EP1290244A1

EP1290244A1 - Method for forming phosphate coatings on nonferrous metals and plated steel sheets

Info

Publication number: EP1290244A1
Application number: EP01928375A
Authority: EP
Inventors: Hirokatsu Bannai; Yasuhiko Nagashima; Kensuke Shimoda
Original assignee: Henkel AG and Co KGaA
Current assignee: Henkel AG and Co KGaA
Priority date: 2000-04-10
Filing date: 2001-04-06
Publication date: 2003-03-12
Also published as: JP2001295063A; EP1290244A4; WO2001077411A1; AU2001255239A1

Abstract

The present invention comprises providing a phosphate coating of at least 50 % Zn2Fe(PO4)2•4H2O on metal having a nonferrous surface by contacting the metal with a phosphate treatment bath that contains zinc ion, phosphate ion, and hydroxylammonium ion and at least 17 weight parts ferrous ion for each 100 weight parts zinc ion. This method can be run on the metal after the metal has been brought into contact with a surface conditioning bath containing divalent or trivalent metal phosphate particles no larger than 5 νm. The hydroxylammonium ion concentration is preferably 0.2 to 5 g/L, the phosphate ion concentration is preferably 5.0 to 30 g/L, the zinc ion concentration is preferably 0.5 to 5.0 g/L, and the ferrous ion concentration is preferably 0.2 to 5.0 g/L.

Description

METHOD FOR FORMING PHOSPHATE COATINGS ON NONFERROUS METALS AND PLATED STEEL SHEETS

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for forming phosphate coatings on the surfaces of metals that lack iron at the surface, e.g., galvanized steel sheet, magnesium alloys, and aluminum, wherein the phosphate coating is strongly adherent to films applied by painting and exhibits an excellent post- painting corrosion resistance.

2. Background Art

There are many instances in which a phosphate coating is formed on the surface of metal that will be painted prior to its final use. This is done, for example, with the metal used for car bodies. The phosphate coating is applied in order to increase adherence of the paint film and improve the post-painting corrosion resistance. In the case of ferriferous metals such as steel sheet, the phosphate coating is typically formed on a ferrous surface by immersion in a phosphate treatment bath containing zinc ion and phosphate ion. The term "ion" as used throughout is intended to encompass a plurality of ions.

Galvanized steel sheet has recently become the most prominent metal stock for uses requiring subsequent painting, for example, in making car bodies. In this case the phosphate coating is formed on the surface of a zinc plating layer. However, the use of the prior-art phosphate treatment baths containing zinc ion and phosphate ion on galvanized steel sheet has resulted in an unsatisfactory paint film adherence and an inadequate post-painting corrosion resistance. It is known that the addition of nickel ion and manganese ion to phosphate treatment baths enables the formation of high-quality phosphate coatings even on the surface of the zinc plating layer. This technology is being widely practiced at the present time. But, as environmental considerations have led in recent years to regulations on heavy metal ions in wastewater, the nickel ion has been taken up as a target of these regulations and the use of a nickel ion- containing phosphate treatment bath now requires special treatment of the nickel ion in the wastewater.

Japanese Laid Open (Kohyo or Unexamined) Patent Application (PCT) Number Hei 7-505445 teaches a nickel ion-free phosphate treatment bath that contains 0.2 to 2 g/L zinc ion, 0.5 to 25 mg/L copper ion, and 5 to 30 g/L phosphate ion. This phosphate treatment bath suffers from difficulties in managing the copper ion concentration due to the low copper ion content and the narrow range of the permissible copper ion content.

Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 1-123080 (123,080/1989), although completely silent with regard to both (i) the problems with phosphate coatings on zinc-plated materials and (2) the solutions thereto, does describe an hydroxylamine-containing phosphate treatment bath for the purpose of obtaining phosphate coatings having spherical or columnar crystals. This patent teaches that the bath can also be used for zinc-plated material and teaches that 0.001 to 0.5 weight% ferrous ion may additionally be present in the bath. This patent, however, does not teach using a ferrous ion-containing treatment bath on zinc-plated material. Moreover, this patent teaches that in the case of zinc-plated materials, the phosphate coating product contains plate-shaped crystals and not the desired spherical or columnar crystals.

