Passivation composition based on mixtures of phosphoric and phosphonic acids

Abstract

The present invention provides an aqueous passivation composition for the treatment of zinc or zinc alloy coatings, comprising: i) phosphoric acid; ii) at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, iii) at least one divalent metal cation (M.sup.2+); and, iv) at least one water-soluble or water-dispersible fluoroacid or a salt thereof, wherein said fluoroacid is defined by the following general empirical formula (II):
H.sub.pT.sub.qF.sub.rO.sub.s(II)
wherein: each of q and r represents an integer from 1 to 10; each of p and s represents an integer from 0 to 10; and, T represents an element selected from the group consisting of Ti, Zr, Hf, Si, Sn, Al, Ge, and B.

Claims

1. An aqueous passivation composition for the treatment of zinc or zinc alloy coatings, said composition having a pH of less than 3 and comprising: i) phosphoric acid; ii) at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, wherein said polyphosphonic acid has a general formula (I): ##STR00003## in which: n is at least 2; and, Z is a connecting organic moiety having an effective valency of n, said polyphosphonic acid being characterized in that at least two phosphonic groups are separated by an alkylene bridge having 1 or 2 carbon atoms (C.sub.1-C.sub.2 alkylene), wherein the composition has a molar ratio of phosphonate groups to phosphoric acid (H.sub.3PO.sub.4) in a range from 2:1 to 1:1; iii) at least one divalent metal cation (M.sup.2+); and, iv) at least one water-soluble or water-dispersible fluoroacid or a salt thereof, wherein said fluoroacid is defined by general empirical formula (II):
H.sub.pT.sub.qF.sub.rO.sub.s(II) wherein: each of q and r represents an integer from 1 to 10; each of p and s represents an integer from 0 to 10; and, T represents an element selected from the group consisting of Ti, Zr, Hf, Si, Sn, Al, Ge, and B.

2. The composition according to claim 1, wherein in general formula (I) n is an integer from 2 to 5.

3. The composition according to claim 1, wherein said polyphosphonic acid is selected from the group consisting of: aminotris(methylene phosphonic acid) (ATMP); 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP); hexamethylene diamine tetra(methylene phosphonic acid) (HDTMP); diethylenetriamine penta (methylene phosphonic acid); diethylenetriamine penta (methylenephosphonic acid (DTPMP); and mixtures thereof.

4. The composition according to claim 3, wherein said polyphosphonic acid comprises 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP).

5. The composition according to claim 1 wherein the composition has a molar ratio of phosphonate groups to phosphoric acid (H.sub.3PO.sub.4) in a range from 1.75:1 to 1.25:1.

6. The composition according to claim 5 wherein the molar ratio of phosphonate groups to phosphoric acid (H.sub.3PO.sub.4) is in a range from 1.6:1 to 1.4:1.

7. The composition according to claim 1, wherein said at least one divalent metal cation (M.sup.2+) is selected from the group consisting of: Mg.sup.2+; Ca.sup.2+; Co.sup.2+; Ni.sup.2+; Sr.sup.2+; and Ba.sup.2+.

8. The composition according to claim 7 comprising Mg.sup.2+.

9. The composition according to claim 1, wherein the composition has a total concentration of divalent metal cations (M.sup.2+) in a range from 0.01 to 1 moles/litre.

10. The composition according to claim 9, wherein the total concentration of divalent metal cations (M.sup.2+) is in a range from 0.01 to 0.5 moles/litre.

11. The composition according to claim 1, wherein in formula (II): T is selected from Ti, Zr, or Si; p is 1 or 2; q is 1; r is 2, 3, 4, 5 or 6; and, sis 0, 1, or 2.

12. The composition according to claim 1, wherein said at least one fluoroacid is selected from the group consisting of fluorotitanic acid (H.sub.2TiF.sub.6); fluorozirconic acid (H.sub.2ZrF.sub.6); fluorosilicic acid (H.sub.2SiF.sub.6); fluoroboric acid (HBF.sub.4); fluorostannic acid (H.sub.2SnF.sub.6); fluorogermanic acid (H.sub.2GeF.sub.6); fluorohafnic acid (H.sub.2HfF.sub.6); fluoroaluminic acid (H.sub.3AlF.sub.6); and combinations thereof.

