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iron splashers a few holes were corroded completely through the metal. That crystal structure or special orientation or localized eutectics may account for the selection of beginning spots of corrosion is quite possible. After once starting, the removal of corrosion product by rapidly moving water seems expected. It would be quite ridiculous to question the conclusions on erosion of propellers by the able research experts, the engineers and shipbuilders,* who have conducted so much successful experimental work on this particular part of the subject of corrosion. That the special action on high-speed ship propellers is primarily connected with cavitation seems perfectly proved. In experiments on the mechanical effects of suddenly closed vacuum spaces in water, where the energy was instantaneously transmitted to small metal area8, thin sheet metal was punctured by the blow, and with thicker metal mechanical deformation occurred. Competent mathematical work in this connection has also shown that the forces available may be adequate to cause the simple physical destruction of the metal unless this is of unusually good resisting quality, better than iron, brass, or some bronzes. In those cases where the erosion showed dents such as could be produced by a round-nosed hammer, there certainly seems to be evidence enough that the rapid closing of vacuous spaces accounts for this type of propeller erosion. It is probable, however, that much erosion, even though it may be localized and on propellers, is due to the chemical attack of the metal by which compounds are formed. These are in turn easily eroded away. That the cavitation, or mutual action of air and mater, should be more effective in removing such compounds than the water alone a t the same velocity may be explained by the higher velocity of the water falling through air or vacuous spaces, such as occur with propellers, or even a t a jet when water flowing in a pipe carries with it bolts of air. Between the extreme case of eroded propellers and simple chemical corrosion there is a large field of destruction of metals in which the action commonly spoken of as mechanical erosion is more probably chemical corrosion, with subsequent removal of the new compound by erosion. In some cases it will be found by removing the chemical reactive agent that the harmful effect is also removed. 2
Trans. I n s f . Na0al Archifecls, 61, 223 (1919).
Kew Studies on Airplane Dusting of Cotton In the studies that the Bureau of Entomology has been making on boll-weevil control by means of airplane dusting, much attention has been given to the mechanical features, and numerous devices for distributing the poison from the plane have been developed and perfected. Different types of planes require quite different equipment. Special planes are also being developed and extensive plans are under way for more widespread use of this method. The airplane dusting has led to many interesting and important developments, such as the possibility of dusting by daylight. Heretofore, the poison has been applied a t night when moisture on the plant caused the dust t o adhere. It has been found that, by charging the dust with electricity of a polarity opposite to that of the plant, the dust, as i t settles over the field, is attracted and adheres to the plant, covering its leaf and stem surfaces completely. An extensive cooperative project is under way, with the Bureau of Public Roads on the mechanical side and the Bureau of Standards on the electrical side, thoroughly to work out the principles involved and their possible commercial application. A thorough study has been made of the different types of calcium arsenate in relation to their effectiveness for weevil control. This is being correlated with methods of manufacture and several new methods have been studied. I t is hoped that when this study is complete a much more efficient calcium arsenate will be available, and furthermore that it will be possible to eliminate materials of low efficiency which have been responsible for erratic results by the farmers in the past.
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The Corrosion of Glass Surfaces By George W. Morey GEOPHYSICAL LABORATORY, WASHINGTON, D. C
To the power of resisting the corrosive action of atmospheric agencies and aqueous solutions possessed by glass may be ascribed the extensive use of glass as a container, and in its other uses this property is also essential. The necessity of producing a glass resistant to corrosion definitely limits the compositions which may be employed, and makes impracticable some otherwise desirable glasses. The mechanism of the corrosion of glass by water is complex, involving in its initial stages a probable miscibility of glass, regarded as an undercooled liquid, with water, and in its later stages the complete decomposition of the silicates. A t ordinary temperatures these processes require a long time for their completion, but at higher temperatures they can be followed in their various stages to the end. In interpreting the results obtained by the various methods of testing glass, it is necessary to bear in mind that the term “solubility” has no meaning in connection with such complex processes, and the testing methods employed merely afford a measure of the rate at which the reactions take place. Such rates of reaction are peculiarly susceptible to change in the conditions of experiment, and great care must be taken to specify these conditions. The effect of various oxides on the resistance of glass to corrosion is briefly outlined.
