A Radioisotope Study of the Nickel Dip

Radioisotope tracer studies of the deposition of nickel during nickel dipping of steel and of the effect of this nickel deposit on the reduction of cobalt ions to metallic cobalt during enamel firing showed the following: (1) The deposition of nickel on the metal surface during the dipping and the deposition of cobalt at the enamel-metal interface during firing were both markedly affected by the type of cleaning given the steel and by the mechanical deformation of the metal surface. (2) The nickel from the nickel dip remained as metal at the interface during firing of the enamel. (3) The presence of the nickel deposit had little or no effect on the deposition of cobalt at the interface during firing.     

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.     

Oxide Adherence and Nickel Flashing in Vitreous Enameling

The influence of iron deposition and nickel flashing on the adherence of oxides to enameling-grade sheet steel has been examined. Nickel flashing promotes oxide adherence, and a correlation between oxide adherence and adherence of a directly applied titania self-opacified cover-coat enamel has been established. Further work is necessary, however, to obtain a satisfactory enamel surface texture under these conditions before the direct application of a cover-coat enamel can become a commercial possibility. Similarly, good adherence of a titania self-opacified cover coat applied directly to steel has been promoted by nitric acid pickling with and without a nickel-flashing treatment. Finally, the iron content of a hydrochloric acid pickling bath and the amount of hydrogen absorbed by the steel during pickling have been shown to influence the amount of nickel deposited on the steel during flashing.     

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.     

MECHANICS OF ENAMEL ADHERENCE, II

The effect on adherence of the variation in the amount of cobalt oxide, nickel oxide, and manganese oxide, and the effect of atmosphere, air, oxygen, and nitrogen on ad- herence were studied. Enamels of same base but with varying amounts of the special oxides were made up, fired in the various atmospheres, and tested for adherence by drop- hammer impact methods and by flexure methods. A number of photographs of metallographic sections showing contact line between enamel and metal are given. General conclusions are (1) that adherence is not a function of the roughness of contact line, (2) that the adherence promoting effect of cobalt oxide is higher at 1.25% than at 2.5% or 5.0%, and (3) that the effect of oxygen and nitrogen in general is to decrease the adherence as measured by impact strength.     

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.     

Nature of Adherence of Porcelain Enamels to Metals

An investigation of the fundamentals of the adherence of porcelain enamels to metals indicated that good adherence is the result of metal-to-metal bonds between the atoms in the base metal and the proper metallic ions in the enamel. To accomplish this type of bond, the enamel must meet certain chemical and thermodynamic requirements: (1) The enamel at the interface must be saturated with an oxide of the metal and (2) this oxide must be one which, when in solution in the glass, will not be reduced by the metal. In the case of iron, the oxide is FeO. Many of the phenomena observed in commercial enameling were investigated and found to be related to adherence, but not essential for its development. An example is the precipitation of metallic particles in the enamel. Much of the complexity in commercial enameling arises from the limitations imposed by practical considerations. For example, because enamels usually are fired in air, the heavy scale developed during the early stages of firing must be removed before adherence can be developed. Likewise, as the conditions of the enamel-metal interface change rapidly during firing, "adherence-promoting oxides" are used to help maintain the necessary conditions for the time required in commercial enameling. Surface roughness, although not necessary for excellent adherence, was found to improve the apparent adherence when the bond between the enamel and the metal was relatively weak.     

Standardizing Shop Practices Relative to Making and Application of Steel Ground Coat Enamels. I

I The effect of manganese dioxide on the physical properties of ground coat enamels is: (1) to give a good gloss; (2) to give a harder enamel; (3) to reduce the coefficient of expansion of the glass in which it is used; (4) when used with cobalt a uniform color desirable in stove work is obtained; (5) no difference in the adhesion of the ground coat between enamels using manganese, cobalt, or combinations of dioxides is apparent; (6) no effect on chipping is apparent; (7) while not absolutely essential in ground coats it is very desirable for use from an economy stand point. II. A theory of ground coat adherence based on the fact that part of the steel is oxidized to iron oxide which is partly absorbed by the fused enamel and into which some of the enamel penetrates is set forth, and an explanation given of various effects obtained. III. Most of the things which should be done to maintain control in making and firing of ground coats for sheet steel are emphasized. IV. The method of control for application of ground coat enamel by weighing the amount of dry enamel on a sheet of metal of known area is described. This is used to maintain a uniform consistency of the enamel. Other essential features which have in the past caused trouble to many enamelers are mentioned.     

