1494
INDUSTRIAL AND ENGINEERING CHEMISTRY
tween active and passive metal, with high electromotive force differences and attendant high current densities at the restricted activated metal areas. It is probable that, in inhibited nonoxidizing acids, pitting is analogously the result of cells set up between active metal and metal ennobled by presence of inhibitor. Active metal may be the result of contact of external substances like glass supports with the metal surface which prevents contact of inhibitor. At these areas, acting as anodes, the metal in carbon-monoxide-saturated solutions is approximately 0.1 volt more active and corrodes preferentially to form a pit nucleus.
Summary Stainless steel (18-8) immersed in dilute hydrochloric acid reacted only slightly on passage of illuminating gas through the acid. Carbon monoxide was found responsible. Other inhibitors were examined which were, in general, less effective than carbon monoxide. In ferric chloride, using the same inhibitors, no decrease in corrosion was observed. Carbon monoxide was found to inhibit corrosion in hydrochloric acid a t room temperature without pitting up to and including 6.3 N acid in 8-hour tests. In 9.0 N acid, pitting and contact corrosion took place. In 3.1 N acid, carbon monoxide was an effective inhibitor above 60' C. but below 90" C. Potential measurements showed that 18-8 assumes a noble potential or can be passivated in hydrochloric acid if carbon monoxide is present in the absence or presence of oxygen. It was found that inhibitors in general change the potential of mild steel and 18-8 to more noble values. This is explained by assuming that carbon monoxide or a similar inhibitor can decrease hydrogen concentration a t the metal
VOL. 32, NO. 11
surface by displacing lattice-dissolved hydrogen or by adsorption which limits production of hydrogen by galvanic action. Decreased lattice-dissolved hydrogen, as demonstrated by other measurements, increases nobility of the metal. This increase in nobility could, therefore, take place whether or not the inhibitor adsorbed on cathodic in preference to anodic areas. The reasons for observed pitting of 18-8 in inhibited hydrochloric acid are similar to those for pitting in ferric chloride.
Acknowledgment The author expresses his appreciation for the advice and support during this investigation by members of The Corrosion Committee a t the Massachusetts Institute of Technology appointed to supervise the research on pitting of stainless steels for The Chemical Foundation.
Literature Cited (1) Bannister, L., and Evans, U., J. Chem. SOC.,1930, 1361. (2) Burns, R. M., Bell System Tech. J., 15, 20 (1936); J. Applied
Phys., 8, 398 (1937). (3) Chappell, E., Roetheli, B., and McCarthy, B., IND.ENG.CHEM., 20, 582 (1928); Mann, C., Lauer, B., and Hultin, C., Zbid., 28, 159. 1048 (1936); Thiel, A., and Kayser, C . , 2. physik. Chem., A170, 407 (1934). (4) Grave, E., 2.physik. Chem., 77, 513 (1911). (5) Mears, R. B., Trans. Faraday Soc., 35, 467 (1939). (6) Uhlig, H. H., Metals Tech., 7, No. 5 (1940); Am. Inst. Mining Met. Engrs., Tech. Pub. 1150. (7) Uhlig. H. H., and Wulff, John, Trans. Am. Znst. Mining Met. Engrs., 135, 495 (1939); Uhlig, H. H., Metals Tech.. 6, No. 7 (1939), Am. Inst. Mining Met. Engrs., Tech. Pub. 1121. PRESENTED before the Division of Physical and Inorganic Chemistry a t t h e 98th Meeting of the American Chemical Society, Boston, Mass.