13. The composition according to claim 12, wherein said at least one fluoroacid is selected from the group consisting of fluorotitanic acid (H.sub.2TiF.sub.6); fluorozirconic acid (H.sub.2ZrF.sub.6); and, fluorosilicic acid (H.sub.2SiF.sub.6); and combinations thereof.

14. The composition according to claim 1, wherein said at least one fluoroacid is present in an amount such that the composition has a molar ratio of phosphoric acid (H.sub.3PO.sub.4) to the element (T) of said fluoroacid in a range from 20:1 to 2:1.

15. The composition according to claim 1 further comprising at least one -hydroxycarboxylic acid represented by the general formula (III):
R.sub.1CH(OH)COOH(III) wherein: R.sub.1 represents a hydrogen atom, a C.sub.1-C.sub.4 alkyl group, a C.sub.2-C.sub.6 alkenyl group, a C.sub.1-C.sub.6 alkoxy group, a C.sub.3-C.sub.6 cycloalkyl group or a C.sub.6-C.sub.10 aryl group.

16. The composition according to claim 1 being substantially free of peroxide and persulphate compounds.

17. The composition according to claim 1, wherein said at least one fluoroacid is present in an amount such that the composition has a molar ratio of phosphoric acid (H.sub.3PO.sub.4) to the element (T) of said fluoroacid in a range from 10:1 to 4:1.

18. A process for imparting a passivate film to a substrate comprising at least one zinc or zinc alloy coated surface, comprising: contacting the at least one zinc or zinc alloy coated surface of the substrate with the aqueous composition of claim 1 at a temperature ranging from 20 C. to 90 C. for a period of time sufficient to form a passivate film thereon.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The Passivation Composition

(2) Component (i)

(3) The composition comprises by necessity phosphoric acid. The added amount thereof is that required to adjust the pH of the passivation composition to a value of less than 3, in particular to a pH of from 1 to 3 or from 1.2 to 2.8.

(4) Component (ii)

(5) A second required component of the composition of the present invention is constituted by at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, wherein said polyphosphonic acid has the general formula (I):

(6) ##STR00002##
in which: n is at least 2; and, Z is a connecting organic moiety having an effective valency of n, said polyphosphonic acid being characterized in that at least two phosphonic groups are separated by an alkylene bridge having 1 or 2 carbon atoms (C.sub.1-C.sub.2 alkylene).

(7) In particular embodiments, n is an integer from 2 to 5 or, preferably, either 2 or 3. Most desirably, said polyphosphonic acid is selected from a group consisting of aminotris(methylene phosphonic acid) (ATMP); 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP); hexamethylene diamine tetra(methylene phosphonic acid) (HDTMP); diethylenetriamine penta (methylene phosphonic acid); diethylenetriamine penta (methylenephosphonic acid (DTPMP); and, mixtures thereof. A particular preference for the use of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) should be noted.

(8) Suitable water soluble salts of the aforementioned polyphosphonic acids include the sodium, potassium, calcium, magnesium, ammonium, triethanolammonium, diethanolammonium and monoethanolammonium salts.

(9) The polyphosphonic acids or the water soluble salts thereof are preferably included in the compositions in an amount such that the molar ratio of phosphonate groups to phosphoric acid (H.sub.3PO.sub.4) in the composition is in the range from 2:1 to 1:1, more preferably in the range from 1.75:1 to 1.25:1 and most preferably from 1.6:1 to 1.4:1. Compositions meeting these molar ratio conditions have been found to be effective and stable without promoting substantial etching of the coated substrates to which they are applied.

(10) Component (iii)

(11) The passivation composition further contains at least one divalent metal cation (M.sup.2+). In preferred embodiments, said at least one divalent metal cation (M) is selected from the group consisting of: Mg.sup.2+; Ca.sup.2+; Mn.sup.2+; Co.sup.2+; Ni.sup.2+; Sr.sup.2+; Ba.sup.2+; and, Zn.sup.2+. The foregoing metal ions or mixtures thereof are most conveniently introduced into the composition as metal oxides, metal hydroxides and/or soluble and compatible metal salts, including but not limited to sulfate and halide salts. The use of nitrate and fluoride salts for this purpose is not preferred, however.

