ALKALINE RESISTANT PHOSPHATE CONVERSION COATINGS AND METHOD OF MAKING

Abstract: A method for increasing the resistance to alkaline dissolution of a phosphate conversion coating on a corrodible metal substrate. The substrate is exposed to the phosphating solution by spraying or dipping to chemically effect a reaction with the substrate. The solution contains 84-94 molar percent of the total metal cations of a first layer-forming divalent metal cation, the metal cation having a hydroxide which has a lower solubility in an alkaline solution than iron or zinc hydroxide and is preferably selected from the group consisting of nickel, cobalt, magnesium and lanthanides, and a second layer-forming metal cation in the form of zinc present in an amount of .2-.6 g/l as a Zn<+2>. The molar ratio range of the first and second metal cations is in the range of 5.2:1 to 16:1, and the first metal cation is present in the solution in an amount of at least 1.0 g/l. The deposited coating has a first divalent metal cation present in an amount of at least 15 mole percent of the total divalent cations, and a second divalent cation present in an amount of at least 25 % by weight of the coating; the coating preferably has a uniform weight of less than 1.3 g/m<2> (120 mg/ft<2>).

Description: ALKALINE RESISTANT PHOSPHATE CONVERSION COATINGS AND METHOD OF MAKING BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT Zinc phosphate conversion coatings are applied to metal surfaces to provide a base for paint and to inhibit the undercutting of paint in a corrosive environment. Initially, adherency was the main consideration in selecting a conversion coating. Any improvement in the resistance to corrosion came as a result of a tighter, more adherent coating system (see U.S. patents 3,144,360 and 3,520,737). In U.S. patent 3,810,792 to Ries et al, a high content of nickel in a phosphate coating was used for the purpose of obtaining a closed, dense phosphate layer within a short contact time during spraying or bath . immersion for deep drawing applications. The Ries patent gives examples only for 100% nickel phosphate and for nickel phosphates of about 68-69 mole percent. Nowhere in the Ries patent is there any evidence offered that there is any discontinuity in the phosphate properties as the film content varies, unrestricted practice of the Ries disclosure will not achieve an improvement in alkaline resistance because (a) Ries indiscriminantly uses too wide a range of nickel that blindly wanders through compositions that offer no hope of alkali resistance improvement, and (b) none of Ries' examples exhibit a quantum-jump in alkaline dissolution resistance and in corrosion resistance after painting, which Ries would have noted if he had discovered such. Use of Layer-Forming Metals in Phosphate Conversion Coatings The method by which these initial coatings were applied included spraying or immersion of the product to be coated with or in a bath solution, the solution containing O PI * layer-forming metal cations, phosphate ions, and oxidizing agents. Typically the layer-forming ions comprised zinc and, in some cases, calcium, iron or manganese, used singly or in combination. The phosphate ions were typically introduced by use of phosphoric acid. The oxidizing agents were commonly inorganic compounds, often consisting of a salt of the above-mentioned metals or of sodium or ammonium. Much of this art, as used today, remains the same. Nickel has been introduced to the phosphating solution; it is consistently visualized as an element useful as a layer- forming metal for phosphate coatings of a particularly homogeneous, continuous structure on steel, zinc and, occaionally, aluminum. It is clear from the prior art that nickel was not added for the purpose of enhancing corrosion resistance as a primary goal. Except for the Ries patent, nickel was added to the phosphate solution in relatively small amounts, consistently under 6% by weight of the solution. In cases where nickel was present along with zinc ions, it was added in an amount providing a nickel to zinc ratio of 1:1 or less (see U.S. patents 4,053,328; 3,723,334; 4,110,128; 4,153,479; and 4,231,812). Increase In Road Salts Cause Art to Examine Role of Phosphates in Preventing Corrosion However, the need for phosphate coatings of consistently higher quality and much greater corrosion resistance has come into sharp focus. The magnitude of the corrosion problem encountered with automobiles is due to the increased use of road deicing salts. The use of road salt in the snow belt areas of the U.S. and Canada has increased rapidly from about one million tons per year in the late 1950 's to over 10 million tons at present. For many years it was not understood why a scratch, in the paint and phosphate film on the exterior of a car body produced corrosion failure over an area much greater than the width of the scratch itself. In a classic study by R.R. Wiggle, A.G. Smith and J.V. Petrocelli, published in The Journal of Paint Technology in 1968, they explained, in their paper entitled "Paint Adhesion Failure Mechanisms on Steel in Corrosive Environments", that alkaline dissolu¬ tion of the phosphate film is the cause of undercutting of the paint film. The alkaline environment results from the cathodic reduction of oxygen to hydroxyl ions (forming alkali-sodium hydroxide). The appropriate reactions are explained in detail in Figures 1 and 2 of a paper entitled "Paint Failure, Steel Surface Quality and Accelerated Corrosion Testing" by V. Hospadaruk, J. Huff, R.W. Zurilla and H.T. Greenwood, published in Society of Automotive Engineers Transactions, Section 1, Volume 87, 1978. Simply explained, the damaged or scratched paint area begins to rust where the paint is missing within the scratch. Iron oxide is formed from the base metal by an anodic reaction in an electrolyte of water and ions of sodium chloride. The dissolution of the iron to. form ferrous ions (Fe*2 + ) is attended by the generation of elec¬ trons. Oxygen and water, which permeate the paint film in the region (which becomes cathodic) adjacent to the anodic area, then react to form hydroxyl ions. Accumulation of hydroxyl ions results in the generation of liquid of very high basicity having a pH as high as 12.5 or more. Con¬ ventional zinc phosphate is soluble in this high basic liquid. Therefore, a disbond between the paint film and the substrate (sheet steel) in the car body results. If the paint is removed, as by pulling a tape applied to the scratched area, the paint adjacent the scratch will be removed but the underlying steel surface is bright and shiny. This is due to the fact that the high pH liquid is an inhibitor for the formation of red rust. Of course, without tape, in actual use, the paint would have flaked off eventually where it had been undercut by alkaline dissolution of the phosphate film. The steel surface will then begin to rust by the same mechanism common to rusting of bare steel. Ability to Prevent Corrosion Depends on Surface Carbon Contamination Steel surface cleanliness, particularly contamina¬ tion of automotive sheet steel by carbonaceous deposits, plays a major role in susceptibility to corrosion. The presence of even a very thin layer of carbon deposits on the steel as received from the steel mill is effective in preventing the formation of a phosphate conversion coating adequate for the best paint adhesion in the presence of an electrolyte which can support the cathodic reduction of oxygen to hydroxyl ions as described above. The reason for this is that the "pores" (either bare spots or very thin areas in the zinc phosphate conversion coating caused by the existence of carbon) act as reactive sites for ini¬ tiation of the oxidation and reduction reactions. Con¬ vincing experimental evidence for this mechanism is given in the paper by R.W. Zurilla and V. Hospadaruk entitled "Quantitative " Test for Zinc Phosphate Coating Quality" published in Society of Automotive Engineers Transactions, Section 1, " Volume 87, 1978. This work was a breakthrough in the understanding of the parameters that control the quality of zinc phosphate coatings as a substrate for paint. This paper established that zinc phosphate coatings are porous and that a porosity of 1.0-1.5% is consistently encountered with substrates that have high surface carbon contamination. It is difficult to modify rolling mill practices to eliminate such surface carbon contamination and thus deleterious porosity in phosphate films must be overcome. The porosity in such phosphate coatings is deleterious because it supports the electrochemical corro¬ sion activity of the substrate. The phosphate film is then subjected to dissolution by the alkali (NaOH) present about the paint scratch or areas where corrosion is initiated. OMPI Attempts to Reduce Corrosion Sensitivity . The main attempts by the prior art to reduce the corrosion sensitivity of phosphated metals have included (a) the introduction of inhibitors into the paint applied over the phosphate coatings to protect against corrosion; and (b) the use of an inhibitor rinse, such as chromic acid, which has been only partially successful in reducing the corrosion sensitivity; and (c) a tighter coating system to prevent the lifting of the phosphate coatings during use. These approaches have not significantly reduced the alkaline sensitivity of phosphate films. SUMMARY OF THE INVENTION The invention relates to a method for increasing the resistance to alkaline dissolution of a phosphate con- version coating on a corrodible metal substrate