Patented June 20, 1944 PROCESS-OF PREPARING FRITTED FOUN-] DATIONS FOR THE ENAMELING OF ME- TALLIC ARTICLES Karl Frank, Bad Soden in Taunus, Germany; ested in the Alien Property Custodian No Drawing. Application April 19, 1938, Serial v N0.'202,907. In Germany April 22, 1937 4 j 2 Claims. (Cl. 20438) The present invention relates to aprocess of preparing a fritted foundation for the enameling of metallic articles. w v
The best adhesion between iron and enamel has hitherto been obtained by the use of a fused foundation containing an adhering oxide. This process is chiefly usedin the case of wrought-iron and for small pieces. In the case of cast iron the top enamel. powder has to be appliedto the fused foundation in the hot condition which, of course, is only possible withsmall objects; large pieces radiate too much heat.
When a fused foundation is used, the resistance of the enameled objects to sudden temperature changes is, however, relatively small. As soon as such an ename1 is quenched with icewater from a temperature of 90 C. it shows capillary cracks. Enameled objects of wrought-iron are somewhat less sensitive than enameled cast iron objects, owing to the adhering oxides, but even their resistanc to sudden temperature changes is generally unsatisfactory in practice.
If instead of a fused foundation a fritted foundation is used, the resistanceto sudden tem perature changes of the enamel may be increased considerably. The adhesion between the fritted enamel and "the. iron, however, is considerably smaller than that of'an enamel with a fused foundation so that more or less large pieces of enamel break away from the coating without any visible reason, whereby th objects become useless.
Attempts have been made to improve th 'adhesion of fritted enamels, and of enamels in general, by roughening the metal surface, especially iron, either by a mechanical treatment, for instance by a prolonged blast of quartz sand or steel sand, or by a suitable pickling. The sand blast attacks the soft constituents more than the hard constituents, for instance the cementite, so that later when the enamel is fired a disintegration and loosening of the enamel may set in where the cementite crystals are situated. When pickling is used it is inevitable that removal of iron by dissolution leaves carbon at the surface, for instance in the form of graphite; this also entail a defective enameling.
Now I have found that a fritted foundation may be prepared without the drawbacks named above. According to the invention the metal surface is roughened, not ashitherto usual by re moving the soft constituents by chemical or mechanical method, but by applying a new. metal layer on the metal surface to be enameled, preferably freed from oxide in the usual manner.
The new metal layer 'isapplied in such a manner that as rough a surface as possible is obtained.
For applying the'metallic intermediate layer there exist various possibilities. Th metal to be applied may, for instance, beheated-to a high temperature and then applied by spraying." Care must be taken by suitably guiding the flame and, if desired, by using a protecting gas that on the one hand an oxidation of themetals and on the other hand an undesired absorption of hydrogen, carbon and the like is avoided. Furthermore, the spraying device must be constructed in such a manner that a coating as rough as possible but of good adhesion is obtained.
Another method for applying a metallicin termediate layer on the metal surface consists in depositing metal from liquid or gaseous metal compounds. For this purpose there are preferably used carbonyls, for instance for enamels on iron the carbonyls of iron or of nickel. These carbonyls are suitably decomposed under a reduced pressure at a raised temperature, for in-,, stance between C. and 200 C. It 'is advisable to choose such a temperatur for producing these deposits that the carbonyls will not decom pose in contact with any ceramic substances or the like which may be present'but only with the iron surface owing to the metallic action of the iron. Especially suitable temperatures for iron carbonyls are those below 180 C. The temperature and also the pressure and the other conditions, however, may vary according to the desired roughness of the coating or to the presence. of small quantities of a protective gas. Instead of by decomposingcarbonyls the meta1 may be applied by decomposing other metal compounds, for instance by a reduction of ametal halide or the like. i A third method of applying a metallic intermediate layer is the galvanic separation of a metal on the object to be enameled. In this procedure the electrolysis i conducted in such a manner-that the deposit is as coarse-grained as possible. This may be attained .by a high .am-
perage. At the same time" any hydrogen dis- 7 charge should ice-avoided as far as possible, for instanceby continuously moving the electrolyte or by heating.
For iron objects deposition of iron or nickel by one of the aforesaid methods is preferable.
The rough metallic layer maybedeposited either directly on the quite clean metalwhich for this-purpose has been freed from rust, for in stance by pickling, or by a mechanical treatment, or through a porous intermediate layer which previously has been fritted or even through a porous non-fritted layer resistant to water and acid which may subsequently be fritted.
