Abstract:
A process for producing a strontium chromate corrosion inhibitor includes the steps of providing an aqueous slurry containing dispersed particulate strontium chromate, adding to the slurry a phosphoric acid or a neutral or acidic soluble salt of a phosphoric acid selected from the group consisting of ortho-phosphoric, pyro-phosphoric, tri-phosphoric, polyphosphoric, metaphosphoric, polymetaphosphoric, cyclic tri-metaphosphoric acid, and phosphorous acid, and, filtering and drying the resultant material whereby particulate strontium chromate of reduced solubility is produced.

Description:
The present invention relates to novel modified pigment grade strontium chromate corrosion inhibitors characterized by reduced solubility in water and undiminished CrO 4   −− content. 
     BACKGROUND OF THE INVENTION 
     Several pigment grade inorganic and organic chromates are known to the art and employed in organic based protective primer applications, most notably: SrCrO 4  and Ca ++ , Ba ++ , Zn ++ chromates. 
     Many chromate corrosion inhibitor pigments used to be ubiquitously applied in the paint and coating industry, worldwide as well as in the United States. Due to environmental concerns, however, most of them have been practically eliminated from general industrial use during the past two decades and replaced with less effective phosphate-, molybdate-, borate- or silicate-based products, which, notably, also represent all of the available possibilities offered by inorganic chemistry for this purpose. The elimination of chromate inhibitor pigments from the paint and coating industry appears to be a compromise allowed primarily by improvements in coating science and resin chemistry. Notable exceptions are high-performance organic coatings, especially aircraft and coil primers, which possess no significant barrier capacity and consequently, require the inhibitor efficiency of chromates without any possibility of compromise. Not surprisingly, the corrosion inhibitor pigment of choice for aircraft and coil primers is SrCrO 4 , a preference entirely justified, considering this product&#39;s remarkably advantageous pigment qualities, as subsequently discussed. 
     Corrosion inhibitor pigments function as reservoirs of inhibitor species built into organic primers. In general, anions are the active inhibitor species of corrosion inhibitor pigments, which, in the case of chromates, is CrO 4   −− . It will be apparent, that high CrO 4   −−  concentration per unit volume of pigment, i.e., high “specific inhibitor capacity”, is the desirable property of pigment grade chromates. This is particularly true in view of the fact the average thickness of high performance aircraft and coil primers is approximately 10 to 20 microns. 
     Understandably, high inhibitor capacity is expected to be concentrated in the limited volume available for corrosion inhibitor pigments in high performance organic primer applications. This concept is quantified by 
     
       
         e isp =c i   ·g   sp   /w   m . 
       
     
     e isp  represents the specific inhibitor capacity of corrosion inhibitor pigments, where Ci and gsp are, respectively, the weight ratio of CrO 4   −−  in the pigment and the specific gravity of the pigment, and Wm is the molar weight of the inhibitor species, in this case of CrO 4   −− =116. It will be apparent that a combination of high CrO 4   −− content and high specific gravity results in high specific inhibitor capacity, as it can be seen by comparing the cases of CaCrO 4  and SrCrO 4  presented in Table 1. Due to higher specific gravity, the specific inhibitor capacity of SrCrO 4  (18.4 mmol/cm 3 ) is higher at only 57% CrO 4   −−  content than at 74% CrO 4   −−  content of CaCrO 4  (e isp =13.1 mmol/cm 3 ). Specific inhibitor capacity is not informative, however, with respect to inhibitive activity of corrosion inhibitor pigments. Corrosion and its inhibition are aqueous, dynamic processes and consequently, in coating applications, the kinetic availability of inhibitor species is essentially a function of the solubility of pigment grade corrosion inhibitors. It will be apparent that corrosion inhibitor pigments must possess effective, however limited, solubility in water, as well as to dissociate and hydrolyze as ordinary electrolytes: 
     
       
         SrCrO 4 ⇄Sr ++ +CrO 4   −− +H 2 O⇄HCrO 4   − +OH −   1. 
       
     
     The corrosion inhibitor activity of pigments can be conveniently qualified by I i =n·c isat /c icrt , the inhibitive activity parameter, where c isat  and c icrt  are the is pigments&#39; solubility and, respectively, the critical concentration of the related inhibitor species, in this case CrO 4   −− . A stoichometric parameter, n=1 in the case of alkaline earth chromates. As for c icrt , it characterizes distinct inhibitor anionic species and it is defined as the minumum concentration of anionic species necessary to maintain metal substrates in a passive state when in contact with aqueous phases. It will be apparent that I i &lt;1 indicates low inhibitor activity of pigments and consequently, are unacceptable materials. Similarily, high values of I i  are not acceptable, because highly soluble pigments promote degradation of organic coatings by osmotic blistering as well as by leaching, the latter resulting in increased porosity and increased water absorption in organic coatings. Consequently, it appears that organic coatings impose a limited window or range of solubilities for corrosion inhibitor pigments. This range is estimated to correspond to 1&lt;I i &lt;100 and preferably 1&lt;I i &lt;10. As can be seen from Table 1, SrCrO 4  is characterized by I i ˜5 and corresponding solubility of about c isat =5 mmoles/L. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE A 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 ei sp   
                   
               
               
                 General 
                   
                   
                 c isat   
                   
                 mmol/ 
               
               
                 Formula 
                 c i   
                 g sp   
                 mmmol/L 
                 pH 
                 cm 3   
                 I i   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 CaCro 4   
                 0.74 
                 2.05 
                 141 
                 11.0 ± 
                 13.1 
                 141 
               
               
                   
                   
                   
                   
                 3 
               
               
                 SrCro 4   
                 0.57 
                 3.79 
                 5 
                 8.3 ± 
                 18.6 
                 5 
               
               
                   
                   
                   
                   
                 0.5 
               
               
                 K 2 CrO 4 *3ZnCro 4   
                 0.53 
                 3.35 
                 11 
                 6.8 ± 
                 15.3 
                 11 
               
               
                 *Zn(OH) 2 *2H 2 O 
                   
                   
                   
                 0.2 
               
               
                 BaCrO 4   
                 0.46 
                 4.4 
                 0.2 
                 7.8 ± 
                 17.3 
                 0.2 
               
               
                   
                   
                   
                   
                 0.5 
               
               
                 Zn 5 CrO 12 H 8   a   
                 0.2 
                 3.45 
                 0.2 
                 7.2 ± 
                 5.95 
                 0.2 
               
               
                   
                   
                   
                   
                 0.2 
               
               
                 PbCrO 4   
                 0.36 
                 6.12 
                 1.3 × 
                   
                 18.9 
                 1.3 × 
               
               
                   
                   
                   
                 10−3 
                   
                   
                 10−3 
               
               
                 Na 2 Cro 4   b   
                 0.71 
                 2.7 
                 5.4 × 
                   