and thereby decrease corrosion sensitivity. The method employs an unusually critical narrow range of select layer-forming metal cations that form a unique mixed-metal phase phos¬ phate that imports great resistance to alkaline dissolution. The coating is deposited by chemical reaction between the substrate and an acidic aqueous solution containing first and second layer-forming divalent metal cations and phosphate ions. The method is characterized by (a) selecting the first divalent metal cation to be a transition metal or lathanide having a hydroxide which has a lower solubility in an alkaline solution than iron or zinc hydroxide; (b) selecting the second divalent metal cation as zinc; and (c) critically controlling within a narrow range the amount of first and second divalent metal cations present during the chemical reaction so that the deposited coating has a first divalent metal cation which is at least 15.0 mole percent of the total divalent metal cations and a second divalent metal cation content of at least 25% by weight of the coating. O PI The first divalent metal cation is selected from the group consisting of nickel, cobalt, magnesium and lanthanides (advantageously the cation is exclusively nickel) and is controlled to be 84-94 mole percent of the total divalent cations present in the solute. Zinc is preferably present in the solute in an amount of at least 0.2 g/1 as Zn +2 of said solution (advantageously 0.2-.6 g/1 as Zn +2 or 0.79-2.38 g/1 as Zn^PO.^-* The deposited coating will preferably be constituted substantially of a continuous nodular mixed-metal phosphate advantageously in the form of Zn2Ni(Pθ4)2« H2θ, but having some slight nickel variation within a narrow range. Advantageously the coating is deposited in a substantially uniform weight of less than 1.3 g/m 2 (120 mg/ft 2 ). The substrate is preferably exposed to the phos- phating solution for a sufficient time and at a sufficient temperature and pH (i.e., 30-120 seconds, 100-1400°F, 2.5-3.5 pH) to chemically react and deposit a coating of phosphate on the substrate, after which excess solution is removed from the coated substrate that has not been deposited as a coating. The molar ratio range of the first and second metal cations is in the range of 5.2:1 to 16:1, and the first metal cation is present in said solution in an amount of at least 1.0 g/1 of the solution. The invention also comprehends a method for coating a phosphate film onto the surface of an alkali cleansed metal article by applying thereto a phosphate coating solution. The improvement is the deposition of a phosphate film in an average coating weight of less than 1.3 g/m 2 (less than 120 mg/ft 2 ) i.e., 6.5-1.3 g/m 2 (60-120 mg/ft 2 ), having at least 15 mole percent (13.7% by weight) nickel, the film providing at least a doubling of the resistance to salt spray corrosion for any metal article coated with known phosphate films and painted. The phos- phate film results from the use of an acidic aqueous coating solution having an oxidizing agent content and a pH effective to chemically react with the article and a solute content consisting essentially of: (a) divalent layer- forming metal cations consisting of 84-94 mole percent nickel of the metal cations and zinc in an amount of 0.2-0.6 g/1 of the solution as Zn +2 ; and (b) phosphate ions in an amount at least sufficient to form dihydrogen phos¬ phate with said metal cations. The article is comprised of a metal selected from the group comprising iron, carbon and low alloy steel, aluminum and zinc. This process is par- ticularly advantageous when employed on metal articles which carry a total surface carbon content greater than 0.4 mg/ft 2 . The phosphating solution preferably possesses a total acid content of 10-40 points, a free acid content of 0.5-2.0 points, and a total acid/free acid ratio of 10-50. When the process is applied to the coating of zinc metal articles or substrates, the solution preferably contains a fluoride selected from the group consisting of a simple fluoride, fluoroborate, fluorosilicate, or other complex fluoride. It is also preferable that the phosphate solu¬ tion be maintained at a pH of 2.5-3.5 when nitrite or other oxidizing agent is present in sufficient amount. The tem¬ perature is maintained at 100-150°F (38-65°C) during the phosphating contact. The exposure of the metal article to such solution should be preferably for a time of 30-120 seconds. It is desirable that the nitrite, used as an accelerator, be used in an amount of 0.5-2.5 points or 0.03-0.14 g/1 of solution as NaNθ2« It is advantageous if a concentrated phosphate solution is employed to replenish the phosphate bath as it is used throughout a series of article coatings. The bath will become enriched with nickel, since more zinc than nickel is contained in the phosphate coating. The re¬ plenishment solution or concentrate is preferably formu- lated to consist essentially of about 18 mole percent nickel cations and 82 mole percent zinc cations (16.5 nickel-83.5 zinc weight percent). Alternatively, a portion of the nickel in the principal coating solution may be displaced by a divalent layer-forming metal cation selected from the group con¬ sisting of cobalt, magnesium, or a lanthanide. The product resulting from the practice of the above process is particularly characterized by a phosphate film in which the predominant structure is a mixed-metal phase phosphate where one of the metals is zinc and the other metal is a transition metal or lanthanide having a hydroxide which has a lower solubility in an alkaline solution than iron and zinc hydroxide. The film preferably has a nickel content of at least 15 mole percent and the mixed-metal phosphate is nickel/zinc phosphate in a con¬ tinuous nodular crystalline structure preferably of the approximate form of Zn2Ni(PQ4)2. wherein the nickel is about 15% of the molecular structure of such crystalline phase. The phosphate coating provides more resistance to corrosion and will double the salt spray life of any painted and phosphated metal article at any specific phosphate solution chemistry by use of the prescribed high nickel content in the bath solution and ultimate nickel/ zinc phosphate film. SUMMARY OF THE DRAWINGS Figures 1-3 are graphical illustrations showing respectively: (1) alkaline sensitivity of coatings made by use of high and conventional nickel zinc phosphate baths, (2) nickel in the coating as a function of nickel in the bath, and (3) salt spray life as a function of nickel in the phosphate coating. Figure 4 is a composite of photographs (4a through 4h) showing taupe spray painted steel panels, each scratched and subjected to salt spray corrosion tests; the panels having varying surface cleanliness, and having been phosphated with a variable nickel content in the bath, in the range of 33-77 mole percent, and variable zinc content. Figures 5-8 are scanning electron microscope photographs of the crystalline structure of coatings (at 1500X magnification) corresponding to panel photographs 4a, 4c, 4e and 9c, respectively. Figure 9 is also a composite of photographs (9a-9h) showing taupe spray painted steel panels, each scratched and subjected to a salt spray corrosion test, and having been phosphated in phosphate baths that were varied incrementally in nickel content (81-85 mole percent). Figures 10-13 are scanning electron microscope photographs of the crystalline structure of coatings (at 1500X magnification) corresponding to panel photographs 9g, 14a, 14c and 14e. Figure 14 is similar to Figures 4 and 9, showing corrosion tested panels corresponding to phosphating baths with nickel contents of 90.5-92.3. Figure 15 is a scanning electron microscope photograph of a coating (at 1500X magnification) prepared in a phosphate bath containing in excess of 95 mole percent nickel. Figure 16 is a composite of photographs (Figures 16a-16d) of salt spray tested taupe spray painted galvanized steel panels. Figure 17 is a graphical display of infrared coatings prepared in phosphate baths having various nickel contents. Table I is a tabulation setting forth for each panel used in Figures 4, 9 and 14: (a) the bath composition used, including the amount of zinc and nickel by mole percent of the combined Ni and Zn, weight percent of nickel or zinc as percent of the combined nickel and zinc, weight in grams of each solute element per liter, and weight percent of each solute element of the total bath solution; (b) the coating composition, including mole percent nickel and zinc of the total Ni and Zn, weight percent of nickel and zinc of the total nickel and zinc, and weight percent in the coating of principal ingredients. OMPI Table II is a tabulation setting forth the physical characteristics of the coatings for the panels listed in Table I. DETAILED DESCRIPTION It is common practice in the automotive industry to apply an acidified zinc phosphate solution to metal car bodies and structural parts to form a zinc phosphate conversion coating on the metal surfaces. This is usually applied by spraying or dipping of the metal car bodies and parts. The phosphate coating has served to provide greater adherence of the paint system, applied thereover, which in turn has resulted in an improvement in resistance to corro¬ sion. This invention provides a dramatic increase in re¬ sistance to alkaline dissolution which is exhibited by a quantum leap in the life of the coating system when sub¬ jected to accelerated salt spray corrosion tests. Corrosion Reactions Corrosion in an aqueous electrolyte is an electro¬ chemical processes involving