C o u r i e s y , Inland Steel Company
COXTISUOCS SHEET ASD STRIPMILL
CHEMISTRY AND VITREOUS ENAMELS EiMERSON P. POSTE 309 hlccallie A4venue.Chattanooga, Tenn.
means of avoiding misunderstanding in this regard, the industry has made use of two other terms, “porcelain enamel” and “glass enamel”, suggestive of similarity in appearance and in properties to products in two other fields of ceramics. At the turn of the century vitreous enameling in the United States mas a relatively insignificant industry. A few plants were manufacturing cast iron sanitary narc, and a beginning had been made in the production of sheet iron hollow ware, though much of the latter was being imported from Europe. Early records indicate that the enameling of cast iron mas particularly difficult because of the poor quality of castings, and that for reasonably satisfactory results with sheet iron enameling, the metal stock had to be imported. Probably one of the major factors making possible the tremendous growth of the industry was the devc>lopnient of proper sheet stock, specific attention to cast iron coming later. The particular application of chemistry first to be considered is, therefore, in the allied field of metallurgy.
0 MANY of the contributions of chemistry to modern life have involved the creation of entirely neTv products that we tend to overlook what the science has done to advance some of the older industries. Though the services of chemistry in the fields of textiles and metallurgy are well appreciated, there is probably a less clear conception in the case of ceramics, an industry or art as venerable as any of which we have historical record. Recent developments in one of the ceramic industriesglass-have been called to the attention of the public in a creditable fashion. The closely related field of vitreous enamel has also contributed within the last few decades a product of unusual adaptability. Not long ago it was chiefly evident as sanitary and cooking ware, but the enameled bathtub and dishpan TT-ere the forerunners of a large familystove parts, signs, table tops, refrigerators, washing machines, chemical &-are, and, more recently, extensive architectural features as well as a numerous line of miscellaneous items. This progress has been made possible largely through the application of chemistry t o a rule-of-thumb industry whose roots go far back into the ancient coating of metals with glass as a means of artistic decoration. Vitreous enamel is a silicate coating that has been fused onto a metal base. It should not be confused with certain organic coatings which are also termed “enamel”. As a
S
Sheet Iron Base One of the first points to be recognized uas the relation between enamel and the carbon content of sheet steel. I n one particular case it was learned that a certain type of blistering of the enamel came when the carbon content of the steel was of the order of 0.20 per cent; n-ith 0.15 per cent 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
10
carbon the enamel processed satisfactorily. The trend was to use steel safely below the latter figure in carbon. There were plenty of troubles due to steel defects other than high carbon, however. Nothing is a better criterion of lowcarbon steel than its enameling characteristics. The steel producers attacked the problem from two points of view.
VOL. 32, NO. 1
sodium oxide content is usually of the order of 2.0 per cent, and the cleaning time varies from 10 to 25 minutes. The use of cold hydrochloric acid for pickling was quite common when the preliminary treatment had been scaling and annealing, but with the advent of chemical cleaning there was, in general, a change to sulfuric acid. It is used a t
.20
i
pB
/5
:
$
RELATIVE ACTIONOF ACID ON A 2-GRAMEI~AJIEL SAMPLE (Powdered frit test)
f ./o
3
3
U
Courtesy, Transactions of the Amerzcan Ceramic Society
.05
0
/O
20
30
+O
50
60
70
80
90
100
F E R Z ~ Acm T
One group strove to produce a low-carbon sheet of special quality, the other introduced stock of the ingot-iron type. Both met with marked success, and made available the various grades required for the several methods of fabricationspinning, drawing, pressing, and welding. Today the cases of faulty enameling stock are rare, in spite of demands on the metal that would have been past hope a few years ago. Another branch of metallurgy that has played a major role is fusion welding. Gradually the oxyacetylene process met the increasing demands of the industry and has now been so perfected as to materials and methods that range and refrigerator bodies are being fabricated as units prior to enameling. Arc welding, applicable to the fabrication of heavier units such as chemical mare, was not successfully used for seams to be enameled until the development of the coated electrode. The bare electrode produced oxides and nitrides in the weld, and unfavorable metallographic structures which did not permit satisfactory enameling. mith protection from the atmosphere and other advantages characteristic of coated welding wire, the use of arc welding has become common practice for the fabrication of parts from heavy stock.
