Abstract:
A method of preparing a (Me)bis-hydrogen cyanamide composition with enhanced corrosion preventive activity, wherein Me is a divalent metal selected from the group consisting of nickel and cobalt or mixtures thereof. The method includes precipitating Ni(OH) 2  or Co(OH) 2  or mixtures thereof, in situ in a liquid reaction medium, containing a soluble cobalt or nickel salt and sodium hydroxide and a stoichiometric excess of H 2  NCN and immediately reacting the resultant precipitate with H 2  NCN. Precipitation of coprecipitated pigment compositions including the aforementioned derivatives and zinc cyanamide is also disclosed.

Description:
FIELD OF THE INVENTION 
     This invention relates to corrosion inhibiting pigment compositions and processes for making the same. More particularly, it relates to such compositions having particular applicability to mirror backing formulations which are based on salt derivatives of hydrogen cyanamide formed with certain transition metals such as nickel, cobalt and zinc. 
     BACKGROUND OF THE INVENTION 
     Silver layers on mirrors are extremely sensitive to the presence and corrosive action of airborne contaminants, e.g., salt particles typical in coastal regions, or H 2  S, NH 3  or acidic contaminants which are always present in domestic or urban environments. These contaminants, in the presence of moisture, are able to promote oxidative processes which occur according to the reaction Ag-e→Ag +   and result in the corrosive disintegration of the mirror&#39;s reflective layer. Specialized corrosion preventive coating systems, known as &#34;mirror backing&#34; coatings are applied in order to extend a silver mirror&#39;s service life. 
     It is known that non-pigmented protective coatings formulated without a corrosion inhibitor pigment component, exhibit limited anti-corrosion protection and consequently do not provide long term service life. Therefore, corrosion inhibitor pigments are critically important functional components of mirror coating systems and basically determine the useful service life of the protected silver surface. 
     It is known in the prior art to use various lead salts, such as lead cyanamide as corrosion retardant pigment components of mirror backing formulations. The specific corrosion retardant activity displayed by these compounds on Ag surfaces is attributed to the presence of lead species and perhaps to their S 2-   scavenging capacity. Due to the excellent performance of mirror backing formulations containing lead compounds, such systems have been widely used for decades by the mirror manufacturing industry. Efforts to develop lead-free alternatives having anticorrosive activity for silver have been spurred by environmental concerns. 
     U.S. Pat. No. 4,707,405/1987 (Evans et al.) discloses the use of metal derivatives of hydrogen cyanamide other than lead cyanamide as corrosion retardant pigment components in mirror backing protective coatings. However that patent limits the concept exclusively to cyanamides formed by Group IIA and IIB elements such as magnesium, calcium and zinc. 
     There are several chemical and physical properties which a pigment grade product must possess in order to function as a component of a paint formulation and/or mirror backing protective coating. Among the required characteristics are limited water solubility, moderate alkalinity and compatibility with resins and solvents used in such formulations. Specifically the capacity to interact with, and provide inhibitive activity against substrate specific electro-chemical oxidant processes, which promote corrosive decomposition of Ag surfaces, are the most important, and are determined by the pigment&#39;s chemical composition and structure. In this sense the disclosures of the above identified U.S. Patent leave a need for further improved pigment systems for use in mirror backing formulations. 
     Zinc cyanamide is known for its valuable pigmentary proprieties and its applicability as anticorrosive pigment in primer formulations recommended i.e. for steel surface protection. In my U.S. Pat. No. 5,176,894 issued Jan. 5, 1993 I disclosed a pigment grade zinc cyanamide which meets the quality requirements for a mirror backing protective coating, i.e., high assay and absence of soluble salts. 
     It is well known in the chemical literature that hydrogen cyanamide, a di-basic acid, forms neutral, as well as basic and acidic salt derivatives with numerous metals, inclusive of the Group IA, IIA elements and some metals of the first, second and third transition series, among others with Co 2+   and Ni 2+ . 
     It is important to note, however, that Ni 2+   and Co 2+   are the only species known to form the bis-hydrogen cyanamide structures symbolized by M (HNCN) 2  M  2+  =Ni 2+ , Co 2+ ) where the M /NCN stoichiometrical ratio is 1:2. 
     Considering that the ═N--C.tbd.N moiety of any cyanamide compound, due to its characteristic structure, is likely to generate the electrochemically active inhibitor species (by interacting with moisture in situ in the protective coating) accountable for substrate specific corrosion preventive activity on silver, it becomes evident that such derivatives characterized by &#34;bis&#34; structure should be preferably employed as inhibitor pigments. In support of this observation it will be noted that the theoretical value in weight % of the &#34;NCN&#34; content for ZnNCN is 37.9 compared to 58.1 for Ni or Co bis-hydrogen cyanamide. 
     Bernard et al. [Compt. Rend. Ser. C 262(3), 282-4 (1966) report that Ni 2+   and Co 2+   species form cyanamide derivatives of bis-hydrogen cyanamide structure and disclose a relevant preparation procedure based on the precipitation reaction involving hexamine-nickelate or cobaltate species and H 2  NCN in ammoniacal medium, as follows: 
     
