Abstract:
An enamel for coating steel articles is disclosed comprising SiO 2 ,  Tsub.2, ZrO 2 , CaF 2 , LiO and Na 2  O with the following ratio of the components, in percent by weight: 
     
       ______________________________________ 
     
     SiO 2           62.17 to 72.29TiO 2           0.2 to 9.54ZrO 2           0.5 to 9.9CaF 2           2.4 to 6.6Li 2  O         0.3 to 5.06Na 2  O         10.48 to 19.05______________________________________ 
     The enamel may also comprise Cr 2  O 3  or Co 2  O 3  in an amount of 0.5 to 2 percent by weight. The oxides ensure the preparation of an enamel of a green or blue color, respectively. The enamel efficiently protects steel articles from aggressive media, i.e. acids and alkalis and the enamel coating has good luster and high continuity, without pinholes.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to enamels for coating steel articles with a view to afford protection to same from the effect of aggressive media such as acids and alkalis. The aforesaid steel articles can be pipes, reactors, heat exchangers and other vessels. 
     The prior art teaches an enamel for coating steel articles comprising SiO 2 , Al 2  O 3 , CaO, Na 2  O, K 2  O, B 2  O 3 , TiO 2 , Li 2  O, ZrO 2 , CaF 2 , CuO and SrO with the following ratio of said components, in percent by weight: 
     
         ______________________________________  SiO.sub.2     50 to 80  Al.sub.2 O.sub.3                0.5 to 6  CaO           0.2 to 5  Na.sub.2 O    7 to 20  K.sub.2 O     up to 5  B.sub.2 O.sub.3                0.2 to 6  TiO.sub.2     up to 5  Li.sub.2 O    0.2 to 5  ZrO.sub.2     0.2 to 10  CaF.sub.2     0.2 to 7  CuO           0.5 to 20  SrO           0.2 to 10______________________________________ 
    
     The disadvantages of the enamel known in the art are the use of a large number of components, as well as a large amount of copper oxide which may result in metallization of the coating and deterioration of its protective properties. In addition, copper oxide is toxic. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to prepare such enamel for coating steel articles which would comprise a considerably smaller number of components and would not require critical and toxic materials for its preparation, having at the same time high protective properties. 
     In accordance with the foregoing and other objects of the invention, there is proposed an enamel for coating steel articles comprising SiO 2 , TiO 2 , ZrO 2 , CaF 2 , Li 2  O and Na 2  O with the following ratio of said components, in percent by weight: 
     
         ______________________________________SiO.sub.2          62.17 to 72.29TiO.sub.2          0.2 to 9.54ZrO.sub.2          0.5 to 9.9CaF.sub.2          2.4 to 6.6Li.sub.2 O         0.3 to 5.06Na.sub.2 O         10.48 to 19.05______________________________________ 
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The enamel of the invention differs from known enamel in that it comprises half as many components and does not contain the critical boron oxide and toxic copper oxide. The enamel has a high acid resistance and alkali resistance and makes it possible to prepare a high-quality coating, without pinholes. 
     It is recommended to use an enamel having the following composition, in percent by weight: 
     
         ______________________________________  SiO.sub.2     68.0  TiO.sub.2     3.23  ZrO.sub.2     4.95  CaF.sub.2     6.32  Li.sub.2 O    2.5  Na.sub.2 O    15.0______________________________________ 
    
     The enamel of said composition has the highest acid and alkali resistance and forms the highest-quality coating (continuous, without pinholes) on steel articles. 
     The enamel may contain in its composition such components as Cr 2  O 3  or Co 2  O 3  in an amount of 0.5 to 2 percent by weight. The oxides make it possible to produce the enamel of a green blue color, respectively. 
     The proposed enamel is prepared in the following manner. 
     First a batch is prepared, the batch components being the following substances: 
     (1) silica sand or pure silicon dioxide; 
     (2) titanium dioxide; 
     (3) zircon concentrate ZrSiO 4  ; 
     (4) fluorspar; 
     (5) lithium carbonate; 
     (6)soda. 
     The content of the components in the batch is determined by the content of oxides in the enamel. 
     The batch components are proportioned, mixed and completely melted in rotary or hearth-type furnaces at a temperature of 1,300° to 1,350° C. The melted enamel is fritted by pouring it in to water or passing through rolls. Thereafter the fritted enamel is dried at a temperature of 100° to 120° C. and a dross is prepared. For this purpose the fritted enamel is charged into a ball mill adding thereto clay in an amount of 5 to 7 percent by weight of the frit, an electrolyte such as ammonium chloride and or ammonium molybdate and molybdenum anhydride, in an amount of 0.5 to 1 percent by weight of the frit, chromium oxide or cobalt oxide, if necessary, in an amount of 0.5 to 2 percent by weight of the frit, and water in an amount of 40 to 50 percent by weight of the frit. The resultant mass is subjected to grinding to a requisite fineness, e.g. that of 1.4 to 1.7 cm (by the method of settling in a Lisenko cylinder). 
     The prepared dross is applied to pre-primed steel articles by a method of dipping or spraying and the enamel coating is dried at a temperature of 20° to 120° C. The dried coating is subjected to calcination at a temperature of 820° to 860° C. in electric furnaces or in induction units. 
     For obtaining a continuous (without pinholes) enamel coating it is necessary to apply the dross to the articles at least three times, drying and calcining the coating after each application. 
    
