Polyphosphate Detergents in Mechanical Dishwashing SOLUBILIZING ACTION OF POLYPHOSPHATES ON CERTAIN METALS LESLIE R. BACON AND EUGENE G. NUTTING, JR: Research and Development Division, Wyandotte Chemicals Corp., Wyandotte, Mich.
S
EVERAL years ago complaints arose from restaurant managers when a new type of stain began to appear on silverware cleaned in mechanical dishwashing machines. The coloration of the stains was somewhat variable but in general appearance brassy, although in some cases colors resembling silver sulfide stains were noticed. Examination in such cases usually showed the virtual absence of sulfides. A further description of the stains and the conditions under which they occurred is being published (1). These films were found to contain copper. Field studies revealed that brass or copper parts of some of the machines involved in the complaints had been seriously corroded, even to the point of failure in as little as 6 months' time. Figures 1, 2, 3, and 4 show several types of corrosion which have been noted. The introduction of sodium hexametaphosphates into mechanical dishwashing compounds was given great impetus by the work of Schwartz and Gilmore ( 4 ) and of Gilmore ( 2 ) . In addition to sodium hexametaphosphate, tetrasodium pyrophosphate and ?odium tetraphosphate were coming into considerable use as early as 1940. Although solubilization of metal anodes in pyrophosphateplating baths is well known to electroplaters, only one reference to simple metallic solution in nonelectrolytic polyphosphate solutions has been found in the literature (3). This recognizes that under. certain conditions sodium hexametaphosphate added to water may increase the amount of lead taken up by the water under alkaline conditions.
about 0.5 mg. per hour, as shown in Figure 5 . The brass strips turned very dark during the test. An indication of the action of Formulas A and C dishwashing compounds containing polyphosphates and of tetrasodium pyrophosphate on zinc, which is a major constituent of brass, is shown in Figure 6. (Zinc also appears in dishwashing machines where galvanized iron is often used for solution tanks.) The compositions of these formulations appear in Table 11. Triplicate 200-ml. solutions of Formulas A and C a t 0.15% and of pyrophosphate a t 0,0670concentrations were placed in 8-ounce bottles and held at 80' C. One strip of zinc sheet 76 X 18 mm. (3 X 3/4 inch) was immersed in each solution. After 2, 4,and 6 hours a zinc strip was removed from each set and weighed. In each case weight losses increased quite rapidly and uniformly. The trisodium phosphate or metasilicate content of Formula A somewhat retarded the solution of zinc caused by pyrophosphate alone in equal (o.0670) concentration. Formula C, which contained somewhat less of another polyphosphate (hexametaphosphate) and more metasilicate, showed much lower zinc losses. A more systematic study of the action of sodium metasilicate in reducing the corrosive action of several polyphosphates was carried out by placing 200-ml. portions of the test solutions in &ounce bottles and maintaining the temperature a t 80" C. in a water bath. One of the usual copper strips was placed in each bottle. At the time intervals indicated in the figures, the solution in one bottle of each series was analyzed for dissolved copper.
TABLE I. SOLUTION OF COPPER IN DETERGENTS F O MECHANICAL DISHWASHING
~
Preliminary Tests Copper Dissolved in 200 MI. of Solution, Mg.
0 05%
1.00% Formula Aa 4.7 14.1 Sodium tetraphosphate 10.2 57.2 Tetrasodium pyrophosphate 12.1 31.4 Sodium metasilicate pentahydrate 0 0.9 Trisodium phosphate6 0 0 0 5 Composed of 40% tetrasodium pyrophosphate, 30% trisodium phosphate (commercial dodecahydrate), and 30'% sodium metasilicate pentahydrate. b Commercial dodecahydrate.
PRELIMINARY SOLUBILIZATION EXPERIMENTS WITH COPPER, BRASS, AND ZINC
All solutions were made up in distilled water unless otherwise stated. Preliminary laboratory tests showed that polyphoephates were capable of marked solubilization of metallic copper as shown in Table I. These tests were run on a commercial dishwashing formulation, which developed notable staining in field service (Formula A of Table I), the separate constituents of this formula, and an alternate polyphosphate.
0.10% 8.4 18.8 17.5 0
Figure 7 shows the rate of solution of copper at 80" C. in 0.15% solutions of mixtures of tetrasodium pyrophosphate and metasilicate in various ratios. Figure 8 shows the rate of solution of copper at 80' C. by sodium tetraphosphate and by mixtures containing sodium metasilicate and sodium tetraphosphate in various ratios, the total concentration of electrolyte being 0.15%. Figure 9 shows the effect of sodium metasilicate and perhaps of crystalline trisodium phosphate on the solubility of copper in hexametaphosphate. The upper curve shows a high rate of solution of copper in 0.06% sodium hexametaphosphate U at 80" C. The lower curve shows the greatly reduced rate of solution of copper in a solution containing the same amount of hexametaphosphate plus 0.06% sodium metasilicate and 0.023'% trisodium phosphate.