HE resistance offered by glass surfaces to the corroding action of atmospheric agencies and aqueous solutions is a property of great practical significance. In a large proportion of the many uses to which glass is put, its power of resisting attack is the chief reason for its preferment over competing materials; in the other uses a high degree of chemical stability is an essential requirement. Glass containers are used for the distribution of commodities ranging from milk to medicines and acids, and here the superiority of glass leaves it without a competitor. More exacting are the requirements of the manufacturing industry; and here, too, the choice of glass as a container is justified only by its resistance to surface attack under extreme conditions. Other uses of glass-for example, in windows and lens s y s t e m s 4 0 not impose such a drastic test, but even here there must be no appreciable amount of surface alteration. That glass must offer great resistance to the common corroding agents is thus a requirement of first importance. Indeed, so essential is this property that Zschimmer makes it a part of his definition of a glass. From the manufacturer’s standpoint, it operates to limit definitely the compositions that may be employed. Many glasses possessing desirable optical or mechanical qualities are impracticable because of their susceptibility to corrosion. The study of the effect of various substances on glass has occupied the attention of many investigators, but in spite of the large amount of work that has been done the subject is still in an empirical state. Nor is this surprising. The problem is not the simple one of determining the solubility of various glasses in water or other agent, that many have supposed. On the contrary, we have here to deal with a complicated process, whose initial stage we do not understand, and whose later stages involve highly complex equilibria under conditions difficult to specify.
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Miscibility of Water and Glass
The early experiments of Schott’ and of Foerster2led to the conclusion that the first step in the attack on glass surfaces by water consists in an actual entering of the water into the body of the glass. This has been confirmed bymany subsequent workers, and appears to be a highly probable hypothesis. It may even be argued that this of necessity takes place. Glass is commonly, and rightly, regarded as an undercooled liquid, a liquid several hundreds of degrees below its temperature of equilibrium with the stable crystalline phases. Water and glass constitute a liquid-liquid system, and is it not probable that these two liquids, instead of having no miscibility, or a very limited miscibility, are miscible in all proportions? The question is not easy to answer conclusively. The molecular mobility in a glass must be limited indeed, and the rate of penetration by water a t ordinary temperatures must be small. The process is greatly complicated by the fact that not only the original anhydrous glass but also the increasingly hydrated product obtained as the process of penetration continues is essentially unstable as regards the formation of crystalline substances. The formation of these more stable phases is greatly accelerated by the entry of water into the glass, and usually the surface of separation between hydrated glass and partially crystalline material is practically coincident with that between anhydrous and hydrous glass. This is not always true, however. In experiments a t higher temperatures the initial stage of formation of a highly hydrated glass without noticeable crystallization has frequently been observed. The well-known observations of Barus3 furnish an excellent example. He found that glass heated under pressure with water took up large quantities of the water; a t 185’ C., the glass swelled enormously, and became white and turbid, while a t 210” C. solution took place rapidly, and a clear liquid was formed. Morey and Fenner,4 studying the equilibrium relations in the ternary system H20-K&3iOJ-Si02 obtained glasses containing from 8 to 25 per cent water, a continuous series ranging from hard, brittle glasses to viscous liquids resembling ordinary water glass. These glasses represented solutions unsaturated and therefore stable a t the temperature of formation, but as unstable a t ordinary temperatures as the usual glass. The natural glasses known as pitchstones, highly siliceous minerals containing up to 10 per cent of water, may also be examples confirming the hypothesis of an actual miscibility between water and silicate glasses. Instability of Glass
It appears probable that, a t high temperatures a t least, water and silicate glasses are miscible to a considerable extent, but the substantiation of this a t low temperatures is difficult. I n addition to the slow penetration of the glass by water, there is the complication resulting from the essential instability of glass, an undercooled liquid only obtainable because of the characteristic reluctance of silicates to assume the crystalline condition. When water enters the silicate liquid, the tendency to form crystalline substances becomes greatly increased. This is especially true a t higher temperatures. I n an extensive study of the effect of water near its critical point (373” C. and 212 atmospheres pressure) on glasses, carried out in cooperation with his colleague, Dr. N. L. Bowen, the writer has been able to crystallize many glasses, obtaining from them numerous crystalline compounds, including many well-known minerals. For example, from a 1
2
* 4
2. Inslrumenlcnk., 9, 86 (1889). Ber., 36, 2915 (1893). Am. J . S c i , [41 9, 161 (1900). J . A m . Chem. SOL, 89, 1173 (1917).
