PROCESS AND COMPOSITION FOR ZINC PHOSPHATE COATING

Abstract: The present invention relates to a composition and process for metal finishing involving the formation of zinc phosphate coatings of desired morphology on a ferrous surface. The inclusion of a hydroxylamine agent in the phosphating bath expands the range of zinc concentrations over which the desired coating morphology is obtained. Zinc and aluminum surfaces can also be coated with this composition and process.

Description: Current day phosphate coating solutions are dilute solutions of phosphoric acid and other chemicals which are applied to the surface of metals; the surface of the metal reacts with the solution and forms an integral layer (on the surface of the metal) of substantially insoluble amorphous or crystalline phosphate coating. Crystal-line coatings result if zinc or similar divalent metals other than ferrous ion are present. Depending on the characteristics of the coating, it may function to enhance corrosion resistance, wear resistance or electrical resistance; as a base for the application of a second coating (e.g., paint); or as a vehicle to retain a lubricant on the coated surface preparatory to cold forming. Certain of these solutions have achieved widespread commercial use. Such solutions typically include phosphate ions, zinc and/or manganese ions and typically one or more of the following ions: nickel, cobalt, copper, nitrate, nitrite, chlorate, fluoborate or silicofluoride. The art has been able to form phosphate coatings since about 1917, and there have been successive discoveries of the effects of the nitrate, copper, nickel, fluoborate, and silico-fluoride ions on the coating ability of such solutions made through the years. Presently, metal surfaces are typically provided with a phosphate coating by being treated in the following process sequence: (1) cleaning; (2) conditioning; (3) phosphating; and (4) post-treating. Rinses are usually employed between steps to avoid drag-in to the next stage. Such processes and solutions for forming conversion coatings on metal surfaces are well known and have been described, for example, in "Metal Handbook", Volume II, 8th Edition, pages 529-693 (1972), the contents of which are specifically incorporated herein by reference. Despite these advances, the best present day formulations are troublesome in certain respects and accordingly there is a continuing demand for still further improvements in the compositions and processes. In particular, known processes are difficult to control, form undesirably high coating weights, cause the formation of scale on processing equipment, and require replenishment as two or more separate additions. Such improvements are still being sought particularly in ferrous-, zinc- and aluminum-based substrates employed in automotive paint-base applications. It is recognized that there are distinct types of crystal morphologies that can be produced on ferrous surfaces by zinc phosphate coating solutions. The crystalline structure may be platelet, columnar or nodular in form when examined with an electron microscope. The platelet structure resembles relatively large plates or flakes of crystalline material. The columnar configuration resembles smaller column shaped crystals, and the nodular configuration resembles uniformly disposed small nodular shaped crystals. The latter two configurations are generally preferred for paint base applications on ferrous surfaces because they achieve equivalent or better performance with respect to paint adhesion and physical tests compared to the platelet configuration. The columnar and nodular coatings are also lower in coating weight which is beneficial where cathodic electropainting is to be employed. It has been recognized that the columnar and nodular forms are obtained by employing baths with zinc concentrations at relatively low levels. See for example U.S. 4,330,345 and U.S. 4,419,199. One of the problems encountered is that if the zinc levels increase in the bath, for example due to dissolution of zinc from galvanized parts or due to process control problems, the form of the coating will rapidly change to the less desirable platelet morphology. It would therefore be desirable to have a coating bath and process which broadens the range of acceptable zinc levels which will still form the desired columnar and/or nodular coatings. Another problem inherent in other phosphating processes is the formation of