The present invention was developed in order to address the problems described above in the prior art. More specifically, the present invention addresses the problem of providing a method that can form a highly paint adherent, highly post-painting corrosion-resistant phosphate coating on the surfaces of metals that lack iron at the surface (e.g., galvanized steel sheet, aluminum, magnesium alloys) and that can do so using a phosphate treatment bath that does not contain the nickel ion or the copper ion with its narrow permissible content range.

SUMMARY OF THE INVENTION

The present invention comprises (1) a method for forming phosphate coatings on nonferrous metals and plated steel sheets. The method is characterized by forming a phosphate coating that contains at least 50% phosphophyllite (Zn2Fe(PO4)2-4H2O) in terms of the P ratio (phosphophyllite ratio, vide infra) on nonferrous metal or plated steel sheet by effecting contact between nonferrous metal or plated steel sheet and a phosphate treatment bath that contains zinc ion, phosphate ion, and hydroxylammonium ion and at least 17 weight parts ferrous ion for each 100 weight parts zinc ion.

The present invention also comprises (2) a method for forming phosphate coatings on nonferrous metals and plated steel sheets. The method is characterized by effecting contact between nonferrous metal or plated steel sheet and a surface conditioning bath that contains at least one selection from divalent and trivalent metal phosphate particles having a particle size no greater than 5 μm, and thereafter forming a phosphate coating that contains at least 50% phosphophyllite

(Zn2Fe(PO4)2-4H2O) in terms of the P ratio (phosphophyllite ratio) on the nonferrous metal or plated steel sheet by effecting contact between the nonferrous metal or plated steel sheet and a phosphate treatment bath that contains zinc ion, phosphate ion, and hydroxylammonium ion and at least 17 weight parts ferrous ion for each 100 weight parts zinc ion.

The present invention additionally comprises (3) a method in accordance with the aforementioned (1) or (2) for forming phosphate coatings on nonferrous metals and plated steel sheet, wherein the method is characterized in that the hydroxylammonium ion concentration is 0.2 to 5 g/L.

The present invention further comprises (4) a method in accordance with the aforementioned (1), (2), or (3) for forming phosphate coatings on nonferrous metals and plated steel sheet, wherein the method is characterized in that the phosphate ion concentration is 5.0 to 30 g/L, the zinc ion concentration is 0.5 to 5.0 g/L, and the ferrous ion concentration is 0.2 to 5.0 g/L.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention employs a phosphate treatment bath that contains zinc ion, phosphate ion, hydroxylammonium ion, and ferrous ion. Phosphate coatings are typically formed on metals having iron at the surface, e.g., steel sheet, using a phosphate treatment bath that differs from the one employed in the present invention by containing zinc ion and phosphate ion but neither the hydroxylammonium ion nor the ferrous ion. When steel sheet is immersed in such a conventional phosphate treatment bath, the surface of the steel sheet is dissolved

2+ by the phosphate treatment bath, which results in the production of Fe ion in the phosphate treatment bath. The phosphate coating obtained under these conditions will contain both hopeite (Zn3(PO_ι)2-4H2θ) and phosphophyllite

(Zn₂Fe(PO4)2 H₂O).

The present invention is directed to the formation of phosphate coatings on plated steel sheets and nonferrous metals that lack iron at the surface.

When such a metal workpiece is immersed in a conventional phosphate treatment bath, that is, a bath that contains zinc ion and phosphate ion but which lacks both hydroxylammonium ion and ferrous ion, the surface of the metal workpiece again is dissolved by the phosphate treatment bath, but in this case without the

2+ production of Fe ion in the phosphate treatment bath. Under these circumstances a phosphate coating of hopeite (Zn3(PO4)2-4H2O) is produced in which phosphophyllite (Zn2Fe(PO4)2-4H2O) is not present.

The present inventors carried out the addition of ferrous ion to a prior-art phosphate treatment bath containing zinc ion and phosphate ion in order to prepare a phosphate treatment bath that did not contain hydroxylammonium ion and carried out immersion of galvanized steel sheet in this phosphate treatment bath. While this phosphate treatment bath did contain ferrous ion, the phosphate coating formed on the surface of the surface iron-free metal workpiece had hopeite (Zn3(PO4)2-4H2O) as its main component, while phosphophyllite

(Zn2Fe(PO4)2-4H2O) was not produced in an amount appropriate to the quantity of ferrous ion addition.

