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So-called Action of Water on Lead,” A n a l y s t , 46 (1921) 270, Rawdon, The utilization of materials that have become familiar “Intercrystalline Brittleness of Lead,” Bur. Standards, Scz Paper 577. through use in other fields is feature of recent chemical 5-“Thermit,” “Dark-Room Metal Ware,” Brzt J Phot , 69 (l922),412 engineering work. As evidence there can be cited the use B--Merlca, 6cIron-NickelAlloys,9~Chem M e t 24 (1921), 376, Johnson, “Nonmagnetic, Flame, Acid, and Rust Resisting Steel,” 16zd , Of Bakelite, hard rubber, stainless steel, chromium-nickel 26 ( I ~ z I )797, , Gillett, “Nonferrous Alloy Progress in 1921,” THISJOURNAL,alloys, etc. Each of these has advantages as a resistant ma14 (19221, 865 terial for certain limited purposes, and finds a corresponding 7--”Protective Metallic Coatings for Rustproofing Iron and Steel,” specific field of application. Pyrex glass, because of its reBur. Standards, Civc. SO (1919); Schuler, “The Covering of Chemical Appasistance to most acids combined with excellent thermal ratus with Metals and Acid Resisting Materials,” Chem. Z t g . , 46 (1921), 315; Watts, “Principles of Alloying to Resist Corrosion,” Trans. A m . Electvoproperties, has a wide field of industrial use with limits set by chem. SOC.,39 (1921), 253. design of parts and possibilities of manufacture. 8--Desalme, “Use of Salts of Tin for Preserving Developers,” Bull. soc. franc. Phot., 131 8 (1921), 192. ENGINEERING PROPERTIES OF PYREX 9-Crabtree and Bullock, “Correspondence,” B r i t . J . Phot., 66 (1919), 446. Before proceeding to a discussion of industrial applications, IO-Marshall, “Pyrex Glass as a Material for Chemical Plant Construcit is desirable to list briefly the various properties which have tion,” THIS JOURNAL, 16 (1923), 141. to be taken into consideration from the engineering standpoint, I1-“Coatings for Dishes,” Bvit. J . Phot., 66 (1919), 574; “White Wooden Trays and Sinks,” Camera Craft, 2s (1920), 367. Specific gravitya 2.25 Specific heata 0.20 Elasticity coefficientn 6230 kg per sq mm. Linear expansion coefficientb (19’ to 350’ C ) 0 0000032 per O C. Thermal conductivitya 0 0027 By A. E. Marshall1 Dielectric strengthb 20 kv. per 100 mil thickness Specific inductive capacity .5 75 to 5.78 3034 S T . PAULS T . , BALTIMORE, MD. Electrical resistivity (volume)c 1014 ohms HEMICAL engineering, considered from a somewhat Electrical resistivity (surface)c 10“ ohms a t 34 per cent humidity 5 X lo8 ohms a t 84 per cent hunarrow angle, is the translation of chemical manufacmidity turing processes from the small or laboratory scale to Corning Glass Works Laboratory. tonnage production in factories. Small-scale experiments General Electric Company Laboratory Bureau of Standards. are usually carried out in glass apparatus, with the result
Pyrex Glass Plant Equipment
C
that corrosion troubles are not given much consideration until the chemical engineer is faced with the design of a small commercial plant incorporating the results of the laboratory work. Chemical corrosion of the materials to be used in construction immediately assumes importance, as the plant equipment cannot be a mere multiplication of the laboratory apparatus. The liter flask of the laboratory has to become a still, retort, or kettle of several hundred gallons capacity, the Liebig or spiral condenser has to be changed to a design capable of handling 50 or more gallons per hour, and 1/4-in. glass tubes used for connections have to be converted into 6- or %-in. pipes. I n each piece of equipment corrosion will enforce limits a3 to materials employed, and the final design will quite likely be a compromise of several materials, each selected with reference to corrosion conditions a t certain points. The introduction of industrial Pyrex a s a plant construction material has enabled the engineer to translate, within limits, laboratory practice directly into plant equipment, and to secure the freedom from corrosion enjoyed by the research chemist. The present limits of manufacture are not final, but are capable of some further expansion, as is indicated by the progression of sizes in the past eighteen months of development work. There is, of course, a limit to the size of Pyrex articles introduced, through structural considerations, but so far the present manufacturing possibilities have not reached the safe limits set by the engineering factors of plant design. FAMILIAR PROPERTIES OF PYREX Pyrex glass has become familiar to most of us as laboratory ware and domestic cooking utensils. The laboratory apparatus has given proof of its resistance to heat and chemical corrosion, and the baking ware has demonstrated ruggedness under constant handling. Sudden heat shock is also a feature of most of the domestic applications, the conditions frequently being more strenuous than in plant uses. A cold pie plate put into an oven heated to 400” F. has to meet a greater thermal shock than the large evaporating dish used on a steam bath in a chemical plant. 1
Consulting Engineer, Corning Glass Works.