oxidation and reduction reactions. The oxidation reaction is the anodic dissolu¬ tion of steel or other metal substrate where ions leave the metal to form corrosion products. The excess electrons left in the metal by the oxidation reaction are consumed in the cathodic reduction reaction. A principal cathodic reaction is the reduction of dissolved oxygen to form hydroxide ions. Another cathodic reaction, which may occur in some cases, is the reduction of hydrogen ions. Both reduction reactions produce an increase in the elec¬ trolyte pH at the cathodic sites. The anodic and cathodic reactions can occur at essentially similar atom sites. However, the reaction sites may become widely separated when differential oxygen or electrolyte concentration gradients are established. After a certain amount of rust is formed at the anodic sites, the cathodic reduction of oxygen for the most part is shifted to the periphery of the rust deposit or scratch, that is, to the zinc-phosphate-coating/steel interface. The anodic oxidation of iron is confined to the rust covered areas. A differential oxygen concentration cell is established due to the restricted transport of oxygen through the rust scale and the relatively easy accessi¬ bility of the adjacent rust-free steel surfaces to the atmospheric oxygen. The important consequence of the differential oxygen concentration cell is the generation of a highly alkaline electrolyte at the phosphate/steel inter- " face. The electrolyte pH can rise to above 12 in a sodium chloride environment, a condition prone to produce under¬ cutting of the primer coating. The undercutting by the alkali sodium hydroxide is due principally to the disso¬ lution of some of the zinc phosphate coating (and to a lesser extent the saponification of the reactive ester groups present in some primer resins). When an undercut paint is broken, the pH of the electrolyte is lowered by salt solutions to more neutral values and the newly exposed metal begins to corrode. Thus cathodically induced adhesion failure of paints is an important precursor to the unrestricted corrosion of the metal. The degree of undercutting a primer coating undergoes in a corrosive environment is dependent on: the nature of imperfections in the paint coating (such as a scratch) the chemistry of the primer and the inhibitor used therein, the amount of contaminants present on the surface of the substrate prior to coating, and the effectiveness of the phosphate coating as a barrier to the lateral spread of the corrosion. This invention has made the phosphate coating considerably more effective in spite of the first three factors. ' JR£ OMPI , WIPO ^ATI Surface Carbon Of these three factors, surface contamina¬ tion is deserving of explanation because it has been one of the most serious obstacles to obtaining a consistent improvement in phosphate coatings. Carbonaceous residues on steel or other metallic substrates to be coated do have a deleterious effect on the corrosion protection afforded by paints and phosphate coatings. It has now been esta¬ blished by the prior art that there* is a corrolation between surface carbon contamination and salt spray per¬ formance. Carbon contamination can, by itself, produce early paint adhesion failure and subsequent corrosion problems. Carbonaceous deposits increase the apparent porosity of zinc phosphate coatings because they interfere with the phosphating reactions and subsequent deposition of the phosphate crystals. An increase in porosity permits a greater generation of hydroxyl ions by the cathodic reduc¬ tion of oxygen at the electroche ically active pore sites. Apparent porosity levels of only a few percentage points can drastically reduce the effectiveness of phosphate coatings. Porosity levels of about 0.5% or less, asso¬ ciated with low surface carbon contamination, are required to inhibit the cathodic undercutting of the primer with conventional phosphate systems. What is needed is a phos- phate coating which is relatively insensitive to alkaline undercutting, irrespective of carbon contamination. That is the purpose of this invention. The research supporting this invention shows that a significant improvement in alkaline dissolution can be obtained with zinc phosphate coatings containing at least 15 mole percent nickel of the Ni and Zn content, and a threshold level of zinc of at least 25% by weight of the phosphate film or coating. The improvement is at least a doubling of the salt spray life after painting when co - pared to coatings without such unique nickel/zinc content. Preferred Process A phosphate film is deposited onto the surface of an alkali-cleansed metal article or substrate by exposing the article or substrate to an acidic aqueous phosphate solution for a sufficient time and at a sufficient tempera¬ ture and pH to chemically react and deposit such film. The solution contains first and second layer-forming divalent metal cations and phosphate ions, the first divalent metal cations being selected from the group consisting of nickel, cobalt, magnesium and lanthanides (all are metals having a hydroxide with a lower solubility in an alkaline solution than iron or zinc hydroxide), while the second divalent metal cation is zinc. The amount of first and second divalent metal cations is controlled in critically narrow ranges to provide a first divalent metal cation content in the resulting film of at least 15 mole percent of the total cation. The prior art, and particularly the Ries patent, demonstrates a lack of awareness of the quantum-jump in corrosion protection that can be obtained for paint systems by the use of phosphate baths having a very narrow range of nickel concentration with a minimum zinc threshold, as disclosed herein. This is borne out by the failure of Ries to mention any deficiencies in the corrosion resistance offered by paint systems traceable to shortcomings of the phosphate coating (see column 1, lines 65-72), and makes no specific reference to improved corrosion resistance as an object of the invention (see column 2, lines 28-30). Ries discloses four examples, the first of which is for a "100% nickel" bath, and the remaining three are for 68.1, 68.1 and 68.9 mole percent (65.8, 65.7 and 66.6 weight percent) nickel, respectively. It is clear that Ries provided these examples to establish upper and lower limits for adherency, and that he did not recognize the unique character of nickel phosphate coatings for use as a paint base in corrosion systems within a more severely restricted range. * 3HE£- OMPI V1PO The Ries disclosure is illustrated clearly in example 1, column 5, lines 50-54, wherein the nickel phosphate coat¬ ings are described as "...very well suited as an adhesion base for subsequently applied lacquer and synthetic resin layers." Clearly, the emphasis is on adhesion of the phosphate to the metal substrate. In connection with example 2, Ries states, "In corrosion tests, they were, as shown by condensed water and salt spray tests, mostly superior to the known zinc phosphate layers, or at least equivalent." As shown by evidence hereafter, nickel con¬ tents of 58-83% and 95-100 mole percent of the divalent layer-forming cations do not provide substantial improve¬ ment in salt spray corrosion resistance. Thus the Ries statement is at best gratuitous and not based on technical data. The process herein is further preferably charac¬ terized by the deposition of a phosphate film in an average coating weight of less than 1.3 g/m 2 (120 mg/ft 2 ) having at least 6% by weight nickel of the total coating that pro- vides at least a doubling of the resistance to salt spray corrosion after painting. The phosphate film results from the use of an acidic aqueous coating solution having an oxidizing agent and a solution pH effective to chemi¬ cally react with the article or substrate. The solute content of the solution preferably consists essentially of (a) divalent layer-forming metal cations consisting of 84-94 mole percent nickel of the total metal cations and zinc in an amount of .2-.6 g/1 of said solution as Zn +2 ; and (b) phosphate ions in an amount at least sufficient to form dihydrogen phosphate with said metal cations. Alternatively, a portion of the nickel content may be substituted by use of a divalent layer-forming metal cation selected from the group consisting of cobalt, magnesium.and lanthanides. In more particularity, the metal article is cleansed by use of an alkaline cleaner maintained at a - ϋlEA OMFI temperature of 100-140°F with a concentration of 2-10 points. The article is subjected to the alkaline cleaner for a period of about 30-120 seconds and then rinsed with water at a temperature of 100-140°F for a period of 30-120 seconds. The alkaline-cleansed metal substrate is then sprayed or immersed in a phosphate bath solution maintained at a temperature of about 100-140°F with a composition modified as specified above. The solution has a total acid content of 10-40 points, a free acid content of 0.5-2.0 points, and a total acid/free acid ratio of 10-50. The pH is controlled to 2.5-3.5 when nitrites are present in the bath in an amount preferably of 0.5-2.5 points. The exposure to the phosphate bath is maintained for about 30-120 seconds, which controls the coating weight to a weight of less than 1.3 g/m 2 (120 mg/ft 2 ). Following such phosphating exposure, excess solution is removed from the article or substrate by a rinsing sequence consisting of a water rinse at ambient to 100°F for 30-120 seconds, an inhibitor rinse which contains a chromate or other dis- solved corrosion