ZINC SULFATE METAL ACID PRETREATMENT

Abstract

A phosphate-free aqueous acidic conversion coating comprising zinc sulfate and sulfuric acid is described. This conversion coating is particularly useful as a primer on metal substrates prior to the application of a cold-forming lubricant or a paint. Conversion coatings described herein avoid the problems associated with phosphate-containing conversion coatings while providing similar benefits to phosphate-based conversion coatings. Optional components of the conversion coating solutions include one or more pH-adjusting neutralizers, coating stabilizers, coating accelerators, coating refiners, etching agents, paint adhesion promoters, and mixtures thereof. Manganese oxide may be used as an additional option instead of zinc oxide.

Description

ZINC SULFATE METAL ACID PRETREATMENT FIELD DESCRIPTION This description generally relates to pretreatment solutions for metals, and more particularly to a phosphate-free sulfate-based zinc oxide treatment solution. BACKGROUND OF THE DESCRIPTION This section provides background information which is not necessarily the prior art to the inventive concepts associated with the present description. In metalworking, it is common practice to apply a lubricating coating to metal before cold forming. It has been standard practice in the industry to first deposit a layer of microcrystalline zinc phosphate onto the metal surface before applying a lubricating coating. Lubricants are typically polymeric in nature, oil-based, or fatty acid soaps, such as sodium stearate. Threaded bolt fasteners are frequently formed using a cold forming process. During cold forming, the wire used to manufacture the bolts is first lubricated before the forming procedures. A popular layered lubricating coating for fasteners involves first depositing a zinc phosphate conversion coating onto the surface of the steel wire and then applying a lubricant. The chemistry and surface structure of the phosphate coating are particularly advantageous to the integrity and performance of the sodium stearate or polymer-based lubricant applied over it. Recently, the fastener industry has been seeking a phosphate-free lubricating process for cold forming. Residual phosphate on the steel surface of the fastener is believed to cause hydrogen embrittlement in heated post-forming operations used to harden the bolt.This condition is believed to reduce bolt strength and cause premature fastener failure. Therefore, there is a desire to develop an alternative zinc phosphate pretreatment technology to conventional phosphate-free pretreatments to mitigate this problem. Furthermore, many environmental regulatory agencies are regulating phosphate-containing compositions, and a phosphate-free metal pretreatment could meet these new environmental regulatory requirements. Consequently, some manufacturing operations, such as cold forming or paint pretreatment, prefer phosphate-free chemistry to meet their wastewater regulatory requirements. Finally, there is a desire to improve under-paint corrosion protection in some applications compared to that provided by conventional zinc or transition metal phosphate pretreatments. In summary, it is desirable to provide: alternative phosphate-free zinc phosphate coatings to conventional ones used for cold forming technology to address phosphate-induced hydrogen embrittlement of steel and steel fasteners; phosphate-free metal pretreatments to meet environmental regulatory requirements; and pretreatments that result in improved corrosion protection under paint. BRIEF DESCRIPTION OF THE DESCRIPTION This section provides a general summary of the present description and is not intended to be interpreted as a comprehensive description of its full scope or all of its features, aspects, and objectives. A conversion coating composition according to the present description is completely phosphate-free and comprises a mixture of sulfuric acid and zinc oxide in an aqueous solution with a pH of 4.0 to 6.0. The conversion coating composition optionally includes one or more of: a pH-adjusting application bath or neutralizer solution to maintain the pH in the range of 4.0 to 6.0; a concentrated stabilizer comprising nitric acid; a coating accelerator; a coating refiner; an etching agent; and a paint adhesion promoter. Preferably, the conversion coating application bath or solution comprises sulfuric acid, zinc oxide, nitric