                 16.1 
                 5 × 102 
               
               
                   
                   
                   
                 104 
               
               
                   
               
               
                   a basic zinc chromate  
               
               
                   b does not qualify as pigment  
               
             
          
         
       
     
     As can be seen, due to its high specific inhibitor capacity (e isp =18.6 mmol/cm 3 , the highest of all corrosion inhibitor pigment grade chromates) and with moderate yet effective solubility (I i ˜5), SrCrO 4  appears to be the most effective and valuable of all chromate corrosion inhibitor pigments. However, due to its solubility, SrCrO 4  is also moderately hygroscopic, an undesirable property of some characteristic finely divided pigment grade products. This generally results in unwanted water vapor absorption from the surrounding air, and consequently, a moderate tendency for agglomeration or clump formation, especially during transport or storage in high humidity conditions. Although clumping does not necessarily alter the initial particle size distribution or fineness of pigment grade SrCrO 4 , it is undesirable, nevertheless, for it adversely affects dispersion procedures and ultimately results in increased energy consumption in paint manufacture. 
     As intuitively expected, the kinetics and extent of the water absorption process by pigment grade SrCrO 4  is a function of the relative humidity (R.H.) of the surrounding air, among other factors. At 60% R.H., the extent of water vapor absorption by SrCrO 4  is negligible and independent of exposure time. 
     In contrast, however, the rate of water vapor absorption is considerable at 100% R.H. and a function of exposure conditions and exposure time. Such conditions can result in a significant increase (4-7% by weight) in the moisture content of pigment grade SrCrO 4 . As also expected, the process is found to be reversible. It is observed, that water absorbed by SrCrO 4  at 100% R.H., desorbs spontaneously at 60% R.H. and ambient temperature, until a corresponding equilibrium value of &lt;0.1% moisture content is established. It is apparent that clumping of pigment grade SrCrO 4  is a consequence of the water absorption process&#39; reversible character and, presumably, it specifically occurs during desorption. 
     DETAILED DESCRIPTION OF THE INVENTION 
     It is a principal object of the present invention to provide a process and a strontium chromate, calcium chromate or zinc (II) chromate product produced by the process having reduced solubility in water with undiminished CrO 4   −−  content. It is a further aspect of the invention that the strontium chromate products display noticeably enhanced corrosion inhibitor performance. In accordance with a further aspect of the invention, the products have a reduced tendency for slump formation. 
     It was discovered pursuant to the present invention, that the natural solubility of chromates such as SrCrO 4  in water (about 5 mmole/L at ambient temperature) can be reduced approximately 40% to 70%, without any negative effect on the pigment&#39;s corrosion inhibition performance. To the contrary, as documented in Example 6, pigment grade SrCrO 4  of reduced solubility produced, according to the present invention, displays incrementally but nevertheless noticeably enhanced corrosion inhibition performance in organic coating applications. 
     It appears that the natural solubility of SrCrO 4 , although moderate (see Table A), is excessive to some extent in comparison to an optimum solubility value. The above consideration is also supported by I i =5 of SrCrO 4  (see Table A), indicating that its c isat  is about 5 times higher than c icrt ˜1 mmol/L of CrO 4   −− , that is the minimum concentration required for maximum possible corrosion inhibitor performance of the species. Considering all of the above, it appears reasonable to state that the solubility of SrCrO 4   −− (or generally, of any pigment grade corrosion inhibitor containing CrO 4   −−  species), should be: 1 mmol/L&lt;c isat &lt;5 mmol/L. 
     In addition to noticeable enhancement of its corrosion inhibitive performance, additional beneficial effects of solubility reduction of pigment grade SrCrO 4  are: improved correlation between pigments&#39; solubility and paint stability as well as reduction of leaching of related ionic species from organic coatings. 
     It is well known with regard to paint stability, that ionic species such as CrO 4   −−  and Sr ++  dissolved in the aqueous phase of water-born paint formulations, adversely affect the Theological stability of latter. In general, direct proportionality can be expected between the solubility of pigments, or concentration of related species, and the extent of this de-stabilizing effect. Consequently, reduction of solubility of pigment grade SrCrO 4  should be beneficial in this respect. Similarly, the leaching rates of CrO 4   2−  species from organic coatings are directly proportional with c isat  of related pigments, in this particular case with the solubility of SrCrO 4 . Leaching, while useful due to its being the only transport process for inhibitor species at discontinuity sites in organic coatings, is also detrimental, however, due to causation of increased porosity and, in general, in diminished protective capacity of pigmented primers. 
     Moreover, it has been observed, that reduced solubility, realized according to the process of the present invention, results also in significant reduction of the clumping tendency of pigment grade SrCrO 4 . 
     A direct correlation was intuitively expected between the solubility (moderate hygroscopic character) and clumping tendency of SrCrO 4 . Indeed, it has been observed, that reduced solubility, realized according to the present invention, results also in significant reduction of the clumping tendency of pigment grade SrCrO 4 . This observation is documented in Example 7. 
     Considering all of the foregoing, it appears desirable to optimize the solubility of corrosion inhibitor pigments, in general. In the particular case of SrCrO 4 , it appears that by reducing solubility, all functional characteristics of this pigment can be improved, inclusive of corrosion inhibitor performance and rheological behavior. As specified earlier however, c isat  or solubility of SrCrO 4  must not be reduced to less than 1 mmol/L. 
     In accordance with the invention, a procedure has been found which is very effective in reducing the natural solubility of SrCrO 4 . This procedure is based essentially on a chemisorption process, which occurs spontaneously on the surface of finely divided SrCrO 4 , when the latter is dispersed in an aqueous solution of any one or more of a diverse group of soluble salts or derivatives of phosphoric acid. 