    
     For a better understanding of the present invention the following examples of producing the proposed enamel are given. 
     EXAMPLE 1 
     A batch of the following composition is prepared as calculated per 100 parts by weight of the enamel: 
     
         ______________________________________silica sand             59.84titanium dioxide        9.54zircon concentrate ZrSiO.sub.4                   7.28fluorspar               6.21lithium carbonate       5.88soda                    25.31                   114.06______________________________________ 
    
     The batch components are proportioned, mixed and melted in a rotary furnace at a temperature of 1,300° to 1,320° C. till complete melting thereof. The melted enamel is fritted by pouring it in to water. Thereafter the fritted enamel is dried at a temperature of 100° to 120° C. and a dross is prepared. For this purpose there are charged into a ball mill 100 parts by weight of the enamel frit, 5 parts by weight of clay, one part by weight of ammonium molybdate, and 42.0 parts by weight of water. The resultant mass is subjected to grinding to a requisite fineness, e.g. that of 1.4 cm (by the method of settling in a Lisenko cylinder). 
     The prepared dross is applied to pre-primed steel articles and the enamel coating is dried at a temperature of 20° C. to 120°. The dried coating is calcined in an electric furnace at a temperature of 860° C. 
     For obtaining a continuous (without pinholes) enamel coating the dross is applied to the articles three times, drying and calcining the coating after each application. 
     The resultant coating (enamel) has the following composition, in percent by weight: 
     
         ______________________________________   SiO.sub.2     62.17   TiO.sub.2     9.54   ZrO.sub.2     4.9   CaF.sub.2     6.21   Li.sub.2 O    2.38   Na.sub.2 O    14.8                 100______________________________________Physical and chemical characteristics of the enamel:______________________________________coeffiecient of linearexpansion (α), in degrees.sup.-1                  87.10.sup.-7calcination range, in °C.                  845 to 915heat resistance, in °C.                  200leachability, in mg/cm.sup.2,of theenamel coating after keepinga steel article with acoating for four hours inboiling aqueous solutions:(1) a 20, 24% aqueous solution    of hydrochloric acid   0.11(2) a 2 N aqueous solution of so-    dium hydroxide         0.47______________________________________ 
    
     The resultant enamel coating has a light gray color and is distinguished by good luster and high continuity. 
     EXAMPLE 2 
     The preparation of the enamel and its application to steel articles is carried out in a manner similar to that set forth in Example 1. 
     The patch composition, in parts by weight (as calculated per 100 parts by weight of the enamel): 
     
         ______________________________________silica sand            60.18titanium dioxide       0.2zircon concentrate     14.72fluorspar              5.7lithium carbonate      5.85soda carbonate         25.27                  111.92______________________________________ 
    
     The dross composition, in parts by weight: 
     
         ______________________________________enamel frit             100clay                    7ammonium chloride       0.5chronium oxide Cr.sub.2 O.sub.3                   2water                   50                   158.5______________________________________ 
    
     The enamel composition, in percent by weight: 
     
         ______________________________________   SiO.sub.2    65.05   TiOI.sub.2   0.2   ZrO.sub.2    9.9   CaF.sub.2    5.7   Li.sub.2 O   2.37   Na.sub.2 O   14.78   Cr.sub.2 O.sub.3                2                100______________________________________Physical and chemical characteristics of the enamel:______________________________________coefficient of linear expansion (α), in degrees.sup.-1 102.10.sup.-calcination range, in ° C.                  840 to 900heat resistance, in °C.                  210leachability, in mg/cm.sup.2, of theenamel coating after keeping asteel article with a coating forfour hours in boiling aqueous so-lutions:(1) a 20, 24% aqueous solution of    hydrochloric acid      0.13(2) a 2N aqueous solution of so-    dium hydroxide         0.48______________________________________ 
    
