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
  • C23C22/34—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
• • 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
  • C23C22/07—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
  • C23C22/18—Orthophosphates containing manganese cations
  • C23C22/182—Orthophosphates containing manganese cations containing also zinc cations
  • C23C22/184—Orthophosphates containing manganese cations containing also zinc cations containing also nickel 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
  • C23C22/07—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
  • C23C22/18—Orthophosphates containing manganese cations
  • C23C22/188—Orthophosphates containing manganese cations containing also magnesium cations

Definitions

• Zinc phosphate conversion coatings are applied to metal substrates to provide a base for paint adhesion and to inhibit the undercutting of paint in a corrosive environment.
• Phosphating compositions typically applied by immersion of the product to be coated in a bath solution or by spraying, commonly have been used in the form of acidic, aqueous solutions typically containing phosphate ions, an oxidizing agent, and divalent, layer-forming metal cations.
• the layer-forming ions typically included zinc used alone or in combination with cations of barium, calcium, cobalt, manganese, magnesium, nickel, lithium, and other metals.
• the phosphate ions are commonly introduced by use of phosphoric acid.
• the oxidizing agents are commonly inorganic compounds, often consisting of the salt of one of the above-mentioned metals or of sodium or ammonia. Much of this art, as used today, has changed little over the years. B.
• U.S. Patent 3,810,792 to Ries teaches a process for applying phosphate coatings on steel, iron or zinc from an aqueous, acidic solution containing divalent, layer-forming metal cations, wherein 59 to 100 mole percent are niclcel cations and the remaining 0 to 41 mole percent are cations other than nickel cations, e.g., zinc cations.
• Example 1 This invention is exemplified in Example 1 with nickel cations comprising 100 mole percent of the divalent, layer-forming metal cations and in Examples 2, 3 and 4 with nickel cations comprising between 68 and 69 mole percent of the divalent, layer-forming cations.
• the amorphous type coating is a mixed phosphate composed principally of zinc and one of the metals selected from ...lithium, beryllium, magnesium, calcium, strontium, cadmium and barium" (column 2, lines 14-17). Hendricks states at line 65 of column 3 that "the chemical analysis of these amorphous type coatings reveals that they are mixed phosphates with the metal of the recited group occurring therein in the ratio of about one-half mole thereof to each mol of zinc" Hendricks further states at line 34 of column 5 that the amount of modifier metal must be adjusted so that lighter metals such as lithium and magnesium nitrates are added in greater quantities than the nitrates of the heavier elements such as barium.
• the minimum concentration for magnesium nitrate is given in the lower table in column 5 as 25.6 gr. /liter or 10 mole percent in a solution containing 3.2 g/l zinc.
• Special coating conditions for an embodiment using magnesium are set forth in column 12 at lines 58-60, i.e., the reader is instructed to operate the process at room temperature and the immersion time kept below one minute.
• Patent 4.231.812 to Paulus et al teaches a process for coating hot metal strips (above 250°C) with a phosphate film by quenching the heated strips in a phosphating bath having a temperature of 80°C or greater and containing one or more phosphates of the type Me (H 2 PO 4 ) n . wherein Me may be zinc, nickel. manganese, or alkali metal. No specific phosphating bath compositions are disclosed.
• U.S. Patent 4.053,328 to Oka et al teaches zinc phosphate solutions containing zinc ions in an amount, preferably, of at least 0.03 percent by weight and nickel ions in an amount of at least 0.01 percent by weight and in the nickel ion to zinc ion ratio of less than 1.89 to 1. This ratio may vary from 1.89:1 to 0.014:1.
• U.S. Patent 4,110,128 to Dreulle et al teaches a phosphate solution containing both zinc and nickel.
• the zinc is present in an amount equivalent to io to 50 grams of anhydrous zinc chloride and 0.5 to 20 grams of hexahydrated nickel chloride per liter of final solution may be added.
• 20 g. of anhydrous zinc chloride are used with 5 g. of crystallized hexahydrated nickel chloride.
• U.S. Patent 4,153,479 to Ayano et al teaches an acidic, oxidant-free, zinc phosphate which contains nickel.
• U.S. Patent 3,723,334 to J. I. Maurer teaches a process for decreasing the scale formation in zinc phosphate composition by adding a carbohydrate.
• Phsophating compositions disclosed include .1-50 g/l zinc and may contain 0.001 to 0.4 wt.% nickel.