(12) In a preferred embodiment of the present invention, the passivation composition comprises magnesium (Mg.sup.2+). This magnesium is desirably introduced into the aqueous passivation composition as one or more of: magnesium oxide, magnesium hydroxide; magnesium sulphate; and, magnesium chloride. A particular preference for magnesium oxide or magnesium hydroxide may be noted.

(13) The total molar concentration of the divalent metal cations (M.sup.2+) in the aqueous composition is conventionally in the range from 0.01 to 1 moles/litre but more typically is from 0.01 to 0.5 moles/litre.

(14) Component (iv)

(15) In accordance with the present invention, the passivation composition comprises at least one water-soluble or water-dispersible fluoroacid or a salt thereof, wherein said fluoroacid is defined by the following general empirical formula (II):
H.sub.pT.sub.qF.sub.rO.sub.s(II)
wherein: each of q and r represents an integer from 1 to 10; each of p and s represents an integer from 0 to 10; and, T represents an element selected from the group consisting of Ti, Zr, Hf, Si, Sn, Al, Ge, and B.
Preferred fluoroacids of empirical formula (II) include compounds where: T is selected from Ti, Zr, or Si; p is 1 or 2; q is 1; r is 2, 3, 4, 5 or 6; and, s is 0, 1, or 2.

(16) Exemplary fluoroacids used in the process of the invention may be selected from the group consisting of: fluorotitanic acid (H.sub.2TiF.sub.6); fluorozirconic acid (H.sub.2ZrF.sub.6); fluorosilicic acid (H.sub.2SiF.sub.6); fluoroboric acid (HBF.sub.4); fluorostannic acid (H.sub.2SnF.sub.6); fluorogermanic acid (H.sub.2GeF.sub.6); fluorohafnic acid (H.sub.2HfF.sub.6); and, fluoroaluminic acid (H.sub.3AlF.sub.6); Preferred fluoroacids are: fluorotitanic acid (H.sub.2TiF.sub.6); fluorozirconic acid (H.sub.2ZrF.sub.6); and, fluorosilicic acid (H.sub.2SiF.sub.6).

(17) Subject to the condition that the salt is water-soluble or water dispersible, one or more of the H atoms of the aforementioned fluoroacids may be replaced by suitable cations, such as ammonium, alkaline earth metal cations or alkali metal cations. The salts of alkali metal cations and ammonium are preferred in this context and mention may therefore be made of the following examples of suitable fluoroacid salts: (NH.sub.4).sub.2ZrF.sub.6; H(NH.sub.4) ZrF.sub.6; (NH.sub.4).sub.2TiF.sub.6; H(NH.sub.4).sub.2TiF.sub.6; Na.sub.2ZrF.sub.6: K.sub.2ZrF.sub.6; Li.sub.2ZrF.sub.6: Na.sub.2TiF.sub.6; K.sub.2TiF.sub.6; and, Li.sub.2TiF.sub.6.

(18) Such salts may be added directly to the composition or may be produced in situ in the aqueous passivation composition by the partial or full neutralization of the acid fluoride or acid oxyfluoride with an appropriate base. It is noted that said base may be organic or inorganic in character: ammonium bicarbonate and hydroxylamine might be used, for instance.

(19) The fluoroacid or salt thereof is typically included in the composition such that the molar ratio of phosphoric acid (H.sub.3PO.sub.4) to the metal (T) of said fluoroacid is in the range from 20:1 to 2:1, preferably from 12:1 to 3:1 and more preferably 10:1 to 4:1. When the level of phosphoric acid is outside the above ranges, the stability of the formulation is diminished: at lower levels of phosphoric acid within the stated ranges, the concomitant loss of stability of the formulation can be mitigated by increasing the amount of divalent metal cations in the composition. When the level of metal (T) falls below the stated molar ranges, the stability of the composition may be substantively affected but a decline in performance in the neutral salt spray (NSS) may be observed.

(20) In an alternative but not mutually exclusive expression, the fluoroacid or salt thereof should be included in the passivation composition such that the molar concentration of the metal (T) in the aqueous composition is conventionally in the range from 0.1 to 1 moles/litre but more typically is from 0.2 to 0.8 moles/litre.