inhibitor at ambient to 120°F for 30-60 seconds, and a deionized water rinse at ambient temperature for 15-30 seconds. Phosphating Solution The phosphating solution must contain nickel cations which constitute at least 84 mole percent of combined metal cations (82.5% by weight) in the solution (or the equivalent metal cations of magnesium, select transition metals or lanthanides, that have hydroxides with lower dissolution than iron or zinc hydroxide in an alkaline solution. However, it is important that the zinc cation of the phosphating solution be at least .2 g/1 as Zn +2 or .79 g/1 as Zn(H204)2- The molar ratio of Ni/Zn is in the range of 5.2:1 to 16:1. Thus, for example, if zinc is 16 mole percent of the nickel/zinc cation content, there SURE O PI * must also be at least .2 g/1 as Zn +2 in solution (pre¬ ferably .2-.6 g/1 or .79-2.38 g/1 as Zn(H 2 P04)2.. The nickel content therefore must be at least 1.0 g/1 of the bath solution (84 mole percent of the nickel/zinc total). This interrelationship between minimum zinc and high nickel content is essential to producing the phenomenon of this invention which is believed to lie in the formation of a unique continuous nodular phosphate structure of the form of Zn2 (Pθ4)2-.4H2θ, where M is magnesium, a transition. metal, or lanthanide whose hydroxide has a lower solubility in an alkaline solution than iron or zinc hydroxide. Such mixed-metal phosphates contain a high content of magnesium, lanthanide, or transition metal, preferably nickel, which provide significantly improved corrosion performance in spite of the presence of considerable surface carbon con¬ tamination (greater than .4 mg/ft 2 ). As shown in Figure 17, infrared spectra establish that the structure of phos¬ phates formed from high nickel baths is different from those coatings formed from baths having less than 84 mole percent nickel. The scanning electron microscope photo¬ graphs further establish that there is an abrupt change in morphology for phosphates formed from baths having a nickel content above 84 mole percent. When the nickel content of the phosphate solution exceeds 94 mole percent, the coatings produced from such phosphate solution become very thin and nonunifor (see Figure 15), and the benefit of improved corrosion perfor¬ mance is decreasesd proportionately. It also is very difficult to achieve the desired ratio of Ni/Zn in the bath without restricting zinc content to a level which is too low, resulting in a failure to form a proper amount of the Zn2 i(Pθ4)2-4H2θ nodular phase. When the nickel content of the phosphate solution is below 84 mole percent, the phosphate coating fails to produce a sufficient amount of the preferred mixed-phase nodular phosphate structure so necessary to obtain the dramatic increase in salt spray O PI corrosion resistance. These coatings do not have the desired alkaline resistance. When the zinc concentration falls below 0.2 g/1 as Zn +2 , the resultant coating will be deficient because a high proportion of iron oxide and iron phosphates will form, lacking the desired resistance to alkaline dissolution. The initial phosphate solution may be conveniently prepared by making up a nickel and zinc phosphate solution concentrate from preferably the oxide or carbonate and concentrated phosphoric acid. The metal ion concentration in each of these separate solutions can be approximately 120-140 g/1, or, more specifically, 475-550 g/1 Zn(H2P04)2» 520-600 g/1 Ni(H2Pθ4)2- These metal phosphate concentrate solutions can be used to make up a fresh phosphate bath by using sufficient quantities of each of such concentrated solutions with water to render the desired bath concentra¬ tions as described previously. The bath will become enriched with nickel during phosphating use, since more zinc than nickel is deposited in formation of the phosphate film. It is desirable to have a separate makeup concentrate which is formulated for replenishment; the replenishment concentrate can contain approximately 18 mole percent nickel and 82 mole percent zinc of the dissolved nickel and zinc cations. Substrate The substrate is preferably selected from the group consisting of iron, carbon or low alloy steel, aluminum and zinc. When the substrate is either zinc or aluminum, the phosphate solution additionally contains 0.1-1.0 g/1 sodium fluoride to enhance the formation of zinc phosphate coating and to prevent precipitation of the dissolved aluminum. Coated Product The resulting coated product is characterized by unusually good resistance to alkali dissolution (see Figure 1 comparing several test panels of the prior art and this invention exposed to a 12.5 pH NaOH solution) and by its excellent chemical bonding to the substrate. The zinc/ nickel phosphate conversion coating is the result of a chemical reaction with the substrate. The structure of the coating of this invention has the morphology of continuous nodules (see Figures 10-13). The product of this invention is particularly characterized by a significantly improved salt spray performance showing little or no paint under- cutting after 500 test hours abruptly occurring when the nickel content exceeds 15 mole percent (at about 13.7% by weight) in the coating (see Figure 3). The amount of nickel in the coating at or above 13.7% by weight can easily be controlled by regulating the amount of nickel in the bath to exceed 84% of the weight of the Ni/Zn in the bath (see Figure 2). The coating has high corrosion resistance even with high surface carbon contamination. Examples A series of test panels for Examples 1-13 were prepared by cutting sheet metal into panels having a size of 4x12". The test panels were exposed to a phosphating solution of known chemistry (see Table I), rinsed and dried. A portion of selected panels was used for deter¬ mination of coating weight composition, morphology and structure of the phosphate coating . The remaining portion from these panels was painted with taupe primer paint (epoxy ester-melamine resin primer). These samples, after baking, were then scribed in an "X" pattern to bare metal and then subjected to an accelerated salt spray test, after which the degree of paint undercutting was observed and/or measured. The salt spray test essentially involves exposing panels to a mist of a 5% sodium chloride solution in an enclosed chamber maintained at 35°C in accordance with the ASTM B117 standard test method. Example 1 A phosphating bath solution was prepared having the following compostion: 2.22 g/1 Zn(H 2 P0 ) 2 1.08 g/1 Ni(H 2 P0 4 ) 2 5.85 g/1 H3PO4 0.13 g/1 NaN0 2 As formulated, this bath had a total acid concen¬ tration of 11.3 points. The bath acidity was then adjusted by the addition of NaOH to a free acid concentration of 0.8 points, resulting in a total acid to free acid ratio of 14. In a bath of this composition, the dissolved nickel consti¬ tutes 33.3 mole percent of the dissolved divalent cations, which is typical of compositions currently used commer- cially in spray applied phosphating systems. Steel panels of two types were selected for phos¬ phating with the above phosphating composition. The first type, designated as Q steel, was a commercially available steel test panel having very low surface carbon contamina- tion, typically in the range of less than 1 mg/m 2 (0.093 mg/ft 2 ). The second type of panel was cut from commercial auto body sheet, identified as F4. This steel was known to have surface carbon values in the range of 1.8 to 6.2 mg/m 2 (0.17 to 0.58 mg/ft 2 ), and to be subject to early salt spray failure in tests with spray paint primers- applied over conventional zinc phosphate. Panels of Q-steel and F4 steel were spray cleaned for two minutes with a conven¬ tional alkaline cleaner having a strength of 4.7 points and a temperature of 60°C (140°F), spray rinsed for 30 seconds with 60°C tap water, and spray phosphated for two minutes with the above phosphate bath heated to 60°C. The panels were spray rinsed for one minute with 20°C (68°F) de¬ ionized water and dried in an oven at 82°C (180°F) for five minutes. None of the phosphated panels were post-treated with an inhibitor rinse. The steel panels that were phosphate coated by this procedure had a uniform, gray appearance and a coating weight of 1.62 g/m 2 (150 mg/ft 2 ). Chemical analysis showed that the nickel content of the phosphate coatings was equal to 3.2% by weight of the total nickel and zinc content of the coating on both the Q and F4 steels. This is equiva¬ lent to 3.5 mole percent Ni. A scanning electron micro¬ scope photograph of the phosphate coating on the Q steel, taken at 1500X, is shown in Figure 5. This structure is typical of the morphology of spray applied commercial zinc phosphate coatings. The phosphate coated Q and F4 steel panels were spray painted with an expoxy ester-mela ine resin based primer. After baking, the paint thickness was approxi- mately 23 y m. Salt spray testing of the painted and scribed panels was carried out in accordance with the ASTM B117 standard. The specification for the epoxy ester- melamine resin based primer employed in these tests stipu¬ lates that 3 mm undercutting of the paint from the scribe line on the tested panels, as determined by taping the entire surface of the panel, constitutes failure. Salt spray testing of the test panels designated Q was discontinued after 480 hours with essentially zero undercutting from the scribe line (see Figure 4a). The test panels designated F4, on the other hand, failed within 96 hours of salt spray testing (see Figure 4b). Example 2 A phosphating bath solution was prepared having the following composition: 4.44 g/1 Zn (H 2 P0 4 ) 2 5.94 g/1 Ni (H 2 P0 4 ) 2 3.36 g/1 H3PO4 0.12 g/1 NaN0 2 As formulated, this bath had a total acid concen- tration of 14. 