Patent Publication Number: US-3880772-A

Title: Composition for ring passivation

Description:
United States Patent [191 Rekertas et a1.  
  11] 3,880,772 [451 Apr. 29, 1975 1 COMPOSITION FOR RING PASSIVATION [76] lnventors: Romualdes Vatslovovich Rekertas,  
 ulitsa V. Putny. 9, kv. 80; Romas Romovich Sharmaitis, ulitsa Antakalne, 93, kv. 24; .Iuozas Juozovich Matulis, ulitsa Maloneii 11, kv. 1, all of Vilnjus, U.S.S.R.  
 [22] Filed: Aug. 8, 1972 [21] Appl. No.: 278,867  
 [30] Foreign Application Priority Data 3.620.777 11/1971 Okabe et a1. 252/387 3.706.603 12/1972 Vesscy et a1 252/387 FOREIGN PATENTS OR APPLlCATIONS 672,751 10/1962 Canada 252/387 Primary Examiner-Benjamin R. Padgett Assistant Examiner-Irwin Gluck Attorney, Agent, or FirmHo1man &amp; Stern [57] ABSTRACT A composition for zinc passivation is essentially a mixture of trivalent chromium compounds, hexavalent chromium compounds, nitric acid salts and boron compounds, contained in the following amounts (in parts by weight):  
 trivalent chromium compound (as calculated for chromium): 5.0-20.0 hexavalent chromium compound (as calculated for chromium): 0.1-2.0 nitric acid salt (as calculated for N0 10.0-25.0 boron-containing compound (as calculated for boron): 1.8-6.0  
 The invention is applicable in metal-working industry.  
 7 Claims, No Drawings COMPOSITION FOR RING PASSIVATION The present invention relates to finishing metal parts and has particular reference to a dry composition for the preparation of a solution for bright dipping and colourless passivation of zinc to increase the anticorrosive properties of the zinc-coated parts such as zinc-based alloys, zinc casting, and especially zinc-plated parts.  
  Zinc is known to rapidly corrode under the effects of atmospheric moisture. In order to lengthen the service life and preserve the esthetic appearance of zinccoated parts, such parts may be passivated in acid solutions of trior hexavalent chromium. Prior art liquid aqueous solutions are known for the colourless zinc; passivation which comprise a trivalent chromium compound and nitric acid, or triand hexavalent chromium compounds.  
  The use of a solution containing a trivalent chromium compound and nitric acid is accompanied by a substantial amount of the metal under passivation getting dissolved, especially in the case when automatic passivation plants are used. Preparation of the passivating solution and the passivation effecting procedure by said method involves great amounts of concentrated nitric acid which presents some difficulties in making said solution and in controlling the composition thereof. A ainc surface passivated in such a solution features good ornamental appearance, however, its corrosionrcsistance is improvedinsignificantly.  
  The use of a solution containing the compounds of triand hexavalent chromium makes it possible to obtain colourless-passive films on the surface of zinc parts, however their corrosion resistance properties is improved inconsiderably. In order to increase the corrosion resistance properties of the passive films, the hexavalent chromium content in the solution may be increased. However. introduction of large amounts of chromium trioxide results in the formation of coloured film on the zinc surface.  
  Said methods are instrumental in obtaining colourless ornamental films on the zinc surface. However. incipient traces of corrosion on the zinc surface when tested with a S-percent neutral solution of sodium chloride, are found to appear as early as within 10-20 hours of the test period.  
  It is an object of the present invention to provide a composition that would be suitable for the preparation of a solution to be used in a colourless zinc passivation process either on manual, aided or automatic plants.  
  It is another object of the present invention to diminish amount of zinc rendered into solution during the passivation procedure.  
  It is another object of the present invention to obtain ornamental&#39;passive films which do not affect the metallic aspect of zinc.  
 It is still a further object of the present invention to hexavalent chromium compound (as calculated for chromium): 0.1-2.0; nitric acid salt (as calculated for N 10.0-25.0; boron compound (as calculated for boron): 1.8-6.0.  
