Observations of onset of sulfide tarnish on gold-base alloys

Observations Joseph

of onset of sulfide tarnish

J. Tuccillo,

New Rochelle,

BALE.,

M.S.,*

and

John

on gold-base

P. Nielsen,

alloys

Ph.D.**

N. Y.

for the purpose lh e various gold alloys used in dentistry differ in composition of meeting the different mechanical requirements of the different types of applications. However, they must all have sufficient noble metal content to be essentially tarnish resistant in the oral environment. In spite of this, clinical dental examples ot: tarnish occur occasionally even in the alloys of high gold c0ntent.l We have received in our laboratory over the last ten years about 80 tarnished restorations submitted by dentists for our examination. On analyzing the tarnished areas with an electron probe, we found the tarnish to be essentially a sulfide intermixed with a carbonaceous material. It has been observed that the tarnish favors regions of subsurface porosity, or regions where a copper redeposit has apparently occurred, probably due to old pickling solution. Tarnish also favors the low gold content alloys, but surprisingly, this is not always true. Some 22-karat gold alloys tarnish in the mouth rather consistently. It is, therefore, of interest to investigate the nature of tarnish when deliberately forced to occur on a series of gold-base alloys which arc generally considered tarnish resistant for dental purposes. “Tarnish,” in this article, will be defined as the visibly detectable discoloration of a metal caused by a thin adherent layer of a reaction product, such as an oxide or a sulfide, induced by a chemical reaction between the metal and the environnlent. In general, such discoloration is the basis for tarnish when caused by mouth fluids.‘, 3 The criterion of tarnish onset is not limited in this article to the unaided “eye” type of detection but will include the detection of discoloration as first appearing at microscopic examination. ‘rhe alloys reported on in this article lvere tested for possible microscopic 9:nrnish produced by a synthetic saliva solution.*-’ After ten days of room-temperature immersion at one minute intervals (the method described below), no microscopic tarnish could be detected. Therefore, synthetic saliva was discarded as a means for _Presented at the Annual Washington, I). C., 1967.

Meeting

of the International

*Manager,

Research and Development,

**Director,

Research and Development,

J. F. Jelenko J. F. Jelenko

Association & Company,

of Dental

Research

in

Inc.

& Company, Inc. 629

630

Tuccillo

and Nielsen

Fig. 1. Gold-silver-copper

ternary scale indicating

J. Prosth. Dent. June, 1971

composition of alloys 1 through 13.

grading alloys as to tarnish propensity. Tarnish in various degrees could be forced to appear on all of these alloys when an 0.5 per cent sodium sulfide aqueous solution was reached starting with zero concentration. This tarnishing solution was selected for the experiments using alternate immersion at room temperature.

PROCEDURE Fourteen alloys were investigated (Table I), and thirteen are indicated as points on a ternary scale diagram (Fig. 1) . The alloy series from No. 1 to 9 represents the variation of silver-copper ratio with a constant atomic gold content, i.e., the gold was at 50 atomic per cent throughout this series. This first series of nine alloys includes weight per cents of gold from 64.0 per cent to 75.6 per cent, a range covering, for the most part, the gold content range of all dental alloys. The four alloys numbered 10 to 13 represent, on the other hand a range in ‘nobility” from 30 to 70 atomic per cent gold, keeping the ratio of silver-to-copper at unity. Alloy 14 was a complex alloy containing 50 atomic per cent gold, the balance was as follows: Cu, 26.0 per cent; Ag, 17.5 per cent; Pd, 5.5 per cent; Zn, 1.0 per cent. All these alloys were single-phase alloys in the “as cast” condition, so that the question of galvanic coupling between phases was eliminated. Fig. 2 illustrates the size of the cast specimen and the surface exposed for study. The initial alloys were prepared by induction melting-150 Gm. charges-and were cast into a 2 x y4 x 4 inch mold. The resulting bars were rolled to a thickness of 0.020 inch and were cut into 1.5 Gm. pieces. For each alloy, 10 Gm. were melted