     The resultant enamel coating has a green colour and good luster and high continuity. 
     EXAMPLE 3 
     The preparation of the enamel and its application to steel articles is carried out in a manner similar to that set forth in Example 1. 
     The batch composition, in parts by weight (as calculated per 100 parts by weight of the enamel): 
     
         ______________________________________silica sand            57.86titanium dioxide       6.25zircon concentrate     13.64fluorspar              2.40lithium carbonate      0.74soda                   32.57                  113.46______________________________________ 
    
     The dross composition, in parts by weight: 
     
         ______________________________________enamel frit            100clay                   5ammonium chloride      0.1ammonium molybdate     0.2molybdenum anhydride   0.4chromium oxide Cr.sub.2 O.sub.3                  0.5water                  47.0                  153.2______________________________________ 
    
     The enamel composition, in percent by weight: 
     
         ______________________________________   SiO.sub.2    62.33   TiO.sub.2    6.25   ZrO.sub.2    9.17   CaF.sub.2    2.40   Li.sub.2 O   0.3   Na.sub.2 O   19.05   Cr.sub.2 O.sub.3                0.5                100______________________________________Physical and chemical characteristics of the enamel:______________________________________coefficient of linear expansion (α),in degrees.sup.-1       95.10.sup.-7calcination range, in ° C.                   830 to 900heat resistance, in ° C.                   200leachability, in mg/cm.sup.2, of theenamel coating after keeping asteel article with a coating forfour hours in boiling aqueous solu-tions:(1) a 20, 24% aqueous solution of    hydrochloric acid       0.12(2) a 2N aqueous solution of sodium    hydroxide               0.45______________________________________ 
    
     The resultant enamel coating has a light green colour and good luster and high continuity. 
     EXAMPLE 4 
     The preparation of the enamel and its application to steel articles is carried out in a manner similar to that set forth in Example 1. 
     The batch composition, in parts by weight (as calculated per 100 parts by weight of the enamel): 
     
         ______________________________________silica sand            72.05titanium dioxide       2.32zircon concentrate     0.74fluorspar              6.48lithium carbonate      6.13soda                   26.38                  114.10______________________________________ 
    
     The dross composition, in parts by weight: 
     
         ______________________________________enamel frit            100clay                   6.0ammonium chloride      0.5cobalt oxide Co.sub.2 O.sub.3                  0.5water                  40                  147______________________________________ 
    
     The enamel composition, in percent by weight: 
     
         ______________________________________   SiO.sub.2    72.29   TiO.sub.2    2.32   ZrO.sub.2    0.5   CaF.sub.2    6.48   Li.sub.2 O   2.48   Na.sub.2 O   15.43   Co.sub.2 O.sub.3                0.5                100______________________________________Physical and chemical characteristics of the enamel:______________________________________coefficient of linear expan-sion (α), in degrees.sup.-1                  99.10.sup.-7calcination range, in ° C.                  845 to 915heat resistance, in ° C.                  220leachability, in mg/cm.sup.2, of theenamel coating after keeping asteel article with a coating forhours in boiling aqueous solutions:(1) a 20, 24% aqueous solution of    hydrochloric acid      0.09(2) a 2N aqueous solution of sodium    hydroxide              0.47______________________________________ 
    
     The resultant enamel coating has a light blue color and good luster and high continuity. 
     EXAMPLE 5 
     The preparation of the enamel and its application to steel articles is carried out in a manner similar to that set forth in Example 1. 
     Composition of the batch, in parts by weight (as calculated per 100 parts by weight of the enamel): 
     
         ______________________________________silica sand            65.59titanium dioxide       3.23zirconium concentrate  7.36fluorspar              6.32lithium carbonate      6.20soda                   25.65                  114.35______________________________________ 
    
     Composition of the dross, in parts by weight: 
     
         ______________________________________enamel frit            100clay                   5molybdenum anhydride   0.5ammonium chloride      0.5water                  45                  151.0______________________________________ 
    
     Composition of the enamel, in percent by weight: 
     
         ______________________________________   SiO.sub.2    68.0   TiO.sub.2    3.23   ZrO.sub.2    4.95   CaF.sub.2    6.32   Li.sub.2 O   2.5   Na.sub.2 O   15.0                100______________________________________Physical and chemical characteristics of the enamel:______________________________________coefficient of linear expansion (α),in degrees.sup.-1       98.10.sup.-7calcination range, in ° C.                   800 to 920heat resistance, in ° C.                   250leachability, in mg/cm.sup.2, of theenamel coating after keepinga steel article with a coating forfour hours in boiling aqueoussolutions:(1) a 20,24% aqueous solution of    hydrochloric acid       0.06(2) 1 2N aqueous solution of sodium    hydroxide               0.42    leachability, in mg/cm.sup.2, of the    enamel coating after its consecu-    tive treatment with a 20,24% aqueous    solution of hydrochloric acid and a    2N aqueous solution of sodium hydro-    xide at a temperature of 96° C. and    with the duration of each treatment    of three hours; said treatment cycles    &#34;acid-alkali&#34; are conducted 6 times                       3.7______________________________________ 
    
     The resultant enamel was tested for acid resistance according to JTE/13 in liquid and vapor phases of 20,24% aqueous solution of hydrochloric acid at the solution boiling temperature for 48 hours. The results of the test are given in Table 1. 
     