         M SO.sub.4 +6NH.sub.4 OH→[M (NH.sub.3).sub.6 ].sup.2+ SO.sub.4.sup.2- +6H.sub.2 O                               1. 
    
     
         [M (NH.sub.3).sub.6 ].sup.2+  SO.sub.4.sup.2- +2H.sub.2 NCN⃡M (HNCN).sub.2  H.sub.2 O↓+(NH.sub.4).sub.2 SO.sub.4 +4NH.sub.4 OH 2. 
    
     
         where M =Co.sup.2+  or Ni.sup.2+ 
    
     Typically the procedure is carried out by introducing H 2  NCN into ammoniacal solution of hexamine-nickelate or -cobaltate and by subsequent agitation of the system for 18 hours at pH=7.5. A similar procedure is disclosed, specified exclusively for Ni 2+   cyanamide in Example #4 of Japanese Patent Nr. SHO 29-8020/12.07.54. The process is performed in one hour by simultaneously introducing nickel sulfate solution and ammonia gas into hydrogen cyanamide solution, at 25°-30° C. and by keeping the pH of the system at 7.5 to 8.5. 
     The NiSO 4  /H 2  NCN=1:1 molar ratio, recommended, quite surprisingly, by this Japanese patent, fails to consider the bis-hydrogen cyanamide structure of the intended product and represents a basic stoichiometrical error (See reaction 2.), which results in particularly low yield (of about 53% in Ni(HNCN) 2   H 2  O) based on the disclosed value, and the correspondingly high amount (practically 50%) of unconverted NiSO 4  dissolved and lost in the process water. 
     As expected, the disclosed value of the obtained product&#39;s nitrogen content (33.2%N) is consistent with bis-hydrogen cyanamide composition, however of a relatively poor quality, which is a direct consequence, as well, of the employed inadequate NiSO 4  /H 2  NCN molar ratio. For the same reason under the final conditions of the process (absence of H 2  NCN, high Ni 2+   concentration, Ph˜8.0) basic divalent nickel salts also precipitate which, by subsequent dehydration are converted into dark colored, inactive inclusions in the final products. 
     In addition to the above-exemplified shortcomings, there are inherent limitations of the manufacturing procedures known by the prior art, all specifically related to the precipitation reaction involving hexamine-nickelate or -cobaltate and H 2  NCN, respectively to the employment of ammonia as pH control reagent. Beside the inconvenience caused by the volatility of NH 3  at the recommended pH value, which require the employment of protective technologies, the following limitations are observed: 
     1. Reaction 2, a typical process of precipitation involving dissolved hexamine nickelate or cobaltate species, reaches an equilibrium which obstructs the completion of the direct reaction (basically the formation of the product by precipitation) to the desirable extent, even at substantial stoichiometrical excesses of H 2  NCN. This undesirable characteristic of the reaction system is a direct consequence of the presence of ammonium salts, soluble by-products formed according to reaction 2, capable to prevent the complete precipitation of Ni 2+   or Co 2+   species as cyanamides. Thus the yield of the process is substantially reduced and the resulting process water (mother liquor and wash water) contains large amounts of irrecuperable Ni 2+   species as well as undesirable ammonium salts. 
     2. As the complete removal of the soluble by-products (usually accomplished by extensive washing) is the critical phase of any corrosion retardant pigment manufacturing process which essentially determines the quality of the final product, processes that use ammonia for pH control, for aforementioned reasons, result in large amounts of non-recyclable, environmentally hazardous process water with high Ni 2+  or Co 2+  contents. 
     SUMMARY OF THE INVENTION 
