Effect of Alkaline Detergents upon Metals Aluminum, Copper, Tin, and Zinc Strips of the metals were immersed in the alkali solutions at 60' * 2' C. and observations were made as to the effect of concentration and immersion time. The range of concentration-immersion time conditions which produced no visible effect on copper and tin was established. After 24-hour immersion the appearance of the specimens is shown photographically and the change in weight is given. The silicates of soda impaired the surface of the metals less than the other alkalies considered with one exception in the be-
havior of sodium carbonate solutions toward zinc. The superior behavior of the silicates seems to be related to the presence of silica in their solutions. Among the sodium silicates themselves, an increasing ratio of silica to sodium oxide seems to give greater protection. Sodium metasilicate (Na2SiOs.5HzO)is the silicate most commonly used for industrial metal cleaning, but the greater protection of the more siliceous silicates is available through the use of liquid or powdered products.
CHESTER L. BAKER Philadelphia Quartz Company, Philadelphia, Pa.
T
HE march of industry toward more rapid The preparation of highly polished metal articles for electroand efficient cleaning methods has encounplating not only requires the surface to be delivered in a chemitered a serious obstacle in the necessity of cally clean condition-i. e., free from oil, oxide, and possible products of reaction with the detergent used-but must not delivering the finished article in an unimpaired condition. have its polish impaired. A rough base surface is apt to result This is particularly true in metal cleaning, for metals as a class in uneven and incomplete coverage with the plating metal, to are subject to both electrolytic and chemical attack. Most give a matt instead of a Dolished d a t e d finish. cleaning baths are good electrolvtes, so that small localized -These examples can indicate only differences in potential on the- surbriefly the diversity of industrial cleanface of the metal being cleaned tend ing in which the effect of the detergent to become galvanically active with and cleaning technic upon metal surresultant corrosion. Most metals are faces must be carefully considered. dissolved in acid cleaning baths-hence While detergent practices which give the preference for alkaline detergents some degree of satisfaction have been whenever it is practical to use them. evolved by most industries, these pracA few of the metals are s u b j e c t t o tices often represent a compromise beattack even by alkaline cleaning baths. tween maximum cleanliness and miniAmong the latter, tin and aluminum mum surface injury. It seems worth present particularly difficult cleaning while to make a study of the factors problems. involved, hoping to lay the foundations The dairy and other food industries for better detergent practice. have long preferred tin and aluminum Starting with the assumption that as metals for direct contact with their an ideal metal cleaning technic must products. Faturally their equipment leave the metal surface unimpaired, it must be frequently and scrupulously is obvious that the composition of the cleaned. A n y d e p a r t u r e f r o m a cleaning solution, the cleaning tempolished surface, through c h e m i c a l perature, and the time that the metal attack or otherwise, m a y p r o v i d e /MMERSION TIME IN HOURS is subjected t o the action of the cleanharboring places for undesirable bacFIGURE 1. MAXIMUM CONCENTRATIO\S OF ing solution are important. That any teria. The use of a noncorrosive deALK~LIES WHICH DID NOT VISIBLY ETCH given combination of these factors is tergent and cleaning technic can often TINPLATE apt to have a different effect on differincrease profits considerably through T h e larger safe concentration-time area with the silient metals is t o be expected. a longer useful equipment life. cates is noteworthy. 1358
NOVEMBER, 1935
IYDUSTRIAL AND ENGINEERING CHEMISTRY
1359