acid, a pH-adjusting application bath or neutralizer solution, a coating accelerator, and a coating refiner.Once applied to a substrate, the conversion coating composition forms a coating comprising zinc sulfate on the substrate surface. This coating is highly beneficial in retaining lubricants used in cold forming processes and in adhering paint to the substrate. These and other features and advantages described here will become more apparent to those skilled in the technique after reading the detailed description. The accompanying drawings are described below. BRIEF DESCRIPTION OF THE DRAWINGS NONE. DETAILED DESCRIPTION OF THE DESCRIPTION In the following description, the details are set out to provide an understanding of the present description. For the sake of clarity, example aspects are described herein to limit the scope of the description to those skilled in the relevant art. Numerous specific details are set forth, such as examples of specific components, devices, and methods, to provide a comprehensive understanding of various aspects of this description. It will be evident to those skilled in the art that specific details need not be discussed herein, such as well-known processes, well-known device structures, and well-known technologies, as they are already well understood by those skilled in the art, and that the example modalities can be incorporated in many different ways and should not be considered as limiting the scope of the description. The terminology used herein is for the purpose of describing particular example aspects only and is not intended to be limiting. As used herein, the singular forms a, one, and the may be proposed to include the plural forms as well, unless the context clearly indicates otherwise. The terms comprise, comprising, including, and having are inclusive and thus specify the presence of stated features, whole numbers, stages, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, whole numbers, stages, operations, elements, components, and / or groups thereof. The method stages, processes, and operations described herein are not to be considered as necessarily requiring performance in the particular order discussed or illustrated, unless specifically identified as a performance order.It will also be understood that additional or alternative / Q / cen / Lznz / q / YiAi stages may be employed. The following abbreviations are used throughout this specification and these claims. The term gram is abbreviated as gm; the term milligram is abbreviated as mg; the term milliliter is abbreviated as mi; the term degrees Celsius is abbreviated as °C; the term degrees Fahrenheit is abbreviated as °f; a millimolar concentration is abbreviated as mM and parts per million is abbreviated as ppm. The coatings presented herein are known in the art as conversion coatings. They are prepared as an application bath or solution that can be applied to a substrate by immersion, roller application, or spray application. The coating solutions are aqueous and acidic. They are completely phosphate-free and comprise at least zinc oxide and sulfuric acid. The coatings, as described herein, also optionally include one or more of the following: pH-adjusting neutralizers; coating concentrate stabilizers; coating accelerators; coating refiners; etching agents; and paint adhesion promoters. These coatings are used to coat a variety of metals, including steel, carbon steel, galvanized steel, other steel alloys, aluminum and aluminum alloys, brass, and copper.In a typical process according to the present description, the substrate surface may undergo initial cleaning and degreasing stages if desired. Then, after rinsing with water, the conversion coating according to the present description is applied by either a dip coating stage, roller application, or spray application. The dip application is typically carried out for an exposure time of 5 to 15 minutes, and preferably 5 to 10 minutes. The spray application can be done for 30 to 90 seconds, preferably 60 to 90 seconds. The substrate is then preferably rinsed with water, either city water or deionized water, to remove any non-adhering coating solution before the subsequent processing stages.The following processing stages may include application of at least one cold forming lubricant, painting, corrosion protection, or any combination thereof. The present conversion coating solution designs can be