scale on heat exchangers and on the piping and related equipment used to circulate and/or apply the solution to the parts which must be periodically removed to maintain both heating efficiency and/or coating quality. Moreover, many processes currently employed use nitrite as accelerator and therefore require the replenishment of the phosphating solutions from two separate replenishing packages because nitrite would decompose in the concentrated acidic replenisher. Finally, modern phosphating processes must be capable of successfully forming a phosphate coating on galvanized and aluminum surfaces in addition to ferrous surfaces. EP-A-0 261 597 discloses a method of phosphate conversion coating metal subtrates from a group consisting of steel, zinc-coated steel and aluminum. The method employs an aqueous zinc phosphate coating solution and forms a crystalline coating. The zinc concentration in the solution is between 0.03 and 0.1 % by weight. The coating solution may contain hydroxylamine sulfate. Example 15 also discloses a zinc-to-phosphate ratio of 0.12 and that the coating solution used contains mangangese, nickel, nitrate and fluoride ions. The coating solution may be sprayed on the substrate and was applied at a temperature of between 38-60 °C. An opitional embodiment of the method is to treat the metal surfaces with a titanium-containing conditioner prior to the phosphating treatment. After the phosphating treatment the metal substrate was treated with a chromium-containing sealing solution. EP-A-0 261 597 was published on March 30, 1988. Hydroxylamine agents have been disclosed for use in certain zinc phosphate solutions. For example, U.S. Patent No. 2,743,204, discloses a metal (iron, zinc and manganese) phosphate coating solution having a pH of about 1.9 to about 3.5. The patentee states that the coating weight resulting from such conventional aqueous acidic phosphate solutions may be increased by the addition of small quantities of certain organic chelating agents. Hydroxylamine is recited as one of many specific oxidizing agents which can be used in such coatings. This patent is directed only to heavy phosphate coatings desirable for base corrosion resistance or cold forming purposes and does not contemplate paint base applications. The specific levels of zinc and hydroxylamine exemplified yield platelet morphology and the inventor does not recognize the potential benefits of the hydroxylamine to produce columnar and/or nodular coatings for paint base applications. U.S. Patent No. 2,298,280, discloses the use of hydroxylamine in a coating acid phosphate solution to accelerate the coating action of the solution. Again, the specific levels of zinc and hydroxylamine exemplified yield platelet morphology and the inventor does not recognize the potential benefits of the hydroxylamine to produce columnar and/or nodular coatings. U.S. Patent 4,149,909, discloses iron phosphate coating processes for applying a moderate coating weight on ferrous metal surfaces by spraying or dipping in the solution. The process employs a combination accelerator comprising hydroxylamine sulfate and an oxidizing agent such as a chlorate or a bromate. The resulting amorphous coatings do not relate to the crystalline coatings of a zinc phosphate system. U.S. Patent No. 4,003,761, discloses a process for applying a phosphate coating to a ferric surface by spraying. The patentee states that an improvement in the production of phosphate coatings by spraying acid solutions based on alkali metal and/or ammonium orthophosphate is accomplished by the addition of 0.05 to 1 grams per liter of a short-chain alklolamine and from about 0.01 to 1.5 grams per liter of a non-ionic wetting agent. The patentee further states that oxidizing or reducing agent accelerators can be employed; he includes hydroxylamine salts as one of the many groups of such compositions that may be employed. It is stated that the pH value of the solution is in the range of 4.3 to 6.5, that the duration of treatment for the spray is 0.5 to 5 minutes and that the process can be carried out at temperatures between 40°C and 95°C, preferably 50°C to 70°C. Amorphous coatings result from this process. U.S. Patent No. 2,702,768, discloses that the coating provided by "non-coating phosphate" solutions can be improved by employing hydroxylamine in the solution. "Non-coating phosphates" are specified as being alkali metal phosphates such as sodium phosphate