7 points . The bath acidity was then adj usted OI-.-PI by the addition of NaOH to a free acid concentration of 0.9 points, resulting in a total acid to free acid ratio of 16. In a bath of this composition, the dissolved nickel consti¬ tutes 57.9 mole percent of the dissolved divalent cations. Panels of the two steels designated Q and F4, described in Example 1, were spray cleaned and rinsed as detailed in Example 1. After the rinsing step, they were spray phosphated for two minutes with the above phosphate bath heated to 60°C. The panels were then spray rinsed with 20°C deionized water and dried in an oven at 82°C for five minutes. As in Example 1, none of the phosphated panels were post-treated with an inhibitor rinse. The steel panels that were phosphate coated by this procedure had a uniform, gray appearance and a coating weight of 1.71 g/m 2 (159 mg/ft 2 ). Chemical analysis showed that the nickel content of the phosphate coatings was equal to 5.6% by weight of the total nickel and zinc content of the coating, on both the Q and F4 steels. This is equiva¬ lent to 6.2 mole percent Ni. A scanning electron micro- scope photograph of the phosphate coating, taken at 150OX, _ is shown in Figure 6. This structure remains similar to the morphology of spray applied commercial zinc phosphate coatings. The phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example 1, the paint film thickness, after baking, was approximately 23 μm. Salt spray testing was again carried out exactly as detailed in Example 1, with 3 mm undercutting of the paint from the scribe line, again considered as failure. Salt spray testing of the test panels designated Q was discontinued after 480 hours, with essentially zero undercutting from the scribe line (see Figure 4C) . The test panel designated F4, on the other hand, failed within 96 hours of salt spray testing (see Figure 4d). Example 3 A phosphating bath solution was prepared having the following composition: 3.33 g/1 Zn(H 2 P0 4 ) 2 6.80 g/1 Ni(H 2 04.2 0.11 g/1 NaN0 2 As formulated, this bath had a total acid concen¬ tration of 14.1 points. The bath acidity was then adjusted by the addition of NaOH to a free acid concentration of 0.9 points, resulting in a total acid to free acid ratio of 16. In a bath of this composition, the dissolved nickel con¬ stitutes 67.7 mole percent of the dissolved divalent metal cations. Panels of the two steels designated Q and F4, described in Example 1, were spray cleaned and rinsed as detailed in Example 1. After the rinsing step, they were spray phosphated for two minutes with the above phosphate bath, heated to 60°C. The panels were then spray rinsed with 20°C deionized water and dried in an oven at 82°C for five minutes. As in Example 1, none of the phosphated panels were post-treated with an inhibitor rinse. The steel panels that were phosphate coated by this procedure had a uniform, gray appearance and a coating weight of 1.35 <g/m 2 (125 mg/ft 2 ). Chemical analysis showed that the nickel content of the phosphate coatings was equal to 7.3% by weight of the total nickel and zinc content of the coating, on both the Q and F4 steels. This is equiva¬ lent to 8.1 mole percent Ni. A scanning electron icro- scope photograph of the phosphate coating, taken at 150OX, is shown in Figure 7. This structure again remains similar to the morphology of spray applied commercial zinc phos¬ phate coatings. The phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example 1, the paint film thickness, after OMPI baking, was approximately 23 μm. Salt spray testing was again carried out exactly as detailed in Example 1, with 3 mm undercutting of the paint from the scribe line, again considered as failure. Salt spray testing of the test panels designated Q was discontinued after 480 hours, with essentially zero undercutting from the scribe line (see Figure 4e). The test panel designated F4, on the other hand, failed within 72 hours of salt spray testing (see Figure 4f). Example 4 A phosphating bath solution was prepared having the following composition: 3.01 g/1 Zn(H 2 Pθ4) 2 10.07 g/1 Ni(H 2 P0 4 ) 2 0.89 g/1 H3PO4 0.14 g/1 NaN0 2 As formulated, this bath had a total acid concen¬ tration of 14.2 points. The bath acidity was then adjusted by the addition of NaOH to a free acid concentration of 0.9 points, resulting in a total acid to free acid ratio of 16. In a bath of this composition, the dissolved nickel consti¬ tutes 77.4 mole percent of the dissolved divalent-cations. Panels of the two steels designated Q and F4, described in Example 1, were spray cleaned and rinsed as detailed in Example 1. After the rinsing step, they were spray phosphated for two minutes with the above phosphated bath heated to 60°C. The panels were then spray rinsed with 20°C deionized water and dried in an oven at 82°C for five minutes. As in Example 1, none of the phosphated panels were post-treated with an inhibitor rinse. The steel panels that were phosphate coated by this procedure had a uniform, gray appearance and a coating weight of 1.14 g/m 2 (106 mg/ft 2 ). Chemical analysis showed that the nickel content of the phosphate coatings was equal to 11.0% by weight of the total nickel and zinc content of _OKPI the coating, on both the Q and F4 steels. This is equiva¬ lent to 12.1 mole percent Ni. The phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example 1, the paint film thickness, after baking, was approximately 23 μm. Salt spray testing was again carried out exactly as detailed in Example 1, with 3 mm undercutting of the paint from the scribe line, again considered as failure. Salt spray testing of the test panels designated Q was discontinued after 480 hours, with essentially zero undercutting from the scribe line (see Figure 4g). The test panels designated F4, on the other hand, failed within 72 hours of salt spray testing (see Figure 4h) . Example 5 A phosphating bath solution was prepared having the following compos tion: 2. 14 g/1 Zn(H 2 P0 ) 2 9.21 g/1 Ni (H 2 P0 4 ) 2 1 - 42 g/1 H 3 P0 4 0.11 g/1 NaN0 2 As formulated , this bath had a total acid concen¬ tration of 14. 2 points . The bath acidity was then adj usted by the addition of NaOH to a free acid concentration of 0.8 points , resulting in a total acid to free acid ratio of 18. In a bath of this composition, the dissolved nickel consti¬ tutes 81.5 mole percent of the dissolved divalent cations . Panels of the two steels designated Q and F4, described in Example 1, were spray cleaned as detailed in Example 1. After the r insing step, they were spray phos¬ phated for two minutes with the above phosphate bath , heated to 60 °C. The panels were then spray rinsed with 20 °C deionized water and dried in an oven at 82 °C for five minutes . As in Example 1, none of the phosphated panels were post-treated with an inhibitor rinse . -f JR OMP U The steel panels that were phosphate coated by this procedure had a uniform, gray appearance and a coating weight of 1.17 g/m 2 (109 mg/ft 2 ). Chemical analysis showed that the nickel content of the phosphate coatings was equal to 11.8% by weight of the total nickel and zinc content of the coating, on both the Q and F4 steels. This is equiva¬ lent to 13.0 mole percent Ni. The phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example 1, the paint thickness, after baking, was approximately 23 μ m. Salt spray testing was again carried out exactly as detailed in Example 1, with 3 mm undercutting of the paint from the scribe line, again considered as failure. Salt spray testing of the test panels designated Q was discontinued after 480 hours, with essentially zero undercutting from the scribe line (see Figure 9a). The test panels designated F4, on the other hand, failed within 120 hours of salt spray testing (see Figure 9b). Example 6 A phosphating bath solution was prepared having the following composition: 2.06 g/1 Zn(H 2 P0 ) 2 9.56 g/1 Ni(H 2 P0 4 ) 2 1.18 g/1 H3PO4 0.12 g/1 NaN0 2 As formulated, this bath had a total acid concen¬ tration of 14.3 points. The bath acidity was then adjusted by the addition of NaOH to a free acid concentration of 0.7 points, resulting in a total acid to free acid ratio of 20. In a bath of this composition, the dissolved nickel consti¬ tutes 82.6 mole percent of the dissolved divalent cations. Panels of the two steels designated Q and F4, described in Example 1, were spray cleaned and rinsed as detailed in Example 1. After the rinsing step, they were spray phosphated for two minutes with the above phosphate bath, heated to 60°C. The panels were then spray rinsed with 20°C deionized water and dried in an oven at 82°C for five minutes. As in Example 1, none of the phosphated panels were post-treated with an inhibitor rinse. The steel panels that were phosphate coated by this procedure had a uniform, gray appearance and a coating weight of 1.09 g/m 2 (101 mg/ft 2 ). Chemical analysis showed that the nickel content of the phosphate coatings was equal to 12.3% by weight of the total nickel and zinc content of the coating, on both the Q and F4 steels. This is equiva¬ lent to 13.6 mole percent Ni. A scanning electron micro¬ scope photograph of the phosphate coating, taken at 1500X, is shown in Figure 8. The morphology is similar to that of commercial zinc phosphate, except for a finer-sized crystal structure. The phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example 1, the paint film thickness, after