Method of pretreating bands and sheets of steel for one-layer enameling, and electrolytic bath for use in connection therewith
United States Patent J iirging et al.  
 [451 Apr. 29, 1975 METHOD OF PRETREATING BANDS AND SHEETS OF STEEL FOR ONELAYER ENAMELING, AND ELECTROLYTIC BATII FOR USE IN CONNECTION TIIEREWITII Inventors: Klaus Jiirging, Remscheid; Hans Zwach, Wuppertal, both of Germany Assignee: Hoeschst Aktiengesellschaft,  
 Dortmund, Germany Filed: Oct. 4, 1972 Appl. No.: 295,010  
 Foreign Application Priority Data Oct. 6, 1971 Germany 2149808 Sept. 2, 1972 Germany 2243339 US. Cl. 204/34; 204/48; 204/49 Int. Cl. C23b 5/48; C23b 5/08; C23d 5/00 Field of Search 204/34, 48, 49  
 Primary ExaminerF. C. Edmundson Attorney, Agent, or FirmWalter Becker [57] ABSTRACT A method of electrochemically pretreating articles, especially bands and sheets of steel, for a later onelayer enameling process according to which the article to be pretreated is degreased and, if necessary, pickled and in an electrolytic nickel and/or cobalt bath containing a nitrate ion is provided with a nickel and/or cobalt layer.  
 5 Claims, No Drawings METHOD OF PRETREATING BANDS AND SHEETS OF STEEL FOR ONE-LAYER ENAMELING, AND ELECTROLYTIC BATH FOR USE IN CONNECTION THEREWITH The present invention relates to a method of electrochemically pretreating bands and sheets of steel and articles made therefrom for the one-layer enameling process according to which the bands and sheets are degreased, if desired pickled, and in an electrolytic nickel and/or cobalt bath are provided with a nickel and/or cobalt layer.  
  Methods of one-layer enameling steel sheets, for instance methods in which no base enamel is employed, have become known. According to one of these known methods, the metal sheet to be enameled is first electrolytically degreased and after washing in water is pickled chemically or electrolytically in sulfuric acid or in a mixture of formic acid and nitric acid, and subsequently is rinsed again.  
  Following the pickling step, the sheet metal is plated electrolytically with nickel in a nickel bath and subsequently is enameled. If the enameling process is not carried out shortly after the nickel plating step, the metal sheet to be nickel plated is electrolytically coated with a zinc layer which is removed prior to the enameling process. A sufficient adherence between the steel metal sheet and the enamel coat is, however, not obtained by this method.  
  According to a further heretofore known method of enameling metal sheets, likewise an electroplating of the metal sheet with nickel is suggested which is annealed in an oxidizing manner and is subsequently pickled. lnstead of nickel, also cobalt or a mixture of nickel and cobalt may be used. The oxidizing annealing at relatively high temperatures and subsequent cooling off in a certain definite atmosphere is, however, relatively expensive and requires considerable equipment so that also this known method is not fully satisfactory.  
  According to still another heretofore known method, the metal sheet is likewise cleaned, washed and briefly treated with an acid. Thereupon from a chemical bath, a nickel and/or cobalt layer is deposited upon the metal sheet, whereupon the sheet is rinsed, is heated in an oxidizing atmosphere, and zinc is deposited upon the nickel and/or cobalt layer which zinc is again removed by the user or in the enameling plant. The drawback of this known method, similar to the before mentioned method, consists in the necessary oxidizing annealing, on the one hand, and, on the other hand, in the fact that the baths to be employed are as to their composition rather unstable. ln addition thereto, this method as well as the other heretofore known methods will operate satisfactorily only when the surface of the metal sheets to be enameled are roughened by the removal ofa certain quantity of material from the steel surface which quantity is generally within the range of from to 40 gr/m Furthermore, with this and other known methods, the time required is too long to be able economically to carry out a continuous method for the pretreatment involved. Therefore, aside from a few exceptions, these known methods have not been adopted in practice to any material extent.  
  It is, therefore, an object of the present invention to provide a method of the above described general character by means of which it will be possible in a very simple manner to pretreat bands or sheets to be enameled in a continuous way, while at the same time the method according to the present invention will be such that it can be practiced and controlled in a simple manner so that also the required installations for carrying out the method can be kept simple.  