light etching in the more dilute solutions to a severe etching in the more concentrated ones. DID NOTETCHTIN PL.4TE AT 60' c. I n the silicate-treated pieces this gradation re-Max. Concn. at Exposure Time of: 0 . 5 hr. 1 hr. 1 . 5 hr. 2 hr. 3 hr. 4 hr. 8 hr. 24 hr: mailled even after 24 hours Of exposure; in the pieces treated with the other alkalies, it largely % % % % % % % % Alkali: disappeared. Na3P04.12HzO 0.20 0.01 0.01 0.01 0.01 0.01 0.01 0,005 NazCO:, 0.01 0.005 0.005 0 , 0 0 5 0.005 0 . 0 0 5 0 , 0 0 5 0,005 The maximum concentrations of the various NaOH 0.005 0.005 0.005 0 . 0 0 5 0.003 0.005 0.005 0 , 0 0 5 alkali solutions which did not visibly attack the NatSiOa.5H;O 0.25 0.15 0.15 0.15 0.15 0.15 0.15 0.05 tin in the various exposure times are given in Sodium silicate: C BW y::: y::: "0:"s: :; Table I. Figure 1 present's graphically the safe U 8.00 8.00 8.00 8.00 8.00 8.00 5.00 5.00 area of concentration and exposure time for each K 50.0 45.0 40.0 35.0 30.0 30.0 30.0 25.0 alkali. Knowledge of these areas should be useN . . 80,O 60,O 40,O 30;O 30,O 3040 S ful in selecting the right alkali for a given cleana N o visible effect at any concent,ration. ing process. It is noteworthy that of the strong alkalies onlv sodium m e t a si 1i c a t e (Na2Si03.5H20j a n d - t r i s o d i u m p h o s p h a t e (NaaPOaThe present work has been restricted to the effect of the 12H20) permit the use of effective concentrations, and the more common alkalies upon tin plate, aluminum, copper, and latter for an immersion time of 30 minutes or less. Where a zinc a t a single temperature, 60" C. The effects of the varihigher sodium oxide concentration or a longer exposure is reables of concentration and inmersion time have been studied, quired, the use of one of the liquid silicates of soda is indicated. and an attempt made to present the observations in such a The loss in weight suffered by the pieces when immersed in their respective solutions for 24 hours a t 60" * 2' C. is manner that they may be useful in fixing certain limits in industrial cleaning practice. Further work with mixed alkashown in Table 11. Some irregularities occur and are probably to be explained by irregularity in the original tin plate lies as well as a t other temperatures is under wag. or faulty technic. I n the main, the losses in weight are conProcedure sistent and correspond to the appearance of the treated pieces. The metals used in this work were obtained in KO. 22 E. S. From these data it appears that some loss of tin may occur without visible etching, although severe etching is usually Standard gage sheets and represented in each case a highgrade commercial product; hot-dipped pure block tin plate, accompanied by heavy loss of metal. virgin aluminum, commercial sheet zinc, and electrolytic The data of Table I1 permit valuable comparisons between copper. The metal was in each case cut into strips measurthe alkalies and concentrations used. It is not safe, however, ing 0.75 X 2.5 inches (1.9 2: 6.35 cm.) to assume that, because a given concentration of any particuThe sodium hydroxide, sodium carbonate, trisodium phosphate (TSP), and sodium metasilicate (Metso) were of c. P. quality. The percentIMM:RSED TABLE11. WEIQHTLOSTFROM TISSURFACE: age composition of the liquid silicates used is as IN ALK.4LI SOLUTIOSS FOR 24 HOURSAT 60 * 2 c:. follows :
TABLEI. MAXIMUMCONCENTRATIONS OF ALKALIESWHICH 7
i:::
;;::
Silicate
Nag0
SiOr
Ratio SiOt/Na,O
BW
19.4 18.0 13.8 11.0 8.9 6.4
30.6 36.0 33.7 31.2 29.0 24.7
1.58 2.00
c
U K N S
2.44 2.84 3.22 3.86
A series of solutions was prepared of each alkali; 100 ml. of each in a tightly stoppered glass bottle was brought to 60" * 2" C. in a Freas constanttemperature oven. A numbered metal strip that had been carefully cleaned, dried, and weighed was immersed in each solution; the bottles were again stoppered and returned to the oven. At regular time intervals the immersed strips were examined, and a record was made of all pieces which showed visible change. At the end of 24 hours of immersion the pieces were removed, thoroughly washed in distilled water and then alcohol, dried, and weighed.