• Example 3. the only example containing both zinc and nickel, the quantity of Zn ++ is given as 0.14 and the quantity of Ni ++ is given as 0.03. No unit of measurement is given for this example. Assuming that the unit either wt.% or g/l, the nickel component comprises less than 50 mol.% of the zinc/nickel component of the bath.
• U.S. Patent 2.554.139 to R. F. Drysdale teaches acidic phosphate compositions 2. 3.
• U.S. Patent 4.265,677 to Muller et al relates to a special phosphating solution to be used prior to cathodic electropainting and is concerned with-the ratio of zinc to fluoroborate.
• a zinc/nickel phosphate solution is disclosed in Example 1 which contains 0.69 g/l:Zn and 0.38 g/l:Ni.
• U.S. Patent 3,520,737 to Gerassinoff et al teaches a method for applying an aqueous acidic zinc phosphate solution which may contain such as a metallic catalyst nickel, cobalt, or copper in the form of soluble salts in small amounts such as 0.0025% nickel. Somewhat larger amounts of copper and/or nickel and/or cobalt. For heavier coating weights, they advocate use of these metals in amounts exceeding 0.001%. It is also disclosed that when these metals are present in greater amounts of 0.0055%-0.0165% autocatalytic nitrite-formation is promoted.
• Oppen et al U.S. Patent 4,264,378, teaches a process for preparing metal surfaces with a phosphating liquid, containing at least one metal cation of valence two or greater (calcium, magnesium, barium, aluminum, zinc, cadmium, iron, nickel, cobalt and manganese), and contains at least one ion selected from molybdate. tungstate. nandate, niobate, and tantalate ions.
• a phosphating liquid containing at least one metal cation of valence two or greater (calcium, magnesium, barium, aluminum, zinc, cadmium, iron, nickel, cobalt and manganese), and contains at least one ion selected from molybdate. tungstate. nandate, niobate, and tantalate ions.
• a bath containing 6.5 g/l zinc and 5.5 g/l nickel, i.e., the nickel component comprised less than 50 mol.% of the combined
• 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 electrons.
• Accumulation of the hydroxyl ions results in the generation of liquid of very high basicity having a pH as high as 12.5 or more.
• Conventional zinc phosphate is soluble in this high basic liquid.
• the paint is undercut, and a disbond between the paint film and the substrate (sheet steel) in the car body results. If the paint adjacent to the scratch is then removed, as by pulling a tape applied to the scratched area, 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 taping, in actual use, the paint would have flaked off eventually where it has been undercut by alkaline dissolution of the phosphate film. The steel surface will then begin to rust by the mechanism common to rusting of bare steel.
• 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. (b) the use of an inhibitor rinse such as chromic acid, which has been only partially successful in reducing the corrosion sensitivity, and (c) the use of finer phosphate crystal structure to provide a more uniform coating. These approaches have not significantly reduced the alkaline sensitivity of phosphate films.
• This invention relates to a method for increasing the resistance to alkaline dissolution of a phosphate conversion coating on a corrodible metal substrate, thereby decreasing corrosion sensitivity, and to products of said method.
• the coating is deposited by chemical reaction between the substrate and an acidic, aqueous solution containing two or more layer-forming, divalent metal cations and phosphate ions.
• iron cations will enter the film whenever the substrate to be coated is a ferrous metal.
• Cations from other contaminating sources also may enter the coating. All statements herein with respect to cation composition of a deposited phosphate coating will be made exclusive of iron and cation contaminating sources.
• first divalent metal cations whether derived froa a single metal or from a mixture of metals.
• the method is characterized by (a) selecting the first divalent metal cation from cations of magnesium, cations of transition metal having a hydroxide which has a lower solubility in an alkaline solution (NaOH) than zinc hydroxide and from a mixture of any of such cations; (b) selecting zinc cations as the second divalent metal cations; 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 comprises at least about 15.0 mole percent and not greater than about 43 mole percent of the total first and second divalent cations in the coating.
• the zinc cations which comprises at least about 15.0 mole percent and not greater than about 43 mole percent of the total first and second divalent cations in the coating.
• the zinc cations which comprises at least about 15.0 mo
• solubility of the hydroxides of the metal exemplified herein in a pH 12.50 NaOH solution in units of micromoles per liter are zinc 66.0, nickel 1.91. manganese 0.26, cobalt 0.25, and magnesium 0.009. These values were calculated from the solubility equations separately set forth for the different metals in "Atlas of Electrochemical Equilibrium in Aqueous Solutions" by M. Pourboix. 2nd edition, copyright 1974, published by National Association of Corrosion Engineers, Houston, Texas.