The tests were made by placing 76 X 25 X 0.84 mm. (3 X 1 X 0.033 inch) copper strips in 200 ml. of solution, boiling for a short time, and allowing to stand for about 8 hours. Copper determinations were made by acidification with acetic acidt addition of excess potassium iodide, and titration of the resulting solution with 0.01 N sodium thiosulfate, using starch as indicator. The results, which were reproducible to within about 1 0 . 2 5 mg. of copper, coupled with the other known facts, indicated that copper was dissolved from brass, bronze, or copper parts of dishwashing machines by the polyphos hate constituents of the newer washing compounds rather than gy sodium metasilicate or trisodium phosphate. Formula A next was proved to attack brass. Yellow brass strips 76 X 18 X 0.79 mm. (3 X 3/4 X 1/38 inch) were immersed in replicate 200-mI. ortions of 0.15% solutions of Formula A maintained a t 80" One of the portions was analyzed for copper a t hourly intervals. The copper content increased by
8.
146
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1952
147
tion of the test solution loranother %hour period and the test so continued lor fivo exposures. Po!yphonnhatc . . ~.. .~~ ~~.Nuniinsi Commercial . _ _ _ _ ~ Grade Mntcriala .. .. Table IV records the data obidenlilyMoiceulsr PolyTrisodium Sodium Sodium meteing Code ratio, phoar>haie, phorphate.. carbonate. si1ioala.tuined lor yellow brass. All iorLetter 1rtts:m N s 1 0 to PzO, % 1211r0. % % 5HzO. % Othai materiala. 3 '6 mu189 showing the more severe A R 2 40 0 30.0 30.0 action on brass dissolved someH T 1.5 22.1 27.5 50.0 C U 1 31.2 2,*.7 4.5 39.2 what less copper from capper 3.4 58.6 D 1 356 P6.Y I' 1.lUb 41.6 0.6 56.1 strips in parallel tests. Where 2 23a 22 3 as.0 41.0 P the IOSRCX were low the differ0 1.18b 22.0 39.8 38.2 fl 1.02b 27.X 38.1 35.1 cnces lound were insignificant. T i b 34 n 5 . 0 60.0 Sodium thioaulfatc I I J X 1.67 37.5 57.5 wetting agent 5 Omitting Formula M from con60.0 K X 1.87 40 0 sideration beesuse oi its un459 28.2 L R 2 0 19 0 4 8 M 2 16 7 ~ 0 16 7 30.6 7.2 'odium tetrnhorale usual composition, ail ~ormuias (0.6lirO)-a8.5 contain substantial proportions " SO0 ?''hie 111. b 1 3 nnaiylis. ~ I D m a wensel them value8 rearcsent nlixtuilra of p d y ~ l m ~ h a t e01 s . There ratio erceoda 2 80 I . of both polyphoBph,hu~s mixtorrs w i t h iiuseiwited orthophusphater. metwilicate; a low order 01 activity is shown by several lormulas; B low order of activity is generally associated with it high metasilicate to polyphosphate ratio; 01 the 10 formulas,5 in the case 01 brass and 7 in the (Kt8e of copper show a msnimurn corrosion at tho iritermediateconcentratian of 0.3%; and no formula shows B condinuous increase in ~ o l u bilizing sotion with inoreasing concentration. Formula M i s exceptional both a q to composition nnd high corrosion 8ctivit.y. Borax rwt,hcr than polyphosphate may be responsible but a t the eame time the iormulta is Ion, in silicate. Table V shows the results of similar teats on brasa carried out with individual polyphosphates. Similar results were obtained for oopper. These teat8 Ehow B higher love1 of corrosion in genersl than Table IV lormulations containing polyphosphates; an i n c p a e in corrosion with concentration without exception; only insignificant diKmnees in the activitieR of the polyphosphstw; find an approxirnlttcly linear increase in wcight 1088 with OTHER SULUBILIZATION EXPERIMENTSON BRASS ARUCOPPER time in :dl P B S ~ H . Tho solubilizing action on copper and t i r ~ s shy numerous additional commercial formulations whose compositions appear irr Tahlc I1 was investigated. Tahle 111eupplics additional datil on the varioua phosphates used. The corrosion test8 were rniide at 0.15, 0.30, and 0.50% comentrntions by weight. For each inciividual tcst, 250 ml. of the test solution were plaetid in :%n P-ounce bottle in a bath st 80" C., and B weighed 76 X 18 X 0.84 nim. ( 3 X a/, X 0.033 inch) copper or p?liow hr:mY st,rip \'-as placed in the solution. A t the end of 2 houru the strip was removed, rinsed in dirtillcd water, quickly dried with a paper towel, and reweighed. The strip WILS then placed in A fresh 2.io-mI. porTABLEIt.