(21) Adjunct Ingredients

(22) The presence of other complex fluoride anions in the passivation composition is not precluded and mention in this regard may be made of: fluoroindates (e.g. InF.sub.4.sup.1); fluorophosphates (e.g. PF.sub.6.sup.1); fluoroarsenates (e.g. AsF.sub.6.sup.1); fluoroantimonates (e.g. SbF.sub.6.sup.1); fluorobismuthates (e.g. BiF.sub.6.sup.1); fluoro sulfates (e.g. SF.sub.6.sup.2); fluoroselenates (e.g. SeF.sub.6.sup.2); fluorotellurates (e.g. TeF.sub.6.sup.2 or TeOF.sub.5.sup.1); fluorocuprates (e.g. CuF.sub.3.sup.1); fluoroargentates; fluorozincates (e.g., ZnF.sub.4.sup.2); fluorovanadates (e.g. VF.sup.2); fluoroniobates (e.g. NbF.sup.2); fluorotantalates (e.g. TaF.sub.7.sup.2); fluoromolybdates (e.g. MoF.sub.6.sup.3); fluorotungstates (e.g. WF.sub.6.sup.1); fluoroyttrates (e.g. YF.sub.6.sup.3); fluorolanthanates (e.g. LaF.sub.6.sup.3); fluorocerates (e.g. CeF.sub.6.sup.3 or CeF.sub.6.sup.2); fluoromanganates (e.g. MnF.sub.6.sup.2); fluoroferrates (e.g. FeF.sub.6.sup.3); fluoronickelates; and fluorocobaltates. Such anions may be included in the form of water-soluble or water dispersible salts, in particular the ammonium, alkaline earth metal or alkali metal salts.

(23) When present, said complex fluoride anions should be included in the composition in an amount up to 0.1 moles/litres, for example up to 0.05 moles/litre.

(24) The presence in the passivation composition of free fluoride ionsnot bound in complex formis also not precluded as the fluoride anions can act as accelerators in the formation of passivation coatings and are present at the interface between the conversion coating and the metal matrix. Such free fluoride anions can be included through the addition to the passivation compositions of, for example: hydrofluoric acid; alkali metal fluorides, such as sodium fluoride; alkali metal hydrogen fluorides, such as sodium hydrogen fluoride; ammonium fluoride; and, ammonium hydrogen fluoride.

(25) This aside, the presence of free fluoride ionsnot bound in complex formis not preferred. Despite the utility of the fluoride species in the passivation compositions, the environmental release of fluoride is problematic as documented in https://www.cdc.gov/niosh/. Thus, it is preferred that the passivation composition be substantially free of free fluoride anions.

(26) In addition to the aforementioned phosphoric acid, the passivation compositions may comprise one or more further mineral acids: the use of nitric acid is not precluded but is not preferred; conversely, the addition of phosphonic or sulphuric acid is considered to be particularly suitable. The above recited pH of the passivation composition is somewhat determinative of the added amount of such acid(s). Within that pH constraint, the presence of phosphonate or sulphate ions in the treatment bath in concentrations of up to 5% by weight and, more particularly, between 0.1 and 3% by weight can be advantageous.

(27) The composition of the present invention may optionally comprise at least one -hydroxycarboxylic acid represented by the general formula (III): R.sub.1CH(OH)COOH(III) wherein: R.sub.1 represents a hydrogen atom, a C.sub.1-C.sub.4 alkyl group, a C.sub.2-C.sub.6 alkenyl group, a C.sub.1-C.sub.6 alkoxy group, a C.sub.3-C.sub.6 cycloalkyl group or a C.sub.6-C.sub.10 aryl group.

(28) Suitable -hydroxycarboxylic acids include but are not limited to: glycolic acid; lactic acid (2-hydroxypropanoic acid); 2-hydroxybutanoic acid; 2-hydroxypentanoic acid; 2-hydroxyhexanoic acid; glucuronic acid; citric acid; mandelic acid; galacturonic acid; ribonic acid (2,3,4,5-tetrahydroxypentanoic acid); gluconic acid (2S,3S,4R,5S)-2,3,4,5,6-pentahydroxyhexanoic acid; tartronic acid; tartaric acid; and, malic acid.

