THE EFFECT OF FURNACE ATMOSPHERE ON THE FIRING OF ENAMEL1

It is shown that in firing ground coat and other one coat enamels, the atmosphere of the furnace plays an important part. Oxygen in the muffle penetrates the enamel during the early stages of the firing forming a film of iron oxide on the surface of the steel. This film, whether applied before enameling or formed incidentally to firing, partially dissolves in the fused enamel giving the intimate bond between metal and enamel. Ground coat enamel fired in an atmosphere of nitrogen was found not to adhere to the steel. Approximate values are given for the proportion of furnace space to enameled surface to insure sufficient oxidation when residual air is depended upon to supply the oxygen.     

Effect of Oxygen Content of Furnace Atmosphere on Adherence of Vitreous Coatings to Iron

A series of porcelain enamel ground coats of the same basic composition but with various amounts of cobalt oxide was fired on enameling iron in atmospheres consisting of various oxygen-nitrogen mixtures. The effect of the oxygen content of the atmosphere on adherence was determined and the effect on interface roughness was estimated from an examination of metallographic cross sections. A decrease in the amount of oxygen in the firing atmosphere necessitated an increase in the amount of cobalt oxide in the enamel if optimum adherence was to be secured. Ground coats containing 3.2% cobalt oxide or more developed a weak but definite bond in oxygen concentrations as low as 0.02 mole %. A qualitative correlation was found between adherence and surface roughness regardless of the oxygen content of the firing atmosphere.     

X-ray Diffraction Characterization of the Enamel–Steel Interface

In the enameling of steel, the oxide is generally regarded as being completely dissolved by the fusing enamel, with the enamel–metal bond forming directly between oxide-saturated glass and metal. According to this model, the adherence of the oxide layer present on the surface of the steel as the enamel begins to fuse is irrelevant, because none of the original oxide layer remains in the matured enamel–steel bond. This model has not been completely verified, however, and some researchers have presented evidence for the presence of a layer of wüstite (FeO) at the enamel–steel interface on the order of 1 to 4 μm in thickness. Whether such a layer exists has important implications regarding the mechanism of enamel–steel adherence. In the present study, a method was developed to concentrate whatever crystalline material might be present in the interfacial zone to make it more amenable to detection by X-ray diffraction. Through the use of wüstite standards, the present technique was shown to be capable of detecting a layer of wüstite at the enamel–steel interface as thin as 0.3 μm. However, in neither one-coat nor two-coat enameling could a layer of wüstite be demonstrated at the enamel–steel interface. Hence, there does not appear to be a 1-to-4-μm-thick wüstite layer at the enamel–steel interface. If a layer of iron oxide is present at the interface, it must be thinner than 0.3 μm.     

WET PROCESS ENAMELS FOR CAST IRON1

Wet process enameling of cast iron is becoming of importance because of its application to the enameling of stove parts, sanitary fittings and hardware. This paper is a report of an extended investigation of wet process enamels for cast iron, in connection with the use of a ground coat. Compositions used in dry process enameling have served as a basis for the work, which includes a study of ground coats and white cover enamels. The effect of varying methods of preparing the frits, mill additions, and the relation of composition of the enamels to such properties as adherence, texture and opacity have been carefully studied. Ground Coats. —Sintering of most ground coat frits has been found desirable in order to develop best adherence on the castings. Clay gave best results as a mill addition for the groundvt and feldspar were found to be less satisfactory. Excessive additions of clay or flint produced flaking of the ground coat while feldspar tended to develop blistering. About 15 per cent of clay or 10 per cent each of clay and flint are recommended as mill additions for the ground coats. Excessive additions of any one flux were not desirable, and best results were obtained when sodium, lead and boric oxides were combined in more or less definite pro-portions. Sodium oxide in excess of about 10 per cent gave rise to blistering. Boric oxide increased the firing range of the ground coats and was preferable to lead oxide on this account, although it tended to promote crawling of ground coats high in boric oxide if they were applied somewhat heavily. The best ground coats developed are Rg-26, 17, 1, 25 and 18. Cover Enamels. —Cover enamels were smelted in the usual manner. The mill additions consisted of 5 per cent of clay, 8 per cent of tin oxide, and 45 per cent of water, all based on the weight of the dry frit. For best results, the cover enamel must he adapted to the ground coat in refractoriness. Boric acid increased the firing range of the enamels but tended to promote crawling, although this was less pronounced in the case of the more fusible compositions. Boric oxide improved the opacity when suhsituted for such fluxes as sodium and lead oxides. Cryolite increased opacity but additions above 10 per cent, based on the melted weight, tended to promote crawling. Considering all factors, cover enamels R-14 11, 28, 1 and 18 gave best results.     

FACTORS GOVERNING ADHERENCE OF ENAMELS APPLIED TO SHEET IRON *

A series of tests were conducted to ascertain the effect of nickel deposition and firing variations upon the adherence of enamels applied to special enameling sheets. Limits were established for securing optimum adherence of zirconium-opacifid white, antimony-molybdenum-opacified white, and commercial blue ground-coat enamels.     