The inventors also carried out immersion of galvanized steel sheet in a phosphate treatment bath prepared by the addition of both ferrous ion and hydroxylammonium ion to a prior-art phosphate treatment bath containing zinc ion and phosphate ion. Under these conditions, a large amount of phosphophyllite

(Zn2Fe(PO4)2-4H2O) — in this case an amount appropriate to the quantity of ferrous ion addition — was produced on the surface of the surface iron-free galvanized steel sheet, and a phosphate coating was obtained that contained both hopeite (Zn3(PO4)2-4H2O) and a large amount of phosphophyllite (Zn2Fe(PO₄)2-4H₂O).

In another series of experiments, the inventors prepared phosphate treatment baths having different ferrous ion concentrations by adding different amounts of ferrous ion to solutions containing zinc ion, phosphate ion, and hydroxylammonium ion. Galvanized steel sheet was immersed in each bath to give a series of galvanized steel sheets bearing phosphate coatings having different phosphophyllite contents. These sheets were painted and the adherence of the paint films and post-painting corrosion resistance were evaluated. The results of these evaluations showed that the paint film adherence and post-painting corrosion resistance were inadequate at low phosphophyllite contents in the phosphate coating. However, phosphophyllite + hopeite phosphate coatings containing at least 50% phosphophyllite were found to exhibit a substantially improved paint film adherence and post-painting corrosion resistance.

According to the information developed by the present inventors, a phosphate treatment bath that has a relatively low content of ferrous ion relative to zinc ion is essentially unable to form a phosphate coating that contains at least 50% phosphophyllite. Rather, a phosphate coating containing at least 50% phosphophyllite is obtained when the bath contains at least 17 weight parts ferrous ion per 100 weight parts zinc ion.

The preceding can be summarized as follows. A phosphate treatment bath containing zinc ion and phosphate ion, but containing neither ferrous ion nor hydroxylammonium ion, is unable to form a strongly paint film- adherent, highly corrosion-resistant phosphate coating on nonferrous metal or plated steel sheet. Even when ferrous ion is added to such a bath, in the absence of a special inventive measure it remains essentially impossible to form a strongly paint film-adherent, highly corrosion-resistant phosphate coating on nonferrous metal or plated steel sheet. Finally, in the case of a prior-art phosphate treatment bath (zinc ion + phosphate ion) to which hydroxylammonium ion has been added, even with the addition thereto of ferrous ion it still remains — absent a special inventive measure — essentially impossible to form a strongly paint film-adherent, highly corrosion-resistant phosphate coating on nonferrous metal or plated steel sheet.

The present invention comprises the addition of hydroxylammonium ion to the prior-art phosphate treatment bath, but also requires the presence therein of ferrous ion in a specific amount (at least 17 weight parts ferrous ion for each 100 weight parts zinc ion). Use of the inventive phosphate treatment bath enables the formation, even on nonferrous metals and plated steel sheets, of a phosphate coating having a remarkably good paint film adherence and corrosion resistance.

The invention will be explained in additional detail below. The phosphate treatment bath used in the present invention is an acidic aqueous solution that contains zinc ion, phosphate ion, ferrous ion, and hydroxylammonium ion as essential components. The zinc ion concentration in this treatment bath is preferably in the range of 0.5 to 5.0 g/L. A zinc ion concentration below 0.5 g/L causes an inadequate post-painting corrosion resistance due to failure to form a coating in adequate amounts and a low coating weight by the phosphate crystals that are formed. A zinc ion concentration in excess of 5.0 g/L results in a coarsening of the coating crystals and in particular in a decline in the post-painting adherence.

The phosphate ion concentration in the phosphate treatment bath used by this invention is preferably in the range of 5.0 to 30 g/L. It becomes quite difficult to form a regular phosphate coating at a phosphate ion concentration below 5.0 g/L, while exceeding 30 g/L is uneconomical since no additional effects are obtained beyond 30 g/L. The phosphate ion can be supplied to the treatment bath by the addition thereto of phosphoric acid or an aqueous solution thereof, or can be supplied by dissolution in the treatment bath of a phosphate salt, e.g., the sodium, magnesium, or zinc salt.