The chemical properties which affect design are mainly corrosion resistances a t various temperatures. The mineral acids, with the exception of hydrofluoric and phosphoric, have no appreciable action on Pyrex up to their respective boiling points. Initially, there is a very slight surface attack, the figure for constant boiling hydrochloric acid being 0.000006 g., and for fuming sulfuric 0.000002 g. per sq. cm. per hr. This initial action is succeeded by a state of practical stability. I n the case of phosphoric acid, the attack produces crystallization at the glass surface and makes accurate determinations of solubility rather difficult. An average figure arrived a t by the Corning laboratories is 0.0027 g. per sq. cm. per min. Hydrofluoric acid does not attack Pyrex as readily as other glasses, this being a matter of knowledge possessed by all laboratory workers who have tried to etch beakers or flasks. The action is, however, sufficient to prevent any serious consideration of the use of Pyrex plant for hydrofluoric acid or fluoride production. Quite recently a question was raised as to the use of Pyrex in acetic anhydride manufacture, and it was found that no measurable attack occurred in 5 hrs. a t the fuming temperature. PRESEXT LIMITATIONS OF SHAPEAND SIZE Previous mention has been made of limits imposed by present processes of manufacture. Such limits are of considerable importance to the engineer who is considering, as a preliminary stage, the use of a new material for construction work. Without this information changes in the final design may be found necessary in order to bring certain parts within the existing limits of manufacture. Industrial Pyrex is made by various methods, the most important being pressing and blowing. Accessories for plant use, such as tees, tubulated vessels, etc., are produced in very much the same way as the smaller ordinary glass articles made by the laboratory glass blower, except that much higher temperatures are involved. Hemispherical evaporating dishes are examples of pressed ware, the largest available size being 24 in. in diameter.
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More difficult shapes call for a reduction in size-drier trays, for instance, being practicable u p to 12 x 18 x 3 in. Sharp angles are undesirable in pressed ware and should always be avoided. Blowing into molds gives an externally controlled size with a n opportunity to vary wall thicknesses in accordance with the use of the equipment. Parts lor condensers may have a wall thickness of to in., whereas tower sections or large socket pipes can be made s',3 in., or even heavier. The limits of blown Ware are approximately 15 gal. for necked vessels and 12-in. diameter for socket pipes. The limits for pressed and blown articles are, of course, subject to upward revision, as the present sizes represent less than two years' development work. As an example of the enlargemeht of sizes, it is of interest- to remember that less than a year ago a 10-liter Pyrex dish 'was an achievement, whereas to-day the 24-in. dish has a capacity of 30 liters. APPLICATIONS IN
THE
INDUSTRIES
The growth of uses for industrial Pyrex has shown a marked increase in recent months, due no doubt to results of earlier tests with single pieces. Generally, the industrial applications fall within four distinct classifications: 1-Uses 2-Uses 3-Uses 4-Uses
where acid resistance is important where purity of product is essential where control is facilitated by transparency where resistance to thermal shock is desirable
Under Class 1 would be grouped Hart condenser tubes, S-bend nitric acid condensers, pipe lines for acid gases and liquids, acid distillation sets, and experimental towers. Class 2 comprises such uses as pipe lines for liquid food products, evaporating and crystallizing dishes for pharmaceuticals, fine chemicals, etc., tanks for precious metal solutions, and drier trays for alkaloids, biologicals, etc. Class 3 uses are limited but interesting: sight glasses for stills, retorts, etc., gage tubes for tanks, boilers, etc.; sight pipe sections for distilling columns; and sight pipe sections for chamber plants. Class 4 uses merge at times into Class 1, as heat and acid resistance are often companion factors for consideration. The following applications are based largely on