supplied either as a concentrate that is diluted with water by the end user or as a final bath. More frequently, the coating chemical is supplied as a concentrate that the end user dilutes with water as required to form the final bath or spray solution. The dilution factor for the concentrate can be any convenient dilution desired by the manufacturer. For example, the concentrate could be a 10X or 20X concentrate. In the present description, the final bath or spray coatings being applied are entirely phosphate-free, acidic aqueous coating solutions having a pH of 4.0 to 6.0, more preferably 4.5 to 5.5.The pH of the coating solution is determined in part by the ratios of sulfuric acid and nitric acid to zinc oxide used and the amount of pH-adjusting neutralizer used. The bath is initially formed, and then the pH-adjusting neutralizer is added as required to bring the pH to the desired level. In this specification and claims, unless otherwise noted, the concentration of any component means its concentration in the application spray or bath as used and not its concentration in a formulation concentrate that is diluted by the end user. The conversion coating components according to the present invention are preferably presented in a final application bath or spray solution at the levels provided below in TABLE 1. The component levels are given in terms of the mM concentration in the final aqueous coating solution. TABLE 1 provides a preferred range and a much more preferred range for each component in the final aqueous coating solution. Concentrates of the formula can be used to create a starter bath or coating solution and can also be used to replenish an existing bath as components are depleted by the coating substrates. Such use of a concentrate's booster is well known in the technique. / Q / crn / Lznz / q / YiAi TABLE 1 COMPONENT PREFERRED APPLICATION BATH RANGE (mM) PREFERRED APPLICATION BATH RANGE (mM) H2SO4 25 - 100 50-75 ZnO 30- 121 60-91 pH Adjustment Neutralizer Sufficient amount to obtain bath 0 solution pH 4.0 to 6.0 Sufficient amount to maintain bath 0 solution pH 4.5 to 5.5 Coating Stabilizer Solution, HNO3 18 - 75 37 - 57 Coating Accelerator 3-12 5-9 Coating Refiner 1-10 3-7 Etcher 12-51 25 - 38 Paint adhesion promoter 0.5-2.0 0.75 - 1.5 The sulfuric acid in the conversion coating solution and application bath serves at least two purposes. It chemically cleans a portion of the metal substrate, thereby forming metal substrate ions, some of which are expected to be incorporated into the coating. It also reacts with zinc oxide to form zinc sulfate, which is deposited onto the metal substrate as part of the coating. Zinc sulfate has low solubility in the acidic aqueous solution and is deposited by incipient precipitation from the conversion coating solution onto the substrate. Sulfuric acid is used in sufficient quantity to provide the appropriate pH and to form the zinc sulfate coating. Preferably, the sulfuric acid is present in the final bath at a level of 25 to 100 mM, more preferably 50 to 75 mM.Zinc sulfate, formed from the reaction of zinc oxide with sulfuric acid, provides a sacrificial corrosion barrier, enhances paint adhesion, and improves the retention of lubricating coatings on metal surfaces prior to cold forming. This is particularly beneficial because solutions prepared according to this description are phosphate-free while still providing the advantages of zinc phosphate coatings. The zinc oxide is preferably present in the final bath at a level of 30 to 121 inM, more preferably at a level of 60 to 91 mM. The preferred concentrated solution stabilizer is nitric acid, HNO3, preferably the final level of nitric acid in the bath is 18 to 75 mM, more preferably 37 to 57 mM. Exemplary pH-adjusting neutralizers that can be used in this description include: NaOH, CaOH, MgOH, sodium carbonates, calcium carbonates, and mixtures thereof. These are not added when the bath or solution is initially formed; instead, they are added after the application bath or solution is formed to bring the pH to a level of 4.0 to 6.0, preferably 4.5 to 5.5. The initially formed bath has a lower pH than the preferred level, and this pH adjustment brings the bath to the preferred point of incipient precipitation. A variety of coating accelerators can be used in the present conversion coating to aid in coating deposition. It is possible to formulate a coating according