Decreasing the Combustibility of Sawdust J
Relative Effectiveness of Certain Chemicals JOSEPH L. McCARTHY' University of Idaho, Moscow, Idaho
ERTAIN chemicals impregnated in woody tissue may induce fire resistance by one or more of the following means: The chemicals evolve gases (such as ammonia) which largely exclude oxygen from the combustion area; the chemicaIs (such as borax) fuse over the surface of the combustible material and thereby shield it from oxygen; the chemicals undergo endothermic reactions (such as vaporization of water) and thus remove a portion of the heat of combustion. That these several types of protective reactions occur is fairly well established (2, 3, 7, IS). Although a great number of chemicals have been patented as fireproofing agents (O), only a relatively few scientific investigations concerned with their effectiveness have been carried out. Among those dealing with wood, the work of Gillet (2, S), and of Hunt, Truax, Harrison, and Baechler (4, 6, 6, 10, 11, 12) may be mentioned. Plunguian and Jahn (8) studied the fireproofing of fiberboards. The fire-tube ap-
C
1
Present addresa. MoGill University, Montreal, Canada.
paratus, described by Truax and Harrison (10) and used in the United States Forest Products Laboratory tests, as well as those of Jahn and Plunguian, has been of great value in standardizing the method of testing the fire resistance of treated wood and other materials. By this instrument the percentage loss in weight of the sample and the temperature attained above the tube may be ascertained a t time intervals during the test period. The sample is subjected to a constant gas flame during a given portion of the test period. Because of difference in density and surface area of sawdust as compared with sawn boards and fiberboards, it is possible that the relative efficiency of various fireproofing reagents, as found by other investigators for wood and fiberboards, might not necessarily be valid for sawdust. I n fact, Eichengrun (1) pointed out that the same chemical impregnated in such materials as fabric, wood, and paper gives quite different results in each case. Moreover, even if the same order of effectiveness held true, i t is probable that differ-
NOVEMBER, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
The firetube apparatus of Truax and Harrison has been modified in such a manner that the fire resistance of sawdust samples can be reproducibly and rapidly determined. Western white pine has been impregnated with various quantities of several chemicals, and their relative efficiencies in decreasing the combustibility of the sawdust have been determined. The decreasing order of effectiveness of the chemicals studied (based on loss in weight of the sample during the test) is as follows : diammonium phosphate, borax, monoammonium phosphate, zinc chloride, ammonium sulfate, ammonium chloride, and magnesium chloride. The first four are effective in moderately small amounts (8-10 per cent) and may be used to fireproof sawdust. Larger amounts of ammonium sulfate and ammonium chloride are required; magnesium chloride is not suitable as a fire-retarding agent. Whereas borax is a relatively good fireproofing agent when impregnated into sawdust, it is without effect when simply mixed in the dry form in sawdust.
1195
ent amounts of the agent would be required for the sawdust than for the boards. Because of the possible value of sawdust for insulation and other uses, a study of the fireproofing of sawdust is of interest.
Impregnation and Testing of Sawdust Samples Idaho white pine sawdust (Pinus moiaticola D. Don)! prepared at the plant of Potlatch Forests, Inc., at Lemston, Idaho, by disintegrating air-dry planer-mill shavings and sawdust in a hammer mill, was used in this investigation. It had the following screen analysis: Fraction of Total Weight
Particle Size on Standard Screens Mesh Coarser than 20 20-40 40-60 60-80 80-100 Finer than 100 Unaccounted for Total
% 37.96 33.00 11.48 7.00 3.79 4.25 2.52 100.00
-
From a study of the literature the following chemicals were selected as being of probable value in introducing fire-resistance into sawdust: ammonium sulfate, ammonium chloride, monoammonium phosphate, diammonium phosphate, magnesium chloride, zinc chloride, and borax. The white pine sawdust was treated with concentrated aqueous solutions of the fireproofing chemicals. Each sample was thoroughly mixed manually so that the chemical was homogeneously sorbed by the sawdust. The water was then allowed to evaporate
TABLEI. COMBUSTIBILITY OF QAT~-DUSTSAMPLES Weight Loss of Samnle
% Xone
0 (5)
60
c,W 5 0 V
40
3-
9 30 !F Y
s 20 10
+ 5-b
j#
+lo% t150b
"
"
0
I
t
0
FIG-
1.