baking, was approximately 23 μ . Salt spray testing was again carried out exactly as detailed in Example 1, with 3 mm undercutting of the paint from the scribe line, again considered as failure. Salt spray testing of the test panels designated Q was discontinued after 480 hours, with essentially zero undercutting from the scribe line (see Figure 9c). The test panels designated F4, on the other hand, failed within 144 hours of salt spray testing (see Figure 9d). Example 7 A phosphating bath solution was prepared having the following composition: 2.18 g/1 Zn(H 2 P0 ) 2 10.07 g/1 Ni(H 2 P0 4 ) 2 2.08 g/1 H3PO4 0.12 g/1 NaN0 2 As formulated, this bath had a total acid concen¬ tration of 15.6 points. The bath acidity was then adjusted by the addition of NaOH to a free acid concentration of 0.6 points, resulting in a total acid to free acid ratio of 26. In a bath of this composition, the dissolved nickel consti¬ tutes 82.6 mole percent of the dissolved divalent cations. Panels of the two steels designated Q and F4, described in Example 1, were spray cleaned and rinsed as detailed in Example 1. After the rinsing step, they were spray phosphated for two minutes with the above phosphate bath, heated to 60°C. The panels were then spray rinsed with 20°C deionized water and dried in an oven at 82°C for five minutes. As in Example 1, none of the phosphated panels were post-treated with an inhibitor rinse. The steel panels that were phosphate coated by this procedure had a uniform, gray appearance and a coating weight of 1.45 g/m 2 (135 mg/ft 2 ). Chemical analysis showed that the nickel content of the phosphate coatings was equal to 13.1% by weight of the total nickel and zinc content of the coating on both the Q and F4 steels. This is equiva¬ lent to 14.4 mole percent Ni. The phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example 1, the paint film thickness, after baking, was approximately 23 μ . Salt spray testing was again carried out exactly as detailed in Example 1, with 3 mm undercutting of the paint from the scribe line, again considered as failure. Salt spray testing of the test panels designated Q was discontinued after 480 hours, with essentially zero undercutting from the scribe line (see Figure 9e). The test panel designated F4, on the other hand, failed within 456 hours of salt spray testing (see Figure 9f). The improvement in salt spray performance noted for Example 7, compared with Example 6, is an illustration of the importance of the nickel content in the phosphate coating. Note that the dissolved nickel content of the baths described in Examples 6 and 7 are both 82.6 mole percent. However, the higher total-acid to free-acid ratio in Example 7 versus Example 6 resulted in a somewhat higher nickel content in the phosphating coating which, in turn, resulted in improved corrosion performance. Example 8 A phosphating bath solution was prepared having the following composition: 1-63 g/1 Zn(H 2 P0 ) 2 9.00 g/1 Ni(H 2 P0 4 ) 2 2.95 g/1 H3PO4 0.13 g/1 NaN0 2 As formulated, this bath had a total acid concen- tration of 14.1 points. The bath acidity was then adjusted by the addition of NaOH to a free acid concentration of 0.8 points, resulting in a total acid to free acid ratio of 18. In a bath of this composition, the dissolved nickel consti¬ tutes 85.0 mole percent of the dissolved divalent cations. Panels of the two steels designated Q and F4, described in Example 1, were spray cleaned and rinsed as detailed in Example 1. After the rinsing step, they were spray phosphated for two minutes with the above phosphate bath, heated to 60°C. The panels were then spray rinsed with.20°C deionized water and dried in an oven at 82°C for five minutes. As in Example 1, none of the phosphated panels were post-treated with an inhibitor rinse. The steel panels that were phosphate coated by this procedure had a uniform, bluish-gray appearance and a coating weight of 0.90 g/m 2 (84 mg/ft 2 ). Chemical analysis showed that the nickel content of the phosphate coatings was equal to 14.6% by weight of the total nickel and zinc content of the coating, on both the Q and F4 steels. This is equivalent to 16.0 mole percent Ni. A scanning electron microscope photograph of the phosphate coating, taken at _ OMPI 1500X, is shown in Figure 10. This represents an abrupt change in morphology from that shown for the previous examples and suggests an overall change in structure. The phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example 1, the paint film thickness, after baking, was approximately 23 Um. Salt spray testing was again carried out exactly as detailed in Example 1, with 3 mm undercutting of the paint from the scribe line, again considered as failure. Salt spray testing of the test panels designated Q was discontinued after 480 hours, with essentially zero undercutting from the scribe line (see Figure 9g). Salt spray testing of the test panels designated F4 was also discontinued after 480 hours, with essentially zero under¬ cutting from the scribe line (see Figure 9h). Example 9 A phosphating bath solution was prepared having the following composition: 1.07 g/1 Zn(H 2 P0 4 ) 2 9.90 g/1 Ni(H 2 P0 )2 0.96 g/1 H3PO4 0.12 g/1 NaN0 2 As formulated, this bath had a total acid concen- tration of 13.5 points. The bath acidity was then adjusted by the addition of NaOH to a free acid concentration of 0.9 points, resulting in a total acid to free acid ratio of 15. In a bath of this composition, the dissolved nickel consti¬ tutes 90.5 mole percent of the dissolved divalent cations. Panels of the two steels designated Q and F4, described in Example 1, were spray cleaned and rinsed as detailed in Example 1. After the rinsing step, they were spray phosphated for two minutes with the above phosphate bath, heated to 60°C. The panels were then spray rinsed with 20°C deionized water and dried in an oven at 82°C for five minutes. As in Example 1, none of the phosphated panels were post-treated with an inhibitor rinse. The steel panels that were phosphate coated by this procedure had a uniform, bluish-gray appearance and a coating weight of 0.86 g/m 2 (80 mg/ft 2 ). Chemical analysis showed that the nickel content of the phosphate coatings was equal to 15.5% by weight of the total nickel and zinc content of the coating, on both the Q and F4 steels. This is equivalent to 17.0 mole percent Ni. A scanning electron microscope photograph of the phosphate coating, taken at 1500X, is shown in Figure 11. This structure confirms the abrupt change in morphology shown in Figure 10, which suggested an overall change in structure. The phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example 1, the paint film thickness, after baking, was approximately 23 μm. Salt spray testing was again carried out exactly as detailed in Example 1, with 3 mm undercutting of the paint from the scribe line, again considered as failure. As in the previous example, salt spray testing of both sets of test panels (Q and F4) was discontinued after 480 hours, with essentially zero undercutting from the scribe line (see Figures 14a and 14b). Example 10 A phosphating bath solution was prepared having the following composition: 1.71 g/1 Zn(H 2 P0 ) 2 17. 69 g/1 Ni (H 2 P0 ) 2 2.75 g/1 H3PO4 0.11 g/1 NaN0 2 As formulated, this bath had a total acid concen¬ tration of 24. 0 points . The bath acidity was then adj usted by the addition of NaOH to a free acid concentration of 0.8 own points., resulting, in. a total acid to free acid ratio of 30. In a bath of this composition, the dissolved nickel consti¬ tutes 91.4 mole percent of the dissolved divalent cations. Panels of the two steels designated Q and F4, described in Example 1, were spray cleaned and rinsed as detailed in Example 1. After the rinsing step, they were spray phosphated for two minutes with the above phosphate bath, heated to 60°C. The panels were then spray rinsed with 20°C deionized water and dried in an oven at 82°C for five minutes. As in Example 1, none of the phosphated panels were post-treated with an inhibitor rinse. The steel panels that were phosphate coated by this procedure had a uniform, bluish-gray appearance and a coating weight of 0.62 g/m 2 (58 mg/ft 2 ). Chemical analysis showed that the nickel content of the phosphate coatings was equal to 21.0% by weight of the total nickel and zinc content of the coating, on both the Q and F4 steels. This is equivalent to 22.8 mole percent Ni. A scanning electron microscope photograph of the phosphate coating, taken at 1500X, is shown in Figure 12. This structure is similar to that shown in Figures 10 and 11. The phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example 1, the paint film thickness, after baking, was approximately 23 μ m. Salt spray testing was again carried out exactly as detailed in Example 1, with 3 mm undercutting of the paint from the scribe line, again considered as failure. As in Examples 8 and 9, salt spray testing of both sets of test panels (Q and F4) was discontinued after 480 hours, with essentially zero undercutting from the scribe line (see Figures 14c and 14d). Example 11 A phosphating bath solution was prepared having the following composition: Q...