to the present invention without accelerators; however, accelerators increase the coating thickness and deposition rate, which is desirable. The preferred accelerator is hydroxylamine sulfate, (NH2OH)2H2SO4, but other accelerators used in conventional metal pretreatment applications, including, but not limited to, m-nitrobenzene sulfate sodium salt, sodium nitrate, sodium chlorate, 3-aminobenzene sulfonic acid, and mixtures thereof, can also be employed. Preferably, the accelerators are used at a level of 3 to 12 mM, more preferably 5 to 9 mM. A thorough and uniform pretreatment coating is generally required to ensure the reliability of the final layered system, such as cold-form lubricants or paint. The addition of trisodium salts of N-(2-hydroxyethyl)ethylenediamineN,N',N'-triacetic acid (HEDTA), available as Dow's Versenal MR120, a coating refiner, has been found to significantly improve coating deposition as described herein. Coatings deposited with formulations including HEDTA are more complete and heavier on the substrate compared to those produced using a bath that does not contain the coating aid. Examples of other coating refiners that may be used include, but are not limited to, sodium salts of L-glutamic acid, N,N-diacetic acid, nitrilotriacetic acid, and mixtures thereof.The refiners of / Q / crn / Lznz / q / YiAi coating are preferably in the bath at a concentration of 1 to 10 mM, 3 to 7 mM. The use of an etching agent in the bath is very helpful when coating aluminum or aluminum alloy substrates, as these substrates tend to form an aluminum oxide coating on the surface, which must be removed. The aluminum etching agent can be added to the bath and aids in the deposition of these coatings. Examples of etching agents include, but are not limited to, those related to hydrofluoric acid and its salts, including bifluoride (ABE) and tetrafluoroboric acid, as well as mixtures thereof. They are preferably used at a concentration of 12 to 51 mM, and more ideally at 25 to 38 mM. The bath may also include a paint adhesion promoter, especially if the surface is to be painted after the cold forming process. Paint adhesion promoters that can be used include, but are not limited to, hexafluorosilicic acid; nitrates such as sodium nitrate, calcium nitrate, and nickel nitrate; sulfates such as copper sulfate or aluminum sulfate; hexafluorotitanic acid; hexafluorozirconic acid; and mixtures of these promoters. Paint adhesion promoters are preferably used at a level of 0.5 to 2.0 mM in the bath, more preferably 0.75 to 1.5 mM. The following examples are of / Q / cen / Lznz / q / YiAi coating solution formulations that were prepared and tested according to this description. The baths were prepared by dissolving the components in the examples below in water to provide the final concentrations as listed in the example tables. After the bath was formed, the pH was adjusted using the pH adjustment neutralizer to the desired range of 5 to 5.5, and the baths were heated to the desired temperature. In most examples, the temperature chosen was 180°F, and the bath exposure time was set to 10 minutes. / Q / cen / Lznz / q / YiAi EXAMPLE 1 COMPONENT Bath Concentration (mM) H2SO4 65 ZnO 79 HNO3 49 (NH2OH) 2-H2SO4 (Hydroxylamine sulfate, accelerator) 7.8 N-(2-Hydroxyethyl)ethylenediamine-N,N',N' triacetic acid (HEDTA, coating refiner) 3.5 EXAMPLE 2 COMPONENT Bath concentration (mM) H2SO4 65 ZnO 79 HNO3 49 (NHaOH)a'HaSOA (Hydroxylamine sulfate) 7.8 N-(2-hydroxyethyl)ethylenediamine-N,N' ,N' - triacetic acid (HEDTA) 3.5 NH4HF2 Ammonium bifluoride, etching agent 33 EXAMPLE 3 / Q / cen / Lznz / q / YiAi COMPONENT Bath Concentration (mM) H2SO4 65 ZnO 79 HNO3 49 (NH2OH) 2'H2SO4 (hydroxylamine sulfate) 7.8 N-(2-hydroxyethyl)ethylenediamine-N,N',N' triacetic acid (HEDTA) 3.5 (H3O) 2SiF6 Hexafluorosilicic acid, adhesion promoter 1.27 As discussed herein, the present conversion coating solutions or baths are formulated to induce the incipient precipitation point in bath 5 to create a zinc sulfate-containing coating on a metal substrate that is coated in the bath. The treatment bath conditions are intentionally created to encourage the dissolved constituents in the treatment bath to precipitate cooperatively in the form of crystals or as an amorphous solid on the metal surface. The sulfuric acid in the bath initially chemically cleans the metal surface and dissolves the base metal into