Volume 25 Number 6

Table

Sulfide

I. Alloy

,411O)J NO. I

2 3 4 5 6 7 8 9 10 11 12 13 14

composition

in atomic

AU 49.3 50.05 49.8 49.85 50.0 49.7 49.3 50.0 50.0 30.0 40.0 60.0 70.0 50.0

(64.0) (66.0) (67.1) (68.1) (70.0) (71.1) (72.0) (74.1) (75.6) (49.6) (60.5) (77.5) (84.3) (75.0)

per cent (weight

(36.0) (31.0) (26.9) (23.9) (18.0) (13.9) (10.0) ( 4.9) ( 0.0) (31.7) (24.9) (14.1) ( 9.9) ( 3.0)

on gold-base

0.0

( 0.0)

7.05 13.7 18.15 26.5 32.5 38.2 43.9 50.0 35.0 30.0 20.0 15.0

( 3.0) ( 6.0) ( 8.0) (12.0) (15.0) (18.0) (21.0) (24.4) (18.7) (14.6) ( 8.3) ( 5.8) (11.0)

alloys

631

per cent in parentheses) Tarnish

cu

AS 50.7 42.9 36.5 32.0 23.5 17.8 12.5 6.1 0.0 35.0 30.0 20.0 15.0

tarnish

characteristics

Uniform speckled Dendritic attack Dendritic attack Dendritic attack Dendritic attack Dendritic attack Matrix attack Matrix attack Grain orientation dependent Dendritic attack (heavy) Dendritic attack (moderate) Slight attack Very slight attack Slight grain-boundary attack

in a centrifugal casting machine* and were cast into a 677’ C. cristobalite mold at 1,038’ C. metal temperature. The “as cast” specimens were ( 1) mounted in clear lucite, (2) wet-surface ground through 600 silicon carbide grit, and then (3) polished with aluminum oxide of 0.3 ,u particle size in distilled water on a rotating velvet cloth. This is a standard polishing procedure for metallographic examinations of gold alloys. TESTING

APPARATUS

The testing apparatus (Fig. 3) consisted of a plastic vertically rotating wheel, with eight holes 1 inch in diameter along its periphery in which standard metallographically mounted specimens were clamped. The wheel was rotated at 1 r.p.m. with the lower third submerged in the testing medium. The height of the liquid :medium was adjusted to allow each specimen to be alternately immersed and withIdrawn for 15 seconds and 45 seconds per revolution, respectively. As each specimen was withdrawn from the medium, it was mechanically wiped with a cloth to remove any nonadhering deposit. This generally left a thin adhering tarnish layer resembling the tarnish of a dental restoration. A counter was attached to the unit to record the number of revolutions. At intervals, the specimens were removed, washed with soap and distilled water, dried, and examined. Duplicate runs were made for all tests. RESULTS As previously mentioned, after ten days (14,000 immersions) in a synthetic saliva solution medium, no tarnish deposit developed on the microstructure of any of the alloys, consistent with the results reported by Swartz, Phillips, and El Tannir.4 The 0.5 per cent sodium sulfide solution, however, induced tarnish in almost every casting. This was detected as incipient tarnish by a microscopically visible sur“Thermotrol

D-2, J. F. Jelenko

& Company,

Inc., New Rochelle,

N. Y.

632

Tuccillo

J. Prosth. Dent. June, 1971

and Nielsen /-

EXPOSED

SURFACE

Fig. 2. The size and shape of the cast specimen.

Fig. 3, Testing apparatus. product of some kind. Five types of deposit patterns were revealed (depending upon composition), listed, and described as follows: (1) uniform speckled, (2) dendritic attack, (3) matrix attack, (4) grain-orientation dependent, and (5) grain-boundary attack. Uniform speckled. This type appeared in the form of fine dots under ~500 magnification, which started apparently at the subboundaries (Fig. 4), and later the dots spread out to other areas. After extensive attack, the dots increased in density, but their discreteness remained until disguised by heavy overlapping. To the naked eye, the discreteness was never evident. In the early stages (subboundary attack and spreading from this), the specimens would have been rated by normal visual inspection as exhibiting no tarnish and, at a later stage, as exhibiting “uniform” total-coverage tarnish. face reaction

Sulfide

tarnish

on gold-base

alloys

633

Fig. 4. Fifty-fifty atomic per cent gold-silver alloy CA) after 150 minutes exposure in 0.5 per cent sodium sulfide solution and (B) after 500 minutes exposure. (Original magnification xl 00.)