and potassium phosphate, as well as ammonium phosphate. It is suggested that the hydroxylamine be used at a level of 0.1% to 0.5% and at a pH of about 4.2 to 5.8. Again, amorphous coatings are the result. U.S. Patent No. 3,615,912, discloses treating and coating solutions containing alkali- or ammonium-based orthophosphates with hydroxylamine being an optional ingredient. Amorphous coatings result from this process. U.S. Patent No. 4,220,486, describes an alkali phosphate conversion coating solution containing stannous ions and fluoride ions and optionally pyrazole compounds, hydroxylamine compounds and hydrazine compounds at a level of 0.2 grams per liter to about 5 grams per liter. This process does not produce a crystalline zinc phosphate coating. A treatment solution that attempts to control the crystal morphology is disclosed in EP-A-0,175,606, based upon FR-A-8412878. The use of a hydroxylamine agent is not suggested. In none of the prior art is there even a general teaching or suggestion that the use of hydroxylamine critically influences crystal morphology in any fashion. The present invention solves the foregoing problems in producing the desired paint-base coating through the inclusion of a hydroxylamine agent in the zinc phosphate solution, as can be seen from the independent Claims 1, 21 and 30. Further embodiments can be found in the claims being dependent thereupon. It has now been found that where predominantly columnar and/or nodular coatings are desired the inclusion of hydroxylamine, hydroxylamine salts or - complexes in a zinc phosphate solution enhances the process and broadens the range of zinc content at which the desired coating is obtained. Additionally, scaling of heat exchangers and process equipment is reduced and the solution can be replenished by a single package replenisher concentrate. The improved zinc phosphate type conversion coating solution and process of the present invention employ a hydroxylamine agent. The agent, when present in sufficient quantities, alters the morphology of the resulting coating from platelet to columnar and/or nodular and achieves this result over a broadened range of zinc concentrations. The zinc phosphate type conversion coating solutions to which this invention applies includes any such solution which will form a columnar and/or nodular coating on a ferrous surface. Any of the conventionally known additives for such solutions may be present unless they detrimentally affect the formation of a uniform coating of the desired morphology. For example, the presence of nitrite in substantial amounts would adversely affect the permissible range of zinc concentrations. The presence of hydroxylamine increases the maximum permissible zinc to phosphate ratio to about 0.125 to 1 with values as high as 0.27 to 1 being possible. The prior art has generally taught the maximum ratio for so called "low zinc" processes to be 1:12 or only 0.08 to 1. In terms of the zinc concentration, levels as high as 0.2 wt. % are permissible whereas in the prior art processes, platelet morphology results even at zinc levels well below 0.1 wt. % (1.0 g/l). The minimum amount of zinc according to the invention is 0.02 wt.%. For purposes of allowing a safety factor in controlling the process to obtain the desired morphology, a zinc level of from 0.045 to 0.11 wt. % is preferred. The expanded tolerance for zinc is important since control cannot always be tightly maintained in practice, especially where galvanized or partly galvanized parts are being treated in addition to the ferrous parts as claimed. Zinc content may increase as a result of the attack of the solution on the galvanized surface. The hydroxylamine can be added to the coating solution in any suitable form, and from any conventional source. The term "hydroxylamine agent", as used herein, means any compound that provides hydroxylamine or a derivative thereof such as a hydroxylamine salt or complex. Suitable examples include hydroxylamine phosphate, nitrate, sulfate, or mixtures thereof. More preferably, the hydroxylamine agent or source is a coating concentrate formulated with hydroxylamine sulfate ("HS"), a stable salt of hydroxylamine. Hydroxylamine sulfate may be represented by the formula (NH₂OH)₂ . H₂SO₄ or (NH₃OH)₂ . SO₄. Throughout this specification, quantities of hydroxylamine are expressed as hydroxylamine sulfate equivalent. An effective amount of hydroxylamine is to be employed in these