The ferrous ion concentration in the phosphate treatment bath used by this invention will vary depending on the zinc ion concentration, but preferably is in the range of 0.2 to 5 g/L. More preferably, the concentration of ferrous ion in the phosphate bath is in an amount so that the phosphate bath contains at least 17 weight parts ferrous ion for each 100 weight parts zinc ion, even more preferably at least 25 weight parts, and even more preferably at least 75 weight parts. In other preferred baths, at least 100 weight parts ferrous ion for each 100 weight parts zinc ion is present, more preferably at least 150 weight parts ferrous ion, even more preferably at last 200 weight parts ferrous ion, and most preferably at least 275 weight parts ferrous ion. This ferrous ion is a crucial source component for forming phosphophyllite crystals on the surface of nonferrous metals since a nonferrous metal workpiece is unable to supply iron ion. Ferrous ion can be supplied by dissolving, for example, ferrous sulfate, ferrous nitrate, ferrous hydroxide, or ferrous halide, in the treatment bath.

Ferrous ion is readily oxidized by atmospheric oxygen to ferric ion. The ferric ion cannot form a stable solution in phosphate treatment baths and produces an insoluble iron phosphate (sludge) with the phosphate ion; this results in the generation of large amounts of sludge in the phosphate treatment bath. This problem — when it must be addressed — can be addressed by the addition of a weak reducing agent, such as L-ascorbic acid, to the bath. The concentration of the reducing agent in the phosphate treatment bath is preferably in the range of 10 to

500 ppm.

The addition of ferrous ion alone to the phosphate treatment bath is essentially insufficient to produce phosphate crystals with a high phosphophyllite ratio on the surface of a metal workpiece that cannot function as an iron source.

An important consideration for producing high phosphophyllite ratio phosphate crystals is also how to maintain a high ferrous ion concentration at the interface with the metal workpiece. This function is fulfilled by the hydroxylammonium ion, and the ferrous ion and the hydroxylammonium ion are essential and indispensable components in the present invention. The hydroxylammonium ion is believed to exercise a stabilizing function on the ferrous ion at the interface with the metal workpiece and also to accelerate the reactions that form the phosphate coating of this invention.

The hydroxylammonium ion concentration in the phosphate treatment bath used in this invention is preferably in the range of 0.2 to 5 g/L. A concentration of hydroxylammonium ion below 0.2 g/L results in inadequate conversion reactions and also strongly impairs the ability to produce a phosphate coating that has a high phosphophyllite ratio. Concentrations in excess of 5 g/L are uneconomical because no additional effects occur beyond 5 g/L. The hydroxylammonium ion can be supplied by dissolving, for example, hydroxylamine sulfate, hydroxylamine phosphate, or hydroxylamine hydrochloride, in the treatment bath.

An etchant may be added to the phosphate treatment bath used in this invention in order to induce a uniform etch of the surface of the metal workpiece. Usable as this etchant are the fluoride ion and complex fluoride ions such as the fluosilicate ion. These ions can be obtained from such fluorine compounds as, for example, hydrofluoric acid and fluosilicic acid and their metal salts, e.g., the sodium and potassium salts.

The phosphate treatment bath of this invention may also contain one or more metal ions selected from the manganese ion, magnesium ion, cobalt ion, and calcium ion. These ions can be furnished to the treatment bath by dissolution therein of, for example, the oxide, hydroxide, carbonate, sulfate, nitrate, or phosphate of the particular metal. The concentration of the added metal ion in the phosphate treatment bath is preferably in the range of 0.1 to 2 g/L.