NOMINAL COMKMTIOY O F CoMmucihi. D~SHWASIIINQ FORMULATIONS TESTED ,~~
Figure 1. Corroded Pump Impeller
~~
Figure 2.
Corroded Water Distributer Head
Vol. 44, No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
148
which have appeared in Table V. Although different samples were involved, the agreement k considered satisfactory.
Figure 3. Severely and Moderately Corroded Vulve Parts
DISCUSSION
.Figure 4.
That copper, brass, and einc are readily dissolved from the metallic state by polyphoaphates generally k well eatahlkhed by these studies. On occasion, solutions of faint but distinct blue coloration resulting from the corrosion of copper have been obaerved; this coloration could be enhanced by the addition of ammonia to form the Cu(NII&++ ion complex. The maximum corrosion rates ohserved for plyphosphates alone are equivalent to 0.0123 and 0.0115 inch per year for copper and brass,
Brass Tube between Pump and Spray licads Io ac.rieo 3 ye'va
ZN ~ o l . ~ P l O NOF s TABLE IV. WEIGHTL a s s ~ s01 Bmss STRIPS COMMERCIAL Drsmvastu~oCOMPOUNDS CONTAINING I'OLYPHOSPBATTES
Temperstirre 80' C.. 250 MI. of Solution Citmuistivc Weight Lms. M g . . . . .__.. ConoentrRDishwaehing Formulation B
tioil.
?&
0.15 0.30
0.50 D
E F
0.15 0.30 0.50 0.15 0.30 0.50 0.15 0.30
0.50 G
0.15 0.30 0.50
H I
I
K
0.15
0 30 0.50 0.15 0.30 O.EO 0.15 0.30 0.50 0.15
0.30
0.50
L
0.15
0.30 0.50
2 houra 0.4
Ill;
0.4 0.6 +0.2* 0.7 0.7 +O.l
0.7 0.6 0.6 0.8 1.2
0.8 0.7 1.4
0.5 0.2 ;.5 1.6
0.5
0
0.4
0.5 0.1 0.6 0.9 0.4 1.2
1.3 L3
4
houra 0.5 ;.4
1.1 0.6 0
0.9 0.7 0.1 0.8
0.7
1.1 1.4 1.7 1.7 1.5 1.8 1.5 0.8 0.2
+o.i
1.6
0.4 0 0.4 0.1 0 1.0 1.7 1.2 1.8 2.9
3.0
6 hours 0.6 0.7 0.2 0.3 0.7
0 1.1 1.0 0.1
1.0 1.5 1.6 1.8 2.6
2.5
1.4 2.8 2.4 0.5
0.4 0.3
1.7 n.6 0.1 0.6
0.3 0.3 1.2 2.6
2.1
1.9 5.1
5.3
8 hours 0.5 0.7 0.2 0.7 0.6 0.3 1.2
0.7 0.4
I 1 2.2
2.0 2.0 3.6 3.3 2.0 3.5 3.1 0.6 0.6
0.5 1.6
0.8 0.4 0.7 0.4
0.3 1.2 3.1 3.1 2.3 6.7 7.4
10
TABLE v.
WElQHT Losses O P B R A SSTsE's IN POLYPHOSPXhTlP Sor.nr,onm A T ROO C.
250 MI. Of Solution
~
IlDurJ
0.6 0.8 0.3 0,7
PO1 phos&.te
T
0.7
0 1.0
W
1.1
0.1 1.4 2.5 2.1
X
Cumulative Weight L m . Mg.. caoeentration, 2 4 6 8 10 hours how8 homo hours hours % 0.05
0.15 0.50
0.05 0.15 0.50 0.05 0.15
n..m ~
Y
2.4 4.5
~~
0.05
0.15 0.50
1.0 1.8
1.8 2.7
1.Q
3.3
0.2
0.8 0.9
0.3 2.4 0.8 0.Q 1.8 1.0 1.3 2.5
4.0
1.6 1.8
3.0 1.5 2.4 4.1
3.4
3.7
2.5 3.8 5.1
4.8
4.6
6.8
K?