(29) In a preferred embodiment, said at least one -hydroxycarboxylic acid is selected from the group consisting of: glycolic acid; gluconic acid; lactic acid (2-hydroxypropanoic acid); 2-hydroxybutanoic acid; 2-hydroxypentanoic acid; and, 2-hydroxyhexanoic acid. More particularly, the -hydroxycarboxylic acid(s) of the coating composition should comprise or consist of gluconic acid.

(30) For completeness, it is again noted that the above recited pH of the passivation composition is somewhat determinative of the added amount of such -hydroxycarboxylic acid(s). When added within that pH constraint, the -hydroxycarboxylic acid(s) should conventionally be included in the aqueous passivation composition in an amount up to 0.1 moles/litres, for example up to 0.05 moles/litre.

(31) It is considered that the corrosion-protection performance of the disclosed passivation compositionsand resulting passivate filmscan be enhanced by the incorporation of a transition metal salt and/or a transition metal complex therein. Considered particularly useful in this regard are the salts or complexes of transition metals selected from the group consisting of Ce, Ni, Co, V, Fe, Zn, Zr, Mn, Mo, W, Ti, Zr, Hf, Bi and the lanthanides.

(32) Whilst said transition metals may be present in the complex fluoride anions mentioned hereinabove, such transition metals may alternatively or additionally be included in the composition as complexes with other ligands and/or as salts with further anions, provided said salts are at least partially soluble in water. As examples of anions, there may be mentioned: oxide; hydroxide; sulphate; chloride; iodide; citrate; lactate; succinate; formate; oxalate; malonate; and, acetate. As exemplary ligands for transition metal complexes, there may be mentioned: ethylenediaminetetraacetic acid (EDTA); diethylenetriaminepentaacetic acid (DTPA); hydroxyethylethylenediaminetriacetic acid (HEDTA); nitrilotriacetic acid (NTA); and, methylglycinediacetic acid (MGDA).

(33) The present compositions may further comprise additives which are conventional in this field; in particular, the compositions might comprise: corrosion inhibitors, such as dialkylthioureas, cupric sulphate and copper sulphate; adhesion promoters; non-ionic surfactants; wetting agents; de-foaming agents; sequestrants; lubricants; and, mixtures thereof. As further exemplary corrosion inhibitors mention may be made of the following commercial materials: the Rodine series, available from JMN Specialties, Inc. and Henkel Corporation; the Dodicor series, available from Clariant AG; and, the Armohib series available from Akzo Nobel Surfactants LLC. That aside, any such additives are necessarily minor ingredients of the present compositions and, when used, should only be used in amounts which are not deleterious to the performance of the composition and the coating derived there from.

(34) Exemplary Formulation of the Passivation Compositions

(35) In an exemplary embodiment, which embodiment is not intended to be limiting of the present invention, there is provided an aqueous passivation composition having a pH of less than 3, said composition comprising: i) phosphoric acid; ii) 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) such that the molar ratio of phosphonate groups to phosphoric acid (H.sub.3PO.sub.4) is in the range from 2:1 to 1:1; iii) Mg.sup.2+ and optionally at least one further divalent metal cation (M.sup.2+) selected from the group consisting of Ca.sup.2+, Mn.sup.2+, Co.sup.2+, Ni.sup.2+, Sr.sup.2+, Ba.sup.2+, and Zn.sup.2+, wherein the total concentration of divalent metal cations (M.sup.2+) is in the range from 0.01 to 1 moles/litre; and, iv) at least one fluoroacid selected from the group consisting of fluorotitanic acid (H.sub.2TiF.sub.6), fluorozirconic acid (H.sub.2ZrF.sub.6) and fluorosilicic acid (H.sub.2SiF.sub.6), wherein said fluoroacid is present in an amount such that the molar ratio of phosphoric acid (H.sub.3PO.sub.4) to the metal (T) of said fluoroacid is in the range from 20:1 to 2:1.