     An important aspect of the invention is the use of derivatives of H 3 PO 4  as surface modifying agents for treating strontium chromate and altering the surface characteristics thereof. In accordance with a further aspect of the invention, the derivatives of phosphoric acid used to modify the strontium chromate surface characteristics include alkali metal salts, alkali metal acid salts, and ammonium salts phosphoric acids including orthophosphoric acid, metaphosphoric acid and phosphorus acid. 
     In accordance with an important aspect of the invention, preferred surface modification agents for treatment of strontium chromate include alkali, alkali acid salts, or ammonium salts of acids selected from the group consisting of H 4 P 2 O 7 , H 5 P 3 O 10 , H 3 PO 3  and H 3 PO 4 . 
     In accordance with a further related aspect of the invention, such preferred treating agents include materials selected from the group consisting of: Na 4 P 2 O 7 , K 4 P 2 O 7 , Na 5 P 3 O 10 , K 5 P 3 O 10 , Na 2 HPO 3 , K 2 HPO 3 , Na 3 PO 4 , K 3 PO 4 , NaH 2 PO 4 , KH 2 PO 4 , Na 2 H 2 P 2 O 7 , K 2 H 2 P 2 O 7 , (NaOPO 2 ) 3 , (KOPO 2 ) 3 , NaNH 4 HPO 4 .4H 2 O and KNH 4 HPO 4 .4H2O. 
     As illustrated below for Na 4 P 2 O 7  (tetra-sodium pyrophosphate), one of the preferred surface modifying agents, the chemisorption of diverse ionic phosphate species occurs, presumably, by an exchange mechanism, with the participation of the anionic species: SrCrO 4 /(2SrCrO 4 ) n +nP 2 O 7   4− ⇄SrCrO 4 /(Sr 2 P 2 O 7 ) n +2nCrO 4   −− 2. where typically n=10 −4 -10 −2 . 
     It is reasonable to assume, that chemisorption of diverse ionic phosphate species results in formation of essentially insoluble compounds on the surface of treated SrCrO 4 , and consequently, the equilibrium of Reaction 2 is shifted to the extreme right. The resultant surface species are believed to be chemically similar to the correspondent bulk salts of strontium, such as Sr 2 P 2 O 7  or strontium pyrophosphate, in the above case. 
     Taking into account the above considerations, it appears reasonable to assume that chemisorption of phosphate species radically alters the chemical composition of the surface of SrCrO 4  crystallites, and as a consequence, alteration of some of related physical surface properties can be intuitively expected. 
     An important aspect of the present invention is that chemisorption of diverse phosphate ionic species results in reduction of solubility of SrCrO 4  and as a consequence, in improvement of all functional characteristics of this pigment grade product, in particular, its corrosion inhibitor performance and mitigation of its tendency for clump formation. It has been observed that the chemisorption process (Reaction 2), occurs practically instantaneously. 
     A function also of the SrCrO 4  phase&#39;s specific surface area, a critical stoichiometrical ratio apparently exists between the treating species and SrCrO 4 , above which the process is self-limiting, as might be expected on basis of generally accepted concepts of surface chemistry. 
     For example, in the exemplary case of P 2 O 7   4− , at typical values of specific surface area of SrCrO 4  corresponding to a 2-5 micron average particle diameter, the critical value of the molar ratio was found to be in the range of Na 4 P 2 O 7 /SrCrO 4 =9×10 −4 -1.8×10 −3  above which the chemisorption process is self-limiting. 
     The self-limiting character of the chemisorption process indicates, that at the critical value of the molar ratio, the adsorbent solid phase&#39;s particle surfaces become completely covered by a layer of chemisorbed species, and, essentially, Reaction 2 stops. Due to the practically insoluble character of the chemisorbed Sr 2 P 2 O 7  layer, it was observed that the chemisorption process becomes self-limiting even at the quite low initial bulk concentrations of about 0.5-1.5 mmole P 2 O 7   4− /L. 
     As for distribution, it is reasonable to assume a uniform spread of the chemisorbed layer on the adsorbent&#39;s surface, although the process could preferentially occur at high energy sites such as edges or tips of crystallites. Assuming, however, an insoluble layer of Sr 2 P 2 O 7  uniformly distributed on the surfaces of pigment grade SrCrO 4 , of particles with diameters of 5 micron, the thickness of insoluble layer (at the molar ratio value of Sr 2 P 2 O 7 /SrCrO 4 =1×10 −3 ) is estimated at approximately 10-20 Angstrom. It will be apparent, however, that in the 0&lt;Sr 2 P 2 O 7 /SrCrO 4 &lt;1×10 −3  value range of molar ratios, the corresponding thickness of the chemisorbed layer will be 0-10 Angstrom and the adsorbant&#39;s surface coverage, only partial. 
     The chemisorbed layer may represent 0.005% to 1% of the modified chromate. Generally, the chemisorbed Sr 2 P 2 O 7  layer, at saturation, amounts to about 0.017-0.34% by weight of the resultant surface modified SrCro 4 , and consequently some of the desirable physical characteristics of the latter, such as specific gravity, pH or color, are unaltered. As will also be apparent, the CrO 4   2−  content of surface modified SrCrO 4  produced according to the present invention, is substantially undiminished in comparison to the pigment grade versions heretofore known in the art. 
     Understandably and with no intent, however, to limit the concept, the preferred examples of realization of the present invention are versions of pigment grade SrCrO 4  surface modified to saturation, which can be formally symbolized by the general formula of (Sr x/2 P y O z )n/SrCrO 4 , where n, the stoichiometric coefficient is approximately n=1×10 −4 -2×10 −3 . Sr x/2 P y O z  symbolizes chemisorbed layers consisting of Sr-salts of various oxy anions of phosphorus, organic derivatives and hetero-poly anions included, and x,y,z are correspondent stoichiometric indices. In the significant case of P 2 O 7   4− as a preferred chemisorbed species, the resultant product is symbolized by (Sr 2 P 2 O 7 )n/SrCrO 4 , where n=1×10 −4 -2×10 −3 . 
     Notably, the chemisorbed layers obtained in accordance with the present invention, possess remarkable mechanical stability, which assures the feasibility of the surface modification procedure in industrial conditions. For example, it was observed, pursuant to the present invention, that the Sr 2 P 2 O 7  layer of (Sr 2 P 2 O 7 )n/SrCrO 4 , n=1×10 −4 -2×10 −3 , withstands, without apparent breakdown, drying, and, more specifically, subsequent grinding, which are some of the typical steps used in pigment manufacture. 