1.6
2.2 5.6 2.0 2.6 3.Q
2.5 2.9
2.7 3.5
7.3 2.7 3.5 5.3
8.1 3.0 4.3 6.1
2.3 3.5 5.7
3.1 4.7 7.0
3.a 5.6 7.4
4.2
2 1 4 8 4 n
0.8 0.7
0.3 1.9
0,8
0.4 0.5 0.4 0.5
1.8
4.3 3.6 2.8
8.4 9.4
TABLE VI.
WElQHT CEANQES OF COPPER STRIPS IN %LUTIONs
OP
Vnarous ELECP~~LYTES
2w MI. of o.is%Solution at 800
c.
Cumulative Weight Change,
MR. Compound 2 hours Sodium hydroxide 0.0 Sadium carbonate +0.5* Sodium bicarbonate +0.1 Sodmm metssiiioete.5H,O +0.1 Sodium ohlorids +0.2 Trisalium phosphate anhydrous C0.1 Trisodium ohasohateAZHt0 +0.2 . . Sodium tetraphmphste -1.0 -1.9 Sodium hexsmetsphoszhate Tetiaaodium pyrophhephate -1.i
Ratio.
~
10 hours +0.7 +0.4 -0.6b
-0.2 -0.2
-0.3 f0.1
NaaO to PEOS
-5.5
3.0 3.0 1.5
-6.3
2.0
-5.5
1 .n
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1952 4
I
I
I
I
149
d
B 1 3 n w
3
8 Y
0
e-
0 Q:
w
/
a I-
e)
0
0
/
L
O
'
!
-
O
O
TIME
- HOURS
Figure 5. Rate of Solution of Copper from Brass by a Detergent for Mechanical Dishwashing
TIME
200 ml. of 0.15% solution of Formula A a t 80' C.
- HOURS
Figure 6. Rate of Solution of Zinc by Detergents for Mechanical Dishwashing
/-
5-
,& L I
200 ml. of solution at 80' C. 1. 0.06% solution tetrasodium pyrophosphate 2. 0.15% solution Formula A 3. 0.15 % solution Formula C
1
I
I
I
I
I
4-
Q
IW
3
0
3-
v)
$? 0
a
w
H 4
-0-0-0-
0
1
2
3
TIME
4
- HOURS
5
6
Figure 7. Rate of Solution of Copper by Sodium Metasilicate-Tetrasodium Pyrophosphate Mixtures
-
TIME HOURS Figure 8. Rate of Solution of Copper by Sodium Metasilicate-Sodium Tetraphosphate Mixtures 200 ml. solution at 80' C., total concentration 0.15% 1. 0.00% NaaSiOa.5HaO 0.15% NaeP4Ou 2. 0 06%NarSiOa 5HrO' 0.09% NesP~Oia 3. O b % NaaSiO::SH@,'O.06% NamP4Oi: 4. 0.12% Na&01.5HsO, 0.03% NasP4Oii
respectively, and 0.0036 and 0.0060 inch per year for the detergent formulations of Table IV. Such rates are by no means sufficient to account for the failures of impellers, valves,
etc., observed within short perioqs in the field. The temperature of the tests reported in this paper (SO" C . ) exceeds those of commercial dishwashing practiceusually 55' t o 70' C. Field TABLE VII. EFFECT OF ADDED MATERIALS ON SOLUBILIZATION O F COPPERBY POLYPHOSPHATE experience commonly indicates SOLUTIONS the corrosion problem to be Milligrams aopper diasolved from 76 X 18 X 0.84 mm. strips by 200 ml. solution at 80' C. more acute in communities Tetrasodium Sodium Sodium Pyrophosphate Tetraphosphate Hexametaphosphate water apPolyphosphate 7 0 0 09 0.076 0.06 0.06 0.03 0.09 0.06 0.03 0.06 0.06 proaching zero hardness. All Sodium rnetssificate, 5HaO % 0.06 0.075. 0.09 0.12 0.06 0.09 0.12 0.06 Trisodium phosphate, anh;drous, % 0.023 testa reported here were made Hours in distilled water solutions. 2 1.5 1.7 1.4 0 . 6 0.8 0.4 0.2 1.7 0.3 Other variables which might 2.3 2 . 5 2.8 1.8 0 . 8 1 . 2 0.6 0.3 2.5 0.4 3 well be investigated are dy4 3.1 3.1 3.3 2.1 0.9 1.6 0.6 0 . 4 3.4 0.5 5 3.9 3.4 3.8 2.4 0.9 1.8 0 . 7 0.4 4.2 0.6 namic us. static exposure, 4.6 3.6 4.8 2.6 0.9 1.Q 0.8 0.6 8 composition and metallurgy
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 44, No. 1
dissolved copper down to 1.0 mg. or less in a 6-hour test. An accumulation of practical commercial experience indicates this test offers valuable guidance. SUMMARY
-
TIME-
HOURS
Figure 9. Rate of Solution of Copper by Sodium Hexametaphosphate and a Related Formulation 200 ml. of solution a t 80" C. I . 0 . 0 6 % (NaP0s)e 2. 0.06% (NaP0s)s
0.06% NaaSiOa.5HgO
0.023% NasPO~.lPHpO
Solutions of four polyphosphates have been shown to dissolve copper, zinc, or brass. Corrosion rate data are presented for several metal-polyphosphate combinations a t 80' C. and for several commercial and experimental dishwashing detergents. The inclusion of sodium metasilicate affords a practical means of retarding the corrosive action of polyphosphated dishwashing detergents on dishwashing machine parts. Of the several polyphosphates investigated, pyrophosphate is the most difficult to inhibit. Corrosion of copper, brass, or bronze pumps, valves, and spray nozzles by polyphosphated detergents may seriously reduce the efficiency of dishwashing machines. Many complete failures of machine parts within 6 months have occurred. A brassy tarnishing of silverware washed in certain dishwashing machines is attributed to copper solubilizedfrom machine parts by poorly inhibited polyphosphated detergents and then plated on the silverware by electrolytic action.