Fig. 5

Fig. 6

Fig. 7

Fig. 5. Alloy

No. 6 after approximately

1,000 minutes exposure.

(Original

magnification

x100.)

I’ig. 6. Alloy

No. 7 after approximately

1,000 minutes exposure.

(Original

magnification

x100.)

Fig. 7. Alloy No. 9 (50-50 atomic per cent gold-copper) e.xposure. (Original magnification X100.)

after

approximately

1,500 minutes

Dendritic attack. In certain alloys, the dendrites were attacked first, again by the speckled attack, but the discoloration remained in the dendritic region and did not spread (Fig. 5). Matrix attack. In certain alloys, a reversal occurred in that the dendrites were not attacked but rather only the matrix region around the dendrites, i.e., the continuous region in three dimensions (Fig. 6) . To the inexperienced eye, the distinction as to whether the dendritic region or the matrix was attacked is sometimes difficult. .A rule that aids in making the distinction is to apply the continuity test on crossings 13f microconstituent arms. Dendrites exhibit continuity on the rectilinear arms crossings, whereas the matrix does not. Continuity on curvilinear-type crossing regions represent the matrix (Fig. 6) . Grain-orientation dependent. This type showed certain grains attacked in their entirety while other grains were quite untouched (Fig. 7). It was assumed that this was the result of orientation differences exposed to the surface by the different grains. Grain-boundary attack. In one alloy, the grain boundaries only were attacked,

634

Tuccillo

and Nielsen

J. Prosth. Dent. June, 1971

Fig. 8. Alloy No. 14 after 7,496 minutes exposure in a 5 per cent sodium sulfide solution.

with the attack increasing slightly in width as the tarnish increased spreading over the grains (Fig. 8). In Fig. 9, the different types of tarnish are exhibited schematically.

but

never

DISCUSSION AND CONCLUSIONS It is rather interesting that what was perhaps believed to be a continuous deposit. of the discoloration film producing so-called tarnish is actually a discrete or a discontinuous deposit in every instance, at least in the early stages. Apparently, the principle of concentration cells operates for tarnish as it does in common forms of corrosion. Concentration cells require localized galvanic action. It seems that in the gold alloys studied all the factors for galvanic action are present on a microscopic scale when the alloys are immersed in an electrolyte: potential difference, electron conductivity path, and ion migration path. In castings with the uniform speckled attack, where only silver was alloyed with gold, the concentration cells probably are distributed according to lattice defects. They certainly did not reflect a compositional variation in solid solution, because an electron probe examination of untarnished specimens did not show the discreteness in composition found in tarnished specimens. The lattice imperfections no doubt are anodic to the “good” lattice surrounding them (Fig. 9). Likewise for the silver-rich dendrites, which apparently behaved anodically to the copper-bearing matrix in alloys No. 2 through 6, the dendrites are silver-rich through alloy No. 6. This is even so in the high copper content of the last alloy. This is evident on examining the ternary diagram, where it can be seen that the liquid plus silver-rich solid 2-phase region extends to the liquidus minimum at about 38 per cent copper for this alloy series (Fig. 10).

~9’il-e~ I

Sulfide

‘6”