The inventive method for forming phosphate coatings requires that the metal surface be clean prior to its contact with the phosphate treatment bath. The metal can be brought into contact with the treatment bath without an intervening cleaning step when the surface of the metal is already clean. However, when treatment is to be applied to a metal workpiece whose surface is contaminated with iron particles, dust, oil, etc., the contaminants adhering on the surface must first be removed in a cleaning step using, for example, a water-based alkaline degreaser, an emulsified degreaser, or a solvent degreaser. When a water- based cleaner is used, it will be preferable to carry out a water rinse step after cleaning in order to thoroughly remove cleaner adhering on the metal surface. A typical treatment sequence for the inventive method for forming phosphate coatings comprises alkaline degreasing — water rinse — > contact with the phosphate treatment bath — > water rinse. The alkaline degreasing and water rinses may be run either as single-stage processes or as multistage processes, while the final water rinse is preferably carried out using deionized water.

When the phosphate coating of the invention is to be used as an underpaint coating, it will be desirable to produce a thin, fine, and dense phosphate

2 conversion coating having a coating weight of 1.5 to 5 g/m . In such cases, prior to its contact with the phosphate treatment bath the workpiece is preferably brought into contact with a suitable surface conditioning bath.

The surface conditioning bath used in such cases preferably contains at least 1 divalent or trivalent metal phosphate powder having a particle size no greater than 5 μm. Said divalent or trivalent metal in said metal phosphate powder is preferably at least 1 selection from the group consisting of zinc, manganese, cobalt, iron, calcium, aluminum, and magnesium.

Dipping, spraying, or a combination thereof can be used to effect contact with the phosphate treatment bath used by the inventive method for forming phosphate coatings. The treatment time can be approximately 1 to 5 minutes; this treatment duration can produce a phosphate coating that is entirely satisfactory from a practical standpoint. The temperature of the phosphate treatment bath is preferably in the range of 30 to 60°C.

The inventive method for forming phosphate coatings is directed to the precipitation in large amounts of chemically stable phosphophyllite crystals on the surface of metals that lack iron at the surface. Metals suitable for the practice of the invention therefore include plated steel sheet, such as galvanized steel sheet and zinc alloy-plated steel sheet, and nonferrous metals such as aluminum alloys and magnesium alloys. The inventive method for forming phosphate coatings can also be applied in an entirely unproblematic manner to metals that have iron at the surface, for example, steel sheet such as cold-rolled steel sheet. Application of the inventive method to such metals is not associated with any reduction whatever in the post-painting adherence or post-painting corrosion resistance. This feature enables treatment of the usual types of steel sheet on the same line as used for nonferrous materials and plated steel sheet. In addition, the presence of nickel in the phosphate treatment bath used by this invention again causes no decline whatever in the paint film adherence or post-painting corrosion resistance.

Examples

The advantageous effects of this invention are illustrated in specific detail by the following working and comparative examples of actual treatment. The working examples simply provide examples of the application of the invention and are not intended to in any way limit the applications of the present invention or target materials for the present invention.

1. Metals Phosphate coatings were formed in the working and comparative examples on the following metals. EG : electrogalvanized steel sheet (sheet thickness: 0.8 mm, plating add-on: 20 g/m )

GA : hot-dip zinc alloy-plated steel sheet (sheet thickness: 0.8 mm, plating add- on: 45 g/m²)

MG : magnesium alloy sheet (sheet thickness: 2.0 mm)

I

2. Treatment process

(1) Degreasing: Degreasing used FINECLEANER L4460, an alkaline degreaser from Nihon Parkerizing Co., Ltd.

(2) Water rinse: The water rinse used tapwater and was run by spraying for

30 seconds at ambient temperature. (3) Surface conditioning: Surface conditioning was carried out by immersion for approximately 30 seconds in a surface treatment bath that contained microparticulate metal phosphate. The type, particle size, and concentration of the microparticulate metal phosphate are reported in Tables 1 and 2.

(4) Production of the phosphate coating: After surface conditioning, the workpiece was immersed for 120 seconds in the phosphate treatment bath at 43°C. No water rinse was carried out between surface conditioning and production of the phosphate coating. The compositions of the phosphate treatment baths are reported in Table

1 and 2.

(5) Water rinse: The water rinse used tapwater and was run by spraying for

30 seconds at ambient temperature.

(6) Rinse with deionized water: This step was carried out by spraying for 20 seconds at ambient temperature using deionized water with an electrical conductivity no greater than 0.2 microS/m.

(7) Drying: Drying was carried out for 120 seconds using a hot wind at

90°C.