  Trivalent chromium compounds are the principal passivating agent. In the present zinc-passivating composition trivalent chromium must be present in an amount of from 50-200 weight parts. A hexavalent chromium compound is an auxiliary passivating substance; its concentration in the passivating composition must be within 0.1 to 2.0 weight parts. Nitric acid salts render oxidating and brightening effects; their concentration in the composition, as calculated the N0 ions is to be maintained within 10.0 to 25.0 weight parts. The boron compounds impart polishing and brightening properties to the present composition; their concentration in the zinc-passivating compositions of the invention, as calculated for boron, should be within 1.8 to 6.0 weight parts. One of the optimum variants of the colourless zinc passivating composition may one incorporating the following constituents (parts by weight):  
 &#39; trivalent chromium compound (as calculated for chroobtain colourless passive films possessing an increased corrosion resistance.  
  Said objects have been accomplished by the provision of a composition for zinc passivationwhich, ac-  
 cording to the invention, is essentially a mixture of tri- H O. etc. for instance.  
  As sources of the hexavalent chromium in the proposed zinc-passivating composition chromium trioxide, ammonium chromates. beryllium chromates. potassium chromates, cadmium chromates, lithium chromates, magnesium chromates, sodium chromates, zinc chromates, bichromates of ammonium, beryllium, potassium, cadmium, lithium, magnesium, sodium, zinc. or mixtures of the afore-mentioned compounds may be used.  
  The nitric acid salts present in the proposed zincpassivating composition may be nitrates of ammonium, beryllium, potassium, cadmium, lithium, magnesium. sodium, zinc, or as mixtures of the abovesaid salts.  
  The sources of boron compounds may be orthoboric acid, magnesium orthoborate, polyborates of ammonium, beryllium, potassium, lithium, magnesium, sodium, as well as a diversity of combinations of the abovementioned compounds.  
  The proposed zinc-passivating composition is a mixture of non-volatile solid chemicals and therefore is distinguished for by its simplicity of preparation, transportation, storage and application. The preparations simplicity resides in an easily obtained passivating solution, and correction and control of the composition thereof. Correction of the passivating solution during its use consists in merely adding chemicals to the zincpassivatin g composition.  
  The present zinc-passivating composition may be prepared, in case of necessity, by conventional mechanical intermixing of the parent components thereof at the site of application.  
  The resultant colourless passive films possess good corrosion resistance. The incipient traces of corrosion on a zinc surface exposed to the effects of a S-percent neutral solution of sodium chloride at 35 1 1C may 1 appear only after from 40-60 testing hours.  
 gattrition, therefore passivation may be carried out in manual or automatic plants of both the stationary or the rotary type. I  
  The application of the proposed composition is carried out as follows.  
  The passivating composition is prepared by thoroughly mixing the parent substances until a homogeneous mass is obtained. Then the zinc-passivating composition is dissolved in a definite amount of distilled or de- 7 ionized water taking into consideration that the concentration of the trivalent chromium in the resultant passivating solution should be from 3-6 g/l. Acidity of the freshly prepared solution should be within the pH value of 2.1 i 0.3; during the operation the acidity may be corrected by adding some acid, e.g., sulphuric. Objects made from the fine (unalloyed) zinc or zinc casting, or zinc plated are passivated by dipping or spraying same for from 10120s, optimum 30 s at a temperature of to 35C, optimum C, washed with water and dried.  
  The resultant passive film on the zinc surface is subjected to corrosion tests using the salt fog&#34; method which consists in the continuous exposure of the pas.- sivc film to the effects of fog from the spray of a 5- percent aqueous neutral solution of sodium chloride in a test-chamber at 1C. Corrosion resistance of the film is assessed by time elapsed from the beginning of test until] the appearance of incipient corrosion traces EXAMPLE 1 I A zinc-passivating composition is prepared containing (in parts by weight): chromium sulphate (as calculated for chromium): 10.4 chromium trioxide (as calculated for chromium): 1.1  
  sodium nitrate (as calculated for N0 13.0 boricacid (as calculated for boron): 4.1  
  Further, 40 g of the thus-prepared composition are dissolved in a litre of distilled water. Upon dissolution the pH value of the solution is 1.98. A bright-zinc coated object is taken from the zinc bath immediately after washing with water and. without being brightdipped, is immersed for 30 seconds in the passivating solution, then washed with running water and air-dried. Upon drying the object shows that all surfaces are coated with a uniform colourless passive film. Corrosion of the film was resisted as determined by the saltfog method for at least 60 hours.  