Preparation for Enameling Satisfactory enameling is possible only on sheet that is free from fabrication strains, scale, and grease, and whose surface is properly related chemically to the enamel that is to be applied. Early practice involved scaling and annealing the shapes a t a dull red heat to remove strains and pressing and drawing compounds. The metallurgist has produced stock and welding methods that render annealing unnecessary in most cases, and the chemist has developed a combination of forming compounds and chemical cleaning methods that produces a condition satisfactory for pickling. “Drawing compounds for use prior to enameling should be an emulsion of fatty oils and/or fatty waxes with a bit of mineral oil permissible but Rith no cushion-agent or pigment. The removal of these materials by cleaners involves the proper combination of saponifying and emulsifying agents, with a proper soap as a wetting agent. The average formula, therefore, contains caustic soda, silicate of soda, trisodium phosphate, soda ash and soap” (9). A cleaner of this type is operated just below or a t its boiling point, and the strength of the solution is generally judged by the sodium oxide content. The optimum varies with the type of drawing and pressing compounds used and the nature of a given cleaner. The
a strength of approximately 6 per cent and a temperature close to 150” F. Well cleaned stock will be properly pickled by a treatment of 10 to 20 minutes. The iron content of the pickle is carefully watched and the bath discarded when the concentration reaches 6 per cent. Some plants use organic inhibitors t o reduce the solution of the irod in the acid, but this is not general pract’icesince many feel that etching of the metal surface is essential to proper enameling. Neutralization of the ware after pickling and prior t o enamel application is necessary to prevent rusting and to present a chemical surface condition that is properly related to the enamel being used. Weak, hot solutions of soda ash, or mixtures of soda ash and borax, are in common use, though various excellent proprietary compounds are on the market. In some cases two neutralizing baths are employed, the first a t about 0.4 per cent sodium oxide and the second a t approxiI .O mately 0.2 per cent. If a single bath is used, 0.9 the strength is usually 0.6 kept at about 0.3 per d $ 0.7 cent. Control a t this point is essential if the 0.6 ground-coat enamel is 0.5 to be uniform. k 0.4 Two of the above operations are in some $ 0.3 cases modified by the use of an electric cur0.2 rent. Electrolytic 0.I cleaning involves an alkaline bath with a 0 100 200 x)O 400 500 600 700 direct current, the ware PRESSURE D Y N E S / ~ ~ , ~ ~ , Courtesy, Ferro Enamel Corporation serving the cathode and t h e t a n k the TYPICAL PLASTICITY GRAPHS anode. Less common is electrolytic pickling, though the use of alternating current in this operation is a recent development. Enamelers are coming to recognize the nickel dip as good insurance against many of the troubles incident t o subsequent processing. Following pickling, the ware is given a nickel “flash” by immersion in a bath containing 3 ounces per gallon of nickel sulfate (NiSO,GH,O) and 0.25 ounce per gallon
$
JANUARY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
11
valved. But in connection with cast iron parts for enameling, also cleaned by abrasives, the chemical relation between the iron and enamel has received much attention. In addition to enamel blistering that may be caused by the escape of gases mechanically contained in the iron, there is a type of blistering (possibly more than one) t,hat apparently comes from gases liberakd by the graphitization or other reactions taking place within the casting, due to the heat incidental to burning the enamel.
Cast Iron Base Metal
Courlcsg!. Fcrro Enomel Cmpomlion
INTERIOR OF RADIAXT T U B E BOX-TYPE FURNACE
(Right) Box TYPE MUFSLE FmbFIaED FURNACE WITH V-BOTOM TO INCREASE RADIATING SUICFACE
-
COUVlrtpSY.
Fcno Enamel Coiporalion
of boric acid.