• magnesium or a transition metal having a hydroxide which has a lower solubility in an alkaline solution than zinc hydroxide is of importance in the corrosion resistance of the deposited coatings, i.e., its resistance to alkaline dissolution.
• Divalent or trivalent lanthanides which will go into the aqueous, acidic solution in sufficient quantity or can be made to go into solution in sufficient quantity by means known in the art can be substituted for first divalent metal cations.
• Lanthanides are preferably employed in admixture with first divalent metal cations, e.g., nickel, magnesium, cobalt, and/or manganese cations, with a major amount of first divalent metal cations employed with a minor amount of lanthanide cations.
• the first divalent metal cations are nickel cations and are controlled to be about 84 to about 94 mole percent of the total zinc/nickel divalent cations present in the coating bath.
• Zinc is preferably present in the solution in an amount of at least 0.2 g/l as Zn + of said solution.
• the deposited coating will preferably be constituted of a nodular mixed-metal phosphate which we believe to contain Zn 2 Ni(PO 4 ) 2 .4H 2 O, but with some nickel variation within a limited range.
• the first divalent metal cations are selected from cations of cobalt, cations of manganese, cations of magnesium, or mixtures of cations of two or more of nickel, cobalt. manganese, and magnesium.
• the deposited coating will be constituted of a mixed-metal phosphate of these and phosphate ions.
• the concentration in the coating bath is controlled to be about 84 to about 96, preferably 84 to 94 and most preferably 90 to 94, mole percent of the total zinc/magnesium divalent metal cations present in the coating bath.
• the concentration in the coating bath is controlled to be about 45 to about 96, preferably about 84 to about 94 mole percent of the total zinc/manganese divalent metal cations present in the coating bath.
• the concentration in the coating bath is controlled to be about 65 to about 95, preferably about 75 to about 94 and most preferably about 84 to about 94, mole percent of the total zinc/cobalt divalent metal cations present in the coating bath.
• first divalent metal cations When two or more first divalent metal cations are used, relative concentrations can be determined from the concentration data herein disclosed and demonstrated for individual metals used in conjunction with the examples set forth herein for metal cation mixtures and routine experimentation.
• the substrate is preferably exposed to the phospating solution for a sufficient time and at a sufficient temperature and pH (i.e. 30-120 seconds, 100-140°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 mole ratio range of the first and second metal cations is most prefe y in the range of about 5.2:1 to about 16:1.
• 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 having at least 15 mole percent (13.7% by weight) nickel, the film providing at least a substantial improvement in corrosion resistance for any metal article coated with known phosphate films and subsequently painted.
• the improvement constitutes at least a doubling of corrosion resistance.
• the phosphate film results from the use of an acidic, aqueous coating solution havng an oxidizing agent content and a pH effective to chemically react with the article and a solution 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 at least 0.2 g/l of the solution as Zn +2 ; and (b) phosphate ions in an amount at least sufficient to form dihydrogen phosphate with said metal cations.
• the article is comprised of a metal selected from the group comprising iron, aluminum, zinc and their respective alloys. This process is particularly advantageous when employed on steel articles which carry a total surface carbon content greater than 0.4 mg/ft 2 .
• a portion of the phosphate anions needed to solubilize the first and second divalent metal cations may be other anions such as nitrates, sulfates and other anions known to those skilled in the art.
• 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 10-60.
• the number of points of free acid is the number of ml of 0.1/N NaOH required to titrate a 10 ml sample to a brom phenol blue end point and the number of points of total acid is the number of ml of 0.1/N NaOH required to titrate a 10 ml sample to a phenolphthalein end point.
• the solution preferably contains a fluoride selected from the group consisting of a simple fluoride, fluoroborate, fluorosilicate, or other complex fluoride.
• the phosphate solution be maintained at a pH of 2.5-3.5 and contain oxidizing agents in sufficient amount.
• Oxidizing agents typically used in this art and which are suitable for use with this invention include, but are not limited to nitrite, chlorate, nitrate, peroxide, aromatic nitro compounds, and combinations thereof.
• the preferred temperature is maintained at 100-140°F (38-60°C) during the phosphating contact.
• the exposure of the metal article to such solution should be preferably for a time of 30-120 seconds. In particular applications, higher and lower temperatures and much shorter and much longer exposure times may be preferred.