                       Table 1______________________________________     Leachability of the enamel                   gTest medium mg/cm.sup.2 m.sup.2 · day                               mm/year______________________________________Liquid phase       0.13        0.65        0.09Vapor phase 0.20        1.00        0.14______________________________________ 
    
     The enamel was also tested for alkali resistance according to DIN/51156 in a 4% aqueous solution of sodium hydroxide at a temperature of 80° C. for 48 hours. The test results are given in Table 2. 
     
                       Table 2______________________________________Leachability of the enamel       gmg/cm.sup.2 m.sup.2 · day                      mm/year______________________________________0.44        2.2            0.32______________________________________ 
    
     The resultant enamel has a light gray colour with addition of 1 wt. % of Cr 2  O of light green colour, and 0.5 wt.% of light blue colour and good luster and high continuity. 
     EXAMPLE 6 
     The preparation of the enamel and its application to steel articles is conducted in the same way as in Example 1. 
     Composition of the batch, in parts by weight (as calculated per 100 parts by weight of the enamel): 
     
         ______________________________________silica sand            70.36titanium dioxide       6.76zircon concentrate     0.74fluorspar              6.6lithium carbonate      12.5soda                   17.35                  114.31______________________________________ 
    
     Composition of the dross, in parts by weight: 
     
         ______________________________________enamel frit            100clay                   5.5ammonium chloride      0.5water                  42.5                  148.5______________________________________ 
    
     Composition of the enamel, in percent by weight: 
     
         ______________________________________   SiO.sub.2    70.60   TiO.sub.2    6.76   ZrO.sub.2    0.5   CaF.sub.2    6.6   Li.sub.2 O   5.06   Na.sub.2 O   10.48                100______________________________________Physical and chemical characteristics of the enamel:______________________________________coefficient of linear expansion (α),in degrees.sup.-1       90.10.sup.-7Calcination range, in ° C.                   840 to 910heat resistance, in ° C.                   210leachability, in mg/cm.sup.2, of theenamel coating after keeping asteel article with a coating forfour hours in boiling aqueous so-lutions:(1) a 20, 24% aqueous solution of    hydrochloric acid       0.11(2) a 2N aqueous solution of sodium    hydroxide               0.43______________________________________ 
    
     The resultant enamel has a light gray colour (with addition of 1 wt.% of Cr 2  O 3  of light green colour, and 0.5 wt.% of light blue colour) and good luster and high continuity. 
     EXAMPLE 7 
     The preparation of the enamel and its application to steel articles is conducted in the same way as in Example 1. 
     Composition of the charge, in parts by weight (as calculated per 100 parts by weight of the enamel): 
     
         ______________________________________silica sand            67.95titanium dioxide       1.53zircon concentrate     9.37fluorspar              3.2lithium carbonate      10.29soda                   21.72                  114.06______________________________________ 
    
     Composition of the dross, in parts by weight: 
     
         ______________________________________enamel frit            100clay                   6ammonium chloride      0.1ammonium molybdate     0.2molybdate anhydride    0.4cobalt oxide Co.sub.2 O.sub.3                  1.0water                  47.0                  154.7______________________________________ 
    
     Composition of the enamel, in percent by weight: 
     
         ______________________________________   SiO.sub.2    71.10   TiO.sub.2    1.53   ZrO.sub.2    6.3   CaF.sub.2    3.2   Li.sub.2 O   4.17   Na.sub.2 O   12.7   Co.sub.2 O.sub.3                1.0                100______________________________________Physical and chemical characteristics of the enamel:______________________________________coefficient of linear expansion (α),in degrees.sup.-1      93.10.sup.-7calcination range, in ° C.                  840 to 900heat resistance, in ° C.                  200leachability, in Mg/cm.sup.2, ofthe enamel coating after keepinga steel article with a coatingfor four hours in boilingaqueous solutions:(1) a 20,24% aqueous solution of    hydrochloric acid       0.12(2) a 2N aqueous solution of sodium    hydroxide               0.43______________________________________ 
    
     The resultant enamel coating has a blue colour and good luster and high continuity.