Tarnish-Resistant Silver Alloys K. W. RAY AND W. N. BAKER,D e p a r t m e n t of
Chemistry,
0
State University of Iowa, Iowa City, Iowa
RDINARY sterling silver is an alloy of silver and of the alloys of silver with the various metals, in order to copper containing 92.5 per cent silver and 7.5 per cent discover, if possible, an alloy having the intrinsic value, hardcopper. However, any silver alloy containing 92.5 ness, and malleability of copper sterling silver and, in addition, per cent silver is of sterling quality. The beautiful white having a very high resistance to tarnish. color and high luster of the polished surface of pure silver is EXPERIMENTAL PROCEDURE retained in copper sterling silver and, in addition, the presence of the copper makes the sterling silver much harder than pure The metals used in the preparation of the alloys were pure silver without materially decreasing its malleability. The one silver shot, electrolytic copper, the c. P. grade of zinc, 99 per undesirable property of copper sterling silver is its inability to cent aluminum, ordinary crystalline silicon and chromium, retain the luster of its polished surface in the presence of sulfur and beryllium as an 80 per cent beryllium-20 per cent or any of its compounds. Black silver sulfide is formed which copper alloy. After the investigation of a series of alloys, the destroys the beauty of the metal. This tarnish develops less silver was recovered electrolytically and re-used. readily in pure silver than in Fifty-gram samples of the copper sterling silver, and it alloys were prepared by melting the c a r e f u l l y weighed is possible that a silver alloy The s ilver-z inc-aluminum system containing can be d e v e l o p e d that will metals in small fire clay crufrom 75 to 100 per cent silver is investigated. be e n t i r e l y n o n t a r n i s h i n g cibles in a gas-fired f u r n a c e . under ordinary c o n d i t i on s . A s m a l l a m o u n t of sodium Several alloys of silver with chromium, zinc, An ideal silver alloy, in addicarbonate or borax mas used aluminum, beryllium, silicon, copper, tin, and tion to having the hardness as a flux. The melted alloy barium as binary, ternary, and quaternary alloys and malleabilit,y of copper was cast into steel molds, the are studied. A n attempt is made to nitride the sterling silver, would retain its ingots formed being 1 sq. cm. various alloys with gaseous ammonia at a somein cross section and about 10 high luster a t all times. With silver a t its present low price, cm. in length. what elevated temperature in the hope that a EXAMIXATIO~; OF ALLOYS. such an alloy wou'd find extarnish-resistant case will be formed. The h a r d n e s s , malleability, tensive use. Much less work has been constitution. and tarnish resistdone on the development of ance of all of the alloys made tarnish-resisting silver alloys than their importance would were studied. The hardness of the alloys as cast, and also warrant. As early as 1906, an English patent (a) was after annealing, was measured by both the Brinell and the granted for making tarnish-resisting silver by coating its Rockwell hardness testers. However, since many of the surface with a metal which would alloy with silver and form a!loys were too soft for the standard Rockwell B scale, only a white sulfide. Among the more recent patents issued for the Brinell hardness numbers are given, using 500 kg. prestarnish-resistant silver alloys may be mentioned several held sure and 10 mm. ball. The malleability of the cold alloys was by the Oneida Community Company, Ltd. These include: determined by rolling tests. The alloy to be tested was exposing silver to mercury vapors and obtaining a surface rolled on electrically driven rolls until the piece fractured, or amalgam containing a t least 90 per cent silver ( 3 ) ; exposing until it was rolled to a thickness of about 0.5 mm. Xineteen the silver to either chlorine, bromine, or iodine vapors (11); passes rolled a 1-cm. square ingot to 0.5 mm. in thickness, and