• the nitrite, used as an accelerator be used in an amount of 0.5-2.5 points or 0.03-0.15 g/l of solution as NaNO 2 .
• the number of points of nitride in the phosphate bath is the number of ml. of 0.042N KM n O 4 . required to titrate a 25 ml. sample to a permanent pink color.
• the bath will become enriched with nickel, since more zinc than nickel is contained in the phosphate coating.
• the replenishment solution or concentrate should be formulated to maintain the nickel concentration of the bath in the preferred range of 85 to 94 mol percent of the total zinc/nickel divalent metal cations.
• a portion of the nickel in the principal coating solution may be displaced by cations of one or more divalent, layer-forming metals selected from the group consisting of cobalt, manganese, and magnesium.
• 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 magnesium and/or a transition metal having a hydroxide which has a lower solubility in an alkaline solution than 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 nodular crystalline structure.
• 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 ste'el 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 spectra of phosphate coatings prepared in phosphate baths having various nickel contents.
• Figure 18 is a scanning electron microscope photograph of a coating (at 1500X magnification) prepared in a phosphate bath containing 97.0 mole percent nickel.
• Figure 19 shows corrosion tested panels corresponding to a phosphating bath with a nickel content of 97.0 mole percent nickel.
• Figure 20 is a scanning electron microscope photograph of a coating (at 1500X magnification) prepared in a phosphate bath containing 90.1 mole percent nickel.
• Figure 21 shows corrosion tested panels corresponding to a phosphating bath with a nickel content of 90.1 mole percent nickel.
• Figure 22 is a scanning electron microscope photograph of a coating (at 1500X magnification) prepared in a phosphate bath containing 99.0 mole percent nickel.
• Figure 23 shows corrosiontested panels corresponding to a phosphating bath with a nickel content of 99.0 mole percent nickel.
• Corrosion in an aqueous electrolyte is an electrochemical process involving oxidation and reduction reactions.
• the oxidation reaction is the anodic dissolution 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 electrolyte 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 accessibility of the adjacent rust-free steel surfaces to atmospheric oxygen. The important consequence of the differential oxygen concentration cell is the generation of a highly alkaline electrolyte at the phosphate/steel interface.
• the electrolyte pH can rise to above 12 in a sodium chloride environment, a condition prone to produce undercutting of the primer coating.
• the undercutting by the alkali sodium hydroxide is due principally to the dissolution of some of the zinc phosphate coating (and to a lesser extent the saponification of the reactive ester groups present in some primer resins).
• the pH of the electrolyte will normally be lowered by dilution with water to more neutral values and the newly exposed metal will begin to corrode.
• 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.
• Carbon contamination 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 established by the prior art that there is a correlation between surface carbon contamination and salt spray performance. 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.
• 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 temperature and pH to chemically react and deposit such film.
• the solution comprises first and second layer-forming, divalent metal cations, as aforedescribed, phosphate ions and an oxidizing agent.
• the amount of first and second divalent metal cations is controlled to provide a first divalent metal cation content in the resulting film of at least 15 mole percent of the total cations.
• the phosphate film results from the use of an acidic, aqueous coating solution having an oxidizing agent and a solution pH effective to chemically react with the article or substrate.
• the solute content of the solution preferably contains (a) divalent, layer-forming metal cations consisting of 84-94 mole percent nickel of the total zinc/nickel metal cations and zinc in a minimum amount of about .2 g/l of said solution as Zn +2 ; and
• phosphate ions in an amount at least sufficient to form dihydrogen phosphate with said metal cations or as aforedescribed other anions substituted for a portion of the phosphate ions.
• a portion of the nickel content may be substituted by use of a divalent. layer-forming metal cations selected from the group consisting of cobalt, magnesium and manganese.
• the metal article is cleansed by use of an alkaline cleaner containing a titanium conditioning compound at a temperature of 100-140°F. The article is subjected to the alkaline cleaner for a period of about 30 to about 120 seconds and then rinsed with water at a temperature of about 100 to about 140°F for a period of 30-120 seconds.
• an alkaline cleaner followed by a water rinse which contains a titanium conditioning compound.
• the alkaline-cleansed metal substrate is then sprayed or immersed in a phosphate bath solution maintained at a temperature of about 100 to about 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 total acid/free acid ratio of 10-60.
• the pH is preferably maintained at 2.5-3.5 and nitrites are present in the bath in an amount of about 0.5 to about 2.5 points.