METHODS FOR THE DETERMINATION OF THE FIRING RANGE OF VITREOUS ENAMELS USED IN WET-PROCESS ENAMELING1

Several methods for making firing range studies in case of wet-process enamels are described. The methods used in producing enamel deposits of predetermined weights are given. The use of these methods has shown that enamels used in wet-process enameling of metal vary widely in action under various conditions of heat and a general discussion of the subject is included.     

NICKEL FLASHING AND ITS RELATION TO ENAMEL ADHERENCE*

Nickel flashing as applied to enameling stock is shown to be primarily metallic nickel. The role of nickel in developing enamel adherence is apparently bound up with the retardation of oxidation which the nickel flash imparts in the enameling cycle. The problem of adherence may be considered to be a corrosion phenomenon of the base iron developed by the action of gases and other agents which may be present at enameling temperatures. As such, the degree of adherence is a function of oxygen pressure at the interface when the glass is fused. The equilibria developed are strongly affected by the presence of nickel. Using cover coats directly on iron, the amount of nickel required will vary with each particular enamel composition. Şome evidence as to the nature of the adherence-promoting oxide is presented.     

Role of Nickel Dip in Enameling of Sheet Steel

An investigation was made of the effects of (a) the firing time and (b) the weight of nickel deposited from the nickel-dip solution on the adherence developed by a cobalt-free and a cobalt-bearing ground-coat enamel on both enameling iron and a titanium-bearing low-carbon steel. At 1550°F. the nickel improved the measured adherence index the most during the 2-minute firing. The effect of the nickel, however, decreased markedly with longer firing times. There was an optimum nickel deposit for maximum adherence, an optimum that varied from 50 to about 120 mg. per sq. ft., depending on the type of cleaning used and on the type of enamel applied. The nickel dip reduced the tendency to fish-scale. Furthermore, there was an apparent relation between fish-scaling resistance and good adherence; no specimen with an adherence index of 70 or above showed fish-scaling tendencies. Metallographic studies of the interface of coated specimens showed that nickel dipping of the steel before enameling brought about a surface roughening during the firing operation. A relationship was noted between the degree of roughening and the measured adherence index. The roughening of the steel surface was ascribed to galvanic corrosion.     

On the Adherence of Porcelain Enamel to Sheet Steel

A study of the reactions at the interface of enamels and steel during firing has been made using a reflecting microscope, an electron microscope, an X-ray spectrometer, and a light spectrograph. Much attention has been given to the tendency of a number of oxides, e.g., cobalt oxide and nickel oxide, to develop a typical roughening of the base metal during firing. This roughening appears to be caused by two successive electrochemical reactions at the interface of the enamel and steel. With the aid of the results, a short explanation is given of the various ways in which enamels may adhere to steel.     

Microalloy Precipitation at the Glass–Steel Interface Enabling Adherence of Porcelain Enamel

The adherence of vitreous enamels on steel plates is studied clarifying the mechanisms taking place at the interface during firing. Evidence is provided from electron microprobe analysis for an iron alloy precipitation at the interface, which results from oversaturation of ferrous iron in the enamel glass. Subsequent coalescence of the alloy particles is observed but only after their bonding to the base metal adherence after cupping is achieved. The formation of microalloys requires Co²⁺, Ni²⁺, and Cu²⁺ ions in the enamel glass. A selective dissolution of the base metal by the latter cations does not occur.     

Effect of Firing Schedules on Stress-Temperature Relations in Enamel-Metal Systems

The effect of box and continuous enameling furnace firing schedules on the thermal deflection of enameled iron strips was studied. Effective coefficient of thermal expansion values were calculated from coefficient of thermal deflection data. Results indicate that the effective thermal expansion values for annealed and unannealed enameled iron agree with the expansion data obtained by an interferometer study of the same enamels. Variation in the cooling rate of the enameling furnaces studied is sufficient to produce a marked change in the development of thermal stress in the enameled iron. Residual compressive stresses in the enamel are increased by rapid cooling from firing temperatures. Tensile stress developed in the enamel during reheating is reduced by previous annealing.     

A PECULIARITY OF SHEET-IRON ACID RESISTING COVER COAT ENAMELS*

It has been demonstrated that the small bare spots evident in the first coat of acid-resisting enamel applied directly upon the ground coat are not necessarily the result of a process fishscaling in the ground coat during the firing operation, but are more often the result of reboiling of the ground-coat enamel and lack of film strength in the yet unfused layer of acid-resisting enamel. Reboiling, or other mechanical movements of the ground coat, may promote defects in acid-resisting enamels which would not be visible in those of the nonacid-resisting type, particularly when these enamels are applied directly upon the ground coat. The electrolytes used with acid-resisting enamels are directly connected with the popping condition noted. A choice of the proper kinds and amounts of electrolytes for “setting-up” the enamel can, in some cases, eliminate or minimize this trouble. The use of a nonacid-resisting enamel, properly set up as an intermediate layer between the ground coat and the finish coat of acid-resisting enamel, will also prevent troubles of this kind. The tendency for this peculiar defect to occur depends also upon the type of enameling base metal used and its tendency to reboil.     