Tin Plate I n examining the tinned pieces for etching, a number were usually found in the lower range of concentrations which had not been visibly attacked. There was usually no question as to whether a given solution had caused visible etching or not, although the line of demarcation became sharper with increasing time of immersion. Among the etched pieces, the damage effected by short immersion times graded from a very
Concn. NasPOl of Soin. 12HzO NalCOs
TTeight Lost, Grama per Square Metera----NatNaOH Si035HdI BW C U
% 0.005 0.010 0.05 0.10 0.15 0.20 0.25 0.30 0.40 0.50 0.60 0.80 1.00 1.50 2.00 2.50 3.0 3.5 4.0 4.5 5.0 6.0 7.0 8.0 9.0
0.22 0.43: 0 . 5 ~ ~0 . 4 x e 2.3e 2.Se 2.8e 2.7e 3.le 3.2e 3.0e 2.8e 2.Se 3.2e 2.7e 2.8e 2.8e 2.7e 2.7e 2.6e 2 .7e 2.9e 2.9e 2.7e 2.4e 2.8e 2.4e 2.7e .. 2.7e 2.9e
.. ..
..
2.42 3.5e
3.6e 3.8e 3.4e 3.4e 3.5e 3.7e 5.2e 3.08
..
0.72 0 ., 52 , 2 2.4e 2.6e 2.4e 2.7e 2.8e 2.9e 2.6e 2.9e 2.8e 2.4e 2.7e
..
.. ..
..
S
. . . . . . . . . .
2:i
.
2.3
0:i
.
0:i
.
.
2.5
0.2
0.1
.
z:0 o:i
1. 0.1 . . . . 0 . 2 ..
. . . . . . . . . . . .
. . . .
,.
.
,.
0.2 1.3 2.le 2.3e
0.6
0.3 1.0 1.7e l.7e 2.le 1.7e 1.2e
. . . . . . .
. . . . . . .
.
.
. . . . . .
. . . . . . . . ,.
.
0.2 0.1 0.2 0.4 0.5 0.4 0.2 0.5
. . . . . .
..
,.
.. .. .. .. .. ..
,. .
.. .. ..
0.8
. . . . . . . . . . . .
.
,.
.. , . . . . .
2.5e 2.2e 2.4e
. . . .
.
.
::
,
2.2e 2.88
.
.
o:i
2.4 1.4 2.3 1.6 2.4 1 . 2 2.5e3.2e0.0 2.5e3.5e0.1 2.4e3.le0.2 2.8e 3.3e 0 . 2
..
.
::
. . . . . .
..
10.0 .. .. 12.5 .. .. .. 15.0 .. .. 17.5 20.0 .. .. .. .. 25.0 30.0 .. .. .. .. .. 35.0 .. .. .. 40.0 45.0 .. .. 50.0 .. .. .. 55.0 .. .. 60.0 .. .. .. .. .. 70.0 80.0 .. .. .. .. 90.0 100.0 .. .. 0 x = average value: e = visibly etched.
N
K
.
.
.
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . .
..
.. .. ..
o:i 0.0 0.1 0.2e O.Oe
.. .. .. 0:0
0:0 1.6e 1 . 7 e 0:i 1.6e 2 . 9 e 0:4 1.7e 0 . 2 3.6e 0.2 1.6e 0 . 3 0.4e 0 . 2
ii light cdor in t'igurt Thi? eteliiiig n-a.. always acrxrriipairied iiy the evulutinn of hydrogen and oceurred orer a definite range of higher concentrations. It, was niwt pronoiiniwl in diie rionsiliceous sohitioris. Chemical attack on tho aiuminiirrr was si,sible aliriost iinisieilintcly, i o that this study did riot establisti a time-concentratiori area within whii-h n o attnck ncciined, as did the stwly
slimviiig
\zit,h tiri plate. Tlie I m s in weight siltrered I,? the strips alter 24-hour i n iivrsiou is reciirtlcd iii Tahle IY. It should lie noted, Iiowever, as pointed out below, that in some cases the treated strips carried iui iiirrust.ation so that t,be weight losses giveii do not, in every case represent the total loss nf metal. T l r ~sorliirin iiydronide attacked the aluminurri iiliniedi:it,cl? :~iiii rigormisly at all concentrations wit,Ii the evoliit,ion
ll.lNl5
I1 01 0 05 0~10 0.15
0.20
11.2a 0.30 0 40 ll.50
0 % 0.1
0.3
Y.3d 4.5d 6.Od
0.4
0,s 0, 7 0 6
I 2
d 2 . 2d 10.6d 24.5d :36.ndl 4 3 . Id? 6b.Odo 76.4e 77.4di
0.3d n.4d I.0d
1.6d I ,811
7.0d 9.2d l2.4d
68.7dcr
15.0de
0.1e 0.2e 0.26
0.1
01
0.1 0.1
0.sa
0 0
0 0 0
u,1r
0
.. !I 005 0 01 0 0,5 0.10 0.15 0.20
11 I 0 1 2 ..'J
0 2 0 :i 1.8 1 7 1.x
1.7
1 8 1.8
I i 1 6
0.25
1.7
'. 1.isaea i n weight reprensiit
.