• excess solution is removed from the article or substrate by a rinsing sequence consisting of a water rinse at ambient to about 100°F for about 30 to about 120 seconds, an inhibitor rinse which contains a chromate or other dissolved corrosion inhibitor at ambient to about 120°F for about 30 to about 60 seconds, and a deionized water rinse at ambient temperature for about 15 to about 30 seconds.
• the phosphating solution should contain nickel cations which constitute at least about 84 mole percent of combined metal cations (about 82.5% by weight) in the solution.
• the zinc cation of the phosphating solution be at least .2 g/l as Zn +2 or .79 g/l as Zn(H 2 PO 4 ) 2 .
• the molar ratio of Ni/Zn is advisedly in the range of 5.2:1 to 16:1.
• zinc is 16 mole percent of the nickel /zinc cation content, there must also be at least .2 g/l as Zn +2 in solution.
• the nickel content therefore must be at least 1.0 g/l of the bath solution (84 mole percent of the nickel/zinc total).
• This interrelationship, between minimum zinc and high nickel content in the nickel embodiment is essential to producing the phenomenon of this invention, which is believed to lie in the formation of a mixed-metal phosphate structure containing zinc and either magnesium or a transition metal whose hydroxide has a lower solubility in an alkaline solution than iron or zinc hydroxide.
• infrared spectra establish that the structure of phosphates formed from high nickel baths is different from those coatings formed from baths having less than about 84 mole percent nickel.
• the scanning electron microscope photographs further establish that there is an abrupt change in morphology for phosphates formed from baths having a nickel content above about 84 mole percent.
• 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 phosphoric acid. Concentrates of zinc ions and concentrates of the first divalent metal ions may be prepared separately.
• Separate divalent metal cation concentrates preferably contain 100 to 150 g/l of the first divalent metal ions when the divalent metal is nickel or cobalt, 90 to 140 g/l when the divalent metal is manganese and 40 to 65 g/l when the divalent metal is magnesium.
• a combined concentrate is prepared, one preferably adds 11 to 20 g/l of zinc ions to such separate concentrates.
• These metal phosphate concentrate solutions can be used to make up a fresh phosphate bath by using sufficient quantities of either, a combined concentrate or of each of such separate concentrated solutions with water to render the desired bath concentrations as described previously.
• the ratio of nickel to zinc will increase because more zinc than nickel is deposited in formation of the phosphate film. It is desirable to have a separate concentrate which is formulated for replenishment of the starting concentrations of zinc and nickel in the bath.
• the substrate is preferably selected from the group consisting of iron, aluminum, zinc and their respective alloys.
• the phosphate solution preferably additionally contains 0.1-4.0 g/l in some embodiments preferably, 0.1-1.0 g/l fluoride ion to enhance the formation of zinc phosphate coating.
• 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 substate.
• the zinc/nickel phosphate conversion coating is the result of a chemical reaction with the substrate and has a nodular morphology (see Figures 10-13).
• the product of this invention is particularly characterized by a significantly improved salt spray performance showing little or no paint undercutting 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
• the coating has high corrosion resistance even with surface carbon contamination levels greater than mg/ft 2 . Also, for bimetal interfaces, e.g., steel/galvanized steel couple, the coatings of this invention have shown superior corrosion resistance.
• Examples A series of test panels for Examples 1-13 were prepared by cutting sheet metal into panels having a size of 4 x 12". 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 determination 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.
• taupe primer paint epoxy ester-melamine resin primer
• 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.
• "conventional alkaline cleaning” means that the alkaline cleaner contains a titanium compound.
• the number of points of total alkali is the number of ml of 0.1/N HCl required to titrate a 10 ml sample of cleaner to a brom cresol green end point.
• a phosphating bath solution was prepared having the following composition: 22.2 g/l Zn(H 2 PO 4 ) 2 1.08 g/l Ni(H 2 PO 4 ) 2 5.85 g/l H 3 PO 4 0.13 g/l NaNO 2 As formulated, this bath had a total acid concentration 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 constitutes 33.3 mole percent of the dissolved divalent cations, which is typical of compositions currently used commercially in spray applied phosphating systems.
• 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 conventional 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) deionized 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 about 1.62 g/m 2 (150 mg/ft 2 ).
• the phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melaraine resin based primer. After baking, the paint thickness was approximately 23 micrometers. 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 stipulates 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.
• Example 2 A phosphating bath solution was prepared having the following composition:
• this bath had a total acid concentration of 14.7 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.