     Consistent with the above disclosed considerations, it is a principal object of the present invention to provide inhibitor pigment grade products and coprecipitated pigment compositions characterized by bis-hydrogen cyanamide structures, or containing such products, and procedures to manufacture the same. The pigment compositions produced according to the subsequently disclosed procedures provide corrosion inhibitive activity, specifically on silver substrates, and are suitable to be employed in &#34;mirror backing&#34; paint and coating formulations. 
     Coprecipitated pigment compositions comprising Ni 2+   or Co 2+  bis-hydrogen cyanamides and various transition metal cyanamides; i.e., Pb 2+ , Cd 2+ , Cu 2+ , Ag + , etc., can be obtained according to the subsequently disclosed procedures. However, without any intent to limit the applicability of the present invention, the relevant Examples are limited to Co 2+ , Ni 2+ , and Zn 2+   coprecipitated cyanamides. 
     As subsequently disclosed, Ni 2+   and Co 2+   bis-hydrogen cyanamide, or compositions comprising such derivatives coprecipitated with transition metal cyanamides, more specifically zinc cyanamide, are produced at pigment grade quality (high assay, soluble salt contaminant-free conditions) according to an environmentally compatible procedure which results in heavy metal-free waste waters. 
     Briefly summarized, the invention contemplates a method of preparing pigment grade Ni 2+ , Co 2+   (or Ni 2+  +Co 2+ )bis-hydrogen cyanamide derivatives with enhanced corrosion inhibitive activity on silver. The method includes reacting, &#34;in situ&#34; freshly precipitated Ni(OH) 2 , Co(OH) 2  or mixtures thereof, with stoichiometrical excess of H 2  NCN. 
     Coprecipitated pigment compositions containing bis-Hydrogen cyanamide derivatives and zinc cyanamide are also produced by introducing dispersed and hydrated ZnO into the reaction system and reacting it with H 2  NCN at appropriate stoichiometrical ratios. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     IR Spectra characterizing Ni 2+   or Co 2+   or (Co 2+  + Ni 2+ ) bis-hydrogen cyanamide compounds and coprecipitated pigment compositions based on ZnNCN and the aforementioned bis-hydrogen cyanamide derivatives in 10/1 molar ratio, are presented in FIG. 1 and FIG. 2, respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The above presented limitations, which are characteristic to the manufacturing procedures known by the prior art, are eliminated according to the present invention by producing high quality pigment grade Ni 2+   or Co 2+   bis-hydrogen cyanamide based on the heterogeneous reaction involving insoluble and freshly precipitated Ni(OH) 2  or Co(OH) 2  and H 2  NCN, as follows: 
     
         M SO.sub.4 +2NaOH→M (OH).sub.2 ↓+Na2SO.sub.4 3. 
    
     
         M (OH).sub.2 ↓+2H.sub.2 NCN→M (HNCN).sub.2  H.sub.2 O↓4. 
    
     
         where M =Ni.sup.2+  or Co.sup.2+ 
    
     Such pigment grade quality Ni 2+   or Co 2+   bis-hydrogen cyanamides typically are characterized by high N (or cyanamide) content, virtual absence of insoluble contaminants and light green or light brown color, respectively. 
     Both, Ni 2+   and Co 2+   species are characterized by similar chemical behavior under the conditions of reaction 3 and 4; both Co(OH) 2  and Ni(OH) 2  are insoluble in excess of NaOH or in the presence of soluble by-products of reaction 3, and if freshly precipitated, react instantaneously and quantitatively with H 2  NCN. 
     As above indicated, in contrast to reaction 2, reaction 4 is a diffusionally controlled heterogeneous process which takes place at the M (OH) 2  liquid interface and converts the freshly precipitated solid precursor into the solid final product. It is observed that the reaction plausibly occurs by nucleophile substitution, where the .sup.(-) HNCN anion is actually the reactive species, formed by dissociation favored in alkaline medium, as follows: 
     
         H.sub.2 NCN+OH.sup.- ⃡.sup.(-) HNCN+H.sub.2 O  5. 
    