& V C ~ ~ J I OfBi n ~
c, 1
1 4
.:I
1.6 1.4 I :I 1 .i I .I 1.6 two ~~~
diKaicnr
0.0 0 1
0.5 0 6
0.x ex!?er~iiiPitts. ~~~
~
~
Figiira 2 shoas the appeara~iceof the pieces after iliiinerfor 8 Iiours. The depfree of visible etching is indicated lxlth hy the spangling aiid lip the clearness with which the picres sliow the reHect,ions of the hatclrcd lirics.
him
;Iluminum 111 the work with alumiiiuni it Was f u u ~ iiniiecca~ary d tu stiidy the action of the inwe siliceous silicates. Sudiurri s t w iiduded iii the iist of detergents u ice in cleaning ilmwtie aiuniinum. Ttre sodium stearate was prepwed by dissolving a rveiglieii quantity of triple-pre. I stearic acid in alcohol and carefully iicutraiiaing to pheri~,iplrtiialeiliwith an alcolsilic solution of siidiiini hydroxide. The alciiliol its evaporat,eti at
loo" c.
The ;ippearance of the s t r i p after 24 hours of iriirnersioii ut 60" * 2' C. is shown in Figure 3. The unattaeked pieces siiow- black arid give lbrillinnt reHections of the ruled lines. The sodace of the alurriinum was subject to two different kinds uf visible eliange. First, a darkening of the surface occurred which might assuirie any shade from a light gray to a rich black. This eharrge i w s brwght about over a definite range of concentration lig each of the alkalies wed. In most cases it was produced in the lower range of concentration. This condition is s l r o a n in Figure 2 by the strips which are dark but show little or no reflection. The second visible change comprised extcusive etching of the surface, leaving it diill hut. motallic in appearance, as indicated by the strips
0.211
0.25 0 .:XI 0.40
0.50 0 KO 0.SO 1.00 1.50
2.00
a . .50 8.00 a . 50
4.m 4 ,50 5.M) 6.m
7.m 8.00 IO.W
0.9wk 0.burk
0.9Wk Llwk
1 .9"~k
2 %wk
2.9d
1.5-k 04k 0.Sk
N.%k
0.31.
12.Sk 25.0k 151.411 24.0k 29.8h
0.51 0.81. I.lk
[email protected] 26.81. 21.Yk l0.2k 14.5k 20.0dk
14.2d 34.8d 20.2d
0.4k
14k 1.8k 2.0h 2~5k
2.91. 2 8k 5.5d
7.4d
&Id
11.Dd
3.2wk 0,2k 3.5wk 0.2k 2 . 4 w k 051. 1.5wk 0 . 2 k 0.9u.k 0 . 2 k 0.8luk 0 , 3 k 0.8k 0.8k 0.4k O.5k 0 5k 0,7k 1.2k 0.5k 1.5k 0.7k
0.71
3.lk
O.bk 0.7k 08k 1.2k 1.211 1.211 I.2k
3 0k a.Uk 3.4k 4.Yd 4.2d 5.6d
101
9.Sd
3.4d
3.diuk 3 .5 w k 0 71ak 0.5wk 0.8wk 0.41.