solution. It is expected that some portion of this dissolved metal will be redeposited and incorporated into the precipitated coating on the metal substrate. During the use of the present bath, a sulfate sludge forms over time.In the coating process, zinc oxide reacts with sulfuric acid to form a soluble salt. The addition of nitric acid, a concentration stabilizer, to the bath is advantageous as it increases the stability of concentrated formulations. The example concentrations of the components presented in Examples 1 through 3 are empirically based on coating deposition results that provide corrosion protection to an underlying substrate and enhance the ability of the zinc sulfate coating complex to accept lubricants typically used in cold forming or paint coatings. A conversion coating as cited in Example 1 above was prepared and successfully used as a coating for steel substrates subsequently processed in a cold forming lubricating coating application. The coating according to Example 1 provided adequate interim corrosion protection under the lubricant after the lubricant application process was completed. The lubricant used was a sodium stearate-based reactive lubricating coating. A commercial example of such a lubricant is Formlube 1 from Freibome Industries Inc. (Eli). The conversion coating according to Example 1 has also been successfully used on steel substrates that were subsequently coated with an aqueous polymeric lubricating coating prior to a forming operation.Commercial examples of such aqueous polymeric lubricating coatings include Formlube 47, also available from Freibome Industries Inc. Conversion coatings prepared according to the present description allow lubricants to wet and completely cover the substrate surface. Conversion coatings prepared according to the present description retain the lubricants on the substrate surfaces during transport and cold forming operations. The conversion coating of Example 3 above was used to coat steel honeycomb panels, which were then coated with polyester and epoxy powder paint. These paint-coated panels were plotted and individually placed in an accelerated corrosion test according to ASTM B117 / Q / crn / Lznz / q / YiAi for 500 hours and evaluated according to ASTM DI654. The results showed excellent deformation protection against corrosion.During a 500-hour test, the average deformation was 0.5 mm, with a range of 0.25 to 2.0 mm. However, the loss of paint adhesion at the paint pretreatment interface was unacceptable. The conversion coating made according to Example 1 above was modified in Example 2 by the addition of ammonium bifluoride etching agent when used to coat aluminum or aluminum alloy substrates. It was found that these substrates frequently had an outer coating of aluminum oxide, which ammonium bifluoride successfully removed. The use of conversion coatings according to the present description, which included an etching agent, improved their performance on aluminum and aluminum alloy substrates. Aluminum acid readily forms on these substrate surfaces and is frequently found as a solution barrier to the underlying native aluminum substrate. Other etching agents similar to tetrafluoroboric acid are also capable of attacking this aluminum oxide film and can be used in the present conversion coatings. A rinse with water, city water, or deionized water after application of this conversion coating improves paint adhesion. The preferred method for increasing paint adhesion after application to this conversion coating, however, is to employ a paint adhesion promoter in the conversion coating. Paint adhesion promoters used herein include, but are not limited to: hexafluorosilicic acid (used in Example 3); sodium nitrate; calcium nitrate; nickel nitrate; copper sulfate; aluminum sulfate; hexafluorotitanic acid; and hexafluorozirconic acid. When used, paint adhesion promoters are preferably present at levels of 0.5 to 2.0 mM in the bath, more preferably 0.75 to 1.5 mM.These promoters can be added directly to concentrated formulas or added to the bath. The preferred bath or dew point temperature according to this description is 71 to 80°C (160 to 180°F). As the temperature is reduced to 160°F, the zinc sulfate coating develops a reddish color. Once the bath has been raised to the desired temperature, the pH of the bath is adjusted to a pH of 4 to 6, preferably 4.5 to 5.5. As discussed herein, a variety of