• the dissolved nickel constitutes 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 20oC deionized water and dried in an oven at 82°C for five minutes. As in Example l, 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 about 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 equivalent to 6.2 mole percent Ni.
• a scanning electron microscope photograph of the phosphate coating on the Q steel, taken at 1500X magnification, is shown in Figure 6. This structure remains similar to the morphology of spray applied commercial zinc phosphate coatings.
• Example 3 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 micrometers. 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 panels designated F4. on the other hand, failed within 96 hours of salt spray testing (see Figure 4d).
• Example 3 Example 3
• a phosphating bath solution was prepared having the following composition:
• this bath had a total acid concentration 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.
• the dissolved nickel constitutes 67.7 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 f or two minutes wi th the above phospha te 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-t ⁇ eated with an inhibitor rinse.
• the steel panels that were phosphate coated by this procedure had a uniform, gray appearance and a coating weight of about 1.35 g/m 2 (125 mg/ft 2 ).
• 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 panels 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: 3.01 g/l Zn(H 2 PO 4 ) 2
• this bath had a total acid concentration 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.
• the dissolved nickel constitutes 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 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 about 1.14 g/m 2 (106 mg/ft 2 ).
• the phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example l. the paint film thickness, after baking, was approximately 23 micrometers. 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 failed within 72 hours of salt spray testing (see Figure 4h) .
• Example 5 A phosphating bath solution was prepared having the following composition:
• this bath had a total acid concentration of 14.2 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 constitutes 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 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.
• Example l The phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example l, the paint film thickness, after baking, was approximately 23 micrometers. 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
• a phosphating bath solution was prepared having the following composition:
• this bath had a total acid concentration 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.
• the dissolved nickel constitutes 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 about 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 equivalent to 13.6 mole percent Ni .
• a scanning electron microscope photograph of the phosphate coating, taken at 1500X magnification, 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 micrometers. 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
• a phosphating bath solution was prepared having the following composition:
• this bath had a total acid concentration 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.
• the dissolved nickel constitutes 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 about 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 equivalent 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 micrometers. 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 panels designated F4 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.
• Example 8
• a phosphating bath solution was prepared having the following composition:
• Example 1 0.13 g/l NaNO 2 . As formulated, this bath had a total acid concentration 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 constitutes 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.
• 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 1500X magnification, 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 micrometers. 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 undercutting from the scribe line (see Figure 9h) .
• a phosphating bath solution was prepared having the following composition:
• this bath had a total acid concentration 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 constitutes 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 about 0.86 g/m 2 (80 mg/ft 2 ).
• Example 11 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.
• 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 micrometers. 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: 1.71 g/l Zn(H 2 PO 4 ) 2
• this bath had a total acid concentration of 24.0 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 30.
• the dissolved nickel constitutes 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 60oC. 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 l, 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 about 0.62 g/m 2 (58 mg/ft 2 ).
• the phosphate coated Q and F4 steel panels were spray painted with an epoxy ester-melamine resin based primer. As in Example l, the paint film thickness, after baking, was approximately 23 micrometers. Salt spray testing was again carried out exactly as detailed in Example 1, with 3 mm undercutting of the paint f rom the scribe line aga in considered as failure.
• a phosphating bath solution was prepared having the following composition:
• 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 micrometers. 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: 0.63 g/l Zn(H 2 PO 4 ) 2 12.22 g/l Ni(H 2 PO 4 ) 2 1.10 g/l H 3 PO 4 0.11 g/l NaNO 2 As formulated, this bath had a total acid concentration 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 constitutes 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 var ied f rom about 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 magnification, is shown in Figure 15.
• 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 contaminated with carbon.
• This range of nickel contents which Examples 1 through 12, collectively, have shown to be characterized by a phosphate coating having a microstructure distinctly different from the microstructure of ordinary commercial zinc phosphate coatings, is from about 16 (Example 8) mole percent to about 27 (Example 11) to about 44 (Example 12) to about 95.2 mole percent nickel, corresponding to about 85 to about 92.3 mole percent dissolved nickel in the phosphating bath.
• Example 8 mole percent to about 27 (Example 11) to about 44 (Example 12) to about 95.2 mole percent nickel, corresponding to about 85 to about 92.3 mole percent dissolved nickel in the phosphating bath.
• 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 contaminat ion ; 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 contamination, 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, consistently 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.
• Example 13 will serve to illustrate the benefits obtained on substrates other than steel, particularly on the zinc surface of not-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 substrates in the previous examples.