  It is a further object of the present invention further to simplify and improve the method set forth in the preceding paragraph while simultaneously improving the adherence of the enamel layer applied after the pretreatment.  
 These and other objects and advantages of the inven-- tion will appear more clearly from the following specification in connection with the various examples set forth further below.  
  In deviation from the heretofore adopted theory and practice for pretreating bands and sheets of steel for the subsequent enameling step, according to which it was always necessary to pickle the bands and sheets by a measurable removal of metal prior to the application of the metallic intermediate layers of nickel and/or cobalt to be able to provide a sufficiently good adherence of the enamel coat, the objects of the present invention as set forth above have been realized by adding nitrate ions to the nickel and/or cobalt bath. According to one advantageous method of practicing the present invention, there are added to the nickel and/or cobalt bath from 0.5 to 110 gr, preferably from 25 to gr, of nitrate ions per liter of electrolyte liquid, and a nickel and/or cobalt layer is at room temperature deposited on the band or sheet in a time period of from 50 to seconds at a current density of from 5 to 15 amperes/dm at a pH-value of the electrolyte of from 1 to 5. In an advantageous and simple manner, the electrolytic bath for carrying out the method according to the invention consists of from 20 to gr/liter of nickel, 0.6 to gr/liter of ammonium nitrate, from 15 to 40 gr of boric acid in sulfuric acid or hydrochloric acid solution. lf an intermediate or adhering layer consisting merely of cobalt is to be deposited on the sheet or band, the bath expediently consists of from 20 to 100 gr/liter of cobalt, of 0.6 to 135 g/liter of ammonium nitrate, of from 15 to 40 gr/liter of boric acid in sulfuric acid solution. lf an adhesive layer consisting of nickel and cobalt is required for the subsequent enameling step, a corresponding mixed bath is employed in conformity with the above referred to bath compositions.  
  According to a further development of the invention, the nickel and/or cobalt layer is in a time period of from 5 to 60 seconds deposited at a current density of from 5 to 50 A/dm in the bath having a temperature of from room temperature to boiling temperature. In this way a considerably widened range of the bath temperature and, more specifically, from room temperature or slightly above room temperature to boiling temperature of the bath, is available and a further reduced time range of from 5 to 15 or 14 seconds, and furthermore a likewise widened range of the current density of from 15 or 16 to 50 A/dm is available in an advantageous manner for carrying out the method according to the invention.  
  The method according to the invention will now be explained in connection with the following examples.  
 EXAMPLE 1 a. Electrolytic degreasing in alkaline lye at 80C for 6 seconds. b. Rinsing for 5 seconds.  
 c. Pickling at room temperature in a 10% by weight sulfuric acid for 2 seconds.  
 d. Rinsing for seconds.  
 e. Nickel plating in an electrolyte having the following composition: 135 gr/liter of nickel sulfate 35 gr/liter of ammonium nitrate 25 gr/liter of boric acid in a sulfuric acid solution at a pH-value of 3.5 and at room temperature, a current density of A/dm in 28 seconds.  
 f. Rinsing for 5 seconds.  
 g. Drying.  
 EXAMPLE 2 The method is carried out in the same manner as set forth in Example 1 without the method step cpickling while the method step b-rinsing is replaced by a spray rinsing step with longer duration of the spraying.  
 EXAMPLE 3 The method steps a) to d) and f) and g) of Example 1 are likewise used in this Example. As method step e) there will be carried out a coating or plating with cobalt in an electrolyte composed of 135 gr/liter of cobalt sulfate 35 gr/liter of ammonium nitrate 30 gr/liter of boric acid, and this is carried out in a sulfuric acid solution at a pH- value of 3.5 and at room temperature, and a current density of 10 A/cm in 28 seconds. Method step c) may also in this Example be replaced in the same manner as described in connection with Example 2.  
 EXAMPLE 4 a. Electrolytic degreasing in alkaline lye at 80C for 6 seconds. b. Rinsing for 5 seconds. c. Pickling at room temperature in a 10% by weight sulfuric acid for 2 seconds. (1. Rinsing for 5 seconds. e. Nickel plating in an electrolyte having the following composition: 135 gr/liter of nickel sulfate 35 gr/liter of ammonium nitrate 25 gr/liter of boric acid at a pH-value of 4.5, a temperature of from 40 to 60 C, and a current density of 20 A/dm in 28 seconds.  
  f. Rinsing for 5 seconds. g. Drying.  