(36) In a further exemplary embodiment, which embodiment is again not intended to be limiting of the present invention, there is provided an aqueous passivation composition having a pH of less than 3, said composition comprising: i) phosphoric acid; ii) 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) such that the molar ratio of phosphonate groups to phosphoric acid (H.sub.3PO.sub.4) is in the range from 1.6:1 to 1.4:1; iii) Mg.sup.2+ and optionally at least one further divalent metal cation (M.sup.2+) selected from the group consisting of Ca.sup.2+, Mn.sup.2+, Co.sup.2+, Ni.sup.2+, Sr.sup.2+, Ba.sup.2+, and Zn.sup.2+, wherein the total concentration of divalent metal cations (M.sup.2+) is in the range from 0.01 to 0.5 moles/litre; and, iv) at least one fluoroacid selected from the group consisting of fluorotitanic acid (H.sub.2TiF.sub.6), fluorozirconic acid (H.sub.2ZrF.sub.6) and fluorosilicic acid (H.sub.2SiF.sub.6), wherein said fluoroacid is present in an amount such that the molar ratio of phosphoric acid (H.sub.3PO.sub.4) to the metal (T) of said fluoroacid is in the range from 10:1 to 4:1.
Preparation of the Passivation Compositions

(37) The aqueous passivation compositions are formulated by simple mixing of the various components. If necessary, the passivation composition may be prepared well in advance of its application. However, in an interesting alternative embodiment, a concentrated passivation composition may first be obtained by mixing components with only a fraction of the water that would be present in the passivation composition as applied: the concentrated passivation composition may then be diluted with the remaining water shortly before its introduction into the passivation bath. It is considered that such concentrated passivation compositions may be prepared and stored as either single-package concentratesthat can be converted by dilution with water onlyor as multi-part concentrates, two or more of which must be combined and diluted to form a complete working composition according to the invention. Any dilution can be effected simply by the addition of water, in particular deionized and/or demineralized water, under mixing. The passivation composition might equally be prepared within a rinse stream whereby one or more streams of the concentrate(s) is injected into a continuous stream of water.

(38) Without specific intention to limit the amount of water included in the passivation compositions, it is preferred that said compositions contain from 40 to 90 wt. %, more preferably from 50 to 80 wt. %, based on the weight of the composition, of water. In an alternative but not mutually exclusive characterization, the passivation composition may be defined by a viscosity of from 0.005 to 1 Pa.Math.s (50 cps to 1000 cps), as measured using a Brookfield viscometer at 25 C.

(39) Methods and Applications

(40) Whilst the present invention is concerned with passivating of surfaces of zinc or zinc alloys, there is no intention to limit the base substrate to which that zinc or zinc alloy may have been applied nor the method of such application. As such, suitable base metal substrates may include but not be limited to iron, nickel, copper, aluminium and alloys thereof. Such metals and alloys may be provided in various forms, including sheets, plates, cuboids, spheres, solid cylinders, tubes and wires. Moreover, the plating or coating of zinc or zinc alloy may be applied to such base substrates by: electroplating; galvanizing, including hot-dip galvanizing and thermal diffusion galvanizing; and, galvannealing. By way of example only, the passivation compositions and methods of the present invention may have utility in the treatment of: GALVALUME, a 55% Al/43.4% Zn/1.6% Si alloy coated sheet steel available from Bethlehem Steel Corporation; and, GALFAN, a 5% Al/95% Zn alloy coated sheet steel available from Weirton Steel Corporation.

(41) In accordance with process aspects of the present invention, it is often advisable to remove foreign matter from the coated or plated metal substrate by cleaning and degreasing the relevant surfaces. Such treatments are known in the art and can be performed in a single or multi-stage manner constituted by, for instance, the use of one or more of: a waterborne alkaline degreasing bath; a waterborne cleaning emulsion; a cleaning solvent, such as carbon tetrachloride or trichloroethylene; and, a water rinse, preferably of deionized or demineralized water. In those instances where a waterborne alkaline degreasing bath is used, any of the degreasing agent remaining on the surface should desirably be removed by rinsing the substrate surface with deionized or demineralized water. Irrespective of the cleaning or degreasing agent applied, the so-treated substrate should not be subjected to an intermediate drying step prior to either the passivation treatment or to any subsequent pre-treatment step which precedes said passivation treatment.