the addition of 0.25 to 3 per cent silicon to 90 per cent and its length was increased about 2000 per cent. While the desilver, the remaining material not being specified (4). A crease in thickness was proportional to the number of passes United States patent (1) has also been granted for an alloy through the rolls, the increase in length was not. Twelve which contains, besides silver, 1.5-6 per cent silicon together passes increased the length of the alloy 100 per cent, fifteen with cadmium, aluminum, zinc, and antimony, as well as one passes increased it 250 per cent, and eighteen passes increased (10) for an alloy containing 93 per cent silver, 6.5 per cent zinc, it 1000 per cent. Thus, alloys differing only slight'y in and 0.25-0.50 per cent sodium. It is claimed that the tarnish malleability may vary greatly in percentage elongation. The which forms on this alloy is easily rubbed off. While these results in all cases are given as percentage of elongation propatented alloys may have a higher resistance to tarnish than duced by rolling until fracture occurred. The constitution of the copper sterling silver, they are not entirely stainless by the alloys was studied by means of cooling curves and microscopic appearance. any means, and some of them are of doubtful value. I n taking the cooling curves, the temperature wab autoJordan, Grennell, and Herschman (6) made a rather comprehenske investigation of tarnish-resisting silver alloys, matically recorded a t 15-second intervals by a Bron 11 recordboth binary and ternary. From the results of this work they ing pyrometer using a chromel-alumel thermocouple. An conclude that tin, antimony, cadmium, and zinc increase electric tube furnace lyas used to melt the samples, about 50 tarnish-resistance when added to silver, while other metals grams of the alloy being used for each determination. Firedecrease it. They discovered no nontarnishing alloys and clay crucibles were used. X o reducing gases were used to express very little hope that a stainless silver alloy, especially prevent oxidation, since the alloys oxidized but slightly even a t their melting points. These slowly cooled alloys, after of sterling quality, will ever be found. Leroux and Raub (S) studied silver-copper base alloys con- proper polishing and etching, were used in studying the microtaining zinc, cadmium, and nickel. They found that most scopic structure. The best etching agent \vas found to be of these alloys could be age-hardened, but they do not discuss sodium cyanide with hydrogen peroxide, although dilute nitric acid was used in some cases. the tarnish resistance of these substances. The resistance to tarnish was the main object of the experiThe purpose of this investigation was to continue the study 778
780
iirg alloy was two gliasc, even in tlic liquid
state. The silver phase of the solid alloy contained about 13 per cent aluminum and some scgregated cl,rfiiiiiiiiii-ricli phase. The alloy w a ~ too brittle and porous to have any value. A n alloyoSnickel,al~iminiim,a~~rl silver behaved somewhat, sirnilarly and \Yas not inwstigated f l l T t ~ 1 f T . II~;iiYLr.rrnr-Sii,icf)~-~iL~,Eii B A S EA I.LOYS. .According t.0 the t l m n i a l diagrams givtin in 111ternational Critieal Tables, both silicon and her+ linin forin eutectic alloys with silver. I h w v r , tlicse niet,alsare nirntioned in s m i e of t,lie pateiited t:irnislr-resistiirg silver alloys, lint J C I T ~ M I . Urcnnell, and Ifersoliman report that silicon does not increase the tarnish resistance, nnil t,liat
ncid
U P sSL\Ei< \\ITH
.~!.I,l,UYS
Sincc both beryllimn and silicon are difficiilt to alloy with silver, owing to their higb molbisrg points and their strong tendencies to become covered witit an infusible film of oxide, a sinal1 asnonnt of beryllinsn-sil\w alloy, as mll as a small amount OS a silicon-silver alloy, was fint made at a high temperature in an induction fumact?. BSter analysis, these alloys were used instead of the pure A:iiir~nt.sfor making the alloys to be tested.