The control of p H within a range of 5.5 to 6.0, varying somewhat from one case to another, is essential. The bath is operated a t a temperature range of 150' to 180' F., and the time.of treatment is usually about 5 minutes. I n all well-operated plants the cleaning and pickling department is equipped with a chemical control outfit for the careful volumetric regulation of the several operations involved. A substitute is the use of standard "pills" for determining the strengths of the various solutions. Though not as accurate as titration methods, they serve well if properly used. Theforegoingstatementsas to processing have referred to ware made from relatively thin sheet stock. I n the manufacture of large storage and processing units from thicker metal, preparation for enameling is hy surface grinding and abrasive cleaning, and no chemical considerations are in-
Irons that anneal readily so that the resulting gases pass off before the fusion of the enamel, give satisfactory results, while slowly annealing irons continue t o give off gases a t higher temperature that become entrapped in the viscous enamel. Irons that contain free or hypereutectoid cementite, either throughout the casting or in the thin surface layer that has come to be known as the microchill. tend to cause blisterina. This eau be eliminated in the case of mild microchill b; severe abrasive treatment. Some special irons of the pearlitic type,
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VOI, 32,NO 1
Chemical Raw Materials
Courter", IIet1er XRorneliny IllOS
OXIDE 1 k . M
Hydrochloric-scid-etohed StrUctUrE of iron-enamel interface of a normally fired comrrrercid enamel on sheet steel; etchant attack on ferrous-oxide-rich areas ( X 700).
Chemical processing and control have made conspicuous contributions to vitreous enaxnclina through the raw materials used. Such clieniicals as soda ash,-sodium nitrate, borax, and oxides of tin, lead, aid zinc have been well standardized for some time. By cliemical treatment and control natural minerals, such as silica, feldspar, clay, cryolite, and fluorspar, have more recently been brought to high standards of purity and uniformit,).. Chemical research has produced several substitutes for tlie more expensive tin oxide, originally the ehici white pigment and opacifier. Comj>oundsof antimony and zirconium are now in wide use, either in place of or as partial substitutes for tin oxide. The demand for attractive colors has resulted in tlie solution OS some knotty problems. The color manufacturers have done an excellent job in developing coloring oxides that will give uniform results over wide ranges of enamel composition and processing conditions. Tvro types of coloring compounds are used-those that dissolve in the enamel matrix approaching a clear glass, supported by separate opacifiers if desired, and those that resist solution and contribute color and opacity through fine dispersion in the enamel. I n making these colors the pigmenting constituents are mixed with certain carriers, so clioscn as to give a product that will be properly related to the enamel involved. This mix is calcined, waslied free of soluble matter, dried, and ground to a state of fine subdivision. Careful control of the entire process is necessary to ensure uniiormity.
Enamel Compositions
Ciiuifeag, R .
M . Kin#
DEXDEITEY IK Esawsr. Iron-enamel interface of normally fired eornmercial enamel on sheet stccl; dendrites precipitated in enamel on metal surface.
so stabilized as t.o reaist graphitization by the lieat of enameling, are also successfully enameled. There areother chemical factors connected vith iron composition and blistering that may be involved; they are under study at the present time. With proper fouudry practice, iron of tire Sollowing approximate analysis should give good results for stove plate. For thicker sections and the use of lov-phosphorus pig, adjustments are requircrl : siiioon suiiur Phosphorus
""'
0.075 U.76
Maopsntse Total osrhon GinDhitio carbon
0.70% 3.30-3.40 2.80-2.90
This composition is in line with the present theory that the carbon equivalent
should be 4.3, or slightly above, for store plate.