Properties evaluated and methods of evaluation

(1) P ratio (phosphophyllite ratio) in %: The intensity of the phosphophyllite (100) plane in the phosphate coating and the intensity of the hopeite (020) plane in the phosphate coating were measured using an x-ray diffractometer (Cu tube). The P ratio was calculated using the following formula.

P ratio = [(phosphophyllite (100) plane intensity)/(phosphophyllite (100) plane intensity + hopeite (020) plane intensity)] x 100

(2) Appearance of the phosphate coating: The phosphate coating was visually evaluated. A uniform film was assigned a rating of " + ", while a film exhibiting breakthrough or nonuniformity was assigned

a rating of " x ".

(3) Adherence to paint film: The phosphate coating was electrocoated to a

thickness of 20 μm followed by the application of a middle coating

and a topcoat. This process formed a paint film with an overall

thickness of 100 μm. The resulting sample was immersed for 240

hours in a 40°C hot-water bath that was bubbled with air. The

sample was then removed from the hot-water bath and allowed to stand for 2 hours. A Crosshatch grid (2 mm squares) was subsequently executed in the sample down to the metal surface and pressure-sensitive adhesive tape was applied to the grid and peeled off. The complete absence of peeling was scored with " + + "; some peeling at the cut edges in part of the grid was scored with

" + "; and substantial peeling was scored with " x ".

(4) Resistance to hot saltwater: An electrocoating with a thickness of 20

μm was formed by electrodeposition on the phosphate coating. A

cross was cut down to the metal surface using a cutter, and the resulting sample was immersed for 240 hours in 5% saltwater at

55°C that was bubbled with air. The sample was thereafter

withdrawn from the saltwater bath and allowed to stand for 1 hour. Pressure-sensitive adhesive tape was then applied to and peeled from the cross cut region and the width of paint film peeling from the cross cut was measured and scored on the following scale. + + : maximum one-sided peel width less than 3 mm + : maximum one-sided peel width in the range of 3 to 5 mm

x : maximum one-sided peel width greater than 5 mm

(5) Salt-spray resistance: An electrocoating with a thickness of 20 μm was

formed by electrodeposition on the phosphate coating. A cross was cut down to the metal surface using a cutter, and the sample was thereafter subjected to salt-spray testing for 480 hours using 5%

saltwater at 35°C. After a water rinse following the salt-spray

exposure, the width of corrosion from the cross cut was measured. The following scale was used for the galvanized steel sheets (EG, GA). + + : maximum one-sided rust width less than 4 mm

+ : maximum one-sided rust width in the range of 4 to 5 mm

x : maximum one-sided rust width greater than 5 mm

The following scale was used for the magnesium alloy sheet (MG).

+ + : maximum one-sided rust width less than 5 mm

+ : maximum one-sided rust width in the range of 5 to 8 mm

x : maximum one-sided rust width greater than 8 mm Table 1.

Table 2.

Table 3.

Table 1 covers the working examples of the inventive phosphate treatment bath. All of the phosphate treatment baths in Table 1 contained the hydroxylammonium ion (NH3OH ) and each bath contained at least 17 weight

2+ 2+ parts ferrous ion Fe per 100 weight parts zinc ion Zn . As shown by the results reported in Table 3 for Examples 1 to 8, the phosphate coatings all contained at least 50% phosphophyllite. Also as reported in Table 3, all of the phosphate coatings in Examples 1 to 8 gave very good results for coating appearance, paint film adherence, resistance to hot saltwater, and salt-spray resistance.

Table 2 covers the phosphate treatment baths of the comparative examples. The bath in Comparative Example 1 contained the hydroxylammonium

+ 2+ ion (NH3OH ), but lacked the Fe ion. The baths in Comparative Examples 2

2+ and 3 both contained the Fe ion in high concentrations, but each lacked the hydroxylammonium ion. As may be seen for Comparative Examples 1 to 3 in

2+ Table 3, a bath lacking Fe ion or hydroxylammonium ion gave a phosphate coating that had a low phosphophyllite content, in each case not reaching 50%. The corresponding phosphate coatings had an unacceptable paint film adherence, resistance to hot saltwater, and salt-spray resistance.