/
/
I
/
"
I
I
2 3 4 5 COMBUSTION TIME, MINUTES
RATE OF LOSS IN WEIGHT OF
SAWDWT SAMPLES
BURNING
7c
.Ifin.
"/o
70
100 100 100 100 100 SO 95 95 100 100 100 100 15 100 55 30 70 55 60 95
5.66 4.45 4.00 3.30 4 , I66 3.45 3.33 2.75 4.66
78 74 68 68 77 48 48 43 70 70
72 72 68 68 71 50 49
4.42 3 . I10
4.33 1.37 5.00 2 . !96 2.28 3.92 3.47 2.08 5.28
441
36 28 63
67 69 58 70 27 65 28 26 44 37 32 60
330 272 238 359 166 160 11s 326 292 162 312 29 350 83 57 169 125 58 332
54
72 21 65 28 25 43
44
75
3.75
35
36
131
70
3.47
35
35
121
75 16 10 100 10 15 20 15 25 90
3.28 1.37 1.20 5.25 1.67 1.37 1.33 1.30 1.50 5.11
34 19 16 68 17 20 19 20 20 55
35 24 20 60 19 20 25
111 26 19 35s 27 27 25 26 37 281
22
21 51
a Percentage of impregnated chemical (as formula) based on the weight of air-dried sawdust: figures in parentheses, /ndicate number of experiments carried out under these particular conditions. 1, Rouvh visual estiniate of charring of t h e 6-inch-diameter surface of t h e sam le c hrtming time from a p lication of ignition, flame t o extinction of flame. d Product of t h e flame &ration and the weight loss a t flame duration. e Althoueh mesumably used a s t h e hydrate, the chemlcal was weighed o u t as MgCli. f Sample was dusted and mixed with boras, not impregnated.
_ _
INDUSTRIAL AND ENGINEERING CHEMISTRY
1496
fI Q
z
80
60
c -40
4
iY
2o
I
0
2
4
6
8
IO
I2
14
16
CHEMldL SORBED BY SAWDUST, PER CENT
FIGURE 2 (above). EFFECTOF AMOUNT OF CHEMICAL SORBED UPON THE WEIGHT Loss OF SAWDUST SAMPLES IN BURNING OF CHEMICAL SORBED FIGURE 3 (below). EFFECTOF AMOUNT UPON THE FLAME NUMBER OF SAWDUST SAMPLES
until an air-dry condition was reached (about 5 per cent water), and the samples were tested for combustibility. An amount of solution was used in each case sufficient t o leave a previously calculated amount of dry chemical impregnated in the sawdust as shown in Table I. The degree of fire resistance was measured in a modified Forest Products Laboratory fire-tube tester (8, IO). Instead of the usual screen cage at the bottom of the tube, an 8-inch square of window screen in a light iron frame was horizontally suspended by four wires from the fire tube so that the screen was about 4 inches below the bottom of the tube. The sawdust was centered on this screen by a sheet metal ring so that the sample formed a 6-inchdiameter disk approximately 1/t inch in thickness which was located directly below the fire tube. A standard Bunsen flame (6 inches in height, and maintained constant by adjustment of the gas pressure as shown by a manometer) was held a 6xed distance beneath the sawdust on the screen for exactly 1 minute and removed; then in order to ensure ignition, another standardized flame was played over the upper surface of the sawdust for exactly 10 seconds and also removed. The combustion was allowed to proceed. and the weight loss of the sample per unit time, the time of flaming, and the approximate surface area finally charred was measured in each case. The temperature at the top of the fire tube was not measured. Since it was recognized that the measurements could be only relative and arbitrary, all conditions were standardized and maintained as constant as possible throughout the series of experiments.