Progress in the dovelopment of enamel coropositions was slow in the hands of the enameler who jealously guarded his secret formulas, but with the advent of the technically trained man and tlie accompanying scientific approach, advanconlent became more rapid. From the chemical point of siew early sheet iron enamels were just acid resistant enough to withstand the milder conditions encountcred in the kitchen, but were attacked by strong fruit acids. Cast iron enamels were not so resistant as the sheet iron compositions. A little lemon juice carelessly dropped on the kitchen sink destroyed the glossy suriace. Large equipment found use in the brewing and dairy industries but to only a limited extent in the canning, pharmaceutical, and chemical fields; the MILL ADDITIONS latter made use o i VARkPgLE WACIRER 0.2511r MSWS hizhlv acid-resisting,, cast iron ware from Europe. W i t h t h e imported ware cut off by the war, tlie development of domest i c acid-resisting enamelsbegan. Cast 0 I lNR 2 4 6 E iron and sheet steel PER CENT WACIFIER enameled eouiument was pmducej i;r the Courleay, F e r n Enamel Corporolion chemical trade and REFLECTANCEus. AVUUNT OF allied industrieswhich OPACIFIEX was as resistant to acids as that formerly imported. A critical situation with reference to the manuiacturc of certain explosives, medicinals, and dyestuffswas thus met. The use of acid-resisting enamels has recently carried over into the cooking ware and sanitary fields so that enamels are now available for liouseliold use which are not att.acked by any of the fruit acids or by the hydrocliloric acid that the tile setter may drop on the bathY
"
Y
INDUSTRIAL A I D ENGIXEERING CHEMISTRY
JAXUARY, 1940
tub incidental to the removal of cement from the surface of wall tile. iinother important change in enamel composition is the elimination of lead from wet-process cast iron enamels used for stove parts and similar ware. Enamels of this type fuse a t a lower temperature than do those for sheet iron, and lead oxide has been in common use as a flux. The leadless enamels often substituting barium carbonate for lead oxide, have been more difficult to process and more sensitive to defects in the casting, but recent improvements in the enamel and cast iron, accompanied by modified processing, have combined t o m a k e t h e leadless y 75 en a m e 1s c ornm e r c i a 11y practical. MILL ADDITIONS
/
I /
100Ibr. 71br CLAY FRIT 4 Iba.OPACIFIER 0 251b. MQCO5
1 1
Consistency of
Enamel Slips
One of the most imuortant points in the appiication of wet-process enamel DRY WEIGHT GRAMS PER SQUARE FOOT is the consistencv of the Courtesy, Ferro Enamel Corporatton slip. This property is a determining factor in the REFLECTANCE us. WEIGHT OF ENAMEL production of a smooth, even coat of enamel by dipping or draining, less important, but not to be ignored, in application by spraying. Early methods of control involved the use of borax and sometimes magnesia for setting up ground coats and epsom salts for cover coats. The operator was guided in the use of these additions by the manner in which the slip drained from the hand. Early investigators realized that the basic principles of colloid chemistry were involved and that some accurate means of measuring consistency was needed. The effect on consistency of a variety of electrolytes was investigated, as indicated by apparently appropriate viscometers, but the data obtained did not lead to satisfactory interpretation. It was possible to determine standard requirements for a given enamel under definite operating conditions, but troublesome variables asserted themselves and no fundamental principles were recognized that permitted a scientific analysis. 9 new day arrived when it was recognized that enamel slips follow the laws of plastic rather than those of viscous flow. By the use of simple plastometers it was possible to establish graphs corresponding to correct consistency for enamel slips for various conditions of application. The slope of the graph indicates “mobility” and the intercept the “yield value”. Variations in one or both of these quantities offer means of adjustment. -4rise in temperature increases mobility without change in the yield value; the addition of water increases the mobility and decreases the yield value; the use of more clay increases the yield value without change in mobility, and the addition of magnesium sulfate produces a like effect. Following the recognition of these facts, other electrolytes were studied and the important effect of the solution of sodium oxide, boron oxide, and other oxides from the frit in the mill liquor was appreciated. Various improvements of the early instruments are involved in the plastometers and mobilometers now on the market and in use in research and control. By the application of the principles of plastic flow through the above and auxiliary control methods, the consisteiicy of enamel slips can be adjusted so as t o make use of special enamels previously difficultto work and t o make possible the application of enamel to constructions that could not previously be enameled.
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Burning It is a long step from the coal-fired fire clay muffles of a few decades ago t o the modern furnaces heated by oil, gas, and electricity. This progress has come through improved refractories, the development of heat-resisting alloys, and a better understanding of the application of heat to the burning of enamel. Bearing on the proper maturing of the enamel, two important factors have been recognized--the proper time-temperature cycle and the necessity for correct furnace atmosphere. Several interesting chemical features are involved in the latter. In general, air free from other gases is the ideal furnace atmosphere. It was early recognized that sulfur gases were detrimental to final finish. In muffle furnaces combustion gases carrying sulfur were excluded except in the frequent cases of defects in the muse lining. With the development of certain direct-fired oil furnaces in which the products of combustion come in contact with the ware, it was found that “sulfur scum” did not develop with fuel oils below 0.5 per cent in sulfur content. In this type of furnace it was also required to determine the amount of free oxygen needed to mature a given enamel properly and to control combustion conditions accordingly. It was found that the furnace gases must contain over 5 per cent free oxygen for the proper maturity of the enamel involved.