(42) As therefore intimated above, the present invention does not preclude the pre-treatment of the zinc or zinc alloy surface, independently of the performance of cleaning and/or degreasing steps. Such pre-treatments are known in the art and reference in this regard may be made to: German Patent Application No. DE 197 33 972 A1; German Patent Application No. DE 10 2010 001 686 A1; German Patent Application No. DE 10 2007 021 364 A1; and, US Patent Application Publication No. 2014/360630.

(43) After said cleaning, degreasing and/or pre-treatment steps, an operating bath as hereinbefore described is prepared and the passivation composition is applied to the substrate by, without limitation, immersion, flooding, air-atomized spraying, air-assisted spraying, airless spraying, high-volume low-pressure spraying and air-assisted airless spraying. The minimum contact time of the composition with the substrate is most broadly that time which is sufficient to form the desired passivate film thereon: that contact time can be as little as 1 second or as great as 15 minutes in that instance where the passivation or conversion treatment is being performed on metal that will be cold worked: however, dependent upon the pH and the concentration of the applied solution, a contact time of from 5 to 300 seconds, for example from 5 to 50 seconds, would be more typical. Moreover, the compositions are applied at a temperature ranging from 20 C. to 90 C., for instance from 30 C. to 80 C. or from 40 C. to 70 C.

(44) At the conclusion of the passivation treatment, the article is extracted from the bath and dried using, for instance, ambient air drying, circulating warm air, forced air drying or infrared heating. It is not precluded that the article be subjected to: at least one water rinse to remove residual passivation composition therefrom; and/or, rinsing with a dilute silicate solution based on the aforementioned silicate compounds and having a temperature of from 20 C. to 70 C. The silicate compound can be present in the rinse solution in an amount of from 1 to 40 g/l, for example from 5 to 15 g/l, calculated as SiO.sub.2. The rinsed substrate may be dried after completion of the rinsing step(s) or, if applicable, after each rinse solution.

(45) The composition according to the present invention yields a passivate film that is either colorless, or blue or olive in color, with a flat to glossy finish. The exact nature of that finish is determined predominantly by the base substrate, the zinc or zinc alloy coating, and the immersion time in the conversion coating composition. Zinc or zinc alloy coatings passivated in accordance with the present invention exhibit corrosion protection to 50-96 hours before the observed onset of white rust corrosion, as defined by ASTM B-201. Alternatively or additionally, said zinc or zinc alloy coatings passivated in accordance with the present invention exhibit corrosion protection to 50-96 hours before the observed onset of white rust corrosion (as defined by ASTM B-201) when treated with neutral salt spray (NSS, 5 wt. % NaCl, 95 wt. % H.sub.2O) under steady state conditions in accordance with the procedure of ASTM B-117.

(46) The present invention does not preclude supplementary conversion coatings being applied to the passivate film obtained in accordance with the present invention; indeed such supplementary coatings may further extend corrosion protection and improve the aesthetics of the finished article. Inorganic coatings based on silicates and organic conversion coatings based on epoxy resins might be mentioned as non-limiting examples of supplemental conversion coatings: reference in this regard may be made to inter alia U.S. Pat. No. 5,743,971 (Inoue) and U.S. Pat. No. 5,855,695 (McMillen). These supplemental conversion coatings may be applied by any suitable means known in the art, such as by dipping, spraying, electro-coating or powder coating.

(47) The conversion coating(s) may constitute the topcoat applied to the substrate surface. Alternatively, the conversion coating(s) may serve: as an undercoat for paints, lacquers, inks or powder coatings; as a base to which polymers, such as rubber, may be bound; and/or, as a base to which adhesives or sealants may be applied.

(48) Various features and embodiments of the disclosure are described in the following examples, which are intended to be representative and not limiting.

EXAMPLES

(49) The following commercial products are used in the Reference Examples and Example according to the invention: Codex 661:1-Hydroxyethylidene-1,1-diphosphonic acid (CAS No. 2809-21-4) available from Excel Industries Limited. TD-1355-HM: Polymer resin available from Henkel Surface Technologies PVT Ltd. Fluotitanic Acid: Hexaflurorotitanic acid (H.sub.2TiF.sub.6) available S.B. Chemicals.