ΓI 1.55 g/1 Zn(H 2 P0 ) 2 18.04 g/1 Ni(H 2 P0 ) 2 2.89 g/1 H3PO4 0.12 g/1 NaN0 2 As formulated, this bath had a total acid concen¬ tration of 24.4 points. The bath acidity was then adjusted by the addition of NaOH to a free acid concentration of 0.5 points, resulting in a total acid to free acid ratio of 49. In a bath of this composition, the dissolved nickel consti- tutes 92.3 mole percent of the dissolved divalent cations. Panels of the two steels designated Q and F4, described in Example 1, were spray cleaned and rinsed as detailed in Example 1. After the rinsing step, they were spray phosphated for two minutes with the above phosphate bath, heated to 60°C. The panels were then spray rinsed with 20°C deionized water and dried in an oven at 82°C for five minutes. As in Example 1, none of the phosphated panels were post-treated with an inhibitor rinse. The steel panels that were phosphate coated by this procedure had a uniform, bluish-gray appearance and a coating weight of 0.52 g/m 2 (48 mg/ft 2 ). Chemical analysis showed that the nickel content of the phosphate coatings was equal to 24.8% by weight of the total nickel and zinc content of the coating, on both the Q and F4 steels. This is equivalent to 26.9 mole percent Ni. A scanning electron microscope photograph of the phosphate coating, taken at 1500X, is shown in Figure 13. This structure is similar to that shown in Figures 10, 11 and 12. The phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example 1, the paint film thickness, after baking, was approximately 23 μm. Salt spray testing was again carried out exactly as detailed in Example 1, with 3 mm undercutting of the paint from the scribe line, again considered as failure. ' As in- Examples 8, 9 and 10, salt spray testing of both sets of test panels (Q and F4) was discontinued after 480 hours, with essentially zero undercutting from the scribe line (see Figures 14e and 14f). Example 12 A phosphating bath solution was prepared having the following composition: 0.63 g/1 Zn(H 2 P0 ) 2 12.22 g/1 Ni(H 2 P0 4 ) 2 1.10 g/1 H3PO4 0.11 g/1 NaN0 2 As formulated, this bath had a total acid concen¬ tration of 14.8 points. The bath acidity was then adjusted by the addition of NaOH to a free acid concentration of 0.8 points, resulting in a total acid to free acid ratio of 18. In a bath of this composition, the dissolved nickel consti¬ tutes 95.2 mole percent of the dissolved divalent cations. Panels of the two steels designated Q and F4, described in Example- 1, were spray cleaned and rinsed as detailed in Example 1. After the rinsing step, they were spray phosphated for two minutes with the above phosphate bath, heated to 60°C. The panels were then spray rinsed with 20°C deionized water and dried in an oven at 82°C for five minutes. As in Example 1, none of the phosphated panels were post-treated with an inhibitor rinse. The steel panels that were phosphate coated by this procedure had a very nonuniform, streaked and spotted appearance, and varied in color from light gray to black. Coating weight varied from 0.28 to 0.70 g/m 2 (26 to 65 mg/ft 2 ). Chemical analysis showed that the nickel content of the phosphate coatings was equal to 41.3% by weight of the total nickel, and zinc content of the coating, on the Q steel panels. This is equivalent to 43.9 mole percent Ni. The coating also contained a high iron content, which is undesirable * A scanning electron microscope photograph of the phosphate coating, taken at 1500X, is shown in Figure 15. Although this structure resembles the structures observed for coatings with nickel contents in the range of 13-25% by weight, the nonunifor visual appearance, non- uniformity of coating weight, and high porosity, were judged unsatisfactory for commercial application; there¬ fore no painted test panels were prepared for corrosion testing. Examples 1 through 12 show, collectively, ' that there is a very narrow, sharply demarcated range of nickel contents in nickel/zinc phosphate conversion coatings which will consistently confer outstanding salt spray corrosion resistance upon subsequently painted commercial steel sheet, such as auto body steel sheet, which may be con¬ taminated with carbon. This range of nickel contents, which Examples 1 through 12, collectively, have shown to be characterized by a phosphate coating having a micro- structure distinctly different from the microstructure of ordinary commercial zinc phosphate coatings, is from 14 mole percent to 26 mole percent nickel, corresponding to about 83.5 to 93.0 mole percent dissolved nickel in the phosphating bath. Depending upon the ratio of total-acid to free-acid, slight variations on these limits are possible. For the sake of simplicity. Examples 1 through 12 have dealt with phosphate coatings applied over two steels, one with low surface carbon contamination and one with moderate surface carbon contamination; and with one paint, an epoxy ester-melamine resin based primer applied by spraying. It must be understood that many of the phosphates described in Examples 1 through 12 as well as other compositions not included in those examples were applied to a variety of other substrates, such as steels having various other levels of surface carbon contamina¬ tion, hot-dipped galvanized steel, and aluminum. In addition, phosphated test panels of these substrates were corrosion tested in salt spray after the application of a commercial cathodic electrocoat primer as well as the spray primer referred to in Examples 1 through 12. Again, con- sistently outstanding salt spray performance was observed only when the nickel content in the phosphating bath was in the range of about 84-94 mole percent of the dissolved divalent cations. The following additional example will serve to illustrate the benefits obtained on substrates other than steel, particularly on the zinc surface of hot-dipped galvanized steel, which result from the formation of a phosphate conversion coating having a nickel content within the narrowly restricted range established for steel sub- strates in the previous examples. Example 13 A phosphating bath solution was prepared having the following composition: 1.42 g/1 Zn(H 2 P0 ) 2 13.02 g/1 Ni(H 2 P0 4 ) 2 2.05 g/1 H3PO4 0.47 g/1 NaF 0.08 g/1 NaN0 2 As formulated, this bath had a total acid concen- tration of 18.1 points. The bath acidity was then adjusted by the addition of NaOH to a free acid concentration of 0.7 points, resulting in a total acid to free acid ratio of 26. In a bath of this composition, the dissolved nickel consti¬ tutes 90.7 mole percent of the dissolved divalent cations. Panels of the two steels designated Q and F4, deribed in Example 1, and of hot-dipped galvanized steel, were spray cleaned and rinsed, as detailed in Example 1. After the rinsing step, they were spray phosphated for two minutes with the above phosphate bath, heated to 60°C. The panels were then spray rinsed with 20°C deionized water and OMFI ^ dried in an oven at 82°C for five minutes. As in Example 1, none of the phosphated panels were post-treated with an inhibitor rinse. The steel panels that were phosphate coated by this procedure had a uniform, bluish-gray appearance and a coating weight of 0.92 g/m 2 (85 mg/ft 2 ). Grayish-black phosphate coatings with a weight of 1.67 g/m 2 (155 mg/ft 2 ) were produced on the hot-dipped galvanized steel panels. Chemical analysis of the phosphate coating on the steels showed that the nickel content was equal to 16.2% by weight of the total nickel and zinc content of the coating. This is equivalent to 17.7 mole percent Ni. The phosphate coated Q and F4 steel panels and the phosphate coated galvanized steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example 1, the paint film thickness, after baking, was approximately 23 μ m. Salt spray testing was again carried out exactly as detailed in Example 1, with 3 mm under¬ cutting of the paint from the scribe line, again considered as failure. As in Examples 8, 9, 10 and 11, salt spray testing of both sets of the steel test panels (Q and F4) was dis¬ continued after 480 hours, with essentially zero under¬ cutting from the scribe line. While the galvanized steel panels were not free from undercutting after 360 hours of testing, there was a very considerable improvement over the performance considered as typical of commercial zinc phos¬ phates. Figure 16 illustrates the improvement in 360 hour and 480 hour salt spray performance for galvanized steel with 17.7 mole percent nickel in the phosphate coating (Figures 16a and 16b) vis-a-vis commercial zinc phosphate containing 3.5 mole percent nickel (Figures 16c and 16d). The high nickel phosphate shows a marked advantage in degree of salt spray undercutting of the paint as well as in the progression of undercutting with time. own v . I?0 In a further study done to confirm the observed structural changes in the phosphate coatings illustrated in Figures 5, 6, 7, 8, 10, 11, 12 and 13, phosphate coatings representative of various levels of nickel in the phos- phating bath were examined by infrared analysis, making use of an attenuated total reflectance technique. The results, summarized as a series of absorbance spectra, are shown in Figures 17a-17f. Figure 17a is the spectrum typical of commercial zinc phosphate, corresponding to Example 1. There are only gradual changes in this spectrum with increasing nickel content of the phosphating bath over the range of 33.3 mole percent, corresponding to Example 1, through 82.6 mole percent, corresponding to Example 6. However, beginning with the attainment of a level of 85.0 mole percent, corresponding to Example 8, there is a gradual change in the spectrum, as shown in Figure 17d, confirming the change in structure illustrated in the scanning electron microphoto shown in Figure 10. The change in the spectrum becomes more pronounced with increasing nickel percentage in the phosphating bath, as shown in Figures 17e and 17f, which correspond to Example 9 and Example 11, respectively.