pH-adjusting neutralizers can be used to maintain the bath pH at the desired level, including, for example, sodium hydroxide, calcium hydroxide, magnesium hydroxide, sodium carbonates, and calcium carbonates. Magnesium ions have been observed to create a lighter-colored conversion coating, in contrast to calcium, which is noticeably darker. Sodium ions impart a green tint to the coating. The bath pH affects the weight of the zinc sulfate coating deposited on the substrate surface. A series of baths were created using the conversion coating according to Example 1 at different final pH levels. A series of steel substrates were immersed in each bath for the same period of time, and the weight of the resulting deposited coating was measured. The results for pH and coating weight were as follows: a pH of 3.pH 3 resulted in a coating weight of 104 mg / ft²; a pH of 4.7 resulted in a coating weight of 130 mg / ft²; and a pH of 5.0 resulted in a coating weight of 300 mg / ft². Thus, as shown by the results, increasing the pH from 3.3 to 5.0 resulted in a significant increase in the coating deposition rate according to the present description. Preferably, the coating weight ranges from 100 to 400 mg / ft² on the metal substrate. As an alternative to the acid zinc sulfate conversion coating described above, a skilled person may also use the present description to prepare a manganese salt of sulfuric acid to create an iron manganese sulfate conversion coating. / Q / crn / Lznz / q / YiAi High-strength metal wear contact surfaces in automotive machinery or applications, such as gearboxes, differentials, power transmissions, and pistons, have historically been coated with an iron manganese phosphate coating to act as a break-in layer in combination with oil to enhance wear resistance. This description can be modified to substitute zinc oxide for manganese oxide, creating an iron manganese sulfate conversion coating. This iron manganese sulfate conversion coating can be used in place of the iron manganese phosphate coating of the prior art. Generally speaking, the components of the application bath that can be used are the same; however, some intervals may be expanded and / or changed.For example, the iron manganese sulfate conversion coating typically requires little to no coating stabilizer, such as HNO₃, or at most a coating accelerator. The levels of manganese oxide can also be reduced compared to the amount of zinc oxide used in the previous formulation. An example conversion coating was prepared as described in Example 4 below. It was applied to test plates by immersion at 80°C for 20 minutes. The solution does not require any pH adjustment; it is typically below pH 4.0. / Q / cen / Lznz / q / YiAi / Q / cen / Lznz / q / YiAi Example 4 Component Concentration of application bath mM H2SO4 28 HNO3 2.6 (NH2OH) 2H2SO4 (Hydrazine sulfate) 0.15 MnO 7.4 The above description has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the described modality may be evident to those skilled in the art and fall within the scope of the description. Therefore, the scope of legal protection afforded by this description can only be determined by examining the following 10 claims. The preceding description of the modalities has been provided for illustrative and descriptive purposes. It is not intended to be exhaustive or to limit the description. Individual elements or features of a particular modality 15 are generally not limited to that particular modality, but, where applicable, are interchangeable and may be used in a selected modality, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be considered as a departure from the description, and all such modifications are proposed to be included within the scope of the description. The example modalities are provided so that this description will be thorough and will fully extend its scope to those skilled in the art. Numerous specific details are set forth, such as examples of specific components, devices, and methods, to provide a complete understanding of the modalities described herein. It will be evident to those skilled in the art that specific details need not be employed; these example modalities can be incorporated in many different ways and should not be considered to limit the scope of the description. In some example modalities, well-known processes, well-known device structures, and well-known technologies are not described.