     As a consequence of the above presented considerations, the rate of reaction 4 is determined by the magnitude of the:specific surface area of the hydroxide phase, the .sup.(-) HNCN concentration and, most importantly, the alkalinity of the medium. It will be observed, however, that the effect of the medium&#39;s alkalinity on the reaction rate is a complex one; it does favorably regulate the concentration of the .sup.(-) HNCN anion but also promotes the well known dimerization of H 2  NCN, in this case an undesirable process, competitive with reaction 4. 
     Under the conditions determined by reaction 4, processes could occur, as well, which are undesirable in reference to the object of the present invention and adversely affect the final products&#39; quality. 
     In this respect, it will be observed that under mild basic conditions, Co 2+   species specifically tend to turn into insoluble basic salts of Co(OH)x type (where x=NO 3  --, SO 4  2--), which although are readily convertible into Co(OH) 2  with appropriate excess of NaOH, do not themselves react according to reaction 4. 
     Equally undesirable is the tendency of freshly precipitated Co(OH) 2  to undergo slow oxidation on exposure to air, yielding dark colored and non-reactive Co(OH) 3 , according to the following reaction: 
     
         4Co(OH).sub.2 +O.sub.2 +2H.sub.2 O→4Co(OH).sub.3 ↓6. 
    
     Such coprecipitated non-reactive species are converted by dehydration into darkly colored oxides, which, as inclusion contaminants, degrade the typically light color and reduce the cyanamide content of the final products. 
     The incomplete conversion according to reaction 4 of the Co(OH) 2  or Ni(OH) 2  reactive precursor species, due to less than optimal  -  HNCN concentration in the reaction medium, as well, adversely affects the final product&#39;s quality. 
     The occurrence of these undesirable side reactions are prevented according to the present invention by selecting appropriate process parameters, as subsequently disclosed. 
     In this respect, it is observed, that reactions 3 and 4 according to the present invention are preferable carried out by instantaneously reacting Ni(OH) 2  and/or Co(OH) 2 , freshly precipitated in situ in the reaction medium, with an appropriate excess of H 2  NCN. 
     During precipitation, which is performed in 15 to 30 minutes, the alkalinity and temperature of the reaction medium is kept at pH=8 to 9.0, preferably 8.5, and 15°-25° C., respectively. 
     The above specified optimal reaction conditions are maintained by simultaneous and individual introduction of the raw materials, at appropriate rates, into the reaction medium. The method of this invention also includes the further steps of filtering the reaction product, washing the filtered product with water to a substantially salt-free condition and subsequently drying the product. 
     With respect to the relevant IR spectrums presented in FIGS. 1 and 2, it will be noted that the presence of a narrow, intense band at 3300 cm -1 , as well as the absence of any absorption band at the 775 -1  region, are the specific features characteristic to the bis-hydrogen cyanamide structure. The triplet band situated roughly at the 2000-2200 cm -1  region is typical for cyanamides. 
     COMPARATIVE EXAMPLE 1 
     In order to exemplify the aforementioned inherent limitations of the manufacturing procedures known by the prior art, Ni(HNCN) 2   H 2  O was produced following basically the recommendations disclosed in Example #4 of the above-identified Japanese Patent, with some exceptions, as follows: 
     adequate Ni 2+  /H 2  NCN=1:2,2 molar ratio was applied, correspondent to the bis-hydrogen cyanamide structure of the final product 
     concentrated aqueous NH 3  solution was employed as pH control reagent 
     all three reagent solutions involved were simultaneously introduced into the reactor at appropriate rate and the total volume of the aqueous phase was comparatively reduced to practical values 
     The process was carried out in 30 minutes by simultaneously introducing Solutions (A), (B) and (C) containing the appropriate amounts of raw materials [(A)--394.1 g. or 1.5 moles of NiSO 4   6H 2  O in 900 ml.; (B)--138.6 g. or 3.3 moles of H 2  NCN in 500 ml.; (C)--57.0 g. or 3.3 moles of NH 3  in 500 ml.]into intensively stirred H 2  O at 25°-30° C. 
     A temporary stoichiometrical excess of H 2  NCN was initially generated and kept during the precipitation process, by introducing &#34;ab initio&#34; 10% of Solution (B) into the reactor. Subsequently, all during the precipitation process, Solutions (A) and (B) were introduced at identical rates; however, the delivery rate of (C), the ammonia solution, was adjusted as required to keep the reaction medium&#39;s pH=7.5 to 8.5 as recommended by the aforementioned Japanese patent. 
     The precipitation process was completed by fixing the reaction system&#39;s pH=8.5 and stirring for an additional 30 minutes. Subsequently, the vivid green precipitate obtained was filtered, washed to salt-free conditions and dried at 70° C. for 12 hours. The experimental results are presented in Table #1. 
     