  EXAMPLE 2 Following the conditions of Example 1 the following passivating composition was prepared (in parts by weight):  
 chromium-potassium alum (as calculated for chromium): 10.2  
 chromium trioxide (as calculated for chromium): 1.0  
 ammonium nitrate (as calculated for N0 10.5  
 magnesium nitrate (as calculated for N0 7.3  
 magnesium orthoborate (as calculated for boron): 4.3 The corrosion resistance time of the zinc surface, as  
 determined by the salt-fog method, is 52 hours.  
 EXAMPLE 3 Following the conditions of Example 1 following passivating composition was prepared (in party by weight): chromium nitrate (as calculated for chromium): 10.0 chromium-potassium alum (as calculated for chromium): 10.0 sodium monochromate (as calculated for chromium):  
  0.1 zinc nitrate (as calculated for N0 15.0 ammonium tetraborate (as calculated for boron): 4.2  
  Corrosion of zinc surface was resisted as determined by the salt-fog method, for 43 hours.  
 EXAMPLE 4 Following the conditions of Example 1. the following passivating composition was prepared (in parts by weight): chromium-potassium alum (as calculated for chromium): 5.0 potassium mono&#39;chromate (as calculated for chromium): 2.0 cadmium nitrate (as calculated for N0 10.0 boric acid (as calculated for boron): 6.0  
  Corrosion of the zinc surface was resisted as determined by the salt-fog method, for 48 hours.  
 EXAMPLE 5 Following the conditions of Example 1. the following passivating composition was prepared (in parts by weight): chromium-potassium alum (as calculated for chromium): 8.0 sodiumbichromate (as calculated for chromium): 1.0 beryllium nitrate (as calculated for N0 25.0 sodium tetraborate (as calculated for boron): 1.8  
  Corrosion of the zinc surface was resisted, as determined by the salt-fog method, for 54 hours.  
 EXAMPLE 6 Following the conditions of Example 1, the following passivating composition was prepared (in parts by weight): chromium sulphate (as calculated for chromium): 10.5 lithium chromate (as calculated for chromium): 0.5 ammonium bichromate (as calculated for chromium):  
  0.7 ammonium nitrate (as calculated for N0 15.5 lithium tetraborate (as calculated for boron): 3.4  
  Corrosion of zinc surface was resisted. as determined by the salt-fog method, for 48 hours.  
 What we claim is:  
  l. A composition for passivating zinc comprising a mixture of the following components, in parts by weight of from 5.0 to 20.0 of trivalent chromium compounds, as calculated for the chromium; from 0.1-2.0 hexavalent chromium, as calculated for the chromium; from 10.0 to 25.0 of inorganic salts of nitric acid, as  
 calculated for the N0 radical; and from [.8 to 6.0 of boron-containing compounds, as calculated for the boron.  
  2. The composition for passivating zinc as claimed in claim 1 wherein the mixture comprises a mixture. in parts by weight, of from 8.0 to 13.0 of trivalent chromium compounds, as calculated for the chromium; from 0.7 1.1 of hexavalent chromium, as calculated for the chromium; from 13.0 to 16.0 of of inorganic salts of nitric acid, as calculated for N0 and from 2.5 to 4.5 of boron containing compounds, as calculated for the boron.  
  3. The composition for passivating zinc wherein the mixture consists essentially of the components as claimed in claim 1, and wherein the salts of nitric acid are selected from the group consisting of nitrates of ammonium, of beryllium, of potassium, of cadmium, of lithium, of magnesium, of sodium, of zinc, and of mixtures of said salts.  
  4. The composition as claimed in claim 3, wherein the boron component is an inorganic boron compound.  
  5. The composition as claimed in claim 4, wherein the boron component is selected from the group consisting of orthoboric acid, magnesium orthoborate, polyborates of ammonium, beryllium, potassium, lithium, magnesium, sodium, and mixtures thereof.  
  6. The composition as claimed in claim 1, wherein the trivalent chromium component is selected from the group consisting of chromic sulfates, chromic nitrates, and mixtures thereof.  
  7. The composition as claimed in claim 1, wherein the hexavalent chromium component is selected from the group consisting of chromium trioxide; the chromates and bichromates of ammonium, beryllium, potassium, cadmium, sodium, magnesium, lithium, and  
 zinc; and mixtures thereof.