coating baths. By the term "effective amount", as used herein, is meant an amount sufficient to cause the solution to produce a coating wherein the morphology is predominantly columnar and/or nodular as opposed to platelets. That is, when two substantially identical phosphate coating solutions or baths (differing only in that one contains an effective amount of hydroxylamine and the other does not) are compared, the solution with the effective amount of hydroxylamine agent produces predominant levels of nodular and/or columnar crystals on the surface of the ferrous article while the other does not. The solution employed in the process of the present invention contains a concentration of hydroxylamine agent (calculated as hydroxylamine sulfate equivalent) of from .05 to 5 wt. percent. Hydroxylamine sulfate levels are typically in the range of from .05 to 1 percent, more preferably 0.05 percent to 0.3 percent, still more preferably 0.1 percent to 0.3 percent. It has also been observed that the solutions of the present invention demonstrate reduced scaling under certain conditions. For example, under conditions where certain equipment (such as heat exchangers) is subject to excessive scaling, the substitution of the solutions and method of the present invention can show a dramatic reduction in the rate of scale formation, thus reducing maintenance requirements and improving heat transfer efficiency and coating quality. The solutions and compositions employed in processes and methods of the present invention may also contain ferrous ions either by deliberate addition or through etch-type buildup. Ferrous ions may be present at levels or in amounts up to the saturation point of the ferrous ion in the bath. Amounts or levels of ferrous iron ions within the bath may be typically in the range of 0.001 to 0.5 wt. % or preferably 0.005 to 0.05 wt. %. When present in sufficient concentrations, Fe⁺² increases the range of Zn⁺² which will produce the desired morphology at a given concentration of hydroxylamine agent. This is an added benefit of a hydroxylamine agent since it allows Fe⁺² to remain in solution, whereas in traditional nitrite baths the nitrite oxidizes the Fe⁺² to insoluble Fe⁺³. Phosphate may be employed at its art-established level. Preferred phosphate levels useful in the present invention are typically in the range of from .2 to 5 wt. percent, preferably 0.3 to 2.5 percent. Typically the total acid points of the bath will range from 12 to 37 with 13 - 22 most typical. The free acid points range from 0.1 to 1.0 with 0.3 to 0.4 being most typical. It will also be appreciated that it may be desirable to perform certain other select steps both prior to and after the application of the improved phosphate coating produced by the processes and method of the present invention. For example, it may be advantageous to take steps to see that the part, workpiece or other article to be coated is substantially free of grease, dirt, or other extraneous matter. This is preferably done by employing conventional cleaning procedures and materials. These would include, for example, mild or strong alkali cleaners, acidic cleaners, and the like. Such cleaners are generally followed and/or preceded by a water rinse. It is highly preferred to employ a conditioning step following or as part of the cleaning step, such as those disclosed in U.S. Patent Nos. 3,310,239; 2,874,081; and 2,884,351. These conditioning solutions typically employ condensed titanium compounds and preferably a condensed phosphate. For example, solutions comprising .0003 - .05% Ti (3-500 ppm Ti) and .01 - 2% sodium tripolyphosphate are suitable. In a highly preferred embodiment, such solution employs 3-25 ppm of titanium. The conditioning step serves to provide the surface with nucleation sites which serve to reduce the grain of the subsequent phosphate coating. After the coating is formed by application of this solution of this invention, it is advantageous to subject the coating to a post-treatment solution as conventionally taught. The solution may contain chromium (trivalent and/or hexavalent) or may be chromium-free. Chromium post-treatment solutions would include, for example, 0.025 to 0.1 wt. percent chromium (Cr⁺³, Cr⁺6, or mixtures thereof). Chromium-free rinses typically incorporate organic materials, zirconium, etc. and may also be employed. See for example, U.S. Patent Nos. 3,975,214; 4,376,000; 4,457,790; 4,090,353; 4,433,015 and 