• Example 13 will serve to illustrate the benefits obtained on substrates other than steel, particularly on the zinc surface of not-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 substrates in the previous examples.
• a phosphating bath solution was prepared having the following composition:
• this bath had a total acid concentration 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 constitutes 90.7 mole percent of the dissolved divalent cations.
• Grayish-black phosphate coatings with a weight of about 1.67 g/m 2 (155 mg/ft 2 ) were produced on the hot-dipped galvanized steel panels. Chemical analysis showed that the nickel content of the phosphate coatings 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 micrometers. 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.
• FIG. 16 illustrates the improvement in 360 hour and 480 hour salt spray performance for galvanized steel with high mole percent nickel in the phosphate coating ( Figures 16a and 16b) vis-a-vis commercial zinc phosphate containing a low nickel level ( 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.
• 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.
• Table II is a tabulation setting forth the physical characteristics of the coatings for the panels listed in Table I.
• Figure 17a is the spectrum typical of commercial zinc phosphate. corresponding to Example 1.
• 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.
• Figure 17d there is a gradual change in the spectrum, as shown in Figure 17d, confirming the change in structure illustrated in the scanning electron microscope photograph shown in Figure 10.
• a phosphating bath solution was prepared having the following composition: 0.52 g/l Zn(H 2 PO 4 ) 2
• this bath had a total acid concentration of 20.5 points.
• the bath acidity was then adjusted by the addition of NaOH to a free acid concentration of 0.65 points, resulting in a total acid to free acid ratio of 32.
• the dissolved nickel constitutes 97.0 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. As in Example 12, the steel panels that were phosphate coated by this procedure had a nonuniform, streaked and spotted appearance, and had a brownish color without the bluish-gray cast. Coating weight varied from 0.56 to 0.74 g/m 2 (52 to 69 mg/ft 2 ).
• 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 micrometers. 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 panel designated
• phosphating bath solution having the following composition: 1.75 g/l Zn(H 2 PO 4 ) 2
• this bath had a total acid concentration of 23.1 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 29.
• the dissolved nickel constitutes 90.1 mole percent of the dissolved divalent cations, which is similar to the composition described in Example 9.
• Steel panels of two types were selected for phosphating with the above phosphating composition.
• the second type of panel was cut from commercial auto body sheet, identified as F6.
• F6 This steel was more severly contaminated than the F4 steel referred to in previous examples. It was known to have surface carbon values in the range, of 8.8 to 17.3 mg/m 2 (0.82 to 1.61 mg/ft 2 ), and to be subject to early salt spray failure in tests with cathodic electrocoat primers applied over conventional zinc phosphates.
• Panels of Q steel and F6 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 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 about 1.08 g/m 2 ( 100 mg/ft 2 ).
• the phosphate coated Q and F6 steel panels were cathodically electrocated with an epoxy based primer.
• Salt spray testing of the test panel designated Q was discontinued after 1440 hours with essentially zero undercutting from the scribe line.
• Salt spray testing of the carbon-contaminated test panel designated F6 was also discontinued after 1440 hours with essentially zero undercutting of the paint from the scribe line.
• a phosphating bath having the following composition:
• this bath had a total acid concentration of 19.7 points.
• the bath acidity was then adjusted by the addition of NaOH to a free acid concentration of 0.65 points, resulting in a total acid to free acid ratio of 30.
• the dissolved nickel constitutes 99.0 mole percent of the dissolved divalent cations.
• the phosphate coated Q and F6 steel panels were cathodically electrocoated with the same epoxy based primer used on the panels described in Example 15.
• the paint film thickness, after baking, was approximately 22 micrometers.
• Salt spray testing was again carried out exactly as detailed in Example 1. with 3 mm undercutting of the paint from the scribe line considered as failure.
• salt spray testing on both the Q and the F6 panels was carried out for 1440 hours. Unlike the panels shown in Figure 21, however, neither of these panels passed the test. Failure was evident on both the Q panel, with a low level of carbon contamination, and the F6 panel, with a high level of carbon contamination.
• test panels of 4 x 12" size of the steels designated Q and F6, described in Examples 1 and 15 respectively were spray cleaned for two minutes with a conventional alkaline cleaner having a strength of 4 to 6 points and a temperature of 60°C, spray rinsed for 30 seconds with 60°C tap water, and spray phosphated for two minutes with the respective phosphate bath heated to 57°C.
• the panels were then spray rinsed with 20°C deionized water for one minute and dried in an oven at 82°C for five minutes.