     As expected on the basis of Reaction 2, chemisorbed layers on SrCrO 4 , obtained according to the present invention, also display stability in aqueous media in neutral, moderately acidic or alkaline conditions, wherein they resist dissolution. It was learned, however, that the stability of freshly formed chemisorbed layers is limited. They degrade on extensive contact, for several hours at elevated temperature with aqueous phases. Notably, it was also realized pursuant to the present invention, that chemisorbed layers on SrCrO 4  must be effectively stabilized by drying of the previously filtered products, typically performed at 100-110° C. For example, (Sr 2 P 2 O 7 )n/SrCrO 4 , n=1×10 −4 -2×10 −3  produced according to the present invention, is processed by filtration and subsequent drying at 100-110° C. 
     Stabilized Sr x/2 P y O z  layers of (Sr x/2 P y O z )n/SrCrO 4 , n=1×10 −4 -2×10 −3  type products resist solubilization in water but allow gradual discharge of the host SrCrO 4  by dissolution. In contact with water, with no apparent effect on the dissolution rate, they reduce c isat , the solubility of host SrCrO 4 , as previously mentioned. Δc isat , the extent of reduction of the host&#39;s solubility appears to be directly proportional to the degree of coverage of the host&#39;s surface by the Sr x/2 P y O z  layers. It seems to be dependent on the chemical identity of latter and notably, is maximized at saturation. Δc isat  can be conveniently defined by: %Δc iast =100 (c isat-md −c isat )/c isat , where c isat-md  is the solubility of surface modified SrCrO 4 . Notably, %Δc isat  values are &lt;0, by definition. It is interesting to note that Δc isat  seems to be constant at saturation for each distinct (Sr x/2 P y O 7 )n/SrCrO 4 , n=1×10 −4 -2×10 −3  type product. More importantly, however, Δc isat  of a product appears to remain constant as the solubilization of the host SrCrO 4  proceeds. Thus the discharge by dissolution of the host seems to resemble flow out of a “container. ” In the illustrative case of (Sr 2 P 2 O 7 )n/SrCrO 4 ; n=2.2×10 −3 , where the surface coverage is assumed to be complete at a thickness of 10-20 Angstrom, the host&#39;s solubility is c isat-md   =1.9 mmol/L and Δc   isat =−63, as documented in 1.2 of Example 1. 
     Apparently, the desirable functional characteristics of surface modified pigment grade (Sr x/2 P y O z )n/SrCrO 4  type products are maximized at n=1×10 −4  to 2×10 −3 , that is at saturation of the host surface, which is preferred in the practice of the present invention. It can be concluded, that surface modified products of the present invention, behave like distinct chemical entities, having practically the chemical composition of technical grade SrCrO 4  (with CrO 4   2− , content undiminished) but with significantly reduced solubility in water. 
     The present invention provides an effective process for modifying the surface of finely divided pigment grade SrCrO 4 . As a consequence of surface modification, pursuant to the invention, the natural solubility of the host product is significantly reduced while the functional characteristics, important for pigment grade SrCrO 4 , are enhanced, inclusive of corrosion inhibitor performance and mitigation of clumping tendency. 
     Novel types of corrosion inhibitor pigment grade SrCrO 4  are also provided, which can be generically symbolized by (Sr x/2 P y O z )SrCrO 4 , n=1×10 −4 -2×10 −3 . Here Sr x/2 P y O z  symbolizes chemisorbed layers, as previously described. These novel type products behave like distinct chemical entities with respect to solubility, and generally, in pigment applications. 
     In practice, the surface modification procedure is realized by initially preparing an aqueous slurry of the host product, preferably of finely divided SrCrO 4 . This slurry can be freshly produced by precipitation of SrCrO 4  or, alternatively, previously dried and finely ground product can be re-slurried in water, by intense agitation. Subsequently, the surface of the dispersed solid phase is modified, essentially according to Reaction 2 by chemisorption of diverse P y O z   x− oxy anionic species of phosphorus, which result from dissociation of corresponding soluble salts or derivatives of the same. Practically, surface modification of SrCrO 4  slurry is performed by introducing a soluble oxy salt of phosphorus into the aqueous phase and by stirring it, preferably for 15-30 minutes, at ambient or higher temperatures. With no intent to limit the concept of the present invention, the initial concentration of P y O z   x− , realized in the aqueous phase, is preferred to be in the 0.5-1.5 mmole/L range and the P y O z   x− /SrCrO 4  molar ratio to be 1×10 −4 -2×10 −3 . The process is finalized by filtration and washing to soluble salt-free conditions and subsequent drying of the solid phase at 100-110° C. The formed chemisorbed layer is also stabilized by the these steps. Typically, dry pigment grade products are ground to 100% 325 mesh fineness. 
     The oxy derivatives of phosphorous, applicable according to the present invention, include neutral or acidic Li + , Na + , K + , NH 4   + salts, and in general, any of the salts of: ortho-, pyro-, tri- and ploy-phosphoric acids, meta-phosphoric, poly-metaphosphoric, cyclic trimetaphosphoric acid, and phosphorous acids as well as mixtures thereof. It will be apparent that the list of applicable derivatives also includes the corresponding acids, the relatively soluble acidic salts of Ca 2+ , Sr 2+ , Ba 2+ , Mg 2+ , as well as the alkyl- or aryl-ammonium salts of the same. Notably, various acidic organic esters, primarily of ortho-, pyro-, or tri-phosphoric acid, phosphonic acid, as well as diverse phosphate-functionalized polymers and soluble salts of all the above, are also useful. 
     Examples of phosphorus oxy compounds, preferred in the practice of the invention for surface modification include: Na 3 PO 4 , Na 2 HPO 4  and more preferably Na 4 P 2 O 7 , Na 5 P 3 O 10  as well as the corresponding acids. 
     Examples of other applicable compounds include: Ca(H 2 PO 4 ) 2  .H 2 O, CaH 2 P 2 O 7 , (NaOPO 2 ) 3 , Na 3 H 15 Al 2  (PO 4 ) 8  and NaH 14 Al 3 (PO 4 ) 8 4H 2 O (dubbed as SALP-s in the bakery industry), soluble Na-polymetaphosphate, salts of hetero-poly acids, i.e.: H 3 PW 12 O 40 , H 3 PMo 12 O 40 ; chelating agents and salts: phosphorus derivatives of nitrilotriacetic acid, (HO (O) CH 2 ) 2 —NCH 2 —CH 2 —PO 3 H 2  and alternatives such as N(CH 2 PO 3 H 2 ) or nitrilotris(methylene phosphonic acid) and 1-hydroxyethylidene-1,1-diphosphonic acid, dubbed as HEDPA. 
     Also useful are salts of alkyl acid phosphates, (RO) 2 P(O)OH, (RO)P(O)(OH) 2 , where R—is an alkyl-, aryl- or poly-ethoxylated derivative. Finally, with no intent to limit the concept, however, phosphorilated starch, [(NaO) 2 P(O)—O—CH 2 —C 6 (O)H 3 (OH) 2 —O—]n, is presented as an example of a phosphate-functionalized polymer. 
     Notably, it was also observed pursuant to the present invention, that a similar effect is achievable on other chromates, as well. More specifically, it was experimentally proven that the same procedure, ie., the chemosorption of diverse derivatives of the phosphoric acids, is very effective also in reducing the natural solubility of calcium and Zn(II) chromates. 
    