The phosphate treatment bath in Comparative Example 4 in Table 2

2+ contained neither the hydroxylammonium ion nor the Fe ion. This bath contained the Ni ion and was a phosphate treatment bath as used in the prior art on galvanized steel sheet. While the phosphate coating afforded by this phosphate treatment bath did have an excellent paint film adherence and post-painting corrosion resistance, as noted above such a bath is undesirable from an environmental standpoint due to admixture of Ni ion into the wastewater during formation of the phosphate coating.

The phosphate treatment bath in Comparative Example 5 in Table 2

2+ 2+ 2+ contained both the hydroxylammonium ion and the Fe ion, but its (Fe /Zn ) value was less than 17/100. The phosphophyllite content in Comparative Example 5 did not reach 50%, and the corresponding phosphate coating gave an unsatisfactory paint film adherence, resistance to hot saltwater, and salt-spray resistance.

The prior art has employed the addition of nickel ion to phosphate treatment baths in order to bring about the formation of highly paint film-adherent phosphate coatings with an excellent post-painting corrosion resistance on nonferrous metals and plated steel sheet. The present invention enables the formation, without the addition of nickel ion, on nonferrous metals and plated steel sheets of phosphate coatings whose paint film adherence and post-painting corrosion resistance are equal to those obtained by nickel-modified phosphate treatment. Due to their nickel ion content, the prior-art phosphate treatment baths cause problems from an environmental standpoint due to entry of the Ni ion into the wastewater. The inventive phosphate treatment bath, on the other hand, does not contain components that would cause these environmental problems. This invention therefore enables the formation of highly paint film-adherent, highly post-painting corrosion-resistant phosphate coatings on nonferrous metals and plated steel sheet and does so without causing environmental problems.

While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method for forming phosphate coatings on nonferrous metals and plated steel sheet, said method comprising subjecting nonferrous metal or plated steel sheet to a phosphate treatment bath that comprises zinc ions, phosphate ions, hydroxylammonium ions and at least 17 weight parts ferrous ion for each 100 weight parts zinc ions to form a phosphate coating on the non ferrous metal or plated steel sheet that is comprised of at least 50% phosphophyllite (Zn2Fe(PO4)2-4H2O) in terms of the P ratio (phosphophyllite ratio).

2. The method of claim 1 for forming phosphate coatings on nonferrous metals and plated steel sheet, further comprising subjecting the nonferrous metal or plated steel sheet to a surface conditioning bath that comprises at least one selection from divalent and trivalent metal phosphate particles having a particle size no greater than 5 μm, prior to subjecting the nonferrous metal or plated steel sheet to the phosphate treatment bath.

3. The method of claim 1 or 2 for forming phosphate coatings on nonferrous metals and plated steel sheet, wherein the hydiOxylammonium ion concentration is from 0.2 to 5 g/L.

4. The method of claim 1, 2, or 3 for forming phosphate coatings on nonferrous metals and plated steel sheet, wherein the phosphate ion concentration is from 5.0 to 30 g/L, the zinc ion concentration is from 0.5 to 5.0 g/L, and the ferrous ion concentration is from 0.2 to 5.0 g/L.

5. The method of claim 1-3 or 4 wherein the phosphate treatment bath further comprises etchant comprising fluoride ions; and the phosphate treatment bath further comprises metal ions selected from the group consisting of manganese ions, magnesium ions, cobalt ions and calcium ions.

6. The method of claim 1-4 or 5 wherein the phosphate coating has a coating weight of 1.5 to 5 g/m².

7. The method of claim 1-5 or 6 wherein the nonferrous metal or plated steel sheet is subjected to the phosphate treatment bath for about 1-5 minutes with the phosphate treatment bath having a temperature in the range of about 30°C-60°C.

8. The method of claim 2-6 or 7 wherein the metal phosphate of the phosphate particle of the surface conditioning bath is selected from the group consisting of phosphates of zinc, manganese, cobalt, iron, calcium, aluminum and magnesium.

9. A nonferrous metal or plated steel sheet member coated according to the process of any one of claims 1-8.

10. A liquid composition of matter suitable to form a coating according to any one of claims 1-8.