I

Uniform total coverage

Orientatton dependent

rxl

Dendritic

tarnish

on gold-base

--I

alloys

635

-

Matrix

Groin boundary

Fig. 9. Schematic of the five types of tarnish. The switch to the matrix attack in alloys 7 and 8 was puzzling at first. However, this behavior was found to be consistent with the finding that the silver-rich constituents are the ones attacked by sodium sulfide. For alloys 7 and 8, the solidification occurs, on cooling, through the 2-phase region: liquid plus copper-rich solid, thus the dendrites are copper-rich. This makes the matrix relatively silver-rich; whereas, in alloys 1 to 6, the dendrites are silver-rich. Furthermore, if solidification is rapid, as it usually is in dental casting, there should be a simultaneous deposit of copper-rich and silver-rich regions surrounding the first-formed copper-rich dendrites. This would come about by a delay in solidification on depressing the solidus line. Thus, the matrix in alloys 7 and 8 is probably not only relatively silver rich but heterogeneous in its mixture of silver-rich and copper-rich regions in a eutectic-like mixture. It should be kept in mind that, although there were silver-rich and copperrich regions in the matrix, all the solidification was of a single-phase substance, as in coring. This would tend to make the matrix quite vulnerable to localized concentration-cell type of corrosion attack. The preferential attack for certain grain orientations on alloy No. 9 is apparently again an example of concentration-cell attack where certain orientations are nnodic to others. GLvathmey and Benton? showed this in a rather striking manner by exposing single crystal-copper spheres to a corrosive medium and by developing tarnish decorations in the spheres. The decoration showed symmetrical patterns that the anodic-cathodic relationship was orientation dethereby demonstrating ??endent. The orientation-dependent tarnish occurred only in the low-silver alloys indicating that, when silver is absent in gold alloys, the concentration-cell effect finds another mode for galvanic action,

J. Prosth. Dent. June, 1971

. ./Liquid \

“.

I ox

2

34 IOX

5 zox

6 30%

7

6 40%

Fig. 10. Vertical section of the gold-silver-copper alloys 1 through 9 are indicated along the abscissa.

8 Cd-rich

phase

Cu-rich phase

9 soxcu

ternary.

The approximate

compositions

of

Alloys 10 and 11, which are low in gold content, yielded dendritic tarnish as would be expected. Alloys 12 and 13, which were high in gold content, tarnished so lightly that it was difficult to classify as to tarnish type. The complex alloy, No. 14, containing about 3 per cent silver, showed no tendency to tarnish on the dendrites or the matrix nor did it exhibit the orientationdependent tarnish effect. However, the galvanic action selected the boundaries as the anode, with the grain interior as the cathode, thus preferentially tarnishing the boundaries. The testing of this alloy was the one exception in the procedure in that 5.0 per cent sodium sulfide solution was necessary to force tarnish in a reasonable testing period. The following general observations can be made from the above findings for tarnish of gold-base alloys produced by a sodium sulfide solution : 1. The onset of tarnish can be detected at a very early stage in the microstructure, well before it is detected with the naked eye. 2. Silver rather than copper, at least for sulfide containing media, appears to be the principal element rendering gold-copper-silver alloys susceptible to tarnish. 3. Tarnishing would appear to be fundamentally a discrete or discontinuous cheinical attack over a surface, with the discontinuity being of microscopic order. This is probably true even for tarnish that appears to be continuous on a surface, the apparent continuity being rather an overlapping of numerous microdiscrete tarnish regions. 4. Tarnish, in the early stages at least, occurs in different modes in different specimens on ostensibly single-phase alloys. There always seems to be some anodecathode relationship. Early tarnish on a microscopic scale generally exhibits contrasting tarnish and nontarnish regions.

Volume 25 Number 6

Sulfide

tarnish

on gold-base

alloys

637

References 1. Vines, R. F., and Wise, E. M.: Age Hardening Precious Metal Alloys in Age Hardening of Metals, ,$merican Society of Metals, Cleveland, 1940 (written discussion by E. W. Skinner). 2. Metals Handbook, Cleveland, 1948, American Society for Metals, p. 15. 3. Wise, E. M.: Gold-Recovery, Properties and Appliances, Princeton, 1964, D. Van Nostrand Company, Inc., p. 192. 4. Swartz, M. L., Phillips, R. W., and El Tannir, M. D.: Tarnish of Certain Dental .4lloys, J. Dent. Res. 37: 837-847, 1958. 5. Raub, E., and Engel, H. Z.: Metallk 44: 298-301, 1953. 6. Souder, W.: Standards for Dental Materials, J. Amer. Dent. Ass. 22:1873-1878, 1935. 7. Gwathmey, A. T., and Benton, A. F.: The Directional Oxidation of a Single Crystal rlf Copper by Heating in Air at Reduced Pressures, J. Chem. Phys. 8: 431, 1940. J. F. JELENKO & COMPANY, INC. 170 PETERSVILLE RD. New ROCHELLE, N. Y. 10801