heat-proof qualities: receivers for hot liquids, flat pans for vacuum and atmospheric driers, and tubular condensers for organic liquids. METHODS OF CO?U'STRCCTION AKD USE Most of the items listed above do not involve special construction features. However, in using Pyrex equipment it is essential to remember that, while i t has an extremely low coefficient of expansion, direct flame is to be avoided. Pipe lines call for special designs if flanged joints are involved, and special cements when joints are of the socket or bell and spigot type. Flanged pipe lines cannot be made up by directly bolting two faces together, so an extra split-metal flange is used behind the Pyrex flange and kept out of direct contact by a rubber gasket. The metal flange may be cast aluminium or pressed steel. A gasket between the glass faces is desirable, and this may take the form of a silver-asbestos corrugated gasket similar to the copper automobile type, or one of the special rubberized fabrics which is resistant to the liquid or gas handled in the pipe line. I n the case of cements, it is preferable to adopt mixtures which will remain plastic in use. Hard-setting cements are liable to cause trouble through expansion. Generally, the use of a soft asbestos cord or rope filler a t the base of the socket joint will be found advantageous, and with a plastic cement will give flexibility to the line.
The Effect of Velocity on the Corrosion of Steel in Sulfuric Acid' By W. G. Whitman, R. P. Russell, C. M. Welling, and J. D. Cochrane, Jr. MASSACHUEETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS.
THEORY T HAS been recognized for some time that there is a distinct difference in the processes governing the corrosion of steel under acids and under natural waters. The fundamental equation in both cases, however, as pointed out by TT7hitney,2is undoubtedly Fe + 2H+ f Fe++ + 2H (1) The rate a t which this reaction progresses is dependent on a number of factors, among which are the following: (1) metal, ( 2 ) hydrogen-ion concentration, (3) ferrous-ion concentration, and (4) removal of hydrogen. &/IETAL-The tendency for a pure metal to corrode is determined by its position in the electromotive series. This factor 'is usually of minor importance, being obscured b y such effects as protective or nonprotective oxide or other films, overvoltage to the evolution of hydrogen gas, and the physical condition of the surface. I n commercial steels and irons the presence of elements other than iron or of products of corrosion on the surface of the metal may materially change the overvoltage and the consequent ease of liberating gaseous hydrogen. AcIDITY-Acidity is measured by the hydrogen-ion concentration of the solution, rather than by normality. For example, the hydrogen-ion concentration of 5 N and 2 N solutions of different acids is shown below.3
I
5 N
2 N
Increasing acidity increases the tendency to corrode by increasing the electromotive force for hydrogen gas evolution. The passifying action exhibited by various strong oxidizing acids is generally accredited t o a rapidly formed film of adherent oxide on the metal surface, and has no direct relation t o the acidity of the solution. FERROUS-ION Con-cENTRA4TIoN-The law of mass action shows that increased ferrous-ion concentration should retard the rate of corrosion. I n practice, however, the effect of other factors is so much more important that ferrous ion probably plays a minor role in the process. REMOVAL OF HYDROGEN-The rate a t which hydrogen is removed is, in general, the most important factor determining the progress of corrosion. It is dependent on several important variables, and a study of this factor as affected by, these variables offers the most logical method for investigating the process of corrosion in acids. Hydrogen can be removed in two ways-by evolution of gaseous hydrogen, according to the reaction 2H e Hz, (2) or by depolarization by oxidation, either by dissolved oxygen
+
2H '/z 0 2 HzO (3) or by some other oxidizing agent. The main variables are noted below under the process fvhich they control and will be discussed in the following paragraphs. 1 Published as Contribution No. 79 from the Department of Chemical Engineering, M. I. T. 2 J. A m . Chem. SOC., 25 (19031,394. * Landolt-B6rnstein, Eth ed , p. 1104 (from conductivity data)