 EXAMPLE 5 The method steps a) to d) and f) and g) of Example 4 are applied to this Example. The nickel plating according to method step e) in Example 4 is carried out with the same electrolyte but at a pH-value of 5.0, a temperature of from 10 to 25C, and a current density of 30 A/dm for 20 seconds.  
 EXAMPLE 6 This Example follows the same method steps a) to d) and f) and g) as in Example 4. The nickel plating according to method step e) is carried out with the same electrolyte but at a pH-value of 4.0, a temperature of from 80 to 90C, and a current density of A/dm for 45 seconds.  
 EXAMPLE 7 The method steps a) to d) and f) and g) of Example 4 also apply to this Example. The nickel plating according to the method step e) of Example 4 is carried out with the same electrolyte as in Example 4 but at a pH- value of 4.5, a temperature of from to C, and a current density of 40 A/dm for 10 seconds.  
  With all of the pre-treatments set forth in the above Examples, following the one-layer enameling carried out in a manner known per se, a firmly adhering enamel layer is obtained.  
  As will be evident from the above, the advantages of the method according to the present invention, in addition to avoiding oxidizing annealing steps and an acid treatment of relatively long duration, consist primarily in the simple way of carrying out and controlling the course of the method according to the invention. In view of this simple way of carrying out the method according to the invention, correspondingly simple installations for carrying out the method are obtained which method can also be practiced in installations already in existence. lnasmuch as in conformity with the method of the invention the heretofore required acid treatment of the metal sheets and bands is no longer necessary, the method according to the invention also avoids the problems of waste water refining which problems occur to an ever increasing extent. Furthermore, with bands and sheets pretreated in conformity with the present invention, it is no longer necessary to apply a protective layer against deformation, for instance a zinc layer, because the adhering layer applied to the material during the pre-treatment cannot be mechanically damaged. The adherence of the one-layer enamel coat on the material pretreated in conformity with the method of the present invention, for which material there may be employed steel with carbon contents of up to 0.10% and also steel which has been previously subjected to a decarborization, is extremely good so that the method of the invention may be employed in a wide field.  
  According to the improved method of the invention, it is no longer necessary to cool the electrolyte. Moreover, the method according to the invention also improves the interengagement of the enamel coat with the supporting or base material, the bands. sheets or individual articles of steel is improved so that an excellent adherence is obtained.  
  It is, of course, to be understood that the present invention is by no means limited to the specific examples set forth above but also comprises any modifications within the scope of the appended claims.  
 What we claim is:  
  1. A method of pretreating a steel article for an enameling process, which includes the steps of preparing an electrolyte bath containing at least one element selected from the group consisting of nickel and cobalt, adding non-substituted nitrate ions from inorganic substance to said electrolyte in the amount of 0.5 to l 10 gr./liter of the electrolyte fluid, and depositing upon the steel article a layer containing at least one of said elements from the electrolyte bath upon the article to be pretreated at a bath temperature of from room temperature to boiling temperature at a current density of from 5 to 50 A/dm for a period of from 5 to 60 secends.  
  2. A method in combination according to claim l, which includes the step of pickling the article to be&#39; gr/liter of nickel, from 0.6 to 135 gr/liter of ammonium nitrate and from 15 to 40 gr/liter of boric acid in a solution selected from the group consisting of sulfuric acid solution and hydrochloric acid solution.  
  5. A method in combination according to claim 1, in which the electrolytic bath contains from 20 to gr/liter of cobalt, from 0.6 to gr/liter of ammonium nitrate and from 15 to 40 gr/liter of boric acid in sulfuric acid solution.  
 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. Dated 9 Invent0r(s) Klaus Jur&#39;ging et 211.  
  It is certified that error appears in the above-identified patent and that said Letters Patent are hereby&#39;corrected as shown below:  
 On the Cover Sheet, in item &#34;Hoesohst&#34; should read Hoesoh Signed and Scaled this twenty-first D ay Of October 1975 [SEAL] Attest:  
 RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner uf&#39;Parenls and Trademarks