Claims

1. A phosphate-free aqueous acidic conversion coating solution for metal substrates, characterized in that it comprises: zinc oxide and sulfuric acid; and the conversion coating solution having a pH of 4.0 to 6.0 and optionally comprising one or more of a pH-adjusting neutralizer, a concentrated coating stabilizer, a coating accelerator, a coating refiner, an etching agent, a paint adhesion promoter, and mixtures thereof.

2. The phosphate-free aqueous acid conversion coating solution according to claim 1, characterized in that the zinc oxide is present in an amount of 30 to 121 mM.

3. The phosphate-free aqueous acid conversion coating solution according to claim 1, characterized in that the sulfuric acid is present in an amount of 25 to 100 mM.

4. The phosphate-free aqueous acid conversion coating solution according to claim 1, characterized in that the concentrated coating stabilizer is present in a positive amount of up to 75 mM. / Q / crn / Lznz / q / YiAi 5. The phosphate-free aqueous acid conversion coating solution according to claim 4, characterized in that the concentrated coating stabilizer comprises nitric acid.

6. The phosphate-free aqueous acid conversion coating solution according to claim 1, characterized in that the coating refiner is present in a positive amount of up to 10 mM.

7. The phosphate-free aqueous acid conversion coating solution according to claim 6, characterized in that the coating refiner is selected from the group consisting of N-(2-hydroxyethyl)ethylenediamine salt; N,N',N'-triacetic acid; a sodium salt of L-glutamic acid; N,N-diacetic acid; nitrilotrisacetic acid; and mixtures thereof.

8. The phosphate-free aqueous acid conversion coating solution according to claim 1, characterized in that the etching agent is present in a positive amount of up to 51 mM.

9. The phosphate-free aqueous acid conversion coating solution according to claim 8, characterized in that the etching agent is selected from the group consisting of hydrofluoric acid; salts of hydrofluoric acid; ammonium bifluoride; tetrafluoroboric acid; and mixtures thereof. / Q / crn / Lznz / q / YiAi 10. The phosphate-free aqueous acid conversion coating solution according to claim 1, characterized in that the paint adhesion promoter is present in a positive amount of up to 2.0 mM in the solution.

11. The phosphate-free aqueous acid conversion coating solution according to claim 10, characterized in that the paint adhesion promoter is selected from the group consisting of hexafluorosilicic acid; sodium nitrate; calcium nitrate; nickel nitrate; copper sulfate; aluminum sulfate; hexafluorotitanic acid; hexafluorozirconic acid; and mixtures thereof.

12. The phosphate-free aqueous acid conversion coating solution according to claim 1, characterized in that at least one adhesion promoter is presented in an amount of 0.5 to 2.0 mM and wherein the adhesion promoter is selected from the group consisting of hydroxylamine sulfate; m-nitrobenzene sulfonate sodium salt; sodium nitrate; sodium chlorate; 3-aminobenzene sulfonic acid; and mixtures thereof.

13. The phosphate-free aqueous acid conversion coating solution according to claim 1, characterized in that the solution has a pH of 4.5 to 5.

5. / Q / cen / Lznz / q / YiAi 14. The phosphate-free aqueous acid conversion coating solution according to claim 1, characterized in that the pH adjusting neutralizer is presented in a sufficient amount to maintain the pH from 4.0 to 6.0 and wherein the pH adjusting neutralizer is selected from the group consisting of sodium hydroxide; calcium hydroxide; magnesium hydroxide; a sodium carbonate; a calcium carbonate; and mixtures thereof.

15. A method for coating a metal substrate, characterized in that it comprises the steps of: providing a metal substrate; and exposing the metal substrate to a phosphate-free aqueous acidic conversion coating solution having a pH of 4.0 to 6.0 and comprising zinc oxide, sulfuric acid and, optionally, comprising one or more of a pH-adjusting neutralizer, a coating concentrate stabilizer, a coating accelerator, a coating refiner, an etching agent, a paint adhesion promoter and mixtures thereof, an exposure time to the conversion coating solution that is sufficient to deposit a coating of the coating solution onto the metal substrate.

16. The method according to claim 15, characterized in that the step of providing a metal substrate comprises providing a metal substrate selected from the group consisting of steel; an alloy of / Q / cen / Lznz / q / YiAi steel; a carbon steel; a galvanized steel; aluminum; an aluminum alloy; brass; copper; and mixtures thereof.

17. The method according to claim 15, characterized in that the step of exposing the metal substrate further comprises immersing the metal substrate in a bath comprising the phosphate-free aqueous acid conversion coating solution for a period of time of 5 to 15 minutes at a temperature of 160 to 180°F.

18. The method according to claim 15, characterized in that the step of exposing the metal substrate further comprises spraying the metal substrate with a solution comprising the phosphate-free aqueous acidic conversion coating solution for a time period of 30 to 90 seconds at a temperature of 160 to 180°F.

19. The method according to claim 15, characterized in that the step of exposing the metal substrate further comprises depositing a coating of 100 to 400 mg / ft2 onto the metal substrate.

20. A phosphate-free aqueous acidic iron manganese sulfate conversion coating solution for metal substrates, characterized in that it comprises: manganese oxide and sulfuric acid; and the conversion coating solution further comprising a coating concentrate stabilizer and a coating accelerator.