                       TABLE 1______________________________________Measured Parameters Found Values______________________________________Product&#39;s appearance               vivid green powderYield               191.5 g.; 80.5%Ni as Ni %          40.0N as N %            33.3Ni (HNCN).sub.2.H.sub.2 O %               83.7 calculated, based               on N %H.sub.2 O %         9.8 calculatedNon-soluble impurities,               6.5 calculatedpossible NiO %N/Ni ratio          3.49Recovered Process Water:Volume              2320 ml.Ni.sup.2+  Concentration               5.78 g./l.Total Ni.sup.2+  Content               13.4 g.Recovered wash water:Volume              6500 ml.Total Ni.sup.2+  Content               2.67 g.Total amount of Ni.sup.2+  lost in               16.07 g.process water and wash waterCalculated amount of lost               43.44 g.; 18.2%Ni(HNCN).sub.2.H.sub.2 O______________________________________ 
    
     It is remarkable that 18% of the final product is not recoverable, being dissolved in the process water, even at optimal Ni/H 2  NCN=1:2.2 molar ratio. As above indicated, the low yield and related consequences are due to the fact that the precipitation reaction involving soluble hexamine nickelate or cobaltate species respectively is based on the employment of ammonia as the pH control reagent. 
     The practical embodiment of the present invention is illustrated in 6 examples as follows: 
     EXAMPLE 2 
     Pigment grade cobalt bis-hydrogen cyanamide with enhanced corrosion preventive activity symbolized by the structural formula Co(HNCN) 2  H 2  O x   (where x=0-1) was produced according to the following procedure: 
     A cobalt salt solution (A) was prepared by solubilizing 1 mole (263.06 g.) technical grade CoSO 4   6H 2  O (from Hydrite Chemical Co. of Wisconsin) in 700 ml. water by stirring it at ambient temperature. Water was added to adjust the volume of the Co 2+   solution to 800 ml. 
     Solution (B) containing 2.2 moles of hydrogen cyanamide was prepared by introducing 185.0 g. of stabilized aqueous hydrogen cyanamide solution of 50% by weight concentration (available from Cyanamide Canada Inc. under the trade name of Cyanamide-50) into 100 ml. water of normal temperature and by adjusting the solution&#39;s final volume to 500 ml. 
     A sodium hydroxide solution (C) of approximately 10% concentration containing 2 moles of NaOH was prepared by introducing 160 g. technical grade caustic solution of 50% by weight into 400 ml. cold water, cooling it subsequently to ambient temperature and by adjusting the volume to 800 ml. 
     In order to prepare pigment grade cobalt bis-hydrogen cyanamide, the previously prepared (A), (B) and (C) solutions were introduced simultaneously in approximately 30 minutes (applies to A and C), at identical volumetric rate of approximately 25-30 ml/minute into 400 ml. intensively stirred water at normal temperature. 
     Substantial stoichiometrical excess of hydrogen cyanamide was assured in the reaction system by carefully keeping the reactant solutions&#39; delivery rate constant all during the precipitation as well as by providing some initial excess of that reagent. 
     The slurry&#39;s pH was monitored accurately and kept rigorously between pH=8.0-9.0 and preferably at pH=8.5 by adjusting the NaOH (C) solution&#39;s delivery rate accordingly. 
     The precipitation was completed by stirring the slurry unheated for one hour at pH=8.5 and the process was completed by heating and stirring the slurry at 40° C.±5° C. for 30 minutes. 
     Subsequently, the product was washed to an essentially soluble salt-free condition by repeated sedimentation of the slurry, decantation, fresh water introduction and reslurrying until a conductivity of 300 micromhos/cm of the supernatant was reached; at that stage by Cyanamide-50 addition, a concentration of 0.1% hydrogen cyanamide was generated and the slurry was subsequently stirred for 15-30 minutes. 
     After solid-liquid separation, the pigment grade cobalt bis-hydrogen cyanamide was obtained by drying it at 55°-65° C. for 12 hours to 4-8% H 2  O content and pulverizing it to fineness of 100%+230 mesh. 
     It is important to note that the mother liquor resulting from the first decantation of the slurry, as well as the wash water generated, was colorless and contained only 0.5-1 mg/l dissolved CO 2+ . 
     Total amount of pigment grade product recovered was 154 g. 
     Analytical data obtained on dried product typical for pigment grade cobalt bis-hydrogen cyanamide produced according to the above described procedure are presented in Table 2. 
     