soda-lime glass are obtained excellent crystals of quartz and of wollastonite, the metasilicate of calcium. A similar process probably takes place a t lower temperatures but too slowly for the final end products to be attained, a t least in identifiable form. Another consideration makes the low-temperature reaction more difficult to follow. At higher temperatures we have separation of minerals definitely in equilibrium with the solution, minerals possessing real “solubility” in the aqueous phase. At room temperature it is doubtful if any of these minerals, except quartz, possesses a true solubility. At low temperatures the process is almost exclusively one of decomposition, and there exists a fundamental distinction between the two cases, a distinction important not only for the understanding of the mechanism of the corrosion of glass surfaces, but also necessary to the interpretation of the results obtained in the testing of glasses for their power of resistance to such corrosion. I n a system composed of water and a salt that is not decomposed by water, “solubility” of the salt a t a given temperature has a definite meaning. Coexistence of three phases, solid, liquid, and vapor, in a system of two
\
Y
Ha0
Si& Flgure 1
components, water and salt, leaves but one degree of freedom, and the fixing of any one of the variables, pressure, temperature, or the composition of the solution, fixes the other two. This is not true, in general, in a system of more than two components. As an illustration, the ternary system HzOK2SiOe-Si02, which has been studied4 from 180’ to 1000” C., may be chosen. Figure 1 gives a number of isotherms in this system along each of which pressure changes continuously, but for the purposes of this discussion it is only necessary to state that the experiments were carried out in closed bombs, from which the water could not escape. The isotherms a t 285” C. will be discussed first. At this temperature potassium metasilicate, K20.Si02, has a true solubility in water, represented by the end point of the saturation curve on the H20-K20.Si02 side of the diagram. The actual solid phase a t this temperature is not K20.Si02, but its monohydrate, K20.Si02.H~O. As the proportion of SiOz in the liquid increases, the composition of the solutions in equilibrium with potassium metasilicate monohydrate follows the saturation curve of this compound until it is intersected by the saturation curve of a second compound, richer in Si02,potassium disilicate monohydrate, K20.2Si02.H20. From the fact that the tie-line H10-K20.-
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INDUSTRIAL A N D ESGIiYEERING CHEMISTRY
2Si02 cuts the saturation curve of K20.2SiOz.H20it follows that this compound also has a truesolubility in water a t 285OC. With a slightly greater proportion of Si02 the saturation curve of K20.2Si02.H20 cuts the saturation curve of a still more siliceous compound, K20.4Si02.H~O. At 200" C. conditions have become very different, since, although the metasilicate monohydrate is still stable in contact with its liquid, the disilicate monohydrate is now decomposed by water. As the proportion of SiOt is increased, starting from the metasilicate side of the diagram, potassium metasilicate monohydrate remains the solid phase until its solubility curve is intersected by the solubility curve of potassium disilicate monohydrate, just as was the case a t 285" C . But on further increase in the Si02 content, it is found that the disilicate no longer remains stable until the ratio SiOz: K 2 0 in the liquid becomes the same as in the crystalline disilicatenamely, two. Instead, before that point is reached the saturation curve of the more siliceous compound K20.4SiOz.HzO cuts the saturation curve of the disilicate, and with larger proportions of Si02 the disilicate is no longer stable. If water is added to a glass of the composition of potassium disilicate a t 285' C. most of the glass will crystallize as potassium disilicate monohydrate, and the remainder will form a solution in which potash and silica are in the ratio of 1:2. If water is added to the same glass at 200" C., both the solid and liquid phases obtained will depend on the amount of water added. When a little water is added, enough K2O will be extracted from the disilicate to form the more alkaline solution corresponding to the intersection of the two saturation curves; the remainder of the material will crystallize mainly as the hydrated disilicate, except that the excess of SiOz resulting from the extraction of some K2O from the disilicate will form the more siliceous compound, K20.4SiOz.H20. If more water is added more solution will be formed, more disilicate will be decomposed, and more K20.4Si02.H20formed until a t length all of the disilicate will have disappeared. Thus, by changing the amount of water we have changed the solid phase from one having a potash:silica ratio of a disilicate to one having the tetrasilicate ratio. The disilicate a t this temperature is decomposed by water and can no longer be said to have a true solubility in water. At a slightly lower temperature the metasilicate also is decomposed by water, and from the shape of the isotherms it appears probable that a t room temperature all compounds of potash and silica are decomposed by water. True equilibrium a t ordinary temperatures in any mixture, no matter how alkaline, would probably mean a solution containing a vanishingly small amount of silica, the actual amount depending on the amount of K2O present, in equilibrium with a solid phase of pure quartz, or a crystalline compound of SiOzand water, if any such exists. T o be sure, "water glasses" containing a large amount of silica, can be obtained but such solutions contain the silica in a colloidal condition, and bear no known relation to the true solutions which could coexist with crystalline phases. For example, a potassium water glass can be obtained as a viscous liquid which would not be stable as a true solution below 450' to 500' C. The end result of treating potassium disilicate with water a t the ordinary temperature, if the peptization of the Si02 could be prevented and true equilibrium attained, would be that practically all the Si02 would be left as quartz, and the liquid would consist of a potassium hydroxide solution containing a small amount of silica. But this solution would in no sense represent the solubility of potassium disilicate in water. This system is analogous to those obtained with complex silicates such as glasses, ceramic bodies, and glazes, as well as natural minerals. When such substances are treated with Kater, either at ordinary temperatures or a t such moderate
391