4,157,028. If the post-treated part is to be electropainted the surface is preferably rinsed with deionized water to avoid undue drag-in of chemicals into the paint tank. The phosphate processing conditions and solution parameters are selected to yield a coating weight of 0.77 to 2.22 g/m² (70 to 200 mg/ft²). Contact times commercially available extend from 3 seconds to 2 minutes or more preferably from 5 seconds to 2 minutes with 30 seconds to 2 minutes being common for processing parts on a conveyor. The phosphating solution is typically maintained in the range of 32 to 93°C (90 to 200°F) the specific temperature selected so as to achieve the desired coating weight in the allowed contact time. The composition and process of the present invention may be employed not only on ferrous metals and their alloys as claimed but also on surfaces of zinc or aluminum and their alloys. Single package replenishers containing a hydroxylamine agent have been prepared and found stable under a broad range of temperatures. In order to further illustrate the benefits and advantages of the present invention, the following specific examples are provided. It will be understood that the examples are provided for illustrative purposes and are not intended to be limiting of the scope of the invention as herein disclosed and as set forth in the subjoined claims. EXAMPLE I The following materials were combined in a 18.9 ℓ (5 gallon) bath: 451 g 75% H₃P0₄ 32.4 g Reagent grade HN0₃ 23.6 g Zn0 68.5 g nickel nitrate solution (equivalent to 29.6 g Ni(N0₃)₂) 118 g Na₂C0₃ Standard titrations yielded a total acid of 19.2 points and a free acid of 0.3 points. The bath was aged by spray processing two racks of eight 10 x 30.48 cm (4" x 12") cleaned and conditioned cold rolled steel panels. A rack of test panels was then processed for 1 minute at 44°C (111°F). At this point, the bath contained no hydroxylamine. Nine grams of hydroxylamine sulfate (HS) were then added to the bath, and several racks were spray processed to age the bath. The hydroxylamine sulfate concentration was then adjusted and cold rolled steel panels were processed for 60 seconds at the following HS levels: 0.05 - 0.06%, 0.07 - 0.08%, 0.12 - 0.13%. A bath analysis showed 0.044% Ni; 0.07% Zn; and 1.48% P0₄. Coating morphology and coating completeness are shown in Table I. The results show that hydroxylamine was necessary for coating formation under the processing conditions employed. They also show that increasing the HS concentration caused the morphology to change from platelet to columnar. RESULTS OF EXAMPLE I Solution % HAS Coating Wt. g/m² (mg/ft²) Morphology Coating Completeness 1A 0 Nil --- --- 1B .05-.06 1.81 (163) Platelet Incomplete 1C .07-.08 1.75 (158) Platelet Incomplete 1D .12-.13 1.36 (123) Columnar Complete EXAMPLE II A phosphating bath is prepared containing the following: Zn⁺² at about 0.05%; P0₄⁻³ at about 1.4%; Ni⁺² at about 0.05%; hydroxylamine sulfate at about 0.2%. The total acid is 20.0 points and free acid 0.3 points. A cleaned and conditioned cold rolled steel panel is spray processed at 46°C (115°F) for 60 seconds spraying time at a reduced spray pressure. The resulting coating is nodular and has a coating weight of about 1.27 g/m² (115 milligram per sq. ft.). Additions of zinc acid phosphate are made to the bath, with each addition increasing the Zn⁺² by 0.02%. After the second addition, the crystal morphology is changed from substantially nodular to thick platelet. Adding 12 g of ferrous sulfate to a 19 ℓ (5 gallon) bath causes the morphology to become a mixture of nodules and columnar crystals. At this point, the bath analysis shows Zn⁺² at about 0.09% and P0₄⁻³ at about 1.70%. EXAMPLE III A phosphating bath was prepared to contain the following: Zn⁺² at 0.075%, P0₄⁻³ at 0.83%, Ni⁺² at 0.042%, F⁻ at about 0.08%, Fe⁺² at 0.02%, and hydroxylamine sulfate at 0.5%. The total acid was 22.6 points and free acid 0.8 points. Cleaned and conditioned cold rolled steel panels were spray processed at 54°C (130°F) for 60 seconds spraying time. The resulting coating was nodular and had a coating weight of 1.15 g/m² (104 mg/ft²). The zinc concentration was then raised to 0.085% Zn⁺² using zinc acid phosphate. Coatings produced at the higher zinc level had columnar crystals and a coating weight of 1.27 g/m² (115 mg/ft²). Ferrous sulfate was then added to increase the Fe⁺² to 0.04%. Increasing the Fe⁺² caused the coatings to revert to nodular.