Courtesy, Pfaudler Company
ACID-RESISTIKG ENAMELED STILL,CONDENSER, A X D RECEIVER For furnaces using radiant heat rnm tight muffle walls, electric resistors, or fuel-heated radiant tubes, the problem includes proper ventilation of the burning chamber so as to avoid an excess of certain gases evolved from the enamel during firing. The chief of these is water vapor arising from
14
INDUSTRIAL AND ENGINEERING CHEMISTRY
moisture that may not have been previously removed, or that resulting from the dehydration of the clay and other constituents of the dried enamel film. Cases of defective enamel have been definitely associated with the presence of 4 per cent water vapor, and as much as 10 per cent has been found in extreme instances. There is ample evidence that certain
VOL. 32, NO. 1
other gases are harmful-for example, fluorine compoundbut when allowed to accumulate in a furnace they are usually accompanied by water vapor and there may be some doubt &s to their effect. Some investigators hold that 0.5 per cent of these so-called acid gases can cause trouble with cast iron enamel.
Adherence One of the most interesting problems that has had the attention of the enamel technologist is the cause of adherence of enamel to sheet steel. Experience to date has indicated the necessity of cobalt oxide in the ground coat to assure adherenee. Nickel and manganese oxides are used with the cobalt oxide; the former is thought to promote adherence, and the latter to widen the firing range. No effort to make a light ground coat free from one or more of these oxides has been commercially successful, but there is a lack of agreement as to the manner in which adherence is produced by them. The first theory was to the effect that the cobalt oxide becomes reduced and that the resulting alloy with the base metal produces a structure to which the enamel can adhere. Recent investigators have advanced more specific explanations. One school of thought holds that adherence is related to the formation of metallic dendrites which x-ray studies have indicated to be alpha-iron, cobalt, and nickel. Some of these dendrites as observed in a given section are attached to the metal; others penetrate some distance into the enamel. If these dendrites do not actually promote adherence in a mechanical way, they are at least present in cases of good adherence and absent under opposite conditions. A modification of this thoory is that the dendrites are chiefly metallic cobalt that has been plated out by electrolytic action of the hot iron base during fusion of the enamel. All of the metals between iron and copper in the electromotive series, if dissolved in glasses, am plated out of solution when the glasses are fired as ground coats on steel. On this basis, cobalt and nickel are held to contribute to adherence by being deposited on the base metal, and possibly mechanically in addition by the dendrites formed. In opposition to the above, adherence has been associated with the presence of certain iron oxide films a t the interface between metal and enamel. “Neither gripping, electrolytic reaction or dendrites are the fundamental causes of commercial cobalt ground coat adherence to iron, but the enamel glass isrigidly held to the iron through the medium of a ferrous phase approximately 3 X lo-* inches thick” (4). In a t least one instance the formation of the desired oxide film is regulated by a controlled furnace atmosphere. The most recent statements of the proCourfuy, Friro Enomd Corporolion ponents of these theories are “iron oxides (Top) VPFREOWSENAMEL EXTERIOR, (m&)KITCHEN, (boltom) INTEEIOR and metallic dendrites are precipitated a t COLUMNS (Photo by F. S. Lincoln) the interface; adherence is associated with
JANUARY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
precipitated metal in all cases, but not always with precipitated oxides” (67, and “the degree of adherence seems to depend upon the thickness of the ferrous oxide film between glass and iron. The thicker the film the poorer the adherence b e comes” (5).