    
     The following examples, wherein all parts are given by weight unless otherwise noted, demonstrate further details of preferred processes and products of the present invention. 
     COMPARATIVE EXAMPLE 1 
     Pigment grade SrCrO 4  was produced according to the double decomposition process known to the art, as follows: 
     
       
         Sr(NO 3 ) 2 +Na 2 CrO 4 →SrCrO 4 +NaNO 3   3. 
       
     
     In practice, the process was realized by initially dissolving each 1.0 mole (211.6 g) of Sr(NO 3 ) 2  (commercial quality, assay ˜99%) and 1.0 mole (162.0g.) of Na 2 CrO 4  (commercial, assay ˜99%) in distinct separate 500 ml volumes of H 2 O, at ambient temperature. 
     Subsequently, the Na 2 CrO 4  solution was gradually introduced, in about 30 minutes, into the intensively stirred Sr(NO 3 ) 2  solution and, if necessary, the pH of the system was adjusted to pH =8 to 8.5. The obtained pigment slurry was further conditioned by stirring for 1 hour at 50° to 60° C., after which the pH of the system was adjusted again, if necessary, to the same values as before. 
     After separation by filtration and washing to soluble salt contaminant-free condition, the pigment press-cakes were dried overnight at about 110° C. and subsequently pulverized to 100% +325 mesh fineness. The yield, before grinding, was 192.0 g., 94%. 
     Following common analytical procedures, the pigment grade products were characterized by the quality parameter values subsequently disclosed: 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 COMPARATIVE TABLE 1 
               
             
          
           
               
                   
                 Quality 
                 Determined 
               
               
                   
                 Parameters: 
                 Values: 
               
               
                   
                   
               
               
                   
                 appearance 
                 yellow powder 
               
               
                   
                 specific gravity, according to ASTM 
                 3.85 
               
               
                   
                 chromate content (as CrO 3  % 
                 48.65 
               
               
                   
                 e isp , specific inhibitor capacity* 
                 18.7 
               
               
                   
                 c isat , solubility at ambient temperature, mmol/L 
                 5.17 
               
               
                   
                 I i , inhibitive activity* 
                 ˜5 
               
               
                   
                 pH of saturated leachet, according to ASTM 
                 8.4 
               
               
                   
                 calculated SrCrO4 content, % 
                 99.0 
               
               
                   
                   
               
               
                   
                 *calculated according to formulae presented above.  
               
             
          
         
       
     
     Notably, the obtained values of characteristic parameters such as e isp , c isat  and I i  or related specific gravity, CrO 3 % and solubility, correspond reasonably well with theoretical ones (i.e.: theoretical CrO 3 % =49.11), as well as with values relevant to commercial products, published in the literature. (See also Comparative Table 3) Consequently, the product was considered chemically pure, corresponding to the general formula SrCrO 4 . 
     COMPARATIVE EXAMPLE 2 
     Solubility in water, c isat , at ambient temperature of all pigment grade SrCrO 4  produced pursuant to the present invention (according to Comparative Example 1 and Examples 1 to 3), as well as of selected commercially available SrCrO 4  identified as Sample (a) and Sample (b) pigments (see also Comparative Example 3) employed as controls pursuant to the present invention, was determined according to an experimental protocol, as follows: 20g of SrCrO 4  pigment were introduced into 200 ml deionized water and stirred for 2 hours at ambient temperature, after which the slurry&#39;s temperature was adjusted to 22° ±1° C., by keeping it in thermostated and under static conditions for 2 hours. After decantation, the clear supernatant was filtered and the CrO 4   −−  content of the saturated solution was determined in a 100 ml aliquot by the well known indirect iodometric titration method. 
     The process, as described, was repeated two consecutive times on the same wet pigment sample, the obtained CrO 4   −−  content values averaged, and c isat , the product&#39;s solubility at 22±1° C., was expressed as mmoles of SrCrO 4  dissolved in 1000ml saturated solution. 
     COMPARATIVE EXAMPLE 3 
     This example is presented to introduce two commercially available pigment grade strontium chromate samples, labeled as commercial SrCrO 4  Sample (a) and commercial SrCrO 4  Sample (b), and to compare the same with pure SrCrO 4  produced according to Comparative Example 1. It will be noted, that these 3 varieties of SrCrO 4  were applied as controls in all subsequently presented examples of realization of the present invention. 
     
       
         
               
             
               
               
             
               
               
               
               
             
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 COMPARATIVE TABLE 3 
               
             
          
           
               
                   
                 Determined values/product: 
               
             
          
           
               
                 Quality Parameters: 
                 SrCrO 4 (a) 
                 SrCrO 4 (b) 
                 Comp. Ex. 1 
               
               
                   
               
             
          
           
               
                 appearance 
                 yellow powder 
               
             
          
           
               
                 specific gravity according to 
                 3.79 
                 3.82 
                 3.85 
               
               
                 ASTM 
               
               
                 chromate content (as CrO 3  %) 
                 48.02 
                 47.85 
                 48.65 
               
               
                 e isp , specific inhibitor capacity* 
                 18.2 
                 18.28 
                 18.7 
               
               
                 c isat , solubility at ambient 
                 4.8 
                 5.0 
                 5.17 
               
               
                 temperature 
               
               
                 I i , inhibitive activity* 
                 ˜5 
                 ˜5 
                 ˜5 
               
               
                 pH of saturated leachet, according 
                 8.3 
                 8.1 
                 8.4 
               
               
                 to aSTM 
               
               
                 calculated SrCrO 4  content, % 
                 97.77 
                 97.43 
                 99.06 
               
               
                   
               
               
                 *calculated, according to expressions above introduced  
               
             
          
         
       
     
     As can be seen, commercially available strontium chromate pigments, although of technical grade, can be considered reasonably pure products. 
     EXAMPLE 1 
     Pigment grade SrCrO 4  of a novel kind, characterized by reduced solubility, which can be symbolized herein by the chemical formula: Sr 2 P 2 O 7 /SrCrO 4 /, was produced according to the procedure: 
     
       
         Sr(NO 3 ) 2 +Na 2 CrO 4 →SrCrO 4 +2NaNO 3 +nNa 4 P 2 O 7 →n(Sr 2 P 2 O 7 )/SrCrO 4 +2nCrO 4   −− +4n Na + +2NaNO 3   4. 
       
     
     in two distinct qualities, as follow: 1.1 where n=9.5×10 −4 and 1.2 where n=2.2×10 −3 . 
     The process was performed in similar fashion as described in Comparative Example 1, by initially dissolving 1.0 mole (211.6 g) of Sr(NO 3 ) 2  (commercial quality, assay ˜99%) and 1.0 mole (162.0g) of Na 2 CrO 4  (commercially available assay ˜99%) in separate 500 ml volumes of H 2 O, at ambient temperature. 
     Subsequently, the Na 2 CrO 4  solution was gradually introduced in about 30 minutes, into the intensively stirred Sr(NO 3 ) 2  solution and, if necessary, the pH of the system was adjusted to pH =8 to 8.5. 
     The resultant pigment slurry was further conditioned by stirring for 1 hour at 50° to 60° C., after which the pH of the system was adjusted again, if necessary, to the same values as before. 
     Pursuant to the present invention, pigment grade strontium chromate, conforming the above specified chemical formulas, was produced by introducing, in about 30 minutes, 9.5×10 −4  moles (0.25g) of Na 4 P 2 O 7  in the case of Example 1.1, or alternatively, 2.2×10 −3  moles (0.58g) of the same, in the case of Example 1.2, (as aqueous solution of about 1%) into the intensively stirred slurry and keeping the same conditions for an additional hour. 
     After separation by filtration and washing to soluble salt contaminant-free condition, the pigment press-cakes were dried overnight at about 110° C. and, subsequently, pulverized to 100% +325 mesh fineness. Yields, before grinding, were: 193g (95%) and 195g (96%) for 1.1 and 1.2., respectively. 
     Following common analytical procedures, the pigment grade products were characterized by the quality parameter values shown in Table 1: 
     