It is possible; for instance, first to apply by a usual method a fritted porous layer on the objects to be enameled. Such a. fritted layer may consist, for instance, of about 70 per cent. of silica, the residual 30 per, cent. comprising alkalies, boric'acid, lime and some alumina. Inthis fritted layer the content of silica is so high that the fritting does not lead to complete dissolution in the fiuxing material but only to a fritted but still relatively porous mass. This porous mass adheres to the iron surface, it being possible, owing to an oxidation of the surface of the iron, that an intermediate layer of iron silicate or the like is present. Owing to the porosity of the fritted layer it is now possible to deposit a metal or the iron surface, by the use of carbonyls or galvanically, through the fritted layer and so to obtain a rough surface. If necessary, the material must be pickled carefully before this deposition of metal in order to expose clean metal in thepores of the ,fritted base. The applied metal fills the pores and an especially intimate connection and interlocking of the metal and the fritted layer is pro- 800 C. with evolution of hydrogen chloride. The most suitable solution of this kind is baryta water which reacts with iron chloride with formation of ferric hydroxide and barium chloride. The barium chloride does not melt below 960 C., that is it melts at a temperature above the firing temperature of the top enamel. The barium hydroxide solution for the treatment of the fritted base is best be applied by spraying. 'This operation,
duced. This connection is so intimatethat itis very difficult to removethe fritted layer or a top enamel applied thereon, so that detachment of large pieces is impossible.
' If the layerthrough which the metal is to be deposited is to be a non-fritted layer, the procedure is similar to that applied in the known manufacture of acid-proof self-hardening water glass cements, for instance by admixture of small proportions of such substances as cause the mass to harden even in absence of air. For'instance, a mixture of 100 grams of a frit basis having a high content 'of quartz, consisting of quartz powder and a frit containing alkali not too finely pulverized, with 4 grams of potassium silicofiuorideand cc.- of potassium water glass is stirred together with 40 cc. of water so as to form a slip which "is then applied on the ironsometimes observed: The layer of the frit basis cemented by water glass contains, after the hardening, silicic acidgel which during the galvanic deposition-of 'metal absorbs small quantities of the metal salt used in the electrolysis, for instance metal'chloride or metal sulfate. If after the electrolysis top enamel is applied on the fritted layer and dried and fired, the metal salt decomposes. at the high temperature and bubbles and pores are formed.
It is possible to remove this drawback by treating the frit layer after the metal has been deposited with a solution'containing such cations which react'with the anions of the solution of metal salt used for the electrolysis and form compounds which at the firing temperature subseof gas.
If,for'instance, through the frit layer cemented with water glass iron is applied by the electrolysis of'iron chloride solution the treatment must be with a solution containing cations that form chlorides which still do not decompose at about however,'has the drawback that during the spraying a relatively large quantity of the foundation frit is lost.
This drawback may be avoided if the foundation frit is applied by, pouring or spraying it, as usual, on the metal surface without previous admixture of water glass, and then fixing it. The fixing is preferably performed by spraying on water glass solution together with an addition which causes self-hardening of the water glass, for instance a silicofiuoride. It may be advisable, before spraying on the water glass, to dry somewhat the cast layer of foundation frit. p
The foundation frit may be poured or sprayed on either merely suspended in water or with an addition of any usual organic or inorganic binding agent.
It is also possible to add the self-hardening agent to the foundation frit suspension itself and after the layer has been dried to spray on water glass solution. In thiscase the binding of the water glass with the foundation frit occurs by itself.
There may also be used materials other than water glass for fixing the foundation frit if they combine 7 the particles of the frit with each other and with the metallic surface in'a manner so porous and'so stable that separation does not occur during the subsequent treatment in the galvanic bath.
For the application of the valuable foundation frit casting is particularly suitable for in this case there are practically nolosses of the suspension and the water content .of the latter may be adjusted as desired to suit other conditions without the danger of formation of cracks.
Similar coatings stable to acid and water may also be produced by any desired other process, for instance by mixing the foundation frit named above in the form of a fine powder with a combustible binding agent which is insoluble in water, for instance with varnish lacquer, and applying'it on theobject to be enameled by spraying. In this case the impinging drops of varnish quently applied do not decompose with evolution 1. The processof preparing a metal article to receive a fused enamel coating which consists in first applying a porous waterproof and acidproof fritted foundation to the surface of the article and thereafter galvanically depositing a bonding and acid-proof porous non-fired layer applied on the metal surface and substantially consisting of fritted foundation with an addition of water glass and an alkali metal silicofluoricle which 5 causes the mass to harden in the absence of air.
v KARL FRANK.