Opacity Accompanying the development of more acid-resistant enamels, much progress has been made with regard to opacity. The result is better ware as to appearance and durability,
15
dispersion; certain fluorine compounds promote the formation of crystals of finely dispersed sodium fluoride; zirconium opacifiers produce immiscible phases, with an index of refraction differentfrom that of the matrix. Earlier practice was based on producing a fair amount of opacity in the frit, depending chiefly on opacifiers, such as tin oxide, in the mill. More recently, higher opacity has been produced in the frit and finer milling has increased the effect of opacifiers used. With these superopaque frits and more grinding, i t is now possible to obtain a given net effect with a total enamel thickness much less than that previously required t~produce enamelswith less over-alt opacity.
Uses
Courfes~,F e i r o Enoml Corporofion
VITR?:OUSENAWXMURALPANELS(Designed by Edein 11. Winter)
the latter through the use of thinner coats with correspondingly greater mechanical resistance. This has come about through a more scientific understanding of the properties involved in tho appearance of the finished coat and the manner in which various opacifiers contribute to it. “Opacity” is a combined effectof reflection, refraction, and diffraction of light. Various devices for the individual and collective evaluation of these factors have proved useful tools. Opacity may be produced by the use of finely divided materials which are insoluble in the enamel matrix under commercial conditions, the development of a crystalline phase within the enamel through chemical reaction during firing, or the presence of immiscible phases, either solid or gascous. In a given case the net effect may be a combination of two or all of these factors. X-ray studies have gone a long way in pointing out the mechanics of various types of opacity. For example. tin oxide remains as an insoluble, finely divided
Vitreous enameled equipment plays an important part in many of the chemical industries. Resistance to the action of acids, with the exception of hydrofluoric, affords ample protection against corrosion and the contamination of materials by metals. There has been a steady and substantia1 increase in the maximum size and maximum pressure ratings of enameled vessels for use in severe chemical conditions. Units up to 1700 gallons in size are now availablc for such varied operations as nitration, chlorination, digestion, sulfonation, crystallization, and distillation. Much larger pieces are suitable for the handling and storage of materials of a less corrosive nature. Standard designs are such as to make possible assemblies of wnsiderable complexity. A majority of the new usages which have been found for enameled equipment during the past few years have been in the field of synthetic organics, particularly of the chlorinated type, and for such finished products as plastics, synthetic rubber such as Neoprene, clilorinated solvents, etc., either for the finished products themselves or for intermediates which enter into their manufacture. Many of the everyday chemical and pharmaceutical compounds that make essential contributions to human welfare are most satisfactorily procesmd in enameled equipment. By way of illustration may be mentioned radium salts, vitamin compounds, glandular extracts, sedatives, and antiseptics. The use of glass enamel has also broadened greatly in the pharmaceutical field for synthetics such as sulfanilamide and in the preparation and concentration of biological products such as vitamins. Thus, and in many other ways, modern ceramics and chemistry have worked hand in hand in the development of a product that is an everyday necessity in the household and in the factory. The sanitary and artistic excellence of the kitchen of today would hardly be possible but for this progress. Stores, filling stations, houses, and commercial structures are made more beautiful and durable through the application of the hasic principles of chemistry and kindred sciences in the conversion of an ancient art into one of our most progressive modern industries.
Bibliography (1) Andrews. A. I., “Eiamols”, Champaign, Ill., Twin City Publishing Co., 1935. (2) Anonymous, Cevamie Data Bock. 9, 132-6 (1937); 10. 175-9 (1938). Hsnsen, J. E.,“Manual of Porcelain Enameling”, Cleveland. Enamelist Publishing Co., 1937. Kauts. K., J . Am. Ceram. Soc., 19. 108 11936). Ibid.. 20. 294 (1937). Xing, R . M., Ceramic Age. 30, 55 (1937). Landrum, R. D., and Carter, W. D.. Bibliogisphy and Abstracts of Literature on Enemels. Columbus. Ohio. Am. Ceramio Soc., 1929. Poste. E. P., Ceram. Id., 22, 138-15. 201-18 (1934); 24, 230-46, 300-6 (1935); 27, 444-6 (1936); 28. 152-4 (1937);
(1938); Ceramic Data Book, 8,102-13 (1935). Poete, E.P.,Pmc. Ann. Poieelah EnamelInnst.. 3,15-16 (1938).