       
         
               
               
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Determined values 
                   
               
             
          
           
               
                   
                 Quality parameters: 
                 1.1 
                 1.2 
               
               
                   
                   
               
             
          
           
               
                   
                 appearance 
                 yellow powder 
               
             
          
           
               
                   
                 specific gravity, according to ASTM 
                 3.82 
                 3.92 
               
               
                   
                 chromate content (as CrO 3  %) 
                 48.41 
                 48.25 
               
               
                   
                 e isp , specific inhibitor capacity* 
                 18.5 
                 18.9 
               
               
                   
                 c isat  mmole/L 
                 2.4 
                 1.9 
               
               
                   
                 % Δ c isat * 
                 −53 
                 −63 
               
               
                   
                 I i , inhibitive activity* 
                 ˜2.5 
                 ˜2.0 
               
               
                   
                 pH of saturated leachet 
                 7.8 
                 7.9 
               
               
                   
                 according to ASTM 
               
               
                   
                 calculated SrCrO 4  content, % 
                 98.6 
                 98.2 
               
               
                   
                   
               
               
                   
                 *calculated as above specified  
               
             
          
         
       
     
     Notably, the measured values of some characteristic parameter, such as specific gravity, CrO 3 % and e isp , correspond reasonably well with values characteristic to pure SrCrO 4  (see also Comparative Table 1) 
     It is important to observe, however, that c isat , the solubility and related I i  of n(Sr 2 P 2 O 7 )/SrCrO 4  produced according to the present invention, which are thermodynamic constants of chemical entities, have significantly reduced numeric values in comparison to numeric values characteristic to SrCrO 4  known to the art. It will be noted, that distinctly different, substantially reduced numeric values of these thermodynamic constants, i.e. c isat  and I i , indicate that n(Sr 2 PO 7 )/SrCrO 4  produced according to the present invention, constitutes a novel type of SrCrO 4 , distinctly different from pure or pigment grade SrCrO 4 , heretofore known to the art. 
     EXAMPLE 2 
     Pigment grade strontium chromate, represented by the chemical formula of n(Sr 2 HP 3 O 10 )/SrCrO 4 , where n=1.0×10 −3  was produced in identical fashion as described in Example 1, except that, in this case, 1.0×10 −3  moles (0.37g) of Na 5 P 3 O 10 , ie., sodium tripolyphosphate, was employed in lieu of Na 4 P 2 O 7 . Yield: 194.0g. ˜95%; 
     
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Quality parameters: 
                 Determined values: 
               
               
                   
               
             
             
               
                 appearance 
                 yellow powder 
               
               
                 specific gravity, according to ASTM 
                 3.87 
               
               
                 chromate content (as CrO 3  %) 
                 42.2 
               
               
                 e isp , specific inhibitor capacity (calculated) 
                 18.6 
               
               
                 c isat  solubility at ambient temperature, mmol/L 
                 2.1 
               
               
                 % Δ c isat  (calculated) 
                 −59 
               
               
                 Ii, inhibitive activity (calculated) 
                 ˜2 
               
               
                 pH of saturated leachet, according to ASTM 
                 8.0 
               
               
                 calculated SrCrO 4  content, %. 
                 98.1 
               
               
                   
               
             
          
         
       
     
     Note that the measured values of some characteristic parameters, such as specific gravity, CrO 3 % and e isp , correspond reasonably well with values characteristic to pure SrCrO 4  (see also Comparative Table 1). 
     It will be observed again, however, (see also Example 1), that c isat , the solubility and related I i , which are thermodynamic constants of chemical entities, have significantly reduced numeric values in comparison to numeric values characteristic to unmodified SrCrO 4 , heretofore known to the art. 
     EXAMPLE 3 
     Modified pigment grade strontium chromate, represented by the chemical formula of n(SrHPO 4 )/SrCrO 4  where n=2.5×10 −3 , was produced in identical fashion as described in Example 1, except that, in this case, 2.5×10 −3  moles (0.36g) of Na 2 HPO 4  were employed in lieu of Na 4 P 2 O 7 . Yield: 197.0g, 97%. 
     
       
         
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Quality parameters: 
                 Determined values: 
               
               
                   
               
             
             
               
                 appearance 
                 yellow powder 
               
               
                 specific gravity, according to ASTM 
                 3.85 
               
               
                 chromate content (as CrO 3  %) 
                 48.3 
               
               
                 e isp , specific inhibitor capacity (calculated) 
                 18.6 
               
               
                 c isat , solubility at ambient temperature, mmol/l 
                 3.7 
               
               
                 % Δ c isat  (calculated) 
                 −28 
               
               
                 I i , inhibitive activity (calculated) 
                 &lt;4 
               
               
                 pH of saturated leachet, according to ASTM 
                 8.1 
               
               
                 calculated SrCrO 4  content, % 
                 98.3 
               
               
                   
               
             
          
         
       
     
     It will be noted again, the measured values of some characteristic parameters, such as specific gravity, CrO 3 % and e isp , correspond reasonably well with values characteristic to pure SrCrO 4  (see also Comparative Table 1). %Δc isat =−28 is also noted. 
     EXAMPLE 4 
     This example is intended to demonstrate that stabilized Sr x/2 P y O z  layers of (Sr x/2 P y O z )n/SrCrO 4 , n×1×10 −4 -2×10 −3  -type products resist solubilization in water and perform rather like “containers” which allow discharge of the host SrCrO 4  by dissolution. 
     It is also intended to document that Δc isat  of a distinct product remains practically constant as the solubilization of the host SrCrO 4  proceeds, that is during the discharge by dissolution of the above suggested “container”. 
     For that purpose, the solubility of SrCrO 4  (as the control) produced according to Comparative Example 1 and of (Sr 2 P 2 O 7 )n/SrCrO 4 , n=2.2×10 −3  according to 1.2. of Example 1, was determined several consecutive times, on the same respective samples, following the procedure disclosed in Comparative Example 2. The experimental results are presented below. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 4 
               
             
             
               
                   
                   
               
               
                   
                 extracted product/solubility, mmol/L 
                   
               
             
          
           
