Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/07—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing phosphates
  • C23C22/08—Orthophosphates
  • C23C22/12—Orthophosphates containing zinc cations
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/34—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides
  • C23C22/36—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also phosphates
  • C23C22/362—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also phosphates containing also zinc cations

Definitions

• 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).
• 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
• 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.
• 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).
• 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.
• 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.
• 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.
• 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(P ⁇ 4)2 « H2 ⁇ , but having some slight nickel variation within a narrow range.
• 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.
• a sufficient temperature and pH i.e., 30-120 seconds, 100-1400°F, 2.5-3.5 pH
• 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.
• 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 NaN ⁇ 2 «
• 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).
• 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.
• 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
• FIGS 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
• FIGS. 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.
• 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.
• reaction sites may become widely separated when differential oxygen or electrolyte concentration gradients are established.
• 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).
• This invention has made the phosphate coating considerably more effective in spite of the first three factors.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the metal article is cleansed by use of an alkaline cleaner maintained at a
• - ⁇ lEA OMFI temperature 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 ).
• 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.
• 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.
• 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.
• 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.
• zinc is 16 mole percent of the nickel/zinc cation content
• 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 (P ⁇ 4)2-.4H2 ⁇ , 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 ).
• 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.
• 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(P ⁇ 4)2-4H2 ⁇ nodular phase.
• 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
• 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(H2P ⁇ 4)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.
• the substrate is preferably selected from the group consisting of iron, carbon or low alloy steel, aluminum and zinc.
• 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.
• 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.
• 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.
• a phosphating bath solution was prepared having the following compostion:
• 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.
• the first type designated as Q steel
• 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 ).
• 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).
• a phosphating bath solution was prepared having the following composition: 4.44 g/1 Zn (H 2 P0 4 ) 2
• 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.
• the phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer.
• the paint film thickness, after baking, was approximately 23 ⁇ 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 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:
• 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 ⁇ 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 4e).
• the test panel designated F4 on the other hand, failed within 72 hours of salt spray testing (see Figure 4f).
• a phosphating bath solution was prepared having the following composition:
• 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.
• 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
• 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 ⁇ 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 4g). The test panels designated F4, on the other hand, failed within 72 hours of salt spray testing (see Figure 4h) .
• a phosphating bath solution was prepared having the following compos tion:
• 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.
• 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 .
• 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 ⁇ 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).
• a phosphating bath solution was prepared having the following composition:
• 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.
• 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.
• 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 ⁇ . 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:
• 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.
• 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 ⁇ . 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).
• Example 7 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.
• a phosphating bath solution was prepared having the following composition: 1-63 g/1 Zn(H 2 P0 ) 2
• 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.
• 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.
• 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).
• a phosphating bath solution was prepared having the following composition: 1.07 g/1 Zn(H 2 P0 4 ) 2
• 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.
• 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 ⁇ 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.
• a phosphating bath solution was prepared having the following composition:
• Example 1 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.
• the paint film thickness, after baking, was approximately 23 ⁇ 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.
• a phosphating bath solution was prepared having the following composition:
• 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 ⁇ 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, 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).
• a phosphating bath solution was prepared having the following composition:
• 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.
• 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.
• 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.
• 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.
• a phosphating bath solution was prepared having the following composition:
• 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.
• 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
• 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.
• the paint film thickness, after baking, was approximately 23 ⁇ 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.

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• 1982
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