of metals and tdloys, and electrical leakages from the power supply. Data on the effectiveness of agents added to reduce the corrosive action of polyphosphates on copper have been collected in Table VII. It is much more difficult to inhibit the solubilizing action of tetrasodium pyrophosphate than that of sodium tetraphosphate or sodium hexametaphosphate by the inclusion of sodium metasilicate in the formula. A metasilicate-polyphosphate ratio of 3 to 1 or 4 to 1 for pyrophosphate or nearly 1.5 t o 1 for tetraphosphate or hexametaphosphate appears to be needed to hold
LITERATURE CITED
(1) Bacon,
L. R.,and Nutting, E.
G., Jr., IND. ENQ.CEEM., 44, 150 (1952). (2) Gilmore. B. H.. Ibid.. 29. 584-90 (1937).
ENQ.CHEM.,26,998-1001 RECEIVED February 23, 1951. Presented before the Division of Water, Sewage, and Sanitation Chemistry a t the 119th Meeting of the AMERICAN CAEMI C A L SOCIETY. Cleveland, Ohio.
(Polyphosphate Detergents in Mechanical Dishwashing) METALLIC STAINING OF SILVERWARE LESLIE R. BACON AND EUGENE G. NUTTING, JR.
observed after washing in conventional dishwashing machines. The stain was shown to be a surface deposit of copper, apparently derived from metal dissolved from machine parts by dishwashing solutions containing polyphosphates. Data were presented to show the solubilizing action of several polyphosphate and polyphosphated dishwashing compound solutions on copper, zinc, and brass. I n the present paper the conditions essential to the staining phenomenon and means for its avoidance will be reported more fully. Two typical examples of staining observed in the field may be described. Both followed the introduction of the detergent Formula A (tetrasodium pyrophosphate 40%) trisodium phosphate 30%, and sodium metasilicate pentahydrate 300j0)into service. After 10 days of use in a hotel dishwashing machine, the silverware suddenly began t o come through badly tarnished, appearing burned. I n short order 400 to 500 pieces were thus affected. Careful examination of the water supply and the machine did not disclose the cause of staining. In a large cafeteria, staining developed in a machine used for washing glasses and silverware. The stains were more prevalent on the lower grades of silverplate, especially a t spots where the silver was worn through. After a fresh detergent solution had been used for a short time, staining became evident and nearly
General experience indicates that exposure of the base metal through imperfect plate, whether porous, scratched, or worn through, accentuates staining, particularly in the case of cheaper ware which is lightly plated. Fanlike or circular colored spots at times may be seen radiating from imperfections in the plate. Colors vary, depending on thickness and character of the deposit. Although stains commonly appear metallic or brassy, shaded blues and browns suggestive of sulfide stains have also been seen. The base metal of better grades of silverware is usually nickel silver (German silver) the composition of which is approximately 65% copper, 5 t o 25% nickel, and 10 to 30% zinc. EXPERIMENTAL METHODS AND DATA
CONFIRMATION OF ELECTROCHEMICAL CHARACTER OF STA~N FORMATION. All solutions were made up in distilled water unless otherwise stated. After it was discovered that polyphosphates are capable of solubilizing copper, experiments were carried out to determine the conditions under which the copper would deposit on pure silver. Strips of pure silver 121 X 19 X 1.6 mm. with a 2.4-mm. hole drilled in one end were used. Two com-