               
                 Number of 
                 SrCrO4, Comp. Ex. 1 
                 n(Sr 2 P 2 O 7 )/SrCrO 4 , Ex. 1.2 
               
               
                 extractions 
                 c isat   
                 c isat   
               
               
                   
               
             
          
           
               
                 1. 
                 5.17 
                 3.24 
               
               
                 2. 
                 4.64 
                 1.97 
               
               
                 3. 
                 4.8 
                 1.75 
               
               
                 4. 
                 4.46 
                 1.75 
               
               
                 5. 
                 4.52 
                 — 
               
               
                   
               
             
          
         
       
     
     As can be seen, for n(Sr 2 P 2 O 7 )/SrCrO 4 , n=2.2×10 −3 , the c isat  (as presented in Table 4) as well as the related values of Δc isat =−37; −57; −63; −60;, respectively, were quite constant. 
     It can be concluded, that the surface modified (Sr 2 P 2 O 7 )n/SrCrO 4 , n=2.2×10 −3  obtained according to the present invention, behaves like distinct chemical entity in the solubilization process. 
     EXAMPLE 5 
     Two package epoxy primer formulation was prepared as subsequently described in Table 5, with the intent to test the corrosion inhibitor performance of pigment grade n(SrP 2 O 7 )/SrCrO 4  of reduced solubility, where n=9.5×10 −4 , produced according to 1.1 of Example 1, in comparison with two commercial grade SrCrO 4  pigments coded as Commercial SrCrO 4  (a) and Commercial SrCrO 4  (b), respectively. (See Comparative Example 3) 
     For that purpose, 3 versions of the formulation were produced: 
     1) Control Primer(a), containing commercial SrCrO 4 (a), 
     2) Control Primer(b), containing commercial SrCrO 4 (b) 
     3) Test Primer, containing pigment grade n(SrP 2 O 7 /SrCrO 4 /, n=9.5×10 −4 , according to 1.1 of Example 1. 
     
       
         
               
               
               
             
               
             
               
               
               
             
               
             
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Raw materials 
                 Trade names, suppliers 
                 Parts by weight 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Part I. 
               
             
          
           
               
                 Epoxy resin 
                 Epon 1001-X075 (1) 
                 182.0 
               
               
                 Corrosion Inhibitor 
                 SrCrO4 according to 
                 120.0 
               
               
                 Pigment* 
                 Primer a.), Primer b.) 
               
               
                   
                 and Test primer 
               
               
                 White pigment, TiO2 
                 RLC-535 (2) 
                 23.0 
               
               
                 Filler pigment 
               
               
                 Talc 
                 40-27 Talc (3) 
                 55.0 
               
               
                 Silica, natural 
                 Min-U-Sil 15 (4) 
                 29.0 
               
               
                 Solvent 
               
               
                 Glycol Ether PM 
                   
                 164.0 
               
               
                 Methyl Isobutyl Ketone 
                   
                 41.0 
               
             
          
           
               
                 Part II. 
               
             
          
           
               
                 Hardener 
                 HY-815 Polymide (5) 
                 75.0 
               
               
                 Solvent 
                   
               
               
                 Iso-propanol 
                   
                 245.0 
               
               
                 Toluene 
                   
                 66.0 
               
               
                 Total 
                   
                 1000.0 
               
               
                   
               
               
                 Raw material suppliers:  
               
               
                 (1) Shell Chemical ompany;  
               
               
                 (2) Millennium Inorganic Chemical;  
               
               
                 (3) Harcross Pigments;  
               
               
                 (4) U.S. Silica Company;  
               
               
                 (5) Ciba-Geigy Corporation;  
               
               
                 *SrCrO 4   
               
               
                 a.) and SrCrO 4   
               
               
                 b.) are commercially available pigment grade products.  
               
             
          
         
       
     
     EXAMPLE 6 
     The corrosion inhibitor performance of pigment grade n(SrP 2 O 7 )/SrCrO 4 , n=9.5×10 −4 , according to 1.1. of Example 1, was assessed in comparison with commercial SrCrO 4  (a) and commercial SrCrO 4  (b), by applying each Control Primer and the Test Primer (by wire wound rod technique) on several distinct polished cold rolled steel panels (from the Q-Panel Corporation), at 0.4-0.5 mils dry film thickness and dried overnight at 140° F. After scribing and taping of the edges, several of the Test and of each Control panels were subject to salt spray exposure pursuant to ASTM B-117 and the corrosion inhibitor performance of both Control and the Test Primer applications was comparatively assessed (at increasing exposure time intervals), graded on the “0 (worst) to 10 (best)” scale for blister size and on the “dense, D; medium-dense, M-D; medium, M; Few, F;” descriptive scale for blister frequency, according to ASTM D-714 and the values averaged. Notably, the observed protective performance (against corrosion) of both of the Control and Test Primer applications was attributed to, and considered directly proportional with the correspondent inhibitor pigment&#39;s corrosion inhibitor performance, that is of Commercial SrCrO 4  (a), Commercial SrCrO 4  (b) and n(Sr 2 P 2 O 7 )/SrCrO 4 , n=9.5×10 −4 , respectively. Test results are presented below. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Exposure time to Primer formulation and related 
               
               
                 performance grades: 
               
               
                 salt fog, in hours: Blister size (numeric), blister 
               
               
                 frequency (descriptive) 
               
             
          
           
               
                   
                 Control (a) 
                   
                 Control (b) 
                   
                 Test Primer 
                   
               
             
          
           
               
                   
                 At 
                 In 
                 At 
                 In 
                 At 
                 In 
               
               
                   
                 Scribe 
                 field 
                 scribe 
                 field 
                 scribe 
                 field 
               
               
                   
                   
               
             
          
           
               
                 240 
                 F,8 
                 10 
                 F,8 
                 10 
                 F,8 
                 10 
               
               
                 408 
                 M,6 
                 10 
                 F,6 
                 10 
                 F,8 
                 10 
               
               
                 600 
                 M,5 
                 10 
                 F-M,5 
                 10 
                 F,7 
                 10 
               
               
                 768 
                 M-D,4 
                 10 
                 M,5 
                 F,8 
                 M,7 
                 10 
               
               
                 93 
                 D,4 
                 10 
                 M-D,5 
                 F,7 
                 M,6 
                 10 
               
               
                 1176 
                 D,3 
                 F,8 
                 D,4 
                 F,6 
                 M-D,5 
                 F,8 
               
               
                   
               
             
          
         
       
     