                       TABLE 2______________________________________Analyzed or Tested           Value       Test ProcedureParameter       Found       Applied______________________________________Appearance      Light brown           powderCo as Co %      40.0        ComplexometryN as N %        37.2        KjeldahlCo bis-hydrogen 93.6        Calculatedcyanamide %                 value based on                       N %H.sub.2 O %     5.2         CalculatedN/Co ratio      3.91Non-soluble impurities           1.2         Calculatedprobable Co.sub.2 O.sub.3 %Conductivity    200 ± 25 ASTM D-281-31(micromhos/cm)pH              9.0-9.2     ASTM D-1208-                       84/6.1.1Specific Gravity           2.23        ASTM D-113-84Oil Absorption Lbs/100 Lbs           27-30       ASTM D-281-31Yield           154 0 g.;           96.9% as           monohydrateTotal amount of process water                   Approx. 10 l.(mother liquor &amp; wash water)recoveredCo.sup.2+  concentration in process water                   &lt;1 mg/l.______________________________________ 
    
     EXAMPLE 3 
     Pigment grade nickel bis-hydrogen cyanamide (symbolized by the structural formula Ni(HNCN) 2   H 2  O x  where x=0-1) with enhanced corrosion preventive activity suitable to be employed in &#34;mirror backing&#34; paint formulations was produced according to the procedure pursuant to Example 2, except that Solution (A) was prepared by dissolving 1 mole (262.7) of technical grade NiSO 4   6H 2  O in a similar fashion, as presented in Example 2. 
     Total amount of pigment grade product recovered was 155.0 g. 
     It is observed that the mother liquor recovered from the first decantation of the slurry, as well as the wash water generated, was colorless and contained 0.5-1 mg/l Ni 2+ . 
     Analytical data (obtained on dried product) typical for the pigment grade nickel bis-hydrogen cyanamide produced according to the procedure above described are presented in Table 3. 
     
                       TABLE 3______________________________________Analyzed or Tested           Value       Test ProcedureParameter       Found       Applied______________________________________Appearance      Light green           powderNi as Ni %      38.4        ComplexometryN as N %        36.3        KjeldahlNi bis-hydrogen 91.2        Calculatedcyanamide %                 value based on                       N %H.sub.2 O %     8.3         CalculatedN/Ni ratio      3.96Non-soluble impurities           ˜0.5  Calculatedprobable NiO %Conductivity    200 ± 25 ASTM D-281-31(micromhos/cm)pH              9.0-9.3     ASTM D-1208-                       84/6.1.1Specific Gravity           2.22        ASTM D-113-84Oil Absorption Lbs/100 Lbs           27-30       ASTM D-281-31Yield           155.0 g.;           97.6% as           monohydrateTotal amount of process water                   9-10 l.recoveredNi.sup.2+  concentration in process                   &lt;1 mg Ni.sup.2+ /l.water______________________________________ 
    
     EXAMPLE 4 
     Coprecipitated pigment grade Co-Ni bis-hydrogen cyanamide of 1:1 with enhanced corrosion preventive activity, suitable to be employed in mirror backing paint system formulations, was produced according to the procedure pursuant to Example 2, except that Solution (A) was prepared by dissolving 0.5 mole of each CoSO 4   6H 2  O (131.45 g.) and NiSO 4  6H 2  O (131.35 g.) in a similar fashion, as presented in Example 2. 
     Total amount of pigment grade product recovered (before grinding) was 152.0 g. 
     It is observed that the mother liquor recovered from the first decantation of the slurry was, as well as the wash water generated, colorless and contained 0.5-1 mg/l. Co 2+   and Ni 2+ . 
     Typical analytical data (obtained on dried product) characterizing the coprecipitated pigment grade product obtained according to the above-presented procedure, are presented in Table 4. 
     