temperatures as are obtainable in autoclaves, the compounds are decomposed, just as the potassium silicates are decomposed, and more or less of certain constituents passes into solution. In all these cases the end result, if equilibrium were attained, would be an alkaline liquid, containing small amounts of certain constituents and vanishingly small amounts of others, together with various crystalline phases, depending on the complexity of the original material. But this liquid would have no bearing on the solubility of the substance that had been decomposed, for the term "solubility" has no meaning in such a case. The original state of the material, whether crystalline or glassy, or a mixture of the two, will have no bearing on the end result of such a decomposition, although it will affect the rate of decomposition, and therefore the results observed in any experiment not carried to its ultimate conclusion. Resistivity of Glass The preceding discussion has dealt with the action of water alone, but frequently the resistivity of glass to the action of aqueous solutions is of importance. I n many cases the solutions are dilute enough so that the differences caused by the dissolved material are not significant, but this is not always true. I n regard to the end results, on attaining equilibrium the presence of dissolved salts or of acids or alkalies is not of much significance. Whether or not such substances are present, the decomposition will tend to go on until decomposition of the glass is complete. But the presence of an acid or alkali will often profoundly affect the rate of decomposition, which is the subject of primary interest here, and the effect will be largely dependent both on the composition of the glass and on the dissolved material. Decomposition of most glasses by water results in the liberation of alkali, by the process just outlined for the potassium silicates, and the liberated alkali often accelerates the further decomposition of the glass. For this reason some types of glass, notably those high in silica, or in silica and boric oxide, are more resistant to acid solutions than to water,. and more resistant to water than to alkalies. On the other hand, glasses low in silica are usually more rapidly attacked by acid solutions; indeed, wollastonite glass, Ca0.Si02, and extra heavy lead glasses can be decomposed rapidly enough for analytical purposes by digesting with hydrochloric acid. The action of concentrated solutions of salts and acids is often in harmony with the views of Foerster,j who assumed that the acids exert no direct action on the glass, and that the attack is due exclusively to water. This view leads naturally to the conclusion that a large amount of acid weakens the attack by diminishing the concentration of mater . Many experimenters have worked on the decomposition of glass, and almost as many methods have been proposed as there have been workers in this field, but the results obtained are rarely comparable, and the subject is still in an empirical condition. When the results of these various investigations are studied, it is seen that only the broadest generalizations are possible. Before these generalizations are mentioned, it will be well to review the methods of testing which have been proposed. The methods fall roughly into two classes; first, those in which the glass is tested under service conditions, or some approximation thereto, relying either on an extended time to bring out the differences or on sensitive methods of treatment; and second, those in which the action of the reagent is accelerated by heating. The first group of methods undoubtedly gives the more unequivocal results. Among t h e results of such methods may be included the large amount of experience that has been gained in practice as to the 2. Znsirumentenk., 18, 457 (1898).
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glass compositions stable enough to be used in optical instruments, either with or without protection, as to the types of glass that have proved serviceable as window and gage glasses, and as to those compositions that have proved serviceable for chemical laboratory glass. In the same group of methods are to be included the many tests that have been carried out on the relative resistance to attack by various common laboratory reagents, such as the work of Walker and Smither,Bthe results of which are directly applicable to laboratory work. The methods based on the use of sensitive indicators are also to be included in this class, though the results of such tests are to be accepted with caution. Foremost among these methods is the iodeosine test developed by Mylius. I n this method a fresh surface of known area of the glass is immersed in an ether solution of iodeosine. The alkali in the glass reacts with the dye, forming a soluble salt, which is dissolved in water, and the color compared with a standard. This gives excellent results with the usual soda-lime and potash-lime glasses, but may give wholly misleading results with glasses rich in boric oxide or with glasses containing little alkali but much lead oxide, or barium oxide, whose salts with iodeosine are not soluble in water. Results obtained by treating powdered glasses with water or with dilute acid are also to be accepted with caution. I n all such cases the reactions taking place are decomposition reactions, in which the end results will be complete decomposition of the glasses. What is determined is roughly a rate of reaction, and of a reaction particularly susceptible to variations in the experimental conditions. This is the clue to the lack of concordance in the observations recorded in the literature. The data given refer, not to solubility, but to rates of reaction, frequently determined under poorly defined conditions. The second type of testing method, in which the process is accelerated by increasing the temperature, is subject to the same limitations. I n addition, we have here the possibility that the type of reaction between glass and water may be completely altered, as is the case with potassium disilicates in contact with water. Although such temperatures have been reached in high pressure bombs, it is not probable that they will be reached with complex glasses in the type of autoclaves used in testing, but nevertheless it must be born in mind that the processes taking place a t high temperatures and pressures differ greatly from those that take place under more ordinary conditions. One important difference is the tendency of the silica to pass into colloidal solution. This is marked a t low temperature, but when the temperature is raised to 200"-300"C. there seems to be no tendency to form colloidal solutions, but rather for the silica to crystallize as quartz.