     Based on the above experimental data, it was concluded that pigment grade n(Sr 2 PO 7 )/SrCrO 4 , where n=9.5×10 −4 , of reduced solubility, according to 1.1 of Example 1, under quite discriminate test conditions (salt spray exposure time: in excess of 1000 days), displayed incrementally, however noticeably better corrosion inhibition performance in comparison to Commercial SrCrO 4  (a) and Commercial SrCrO 4 (b), in an epoxy primer application on polished cold rolled steel. Taking into account, however, what is known regarding the corrosion inhibitor action mechanism and performance of SrCrO 4 , the above conclusion appears to be reasonably valid for the other metal substrates or paint systems, as well. It also appears reasonable to conclude, that reduced solubility does not diminish the corrosion inhibition performance of pigment grade SrCrO 4 , as long as the correspondent values of I i &gt;1. 
     As noted from Table 1, the solubility of n(Sr 2 P 2 O 7 )/SrCrO 4 , (where n=9.5×10 −4 ),according to 1.1 of Example l)is c isat ˜2/4 mmol/L and the corresponding I i ˜2.5. 
     EXAMPLE 7 
     Tendency for clump formation during storage of pigment grade n(Sr 2 P 2 O 7 )/SrCrO 4 , where n=9.5×10 −4  per 1.1 of Example 1, n=2.2×10 −3  per 1.2 of Example 1, and of SrCrO 4  according to Comparative Example 1, as well as of commercial SrCrO 4  (a) and Commercial SrCrO 4  (b), was determined according to the following experimental protocol: 
     Statistically significant numbers of pellets of each pigment were produced by compressing 2.5 to 3.0g (w i ) of finely ground pigment grade product in a cylindrical mold of 13 mm diameter, at approximately 6.8 kg/cm 2  pressure for 1 minute. 
     The initial value of compressive strength (F 1 ) was measured on a significant number of pellets representing each distinct pigment grade product, by applying gradually increased pressure on them and observing the critical value at which the disintegration of the pellets started. Conveniently, F 1  was expressed in kg/cm2 units and the results averaged per pigment grade product. Concurrently, a distinct sampling, consisting of significant number of identical pellets representing each distinct pigment grade product, was exposed to 100% relative humidity conditions in a closed system at ambient temperature for 24 hours, after which the weight increase (w) due to water absorption, was determined and averaged. Subsequently, the pellets were kept at ambient temperature and approximately 60% Relative Humidity for 3 to 4 hours, until water desorption was completed (w i  reached), and consecutively, the resultant value of compressive strength (F) was measured and the data averaged. 
     The hygroscopic character and more importantly, the tendency for clump formation during storage of pigment grade strontium chromate was qualified by 
     
       
         % absorbed H 2 O=100 ( w−w   i )/ w   i   
       
     
     and respectively by I cl , the index of tendency for clump formation, arbitrarily defined as 
     
       
           I   cl =( F−F   i ) F   i   
       
     
     It will be apparent that relatively high value of I cl  indicates strong tendency for clump formation of pigment grade products during storage, and specifically, under humidity conditions. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                 Identity of SrCrO 4   
                 H 2 O absorbed 
                 F i   
                 F 
                 I cl   
               
               
                 Pigment 
                 % 
                 Kg/cm 2   
                 Kg/cm 2   
                 % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Commercial SrCrO 4  (a) 
                 ˜1.3 
                 0.5 
                 1.5 
                 200 
               
               
                 Commercial SrCrO 4  (b) 
                 ˜1 
                 0.5 
                 1.2 
                 140 
               
               
                 According to Comparative 
                 ˜1.5 
                 0.7 
                 2.6 
                 270 
               
               
                 Example 1 
               
               
                 According to 1.1 
                 1 
                 0.5 
                 0.7 
                 40 
               
               
                 of Example 1 
               
               
                 According to 1.2 
                 1 
                 0.5 
                 0.6 
                 20 
               
               
                 of Example 1 
               
               
                   
               
             
          
         
       
     
     As seen, I cl , the clumping tendency of pure SrCrO 4  (of Comparative Example 1) and of commercial pigment grades SrCrO 4  (a) and (b) varies from 140% to 270%, presumably as a function of the manufacturing process used. In comparison to SrCrO 4  produced according to Comparative Example 1, pigment grade surface modified products symbolized by (Sr 2 P 2 O 7 )n/SrCrO 4 , n=9.5×10 −4  and n=2.2×10 −3 , (per 1.1 and 1.2 of Example 1) display 6 times, and 13 times, respectively, lesser tendency for clumping. Apparently, there is a reverse proportionality between clumping tendency and the extent of surface coverage of surface modified SrCrO 4  produced according to the present invention. 
     EXAMPLE 8 
     This example is intended to display, in condensed fashion, numeric values of, and relevant specifically to SrCrO 4  according to the present invention, differences in numeric values of some functional parameters or thermodynamic constants, which (by distinct numeric values) define chemical entities. It will be noted that the comparatively reduced values of the thermodynamic constants c isat  and I i , indicate that treated SrCrO 4  produced according to the present invention, constitutes a novel substance, distinctly different from pure or pigment grade SrCrO 4  known in the art. 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 Inhibitor 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Perform- 
               
               
                 Identity of 
                   
                   
                   
                   
                   
                   
                 ance 
               
               
                 SrCrO 4   
                 % SrCrO4 
                 g sp   
                 e isp   
                 % Δc isat   
                 I i   
                 I cl   
                 pigment 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Commer- 
                 97.77 
                 3.79 
                 18.2 
                 ˜0 
                 ˜5 
                 200 
                 Good 
               
               
                 cial 
               
               
                 SrCrO 4  (a) 
               
               
                 Commer- 
                 97.43 
                 3.82 
                 18.28 
                 ˜0 
                 ˜5 
                 140 
                 Good 
               
               
                 cial 
               
               
                 SrCrO 4  (b) 
               
               
                 According 
                 99.06 
                 3.85 
                 18.7 
                 ˜0 
                 ˜5 
                 270 
                 — 
               
               
                 to Com- 
               
               
                 parative 
               
               
                 Example 1 
               
               
                 According 
                 98.6 
                 3.82 
                 18.5 
                 −53 
                 ˜2.5 
                 40 
                 Compara- 
               
               
                 to 
                   
                   
                   
                   
                   
                   
                 tively 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                 enhanced 
               
               
                 1.1 
               
               
                 According 
                 98.2 
                 3.92 
                 18.9 
                 −63 
                 ˜2 
                 20 
                 — 
               
               
                 to 
               
               
                 Example 
               
               
                 1.2 
               
               
                   
               
             
          
         
       
     
     It can be concluded, that the surface modified products of (Sr 2 P 2 O 7  )n/SrCrO 4  (where n=9.5×10 −4  and n=2.2×10 −3  ) type, obtained according to the present invention, behave like distinct chemical entities: they are characterized by the same assay as technical grade SrCrO 4  (% SrCrO 4  &gt;98 and undiminished CrO 4   2− content) and however, by significantly reduced solubility in water (%Δc isat =−53, and −63, respectively).