olr?lEI< MkTALS
The alloys of silver with a large nusnbcr of other mctals, as binary, ternary, and qiiat.ernary alloys, WCTO tried with tire hope tliat a satisfactory tarnish-resisting alloy would lie found. Most of those metals which were reported to increasr the tarnisli resistance of silver (ai well as many metals whieli verv easilv ..I " become nansivc. stieli as chromium and beryllium) were tricd. The c o m p o n e n t metals were chosen with the idea of obtaining solid-solution alloys if possible. Tlie metals tried in tliose alloys were clirominni, niclcel, berylliirrn, barium. zinc, alinninum, silicon, tin, and mercury. SILVER-CHROMIUM 35ASE I ~ L O Y S . silver and chrorninm are practically insoluble in each other in both t,lieliquid and solid state (6). At 465" C. chroininiii is soluljle in silver to tlie extcnt of 3.3 per cent. The solntility dwreascs with decri in t,enipcrature. JUT&UI, (kennell, and t l e ~ ~ c l i ~ n a npra,ctieally confirm this and report their inability to get silver to alloy with much cliroInium. An alloy made by tliem at 1200" C. contained 0.14 per cent chronlirisn. They also tricd to make ternary sil\-er-climiniiim alloys containC D c. s I L V E W ~ 2 . 5'vIN , s, ZINC D. S I L Y ~88, Tin io, ZN; 2 PEN ing 5 per cent of either zinc, tin, antimony, or CENT ( X 100) 2.5 PERCENT( x 100) cadiniiiin, hut the attempts met with only fair Chill-csslfollowed byanoeding st 700" C. Chill-oast nnd odd-worked. nnnealed success, tlie alloys usually containing about 0.2 at 700' C. f m1 hnur: otcheb with POlo? 1 hour: hornoaensoua 01 phase: etched per c e n t of c l l ~ ~ l i i i l l I l lChroniiuin . rileits diwn wy8mide sild hydroqen poroazde. with sodium cyanide snd hydrogen per(Nulo twinned B"Iid.P"hLti"" oryslals.! 'oxide. :Lt, lR15° C, which is a b o n t the b o i l i n.,e . ooint, of all the other metals tried, except silver and tin. Talde I1 sl~owstho hardness, malleability, and tariiisliSince alnminuni and chroniiurn alloy with each otlicr fwniinr liotli solid solutionu and coniponnds, an attempt was made tu :cbility of the silver-silicon-beryllium alloys tcsted. Silicvn make an alumiunm-clironiiussi-sil\.er alloy, Equal amounts of and beryllium have a marked hardening effect on silver alloys, aluminum and chrorniuni were mixed and heated to abollt but even i n small amonnts they render tile a,lloynonmalleable. 1800' C. wit11 silver in an indnetion furnace. Tlie result- l3ery~liunicauses brittleness in sinailrr amounts than does silicon. As tarnish-resisting elenrcnts in silvcr alloys, silicon ~
.
~~
~
MATAS- 'T.bn~iall-
AL(ILITTE'
'I1 96 87.1 98.3
8i.i 92 91 92.5 90.5 98.7 91
* 6
5
... ...
2
.,
.,
... . . ... .. . . . ..
5
5.5
6 6.3
2 1
2
0.1
...
11.4 0.6
1.8
...
1.8 2.3 I.?
5.5
... ...
6
...
8 .3
... .5 .. 3 .,., 8... 0... ... 0.5
6 Giungstion. Pure silver, 4; sterling
RiiYOi,
65
60 103
...
95
0.5
... ... ...
1.
% 60 77
... ...
? 2
.,BI,,ITY~
164
48 80
3s
55
>2000 6 15
8
40 5
s 8
10
2 ?
30 6
>am >2000 >2m
a
4
5 5
SILvEx-TiN BASEALLOYS. At room tcinprature, tin forms a-solid solutionwitlisilverup to 19per cent tin (12). Jordan, Crrennell, and Herscliman found that silver alloys containing tin were almost as tarnish resistant as those containing an rqual amount of zinc. I n addition, tin-silver alloys are harder than zinc-silver alloys. Tablc I11 sliows the hardness, malleability, and tarnisliability of tlir alloys studied. In silver alloys, tin increases the liardness iiiore than an equivalent amount of zinc. At the same time, the malleability decreases faster than in corresponding zinciil>m alloys. Low-tin alloys are more
tariiiJi resistant than corresponding zinc-silver alloy.. Increafing the percentage of tin from 5 to 10 per cent does nut materially affect tarnishableness, while in zinc-silver alloys the tarnish resistance in the alloys studied is proportional to the percentage of' zinc present. TABLE 111. €3IRDNESS, RIALLEABILITY, h S D OF TIN-SILTERALLOY^ COMPOSITIOS
7 -
.lg
Sn
Zn
95 5 . . 90 10 . . 90 5 5 87 5 5 88 10 . . 10 10 80 92 6 . . CI Elongation. h Pure silver, 4.