THE CAUSES AND CONTROL OF FISH SCALING OF ENAMELS FOR SHEET IRON AND STEEL1

Data obtained.—This paper presents measurements of the following factors and properties and their influences upon fish scaling in the case of typical single and three-coat enamels: time and temp. during melting of the frit; fusibility of the enamel; coeff. of expansion of the enamel, and of a variety of representative irons and steels up to 500°C (accuracy about 0.1%); annealing the enamel coat after firing; chem. comp. of the steels and irons; mechanical treatment (rolling, spinning, drawing, and so forth of the stock; microscopic surface structure of the stock; effect of chem. comp. upon thermal expansion of enamel. Incidentally it was found that the formulas of Mayer and Havas for computing the thermal expansion of enamel from its chem. comp. can not be relied upon: Conclusions.—On the basis of these exhaustive tests, the following conclusions have been drawn as to the causes and the control of fish scaling. (A) Causes.—1. The cause of fish scaling lies in the difference of the coefficients of expansion of the enamel and the stock, that for steel being higher than for enamels so that the latter are under a compressive stress. The factors influencing this phenomenon are as follows: (a) Composition of the enamel as affecting the coefficient of expansion of the enamel; (b) Overfiring—thus volatilizing those substances which tend to keep the coefficient high; (c) Lack of annealing enameled ware, the enamel coating of which is a glass and should logically be treated as such. 2. Fish scaling is due secondarily to a number of factors which may affect the strength of the enamel or its adherence to the metal. (a) The physical condition of the surface of the metal as influenced by drawing, spinning, cold rolling or other mechanical treatment; (b) Composition of the glass as affecting its elastic strength; (c) Underfiring, enamel not fused to metals; (d) Cleanliness of the surface as regards removal of drawing compounds, grease, etc. (B) Remedies.—1. (a) Adjusting the composition of the enamel so as to increase its coefficient, e. g., by decreasing the boric acid content; (b) Correct firing of the enamel to avoid unnecessary volatilization of such fluxes as soda and cryolite; 2. (a) Adjusting enamel composition to increase its strength; (b) Correct firing to give proper adherence. 3. Treatment of metal to give best adhesion by (a) cold rolling or other suitable mechanical treatment of the untreated steel; (b) thorough cleaning. (C) Summary.—Broadly stated, then, fish scaling can be absolutely controlled by one or all of the following methods: (1) By the development of an enamel with an expansion fitting the metal; (2) by annealing the enameled ware; (3) by treatment of the metal as cold rolling or some other method of working: (4) by developing an enamel with a wide range of compressional elasticity and applying this enamel to metal treated as indicated above.     

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.     

IRON OXIDE IN ENAMEL GLASS AS DISSOLVED FROM THE METAL BASE*

The iron oxide content of an enamel glass was measured by the use of a predetermined curve representing iron oxide versus the index of refraction. It was found that the iron oxide content of the glass adjoining the interface between the enamel and the iron base rises sharply in the early stages of firing to a value close to the solubility limit of the glass after which it gradually falls to a constant value. The iron content at the surface remains low until that at the interface decreases, whereupon the surface concentration correspondingly increases. The cobalt additions resulted in a lower iron content in solution at the interface, but they increased the dendrite content of the glass. A decrease in the cobalt content resulted in an opposite effect. There was a direct relation between the amounts of cobalt and dendrites and the adherence. The effects of temperature and thickness of application are given, and the results are discussed.     

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.     

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.     