                       TABLE 4______________________________________Analyzed or Tested           Value       Test ProcedureParameter       Found       Applied______________________________________Appearance      Light brown           powderNi.sup.+  &amp; Co %           38.6        ComplexometryN as N %        36.7        KjeldahlNi bis-hydrogen 92.3        Calculatedcyanamide %                 value based on                       N %H.sub.2 O %     7.6         CalculatedNi/(Co + Ni) ratio           3.99Non-soluble impurities           &lt;0.2        Calculated(probable NiO + Co.sub.2 O.sub.3) %Conductivity    200 ± 50 ASTM D-281-31(micromhos/cm)pH              8.6-8.9     ASTM D-1208-                       84/6.1.1Specific Gravity           2.23        ASTM D-113-84Oil Absorption, 27-30       ASTM D-281-31Lbs/100 Lbs.Yield           152.0 g.;           95.7% as           monohydrateTotal amount of process 10-11 l.water recoveredNi.sup.2+  + Co.sup.2+  concentration in process                   &lt;1 mg/l.water______________________________________ 
    
     EXAMPLE 5 
     Coprecipitated Ni 2+   bis-hydrogen cyanamide mono-hydrite/zinc cyanamide pigment system with 1:10 molar ratio, characterized by enhanced corrosion preventive activity on silver substrate and applicable in mirror backing formulations was produced according to the procedure, as follows: 
     A combination (A) of a soluble nickel salt solution-zinc oxide suspension was prepared by introducing 0.1 moles of NiSO 4   6H 2  O (26.3 g.) as 30% aqueous solution, into a previously prepared ZnO suspension of 10-12%, containing 1.0 mole (81.4 g.) of well dispersed and hydrated high grade ZnO at normal temperature and by adjusting the total volume to 600 ml. 
     The above-mentioned ZnO suspension was obtained by introducing, in small increments, 81.4. of finely ground ZnO (Azo 66 grade, with average particle size of 0.25 microns and 99.8% assay, manufactured by Asarco, Inc. was used) into 400 ml. intensively stirred hot water at 75°-80° C. and by cooling it to ambient temperature after one hour. 
     Solution (B) containing 1.32 moles of hydrogen cyanamide was prepared by introducing 110.9 g. Cyanamide-50 into 250 ml. water and by completing the solution&#39;s volume to 400 ml. 
     A sodium hydroxide solution (C) of approximately 16% concentration, containing 10.2  moles of NaOH was prepared by introducing 16.0 g. technical grade caustic solution of 50% by weight into 50 ml. cold water, cooling it subsequently to normal temperature and by completing the volume to 100 ml. 
     Coprecipitated pigment system of Ni 2+   bis-hydrogen cyanamide mono-hydride/zinc cyanamide at 1:10 molar ratio was produced by simultaneous introduction, at identical delivery rate, of solutions (A) and (B) into 200 ml. intensively stirred water, at normal temperature, respectively by adjusting the delivery rate of solution (C) accordingly to keep the reaction system&#39;s pH=8-9.0, and preferably at pH=8.5, all during the reaction. 
     By keeping the delivery rate of Solution (A) and (B) rigorously identical, substantial stoichiometrical excess of H 2  NCN was kept in the reaction system all during precipitation. 
     The reaction was completed by stirring for two hours and adjusting the slurry&#39;s pH periodically to pH=8.5 by diluted H 2  SO 4  addition, and by heating it at 40° C.±3° C. for 30 minutes. Subsequently, the resultant coprecipitated product was processed in a similar fashion as presented at Example 2. 
     The total amount of pigment grade coprecipitated product recovered was 120.5 g. The resultant mother liquor and wash water (total amount of 6.01) was colorless and contained less than 0.5 mg. Ni 2+  /l. Analytical data relevant to such coprecipitated pigment system of Ni(HNCN) 2  H 2  O/ZnNCN with 1:10 molar ratio are presented in Table 5. 
     