Cu
-AI
bi
. . . . . .
. . . . .
.
3
.
.
.
. . . . . 2 ..
. . . . . . . . . . . . 2
?'ARYIaH.4BILITT
BRINELL HARDNESS I\I +LLl:4s CABT ABILITY" %
> 2000 150 > 2000
40 58 47 54 75 78 75
TARNIBU4BILITY ih
CABT~
> 2000
11 12 10 8
40 60 40
6
;
OTHER ALLOYS. Many miscellaneous silver alloys were made and tested. Barium-silver and strontium-silver alloy. were made by electrolyzing a fused bath of thi. alkali earth chloride in an iron dish using molten silver as the cathode. The alloys were hard, nonmalleable, and easily tarnished when the alkali earth metal was present to the extent of 10 per cent or more. These alloys appeared to have no value: either alone or as ternary alloys with tin or zinc. An amalgam of the type used by dentists for filling cavities in teeth, consisting of 50 per cent mercury, 34 per cent silver, 14 per cent tin, and smaller amounts of zinc and copper, appeared to be exceedingly resistant to tarnish, but wa: entirely nonmalleable. When the composition was modified sufficiently to give malleability, the resistance t o tarnish was greatly decreased.
XITRIDIKG WITH GASEOUS ANMOXIA It was thought that perhaps ammonia a t high teniperatures might case-harden silver alloys similar to the manner in which it case-hardens certain steels. Several elements that alloy with silver form stable nitrides (9). illuriiinum nitride, ranging in color from pale yellow to black, forms when ammonia is passed over pure aluminum a t 700" C. Copper, as copper oxide, mill form copper nitride a t 250" C. in ammonia gas, but it decomposes near its temperature of formation. Zinc forms a number of nitrides between 400" and 600" C. which range in color from gray to black. 811 (of these decompose slowly a t the temperature of formation. Tin decomposes ammonia but does not unite with it; silicon forms a number of nitrides which are grayish white powders. Pure silver does not form a stable nitride with ammonia gas, although a nitride is known which is explosive. TABLEIv.
PHYsIC.4L PROPERTIES O F
15 HOERSAT 650'
ALLOYS NI'I'RIDED
C.
FOR
BRINELL TARXISHHARDNESS .4BILITYa
-
aiiiinoiiia was obtained from a cylinder and passed directly into the furnace. After passing though the furnace, the ammonia was absorbed in water. The results of the nitriding under various conditions are given in Table IV. The alloys are softened because of the high temperature. Their resistance to tarnish is not markedly affected, and it is doubtful if any nitrided case is formed except in a few instances. SUMMARY ATD CONCLUSIOXS 1. Sone of the metals studied gave binary alloys with d v e r that were satisfactory in every respect as regards hardness, malleability, and tarnish resistance. 2. illuminum alone did not increase the tarnish resistance of binary silver alloys. It hardened the alloys materially and made them brittle when present in amounts beyond 4 per cent. Its malleable alloys were not as hard as copper sterling silver. 3. Zinc had the greatest tarnish-resisting effect in silver alloys of any of the metals studied. The resistance to tarnisli was nearly proportional to the amount of zinc present. Zinc did not increase the hardness sufficiently to be used alone. The alloys containing up to 25 per cent zinc were malleable. 4. Chromium did not alloy with silver in amounts sufficient to have much effect upon its resistance to tarnish. 