• none of the phosphated panels were post-treated with an inhibitor rinse.
• phosphated Q steel panels were used for determination of composition, coating weight, and x-ray structure of the phosphate coating.
• the mole percent of metal divalent cations, exclusive of iron, in the phosphate coating and coating weight are listed in Tables III to V for each phosphate bath investigated.
• the phosphated F6 steel panels were painted with either a commercial cathodic electrocoat primer or an epoxy ester-melamine resin based spray primer, as indicated with the Tables.
• the samples, after baking, were then scribed in an "X" pattern to bare metal and then subjected to accelerated salt spray, cathodic delamination, and scab corrosion tests. Descriptions of these tests are as follows. Salt Spray Test
• the test was conducted in accordance with the ASTM B117 procedure. It consists essentially of exposing the scribed test panels to a mist of a 5% sodium chloride solution in an enclosed chamber maintained at 35°C.
• This test consists of immersing scribed test panels in an oxygen saturated 5% sodium chloride solution at room temperature and cathodically polarizing the samples with an electronic potentiostat to a constant potential of -1.05V versus a saturated calomel reference electrode. A carbon rod is used as a counter electrode.
• This test procedure produces a highly alkaline pH at the "X" scribe lines by the cathodic reduction of dissolved oxygen which can cause a loss of paint adhesion by chemically dissolving the phosphate coating. The smaller the amount of paint adhesion loss, the higher the resistance of the phosphate coating to dissolution in an alkaline solution; and conversely, the greater the paint adhesion loss, the lower the resistance of the coating to dissolution.
• tests are conducted for a time sufficient to allow differences in performance to be detected, which for this work was 14, 16 or 17 days.
• This test method has been used by R.R. Wiggle. A.G. Smith, and J.V. Petrocelli to study the alkaline resistance of phosphate coatings, published in The Journal of Paint Technology. 1968. entitled “Paint Adhesion Failure Mechanisms on Steel in Corrosive Environments", and by A.G. Smith and R.A. Dickie to study the alkaline resistance of paint primers, published in Industrial and Engineering Chemistry Product Research and Development.
• the kaolin coating simulates dust and dirt contamination, and serves as an agent for retaining salt solution on the normally hydrophobic paint surface. Additional details have been published in Ford Laboratory Test Method (FLTM) BI23-1 and the SAE paper by V. Hospadaruk et al, hereinbefore referenced to. After corrosion testing the painted and phosphated F6 steel panels for the times indicated in Tables III to V, the amount of paint adhesion loss (or undercutting) in millimeters from the scribe line was determined by taping. The results for each phosphate coating in the above corrosion tests are given in Examples 17-50.
• Figure 1 illustrates the improved alkaline dissolution resistance provided over the narrow range of nickel concentrations claimed in the application.
• the laboratory method used to evaluate the alkaline dissolution resistance consists of immersing a preweighed, 4 x 4", phosphate coated steel sample in a minimum of 600 ml of a sodium hydroxide solution of pH 12.50 at an ambient temperature of approximately 75°F for 40 minutes, removing the sample, and rinsing it with deionized water, followed by ethyl alcohol, and drying at 180°F for one minute. The sample is cooled to room temperature and weighed again to the nearest 0.1 mg precision. The sample is then immersed in 100 ml of inhibited.
• Examples 51A and B through 57A and B listed in Table VI make use of alkaline dissolution resistance as a key indicator of improved phosphate coatings containing mixtures of zinc and the various divalent cations.
• the specific phosphate bath compositions investigated are shown for each example. The total-acid, free-acid, and acid ratio values given are after adjustment of the bath acidity by the addition of sodium hydroxide. The mole percent of the dissolved divalent cations is also shown in Table VI.
• the letter A corresponds to a "high" phosphate bath composition of a specific combination of divalent cations other than zinc
• the letter B corresponds to a "low" phosphate bath composition of the same divalent cations other than zinc.
• the letter A corresponds to a "high" phosphate bath composition of a specific combination of divalent cations other than zinc
• the letter B corresponds to a "low" phosphate bath composition of the same divalent cations other than zinc.
• the coatings are characteristically amorphous.
• the corrosion performance was found to be inferior to that of coatings produced with bath formulations having the preferred bath composition range of 84 to 94 mole percent based on results obtained from scab corrosion testing which is considered representative of field corrosion of automobiles.

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  • 1984-01-06 EP EP19840900774 patent/EP0172806A4/fr not_active Withdrawn
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