Claims: A metal finishing process comprising producing a predominantly nodular and/or columnar crystalline zinc-iron-phosphate coating on a ferrous surface by contacting an aqueous zinc phosphate conversion coating solution wherein the zinc/phosphate weight ratio of the solution is less than 0.27, and the zinc concentration is from 0.02 to 0.2 wt. %. with said surface, wherein the desired coating is formed by adding hydroxylamine hydroxylamine salts or complexes providing a hydroxylamine concentration from 0.05 to 5.0 wt. % to the solution to produce said crystalline structure. The process of Claim 1 wherein the hydroxylamine concentration is from 0.1 to 1.0 wt. %. The process of Claim 1 wherein the zinc concentration is from 0.045 to 0.11 wt. %. The process of Claim 1 wherein the solution additionally comprises ferrous ion. The process of Claim 4 wherein the ferrous ion is present in a concentration of from 0.001 to 0.5 wt. %. The process of Claim 5 wherein the ferrous ion is present in a concentration of from 0.005 to 0.05 wt. %. The process of Claim 1 wherein the ferrous surface is subjected to a conditioning treatment with a titanium containing conditioner prior to contact with the phosphatizing solution. The process of Claim 7 wherein the conditioner also contains a condensed phosphate. The process of Claim 1 wherein the solution is substantially free of chlorate and nitrite components. The process of Claim 1 wherein the zinc phosphate conversion coating solution additionally comprises at least one component selected from the group consisting of manganese, nickel, nitrate and simple or complex fluoride ions. The process of Claim 1 wherein the conversion coated surface is subsequently contacted with a post-treatment solution. The process of Claim 11 wherein the post-treatment solution is chromium-free. The process of Claim 11 wherein the post-treatment solution contains hexavalent chromium. The process of Claim 1 wherein the phosphate concentration is from 0.3 to 2.5 wt. %. The process of Claim 1 wherein the solution is sprayed on the ferrous surface. The process of Claim 1 wherein the solution is contacted with the ferrous surface for a time and at a temperature sufficient to produce a coating weight of from (70 to 200 mg/ft²) 0.77 to 2.22 g/m². The process of Claim 1 wherein the temperature of the solution on contact is from (90 to 200°F) 32 to 93 °C. The process of Claim 1 wherein the contact time is from 5 seconds to 2 minutes. The process of Claim 1 wherein the surface is subsequently painted. The process of Claim 19 wherein the painting is accomplished by cathodic electrodeposition. An aqueous zinc phosphate solution wherein the zinc/phosphate weight ratio of the solution is less than about 0.27, and the zinc concentration is from 0.02 to 0.2 wt. %. which produces a predominantly columnar and/or nodular crystalline coating over a broadened range of zinc concentrations when contacted with a ferrous surface comprising hydroxylamine hydroxylamine salts or complexes providing a hydroxylamine concentration from 0.05 to 5.0 wt.% to form said desired coating. The solution of Claim 21 wherein the hydroxylamine concentration is from 0.1 to 1.0 wt. %. The solution of Claim 21 wherein the zinc concentration is from 0.045 to 0.11 wt. %. The solution of Claim 21 wherein the solution additionally comprises ferrous ion. The solution of Claim 24 wherein the ferrous ion is present in a concentration of from 0.001 to 0.5 wt. %. The solution of Claim 25 wherein the ferrous ion is present in a concentration of from 0.005 to 0.05 wt. %. The solution of Claim 21 wherein the solution is substantially free of nitrite. The solution of Claim 21 wherein the zinc phosphate type conversion coating solution additionally comprises at least one component selected from the group consisting of manganese, nickel, nitrate and simple or complex fluoride ions. The solution of Claim 21 wherein the phosphate concentration is from 0.3 to 2.5 wt. %. A stable single package replenishing composition comprising an aqueous solution of zinc, phosphate and hydroxylamine hydroxylamine salts or complexes wherein the net solids concentration is at least 15 wt. %.

Inventors: KRAMER, LINDA S.
Applicants: HENKEL CORPORATION (A DELAWARE CORP.)
Application year: 1988
Main classification: C23C22
IPC codes: C23C22