SOLUBILITY OF A ZIRCONIUM OXIDE OPACIFIER IN ENAMEL GLASSES*

The solubility of a zirconium oxide opacifier was determined by fusion in enamel glasses and then by heat-treating at enameling temperatures and noting the amount of crystallization. An increase in the alumina, zinc oxide, and calcium oxide content in the glasses caused a decided decrease in the solubility of the zirconium oxide opacifier. The fusion temperatures of the enamels were raised by solution of zirconium oxide.     

PRACTICAL CONSIDERATIONS REGARDING ENAMELING FURNACES1

This paper discusses certain practical requirements which appear essential to securing best commercial results from enameling furnaces. An attempt is made to point out the means by which enameling may be developed into an industry in contrast to an art Various types of enameling furnaces are described and their advantages and limitations pointed out. The action of enamels in the furnace is discussed. The relative advantages of various commercial fuels arc considered.     

ELIMINATION OF BLISTERING OF ENAMELED-IRON NUMBER PLATES *

In the manufacture of enameled-iron number plates for dials on pay-station telephones, blisters, formed in the enamel during the low-temperature firing of the printed characters and background, resulted in an average process loss of 14%. Proposed remedies tried without success were (1) the use of different enamels and enameling irons, (2) firing the characters at other temperatures than the standard temperature of 1310°F, and (3) special treatment of the iron surface before enameling, such as deep steelblasting, heating at 1310°F prior to enameling, or copper plating. The blistering was finally eliminated by increasing the length of the heating-up period in the character firing operation.     

Galvanic Corrosion Theory for Adherence of Porcelain Enamel Ground Coats to Steel

The galvanic corrosion theory of adherence between ground-coat enamels and steel was investigated as a part of a broad study of the bonding mechanism between ceramics and metals. The theory, which is outlined in this report, is based on the mechanical anchoring of the enamel into the pits formed by the galvanic attack of the enamel on the steel surface. The theory was first examined from the standpoint of the data on adherence obtained in earlier studies at the National Bureau of Standards. In addition, several experiments were performed which demonstrated that galvanic corrosion of the metal base could occur during the short firing times encountered in enamel processing. On the other hand, certain inconsistencies were observed in the data which indicated that the mechanism of galvanic attack followed by mechanical anchoring was not the only important factor affecting the bond strength.     

PRACTICAL THICKNESS OF PARTIAL COATINGS OF ACID-RESISTANT ENAMEL*

The effect of the firing cycle, fluidity of enamel glasses, water content of enamel slips, method of application, fineness of grind, and opacifiers on the actual thickness of partial acid-resistant enamel coatings, measured by the Shartsis and Harrison method, is discussed. The results of this study appear to explain the failure of highly acid-resistant enamels to produce satisfactory acid resistance in some enameling plants.     

RELATION OF SURFACE OXIDATION TO CERTAIN DEFECTS IN ENAMEL COATINGS ON STEEL, III

Oxide on the surface of enameling stock is found to play an important part in controlling the occurrence of hydrogen-caused defects. Actually, steel with a preformed oxide coating of certain characteristics may be fired directly with a white cover coat without incurring fish scaling, copperheading, blistering, or reboiling. The oxidizing treatment probably removes some of the inherent hydrogen from the stock and then prevents subsequent absorption during firing from the reaction of iron with moisture which remains in the dried enamel coating. The advantages of preoxidation are discussed. All other gases that may evolve from the enamels pass through the enamel without causing defacement. The primary boil simultaneously oxidizes and hydrogenizes the steel; and the pickup of hydrogen, which occurs during firing, is often the portion responsible for subsequent fish scaling and reboiling.     

低温覆铝锌钢板搪瓷的密着性能研究

通过对覆铝锌钢板搪瓷样品的密着测试,研究了搪瓷的密着性能。通过金相、扫描电镜等测试,分析了铝锌钢板搪瓷断面结合的微观结构,通过能谱测试分析铝锌钢板搪瓷的元素分布,探讨铝锌搪瓷的密着机制。结果表明,铝锌钢板搪瓷的密着性能良好,铝锌镀层与搪瓷层之间有明显的过渡层和元素扩散,中间过渡层与金属铝的结合主要通过金属键来实现,中间过渡层与瓷釉层的结合主要是通过离子键和共价键来实现。     

无铅建筑铝搪瓷的密着机理研究

采用扫描电子显微镜、电子探针等分析无铅建筑铝搪瓷界面的显微结构和元素分布，并结合铝搪瓷的密着强度，探讨了无铅建筑铝搪瓷的密着机理。结果表明，密着性能优良的无铅建筑铝搪瓷面存在铝元素与瓷釉元素相互扩散的中间过渡层；中间过渡层与金属铝的结合主要通过金属键来实现，中间过渡层与瓷釉层的结合主要是通过离子键和共价键来实现。