                       TABLE 5______________________________________Analyzed or TestedParameter           Found Values______________________________________Appearance          White-faint green               powderZn as Zn %          52.82Ni as Ni %          4.95N as N %            23.94ZnNCN + 0.1 Ni (HNCN).sub.2.               71.2-CalculatedH.sub.2 O %Oxides (ZnO, NiO) % 21.6-CalculatedH.sub.2 O %         7.2-CalculatedZn/Ni ratio         9.58N/(Zn + Ni) ratio   1.91Conductivity        350-600(micromhos/cm)pH                  8.0-8.5Specific Gravity    2.80______________________________________ 
    
     EXAMPLE 6 
     Co 2+   bis-hydrogen cyanamide mono-hydrite/zinc cyanamide coprecipitated pigment system with 1:10 molar ratio applicable in mirror backing formulations, was produced in an identical manner as presented in Example 5, except that instead of NiSO 4   6H 2  O, Solution (A) contained 0.1 moles (23.6 g.) of technical grade CoSO 4   6H 2  O. 
     The resultant process water was colorless and contained less than 0.5 mg. Co 2+  /l. A total amount of 120.0 g. of coprecipitated product was recovered. The relevant analytical data are presented in Table 6. 
     
                       TABLE 6______________________________________Analyzed or TestedParameter           Found Values______________________________________Appearance          Light brown powderZn as Zn %          52.95Co as Co %          4.86N as N %            24.21ZnNCN + 0.1 Co      72.0-Calculated(HNCN).sub.2.H.sub.2 O %H.sub.2 O %         6.4-CalculatedOxides (ZnO, Co.sub.2 O.sub.3) %               21.6-CalculatedZn/Co ratio         9.82N/(Zn + Co) ratio   1.93Conductivity        350-600(micromhos/cm)pH                  8.0-8.6Specific Gravity    2.78______________________________________ 
    
     EXAMPLE 7 
     Co 2+   and Ni 2+   bis-hydrogen cyanamide mono-hydrite/zinc cyanamide coprecipitated pigment system with 0.05:0.05:10 molar ratio, applicable in mirror backing formulations, was prepared in identical manner as presented in Example 5, except that (A) contained 0.05 moles (131.1 g.) of each, NiSO 4   6H 2  O and CoSO 4   6H 2  O. 
     A total amount of 120.5 g. of coprecipitated product was recovered. The resulted process water was colorless and contained less than 0.5 mg./l. of Ni 2+   and Co 2+   species. The relevant analytical data are presented in Table 7. 
     
                       TABLE 7______________________________________Analyzed or TestedParameter           Found Values______________________________________Appearance          Light brown powderZn as Zn %          52.87Ni and Co %         4.91N as N %            24.20ZnNCN + 0.1 (CO + Ni)               72.0-Calculated(HNCN).sub.2.H.sub.2 O %Zn/(Co + Ni) ratio  9.68N/(Zn + Co + Ni) ratio               1.93H.sub.2 O %         7.2Oxides (ZnO, Co.sub.2 O.sub.3,               20.8NiO) %Conductivity        350-600(micromhos/cm)pH                  8.0-8.6Specific Gravity    2.79______________________________________ 
    
     The corrosion retardant materials of this invention can be incorporated into a mirror backing composition containing an organic film forming resin which is dispersed or dissolved in either water or an organic solvent. The organic film forming resin should be one which is compatible with the thin metallic mirror film and does not promote deterioration and discoloration of the film. Thus, the resin should not contain functional groups which are reactive with the metal. Preferred organic film forming resins include alkyd resins, acrylic resins, acrylic modified alkyd resins, polyurethane oils, vinyl halide polymers or copolymers, epoxy melamine or urea resins, non-oil based urethane, phenol-formaldehyde resins curable by air drying or baking, or any other resin which is compatible to the metallic layers formed on mirror backings. Examples of solvents for such compositions include xylene, MIBK or toluene. The composition can also contain other components such as catalysts, flow control agents, dryers, anti-settling agents, and the like and mixtures thereof.