4.2mk
5.2wk
0.7k
4.3n'k
:3. it011 4 , 6wk i %u$ .5.2rr.i
0.51. 3.Iuk
0.5k O . P ~ 0.51. 0.3k 0.4k 0.6 0 5ii 0 5 0.9k 0,s L0k 0.7 1.71; 1.5 2.5 2 Y ii.5 2.0 a.9 3.3 4.1 a.2d 4.4d r.3d 5.2d
4.5d 6.7d
8.9d 9.0d
2.Xwk
2.3~1. l . l W i
O . h k 0.2wk 0.5wk
1.Itok 1.1 1.1 1.1 1.5
1.6
2.1
2.8 4.0
?OVEMBER, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
hydrogen. At cmcerrtratioris greater than 1 per cent tlie trip was cornpletely dissolved in less than 30 minutes. After !4 hours the sodium hydroxide had apparently dissolved iliiminum ipantitatively a t all concentrations. Sodium carininate attacked the aluminum imrnediately and rigorously at all concentrations u-ith the evolution of hydru:en. Slthongh most of the pieces develnped sonic incrusta.ion, tlie trite loss of metal after 24 liours appParcd to be quan,itntive. All of the solutions acc:nnulated a heavy gelatinous xecipitnte. Triwiiirini pluspliate attacked tire aluminrmi imniediately @vitlrtlic ciwliitiw of hydrogen at ail concentrations above ).G: ; r r cent. After 24 hours 110 ap~irecialilesnioiint of floc vas formed :lid inine of tlie pieces was incrusted. The metal was dissolsed ijuantitalively by the alkali a t all coneontr&tions. TIie etching at the higher concentrations proiluced :Jeep furrmz in tire metal. Sodium stearate attncked the nluminnm with the evokitioli of hydroge~ia t all concentrations n,l)ove0.40 per cent, causing the surface to blacken. At lower concentrations the rnetal SllffeFed spIm?ciable loss in weight and some loss in reflectivity more or less prnportional to the concentration of the solution. In all cases a greasy film of stearic acid was formed on the surface of the metal, and its presence probahly inhibited the attack somewhat, especially a t the higher coiicentrations. Tire sodium metasilicate solntions differed markedly from the nonsiliceous alkalies in that they left the metal entirely unattucked over a definite range of concentrations, Solutions ranging from 0.64 to 5.00 per cent produced no visible attack during 24 hours of immersion, and the weight lost by the metal was either nil or within the range of possible error in weighing. At concentrations belnw 0.W per cent the surface was slightly dulled, arid a t the very low concentrations was blackened, but the metal lost was small. At concentration? above 5.00 per cent the attaok increased with concentration. At 10.00 per eeut the loss in metal was con~derable. The significant fact brought out is that sodium metasilicate can be safely used for cleaning dimniimml at 60" C. over a definite range of concentration. The. behavior of BW silicate was similar to that of sodium metasilicate except that the safe range of concentrations was greater, 0.10 to 10.00 per cent. Only the three lowest concentrations produced any visible effect on the metal nr any significant loss in met.al. C silicate produced perhaps a slightly smaller effect on the nietal than UW. I t is probably safe to assume that the more siliceous silicates would attack tlie metal less than did C brand. tf
Zinc The appearance of the strips after 24 hours of immersion a t 60" * 2' C. is shown in Figure 4. Loss in weight suffered mider thc same treatment is recorded in Table V. The surface of the zinc was subject to tliree different kind3 of visible change. I n the lower range of concentrations of all the alkalies used, vhite patches developed on the metal. These occurred in small circular areas 1or 2 mm.in diarncter, or in large elongated blotches that tended to grow upnwd on the surface. The w-hite material was loosely adherent and flocculent in character. Even after thorough scrubl)ing a white stain rorriained on tlie metal. This inaterial was lorrncd to a great.er extent, the more dilute the solutkin, and even quite readily in disiilled water. It probably was zinc hydroxide. The loss in weight suffered by the metal in the lower alkali concentrations seemed to be roughly proportional to the area of the white patches formed. The higher concentrat.ions darkened the metal in all cases except with sodium carbonate and the U brand sodium silicate (this darkening was accompanied by severe loss of metal). In an intermediate
1361
range of concentration a slight, irregularly distributed dark film developed in solntions of all the nonsiliceous alkalies, and to some extent in tlie siliceous ones. This film, while visible to the eye, did not alipreciably affect the reflectivity of the metal surface.
INUUSI'IUAL AND IhYGINJ3IJl