Degree of Fire Resistance Attained Figure 1 shows the rate of loss in weight of burning untreated sawdust samples (five duplicate experiments). The reasonably close agreement between these rates indicates that the tests are reproducible. The rates of combustion of sawdust samples impregnated with various amounts of borax are also given. The results of the several experiments are listed in Table I, which shows the weight loss of the sawdust, both at the end of the flaming period and at 4 minutes, the duration of the flaming, and the approximate percentage of the sawdust surface charred. The data for each sample are summarized in its flame number, arbitrarily defined as the product of the time of flaming in minutes and the percentage loss in weight during the flaming period; thus the rapidity of the combustion
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and the persistence of the flame are taken equally into account. However, owing to the technique employed in the testing, the results emphasize the effectiveness of the agents in retarding horizontal rather than vertical flame spread. The weight loss of the various samples during 4 minutes of combustion is plotted in Figure 2 against the percentage of chemical sorbed by the sawdust. From this data it is apparent that appreciable amounts of the reagents are necessary to cause any important decrease in the rates of combustion as shown by loss in weight of the samples. When 10 per cent chemical is sorbed, the decreasing order of effectiveness is: diammonium phosphate, borax, monoammonium phosphate, zinc chloride, ammonium sulfate, ammonium chloride, and magnesium chloride. I n considering fireproofing agents it is necessary to take into account the duration of flaming as well as the rate of combustion. Both of these measurements have been combined in the flame number as defined above; in Figure 3 the flame numbers of the various sawdust samples are plotted against the percentage of chemical sorbed in each case. On this basis, when 10 per cent chemical is sorbed, the relative order of decreasing effectiveness is: borax, diammonium phosphate, zinc chloride, monoammonium phosphate, ammonium sulfate, ammonium chloride, and magnesium chloride. These results suggest that borax and zinc chloride are advantageous, particularly in minimizing the flaming characteristics. The order of effectiveness of the several reagents found in this investigation with sawdust is, in general, similar to that found in the experiments of Hunt and co-workers (4, 6, 6, 11, 12) with wood. The importance of impregnating borax into the sawdust was shown in two experiments in which air-dry sawdust was dusted with 10 and 15 per cent of its air-dried weight of borax. The flame numbers of these samples were 350 and 358 in contrast to 29 and 19, respectively, for samples which had been impregnated with the same amount of borax in solution; thus there was no doubt as to the necessity of the impregnation treatment, at least in the case of borax and probably with similarly functioning agents. Obviously the commercial use of these or any other fireproofing agents demands consideration not only of the effectiveness but also the leachability of the agent, its effect,upon decay and bacterial action, corrosion, settling, conductivity, and cost.
Acknowledgment The author is grateful to Potlatch Forests, Inc., of Lewiston, Idaho, for financing the work and also for granting permission to publish the results. The kindness of E. C. Jahii of Syracuse University in criticizing the manuscript is appreciated.
Literature Cited (1) Eichengrtin, A., 2.angew. Chem., 42, 214 (1929). (2) Gillet, A.,Chimie & industrie, Special No., 221 (Feb., 1929). (3) Ibid., Special No.,302 (March, 1931). (4) Hunt. G. M., Truax, T. R., and Harrison, C. A., Proc. Am. Wood-Preservers' Assoc., 1930, 130. ( 5 ) Ibid., 1931, 104. (6) Ibid., 1932,71. (7) Metz, L., 2. Ver. deut. Ing., 80, 660 (1936). (8) Plunguian. M., and Jahn, E. C., Proc. 6th Pacific Sci. Congr., Can., 1934,3923. (9) Rossman, J., Paper Trade J.,91,No. 9,42 (1930). (IO) Truax, T. R., and Harrison, C. A., Proc. A m . SOC.Testing Materials. 29, Part 11, 973 (1929). (11) Truax, T. R., Harrison, C. A., and Baechler, R. H., Proc. Am. Wood-Preseruers' Assoc.. 1933,107. .. (12) Ibid., 1935, 231. (13) Vedenkin, 5. G.,Trans. Gen. Conf. Standardization and M f r . New Bldg. Matls., Moscow, 1932,17.