(50) Aqueous passivation compositions were prepared by mixing the ingredients given in Table 1 herein below:

(51) TABLE-US-00001 TABLE 1 Composition (g) Reference Reference Example Example Example Ingredient 1 2 1 Water 40.7 86.0 67.5 Phosphoric acid (85%) 15.6 6.0 10.0 1-Hydroxyethylidene- 0 6.0 15.0 1,1-Diphosphonic Acid (60%) Chromic acid (H.sub.2CrO.sub.4) 7.8 0 0 Chromium nitrate (Cr(NO.sub.3).sub.3) 25.0 0 0 TD-1355-HM 10 0 0 Magnesium Oxide (MgO) 0 0 2.5 Vanadium Oxide (V.sub.2O.sub.5) 0 2 0 Fluotitanic Acid (50%) 0 0 5 Gluconic acid (50%, 4.0 0 0 Technical Grade)

(52) Based on these tabulated aqueous compositions, the following tests were performed.

(53) Specific Gravity: The specific gravity of the aqueous compositions was measured in accordance with ASTM D891-18.

(54) Standard Test Panel Preparation: Specimens of Advanced Coating Technology (ACT) G-90 hot dipped galvanized steel were mechanically cut into squares of 4 cm4 cm dimensions. Each obtained panel was treated with an alkaline cleaner at 55 C. for 10 seconds, rinsed with tap water at room temperature and then dried by squeegeeing. The panels were then separately coated with a defined wet layer thickness of each passivation composition selected for evaluation using a Chemcoater: duplicate panels were prepared for each passivation composition. The resultant test panels coated with a wet film of the passivation composition were then dried upon heating to a peak metal temperature (PMT) of from 55-60 C. The obtained coating weight of the test panels was determined on a metals basis.

(55) Zinc Dissolution Panel Preparation: Specimens of Advanced Coating Technology (ACT) G-90 hot dipped galvanized steel were mechanically cut into squares of 4 cm4 cm dimensions. Each obtained panel was treated with an alkaline cleaner at 55 C. for 10 seconds, rinsed with tap water at room temperature and then dried by squeegeeing. The panels were then separately immersed for 2 hours in a bath (volume 20 ml) of each passivation composition selected for evaluation. The resultant coated test panels were then removed from the bath. To then measure the amount of zinc which was dissolved during the formation of the conversion coating, a complexometric (or chelometric) titration with EDTA (ethylenediaminetetraacteic acid) was performed on the residual aqueous composition in the bath.

(56) Neutral salt spray (NSS): This test was carried out according to ASTM B117 with a 5% NaCl solution at 35 C. (https://www.astm.org/Standards/B117). The coated panels were disposed in the spray chamber (ERICHSEN Model 606/400 L) at 15-30 from the vertical for 96 hours. The test panels were not allowed to contact other surfaces in the chamber and condensed or corrosion products on their surfaces were not permitted to cross-contaminate each other. Photographic recording of the test panels was performed each 24 hours. After exposure, test panels were rinsed in deionised water to remove salt deposits from their surface and then immediately dried. From a visual inspection of the coated panels at 96 hours: i) coated panels for which less than 5% by area showed white rust were held to have passed said test; and, ii) conversely, coated panels showing >5% by area of white rust were held to have failed said test.

(57) The results of these tests are illustrated in Table 2 herein below.

(58) TABLE-US-00002 TABLE 2 Reference Reference Test Parameters Example 1 Example 2 Example 1 Appearance Dark green Clear blue Colorless pH (25% v/v bath) 0.3 2.3 Total Acidity of Composition 27 30.6 (mg/g KOH) Specific Gravity 1.26 1.15 Composition Stability at Stable, no Stable, no Stable, no pH 8.5 precipitation precipitation precipitation Coating Weight (mg/m.sup.2) 35 25 5-8 on a metals Basis Salt Spray Tests Pass Fail Pass (ASTM B117)

(59) In view of the foregoing description and examples, it will be apparent to those skilled in the art that equivalent modifications thereof can be made without departing from the scope of the claims.