Claims: We claim: 1. A method for increasing the resistance to alkaline dissolution of a phosphate conversion coating on a corrodible metal substrate, said coating being deposited by chemical reaction between said substrate and an acidic aqueous solution containing first and second layer-forming divalent metal cations and phosphate ions, the method being characterized by: (a) selecting said first divalent metal cation to be a transition metal or lanthanide having a hydroxide with a lower solubility in an alkaline solution than iron or zinc hydroxide; (b) selecting said second divalent metal cation as zinc; (c) critically controlling within a narrow range the amount of first and second divalent metal cations. present during said chemical reaction so that the deposited coating has a first divalent metal cation which is at least 15.0 molar percent of the total divalent metal cations and a second divalent metal cation content of at least 25% by weight of the coating. 2. The method as in Claim 1, in which said deposited coating is constituted substantially of a continuous nodular mixed-metal phosphate, and said first divalent metal cation being selected from the group consisting of nickel, cobalt, magnesium and lanthanides. 3. The method as in Claim 2, in which said first divalent cation is nickel and said continuous nodular mixed-metal phosphate is of the form Zn2Ni(Pθ4>2.4H2θ. 4. The method as in Claim 1, in which said conversion coating has a substantially uniform weight of less than 1.3 g/m 2 (120 mg/ft 2 ). 5. A method of increasing the resistance to alkaline dissolution of a phosphate conversion coating on a corrodible metal substrate, comprising: (a) exposing said substrate to a phosphate solu- tion for a sufficient time and at a sufficient temperature and pH to chemically react and deposit a coating of phos¬ phate on said substrate, said substrate having been at least alkali-cleansed prior to exposure, said solution being an acidic aqueous solution with a solute content consisting essentially of: (i) first divalent layer-forming metal cations selected from the group consisting of nickel, magnesium, cobalt and lanthanides, said first divalent metal cation being present in a molar percent of 84-94 of the total divalent layer- forming metal cations in said solute; (ii) second divalent layer-forming metal cations selected as zinc, present in an amount of at least .2 g/1 as Zn +2 of said solution? (iii) phosphate ions in an amount at least sufficient to form dihydrogen phosphate with said divalent metal cations; and (b) removing excess solution from said substrate that has not deposited as a coating. 6. The method as in Claim 5, in which said first divalent layer-forming metal cation is nickel and the molar ratio of said first and second metal cations is in the range of 5.2:1 to 16:1. 7. The method as in Claim 6, in which said first metal cation must be present in an amount of at least 1.0 g/1 of said solution. OMPI 8. The method as in Claim 5, in which said zinc is present in an amount of .2-.6 g/1 as Zn +2 (.79-2.38 g/1 as Zn(H2Pθ4)2). 9. In a method of coating a phosphate film onto the surface of an alkali-cleansed metal article by applying thereto a phosphate solution, said method being character¬ ized by the deposition of a phosphate film having a weight of less than 1.3 g/m 2 (120 mg/ft 2 ) and containing at least 6% by weight nickel of the total film, said film providing at least a doubling of the resistance to salt spray corro¬ sion for any metal article coated with known phosphate films and subsequently painted, said phosphate film being applied by the use of an acidic aqueous solution having an oxidizing agent and a solution pH effective to chemically react with said article and a solute content consisting essentially of: (a) divalent layer-forming metal cations con- sisting of 84-94% nickel of the metal cations, and zinc in an amount of .2-.6 g/1 of said solution as Zn +2 ; and (b) phosphate ions in an amount at least suffi¬ cient to form dihydrogen phosphate with said metal cations. 10. The method as in Claim 9, in which said metal article surface contains a tightly adherent total surface carbon content thereon which is greater than .4 mg/ft 2 . 11. The method as in Claim 9, in which the surface of said metal is selected from the group consisting of iron, carbon or low alloy steel, aluminum and zinc. 12. The method as in Claim 9, in which said aqueous solution has a total acid content of 10-40 points, a free acid content of .5-2.0 points, and a total acid/free acid ratio of 10-50. 13. The method as in Claim 9, in which the phosphate film possesses a hybrid nickel/zinc continuous nodular structure of the form Zn2Ni(Pθ4)2-4H2θ< 14. The method as in Claim 9, in which said cleansed metal article is comprised of zinc, and said phosphate coating solution additionally contains .1-1.0 g/1 sodium fluoride. 15. The method as in Claim 9, in which said surface is comprised of zinc, and the coating solution additionally contains a fluoroborate or fluorosilicate. 16. The method as in Claim 9, in which said coating solution is maintained at a temperature of 100-150°F (38-65°C). 17. The method as in Claim 16, in which said cleansed metal article is exposed to said coating solution for a period of time of 30-120 seconds. 18. The method as in Claim 9, in which a plurality of metal articles are exposed to said coating solution, said coating solution being replenished to com¬ pensate for the depletion of zinc and nickel therefrom by the use of a replenishment solution consisting essentially of 18 mole percent nickel (16.5% by weight) and 82 mole percent zinc (83.5% by weight) of the dissolved cations. 19. In a method for the application of a corro¬ sion resistant coating system on a substrate comprised of steel, iron, aluminum or zinc, wherein an alkali-cleansed substrate is contacted with an acidic aqueous solution containing dissolved divalent layer-forming metal cations selected from the group consisting of nickel, cobalt. c/> . λVju-o lanthanides, magnesium and zinc, said solution con¬ taining at least zinc and one other cation from said group, and dissolved phosphate anions, said contact being carried out for a time and at a temperature sufficient to effect formation of a phosphate coating on said substrate, the improvement comprising: controlling the amount of said one other cation in said solution to 84-94 mole percent (82.5-93.4% by weight) of the dissolved divalent layer-forming metal cations, and the amount of zinc to .2-.6 g/1 as Zn +2 or .79-2.38 g/1 as Zn(H 2 P0 4 ) 2 . 20. The method as in Claim 19, in which the phosphate film resulting from said process contains at least 16 mole percent nickel of the cations in said film. 21. The method as in Claim 19, in which said substrate is zinc and said solution additionally comprises fluoride ions. 22. The method as in Claim 19, in which said fluoride ions are introduced as a simple fluoride, a fluoroborate, fluorosilicate, or other complex fluoride, said fluoride being present in an amount of .1-1.0 g/1 of said solution. 23. The method as in Claim 19, in which said substrate contains thereon a tightly adherent carbon contamination in an amount of at least .4 mg/ft. 24. The method as in Claim 19, in which said coated phosphate film substantially contains a mixed-metal phosphate, continuous nodular structure of the form Zn2Ni(P0 ) .4H 2 0. OMFI .Λ. ~ 25. The method as in Claim 19, in which several of said metallic substrates are coated causing a depletion at some portion of the coating solution, said solution being replenished by use of a concentrated solution con- taining 18 mole percent nickel and 82 mole percent zinc of the dissolved cations. 26. The method as in Claim 19, in which the time of exposure of said solution is about 30-120 seconds. 27. The method as in Claim 19, in which said phosphating solution is maintained at a temperature of 100-150°F (38-65°C). 28. A method of applying phosphate coatings onto a substrate of steel, iron, aluminum or zinc, comprising: (a) exposing said substrate to an alkaline cleaning solution containing 2-10 points alkalinity and heated to a temperature of 100-140°F for a period of 30-120 seconds, followed by water rinse; (b) contacting said alkaline-cleansed substrate with an acidic aqueous phosphate coating solution having a pH of 2.5-3.5 and a solute consisting essentially of metal cations consisting of 84-94 mole percent nickel of the metal cations and zinc in an amount of .2-.6 g/1 of said solution as Zn +2 , phosphate ions in an amount at least sufficient to form dihydrogen phosphate with said metal cations, and an oxidizing agent and solution pH effective to chemically react with said substrate, said contacting being carried out for a period of time to deposit a maximum average coating weight of 1.3 g/m 2 ; and (c) rinsing said coated substrate sequentially with water, water containing an inhibitor, and deionized water, the phosphate coating on said substrate being characterized by the presence of a mixed-metal • phosphate, _O FI wherein one of the metals is zinc and the other metal is nickel in an amount of at least 6% by weight of the phosphate crystal. 29. The method as in Claim 28, in which the nickel content of said phosphate coating is at least 15 mole percent of the total of nickel and zinc. 30. The method as in Claim 28, in which the phosphate coating has said mixed-metal phosphate of the form Zn2 i(Pθ4)2-4H2θ. 31. The method as in Claim 28, in which the total acid content of said solution is 10-40 points, free acid content is .5-2.0 points, and the total acid/free acid ratio is 10-50. 32. The product resulting from the practice of the method of Claim 9, characterized by nickel content of at least 6% by weight of the coating and a salt spray performance showing no paint undercutting after 500 test hours of salt spray solution exposure, and a crystalline content characterized by a mixed-metal phase phosphate of the form Zn2Ni(Pθ4)2-4H2θ. 33. A method for increasing the resistance to alkaline dissolution of a phosphate conversion coating on a corrodible metal substrate, said coating being deposited by chemical reaction between said substrate and an acidic aqueous solution containing first and second layer-forming divalent metal cations and phosphate ions, the method being characterized by: (a) selecting said first divalent metal cation to be a transition metal or lanthanide having a hydroxide with a lower solubility in an alkaline solution than iron or zinc hydroxide, said first divalent metal cation being present in a molar percent of 84-94 of the total divalent layer-forming metal cations in the solution; (b) selecting said second divalent metal cation as zinc; (c) critically controlling the amount of first and second divalent metal cations present during said chemical reaction so that the deposited coating has a first divalent metal cation which is at least 15.0 molar percent of the total divalent metal cations and a second divalent metal cation content of at least 25% by weight of the coating. 34. The method as in Claim 33, in which said deposited coating is constituted substantially of a continuous nodular mixed-metal phosphate, said first divalent metal cation being selected from the group consisting of nickel, cobalt, magnesium and lanthanides. 35. The method as in Claim 34, in which said first divalent cation is nickel and said continuous nodular mixed-metal phosphate is of the form Zn2Ni(Pθ4)2-4H2θ. OMFI e Λ? v/ipo

Inventors: HUFF JOHN, ZURILLA RONALD W, HUFF, JOHN, ZURILLA, RONALD, W.
Applicants: FORD FRANCE, FORD MOTOR CANADA, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S.A., FORD MOTOR COMPANY, FORD MOTOR COMPANY LIMITED, FORD MOTOR COMPANY OF CANADA, LIMITED
Application year: 1982
Main classification: C23F7
IPC codes: C23C22