noteworthy in their deleterious effect on the resistivity of glass to corrosion, and it may be stated as another rough generalization that the power of a glass to resist corrosion is inversely proportional to the alkali content. Of the two oxides, potash usually exerts the greater effect, but a mixture of the two oxides is superior to either alone. This has been especially well brought out by the work of Peddle,' who showed that in the alkali-lead oxide glasses '(to obtain the maximum of durability in an alkali-lead-silicate glass the best proportions in which to mix the alkalies is the ratio of 7 parts of the potash to 3 parts of soda." No explanation has been offered for this remarkable circumstance. Of the three oxides, magnesia, lime, and barium oxide, the last is inferior to the others in the resistivity that it confers upon glass. Although glasses high in barium oxide and excellent in resistivity are well known, the excellence is to be ascribed to other ingredients. Between magnesia and lime there is little choice, each surpassing the other under certain conditions, but the difference never being marked. As far as the writer is aware, no work has been done on a possible advantage of mixed magnesia-lime glasses, similar to the marked advantage found with the mixed sodapotash glasses. Lead oxide is superior to the alkali oxides but slightly inferior to lime from the point of view of resistance to corrosion. However, lead oxide can be introduced in much larger quantities than can lime without devitrification, and the high lead glasses thus obtained are excellent for chemical resistivity, until the very heavy lead glasses are reached, and even these are good as regards resistance to water alone. It seems to be a general rule that glasses complex in composition are superior to the simpler ones, as, for example, the mixed alkali glasses. Certain substances rarely present in large quantities seem to be of special value in this connection, among which are zinc oxide, boric oxide, and alumina. Pure zinc oxide glasses are not highly stable themselves; yet the addition of zinc appears to have a distinctive favorable effect in certain cases. Boric oxide is a constituent of many, if not most of the best glasses; indeed, Pyrex glass, which is distinctly superior in this respect, contains a considerable quantity of boric oxide. Alumina is also of great value in enhancing the stability of glasses, a content of as little as 1 or 2 per cent having a markedly beneficial effect. The dense barium crowns would probably be much inferior glasses were it not for their very considerable content of alumina, but as manufactured they rank with the best.
Effect of Chemical Composition
Manuscript for the National Directory of Commodity Specifications has been sent to the Government Printing Office. It is to be followed by an Encyclopedia of Specifications, giving in loose pamphlet form complete copies of the more important specifications, Dr. A. S. McAllister, of the Bureau of Standards, is in charge of the work. The directory, formerly called the Dictionary of Specifications, is to contain classified lists of all the commodity specifications in general use in the United States, including those employed by the United States Government, by state and city purchasing agents, and by large industries; and those prepared by the American Society for Testing Materials and by the Federal Specifications Board. For each a brief description will be given, and directions for obtaining copies. A cross-indexed finding list will be included. I n qll about six thousand commodities are included. These are divided into ten groups, covering respectively animals and animal products; vegetable food products, oil, seeds, expressed oil, and beverages; other vegetable products except fibers and wood ; textiles; wood and paper; nonmetallic minerals; ores, metals, and manufactures (except machinery and vehicles) ; machinery and vehicles; chemicals and allied products; and miscellaneous.
The correlation of the ability of glasses to withstand the corrosive action of water with their chemical composition is difficult, as is to be expected from the complex character of the processes involved. The one outstanding feature is the superiority of silica glass over all others; indeed it may be said, as a rough generalization, that the chemical resistivity of a glass is proportional to its silica content. Addition of other substances is necessary, however, to lower the temperature required in the melting process to one conveniently attainable in practice, and in glasses used for certain special purposes, such as optical glasses, for the attainment of other desirable qualities. Of the substances copmonly introduced, the alkalies, soda and potash, occupy a special place in the increased fusibility they give to glass, and few glasses are made which do not contain one of these oxides They also are especially 6
Bur. Standards, Tech. Paper 107.
7
J. Soc. Gloss Tech., 5, 195 (1921).
National Directory of Commodity Specifications
