
B : REFERENCE ELECTRODE
0
C = COUNTER ELECTRODE
Fig. l. Schematic representation of the corrosion cell ASTM Standard G 5.
acid - sodium chloride solution was examined to determine the substance loss (Fig. 2). Three specimens per alloy with a size of 3 0 x 2 0 x l mm were immersed together. At intervals of 2, 4, 7 and 14 days a small amount of the test solution was taken to be analysed by atom absorption spectrometry. After 14 days of immersion the specimens were used for SEM examination to study the corroded surfaces.
air
0 1
2
3 time [d]
4
5
Fig. 3. Corrosion potentials of the four alloys in de-aerated 0.1 M lactic acid +0.1 M sodium chloride over a period of 6 days.
Results The results of corrosion potential/time behavior of the 4 alloys in a de-aerated solution of 0.1 M lactic acid plus 0.1 M sodium chloride (pH = 2.3) over a period of 6 days is shown in Fig. 3. The corrosion potential of alloy A became increasingly more noble after immersion in the test solution and reached 0 V (SCE) after 6 days. The electrode potential of alloy B turned slowly to a more negative potential in the initial state and became stabilized at an equilibrium of about -0.25 V (SCE) after 2
0.1 M
lactic acid 0.1 M NaC[
2.3 pH 37oc T sample = 30x 2 0 x l mm size
Fig. 2. Schematic representation of the immersion test.
days. The molybdenum-free alloy C showed a marked trend to a more negative potential than alloys A and B, being - 0 . 8 V (SCE) or less, whereas alloy D became little more passive with time and remained at -0.65 V (SCE) Both alloys C and D produced a significant oszillation of electrode potential indicating intensive surface reactions. The alloys followed nearly the same sequence when recording potentiodynamic polarisation curves (Fig. 4). Comparing the rupture potentials alloy A had the highest value at 0.73 V (SCE) followed by alloy B with 0.25 V (SCE). Alloy C and D showed an active peak and onset of passivation began at - 0 . 2 8 V and - 0 . 2 2 V (alloy D). Alloy C revealed a beginning break down of the passivating film at - 0 . 1 5 V and alloy D at - 0 . 0 7 V (SCE) with the highest corrosion current. Potentiodynamic polarization measurements in different sodium chloride solutions (Figs. 5,6) signified a drastic decrease of rupture potential and a raised corrosion current of alloy B, C and D with increasing halide concentration as was expected. In comparison the rupture potential of alloy A (0.68 V) was little affected by the various sodium chloride concentrations whereas the magnitude of passive corrosion current rose half a decade. A reduction of passivity from 0.25 V to about - 0 . 3 V was observed for alloy B. Alloy C did not form a passive region even in the 0.02 M NaC1
Ni-Cr alloys & corrosion
291
Fig. 4. E-i diagram of the 4 alloys in de-aerated lactic acid/sodium chloride solution (sweep rate/). 1 mV/s).
0.1M lactic acid + 0.1 M NoCl 1.0
":>w 0.6 ~ o.2
W
-0.2 -0.6
0.0
0.2
0.4
0.6
0.8
1.0
i [mA/cm ]
0,02 M NaCI 0,2 M NaCI 1,0 M NaCI
-,,-,,1.0
i
i
i
i
,
i
i
i
-4,--,,-- 0,02 M NaCI 0,2 M NaCI -,,-,,-- 1,0 M NaCI i
ii
1,0
.4,
0.6
0.6
i
,
,
J
i
,
l
i
i
C
0.2 0.2 -0.2 >
-0.2
> -7
-6
-5
-4
IJ.J C) u3
LLJ
-0.6
-3 LU C.)
1.0
-7
-8
ILl
-6
D
-5
i
-/-,
i
-3
i
0.6 0.6 0.2
0.2
-0.2
-0.2
-0.6 -8
-0.6 -'7
-6
- 5
-4
log i [ A / c m 2]
-8
-7
-6
log i
Figs. 5 & 6. Potentiodynamic potential curves of alloys A, B, C, and D in deiaerated sodium chloride solutions. 19"
-5
-4
[ A/cm 2 ]
-3
Geis-Gerstorfer & Weber
292
Loss of Substance 16
[,g/c,,,~]
A
_Loss of Substance B
12
[llg/cm2]
~176176176 C
I .,~.~,~IN
8
2000 1
L
i I
6
D
0
0
0
~
~~176176i Fj,j?TIIp
I
"/"
2
400
400 200
2 3 4 56 8 10 20
time [d]
2 3 4 56 8 lO 20
time [d]
0
I
~ --'
2
200
~
-3 4 56 8 10
(]
20
/
//
/
~" >,.z
----
1
time [d]
c r'lY~ I
2
3 4 56 8 10
time [d]
Figs. 7 & 8. Analytically measured (AAS) substance loss-time behavior of alloys A, B, C, and D in aerated lactic acid-sodium chloride solution.