5 . The barium-silver and strontium-silver alloys studied were brittle and were as readily tarnished as copper sterling silver. 6. Silicon had a marked hardening effect on silver alloys hut could not be used in amounts mucli above 0.5 per cent, or brittleness resulted. It had no marked effect on tarnish resistance. 7. Appreciable amounts of beryllium made silrer alloys hard and brittle, and decreased tarnish resistance. 8. Tin-silver alloys containing 5 per rent tin were the most tarnisli resistant of any 95 per cent silver alloy studied. a1though only slightly better than the corresponding silver-zinc alloy. Increasing the amount of tin beyond 5 per cent did not increase tarnish resistance materially, while increasing the amount of zinc in silver-zinc alloys did increase tarniqh resistance. 9. Xitriding with ammonia a t either 500" or 650" C. n a s of no value for case-hardening or increasing the resistance to tarnish of silvez alloys. Alloys containing aluminum and silicon were darkened, owing to the formation of an aluminum nitride and silicon nitride case, while the color of the other alloys was not materially affected. Cast alloys were softened because of the high temperature. 10. While many of the silver alloys studied were more tarnish resistant than copper sterling silver, and had sufficient hardness and malleability, none was found that could be considered entirely nontarnishing or stainless.
LITERATURE CITED
Before Sfter Before After APPEIR~SCE (11Carpenter and TVhiteley, 2. Metallkundt., 3, 145 (1912). AFTBR COMPOSITIOY-----nitrid- nitrid- nitrid- njtrid( 2 ) Cowper-Coles, S. O., British Patent 26,966 (Kov. 16, 1906). Ag Zn A1 Cu Si Be Sn ing XITRIDING ing ing ing (3) Gray, Bailey, and Murray, U.S.Patent 1,719,365 (July 2, 1929). 60 5 5 Dark gray 91 5 2 2 0 1 . . . . . 60 1-1) Gray, Bailey, and Murray, C. S. Patent 1,720,894 (July 16, 39 48 4 5 Gray 92.5 5.5 1 0 5 . . 0.5 . . .. 90 45 5 6 Grayish 1929). 90.5 6 ,, 3 0 5 brown t5) Hendricks, 2. anorg. allgem. Chem , 59, 414 (1908). 35 6 93.7 6 . 3 . . . . . . . . . . . . . 28 6 White (6) Jordan, Grennell, and Herschman, Bur. Standards, Tech. Paper White 5 5 91 6 . . 3 . . . . . . . . 55 34 348, 459-96 (1927). 29 White 40 11 12 95 5 11 White . . . . . . . . . . . . . 10 58 45 12 90 (7) Korsunsky, M. G., U. S. Patent 1,613,304 (Sept. 27, 1927). 34 White 90 5 ........... 5 47 .~ 10 (8) Leroux, J. A . , and Raub, E., 2. Metallkunde, 23, 58-63 (1931). 87 5 . . 3 5 54 48 White (9) Mellor, J. W.,"A Comprehensive Treatise of Inorganic and 88 2 10 76 67 9 Very light gray Theoretical Chemistry," Vol. 8, pp. 97-137, Longmans, 1928. 80 10 7 10 White . . . . . . . . . . . 10 78 54 (10) Mitchell, W.L., U. S. Patent 1,614,752 (Jan. 18, 1927). sparkle 6 5 Light gray 75 40 92 2 6 (11) Murray, X7. S., U. S. Patent 1,758,293 (May 13, 1930). a Pure silver, 4. (12) Pentrenko, 2. anorg. allgem. Chena., 53, 200 (1907). t . ,
.............
..
...... .........
.......
...
The samples were heated in a cylindrical electric tube furnace. The temperature was kept constant with a LeedsXorthrup controlling potentiometric pyrometer. The gaseous
RECEIVEDJanuary 25, 1932. This paper has been rewritten and condensed from the thesis presented b y W. N. Baker t o t h e graduate college of t h e S t a t e University of Iowa i n partial fulfilment of the requirements for the degree of master of science.