覆铝钢板搪瓷的低温制备及其密着机理研究

研究了覆铝钢板搪瓷的低温烧成工艺,运用扫描电子显微镜观察了其界面形貌,发现在搪瓷层和金属层的界面形成了铁-铝-瓷釉的复合层。X射线能谱仪分析结果表明界面间的元素扩散十分强烈,并由此促进了覆铝钢板与搪瓷层形成良好的密着。     

A TEST FOR THE ADHESIVENESS OF VITREOUS ENAMELS TO METAL1

A method of testing the adhesiveness of vitreous enamels to metal is developed. The test specimens are prepared by clamping together two strips of enameled metal while they are still in the furnace and allowing them to cool slowly in another furnace from a temperature of 500°C. The test consists of determining the force necessary to pull the two metal pieces apart when treated as a continuous strip of metal under tensile stress. As a source of additional data in analyzing results, an estimate is made of the percentage of contact area on each specimen within which failure occurred in the bond. The adhesiveness of an enamel containing cobalt, manganese, and nickel oxides is compared with the enamel minus these oxides. The cobalt enamel gave a much stronger bond than the cobaltless enamel, a result which ie in agreement with industrial experience. The degree of reproducibility is sufficient to permit positive distinction between conditions of adhesiveness which differ considerably.     

Relation Between Base Metal Composition, Surface Composition, Pickling Rate, and Direct-On Adherence

Changes in residual alloy concentration, particularly increases in copper and nickel, which take place in pickling were found to be due to the formation of a pickling film through galvanic plating of these elements dissolved in the acid. Before pickling, the steel surface was actually higher in copper and nickel than was the interior, owing to preferential oxidation of iron during mill processing. Pickling film buildup (such as surface copper) was found to have no direct telation to adherence. Slow pickling steels, however, on which adherence was less easily obtained, built up more copper on the surface; hence, an apparent correlation existed. Fast pickling steels, which gave good adherence with less metal removal than was required for slow pickling steels, showed much more surface roughening for the same amount of metal removal. This increased roughening is felt to be a contributing factor toward good direct-on adherence.     

MECHANICS OF ENAMEL ADHERENCE: XV,INFLUENCE OF COBALT AND NICKEL OXIDES ON METAL PRECIPITATION AT GROUND COAT-IRON INTERFACE*

The method previously described for determining metal precipitation during groundcoat firing on sheet iron has been refined and used in a quantitative study of the comparative influence of cobalt and nickel oxides on this precipitation. The effect of these oxides has been studied separately and in mixtures, and it has been found that nickel oxide has the more pronounced action. This action is evident even when the ratio of cobalt oxide to nickel oxide is 3 to 1.     

Reaction Kinetics of Porcelain Enamel-Metal Systems

The reaction kinetics of the elevated-temperature fusion reactions between various powdered metals and finely divided sodium borosilicate porcelain enamel frits were studied. The powdered metals employed were nickel, cobalt, iron, and magnesium; the metallic ions were copper, nickel, cobalt, and iron. These reaction studies were made as a function of time, temperature, and concentration of the metallic ion in the frit. Quantitative X-ray diffraction was used to determine the amount of metallic ion in each reaction which had been reduced to the metallic state by the added metal. Specific reaction rates were calculated, their temperature dependence was demonstrated by use of the Arrhenius equation, and activation energies were determined from the slope of these curves. The over-all results were in accordance with the general theory of chemical kinetics, with increases in time, temperature, and metallic-ion content resulting in increases in the amount of metallic ion reduced, except for those frits containing the iron ion, which, when coupled with cobalt or nickel metal, resulted in probable reduction of the iron ion to a lower valence, but not to the metallic state. Inversions in the order of magnitude of reaction rates for two different metals with the same metallic ion frequently occurred at certain times and temperatures. Reactions such as Fe + COO (NiO) = FeO + Co (Ni), which might take place at an enamel-metal interface, were strongly favored to the right and very little tendency existed for the reverse to occur.     

Influence of Copper Ions on Adherence of Vitreous Coatings to Stainless Steel

An investigation was made of the effect of copper oxide in promoting the adherence of vitreous coatings to AISI type 321 stainless steel. It was found that copper ions in the coating produced a significant increase in adherence on both pickled and sandblasted surfaces but that the effect of the copper decreased with increased firing temperature and increased firing time for the pickled specimens. X-ray examination of specimens revealed that the copper ions were reduced to metallic copper near the interface during firing; however, examination of sections with the metallographic microscope showed no selective corrosion of the stainless-steel surface. It was observed that the curve obtained when plotting the copper oxide content of the coating against the adherence index was of the same type as that obtained for cobalt oxide on ingot iron.     

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.