A Wiron 88
D
B
C
Micro-Bond NP2
EuPO Cenam
UltraLek
Co
Ni
\
/
C r ~ o Ni
~'--~\~
Cr
~Cr
Mn
Figs. 9 & 10. Percentage distribution of the analysed elements of alloys A, B, C, and D after 7 days immersion.
Fig. 11. SEM micrograph of Wiron 88.
20
Ni-Cr alloys & corrosion
293
Figs. 12 & 13. SEM micrographs of Microbond NP2.
solution. The aggressive chloride attack exhibited a definite increase of corrosion rate for alloy D. Using an immersion test with an aerated lactic acidsodium chloride solution (Fig. 2), substance loss was measured analytically by AAS (3). The substance loss-time behavior is illustrated in Figs. 7 and 8. Alloy A had the lowest substance loss and alloy B showed slightly higher values. Both were under 16 btg/cm2 in 14 days. The predominant elements dissolved wei-e nickel and molybdenum, followed by small amounts of chromium and iron. A large difference in substance loss was observed between the alloys A/B and C/D with up to a
thousand times greater values. From large grains and some Cr and Mo enboth alloys, C and D, nickel was the main dissolved element followed by chromium. No molybdenuym could be found in the electrolyte of alloy D. Conspicuously great quantities of the controversial beryllium were analysed. A percentual distribution of the elements analysed after 7 days of immersion is plotted in Figs. 9 and t0. Alloy A was most corrosion resistant and showed only few pits in the SEM examination (Fig. 11). Alloy B, consisting of an almost single phase structure, with richcd small zones, showed some even attack over the total surface (Figs. 12,
13). Around the small zones the original unattacked surface remained, probably an effect of galvanic coupling. The molybdenum free alloy C was very succcptible to pitting corrosion. Figs. 14 and 15 illustrate the two-phase microstructure of alloy C. After a short time a strong etching was already visible. The beryllium containing Alloy D produces a substantial quantity of NiBe eutectic working as an anode which was preferentially dissolved. Figs 16 and 17 show an attack reaching deep into the structure causing dissolution of the Ni-Bc eutectic but leaving an alumina rich film with a thickness of 0.5 to 1.0 gm.
294
Geis-Gerstorfer & Weber
Figs. 14 & 15. SEM micrographs of Euro-Ceram.
Discussion The possibilities of becoming sensitized to metals are diverse. Taking into account all the quantities of oral metal ingestion, (e.g. the daily food consumption contains 300 to 600 btg nickel [1,4]) the quantities of nickel dissolution from dental nickel alloys have to be discussed in comparison. In this investigation, the relatively corrosion resistant alloys A and B showed a loss of nickel of about 3 btg/cm-"a day, whereas
alloys C and D showed a nickel loss of about 500-1000 gg/cm z a day in the electrolyte used (0.1 M lactic acid/sodium chloride). With regard to the nickel content in food and drinking water, alloys A and B can be considered to be not critical but it is doubtful wheather the substance loss of alloys C and D is harmless. In principle it seems that Ni-Cr alloys in the oral environmerit only cause soft tissue inflammation and sensitization dermatitis if pa-
tients have already been sensitized before or if the substance loss of the used alloy is very high. From this point of view alloys A and B should be preferred to alloys C/D. In the future, systematic investigations should be taken to find an ideal artificial saliva, and the influence of alloying elements on corrosion resistance. Furthermore, the results obtained from studies like this one should be correlated with clinical data.
N i - C r alloys & c o r r o s i o n
295
Acknowledgement - This study was supported by DFG:SFB 175 "Implantologie": Projektbireich.
References 1. Blanco Dalman L. The nickel problem. J Prosthet Dent 1982: 48: 99. 2. Fisher A. Contact dermatitis. Philadelphia: Lea & Febiger, 1973. 3. Geis-Gerstorfer J, Sauer KH, Weber H. In vitro Korrosionsuntersuchungen zum Massenverlust von Nichtedelmetallegierungen. 4. Louria DB. The human toxicity of certain trace elements. A n n Int M e d 1972: 76: 307.
Figs. 16 & 17. SEM micrographs of Ultratek.