PASSIVATION COMPOSITION BASED ON TRIVALENT CHROMIUM

Abstract

An aqueous passivation composition, having a pH of less than 3, comprising: i) a source of trivalent chromium ions; ii) at least one -hydroxycarboxylic acid having general formula (I): R.sub.1CH(OH)COOH (I); wherein: R.sub.1 represents a hydrogen atom, a C1-C4 alkyl group, a C2-C6 alkenyl group, a C1-C6 alkoxy group, a C.sub.3-C.sub.6 cycloalkyl group or a C6-C10 aryl group; iii) phosphoric acid; iv) at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, said polyphosphonic acid having general formula (II): Z[PO(OH).sub.2].sub.n (II); in which: n is at least 2; and Z is a connecting organic moiety having an effective valency of n, said polyphosphonic acid including at least two phosphonic groups separated by an alkylene bridge having 1 or 2 carbon atoms (C.sub.1-C.sub.2 alkylene); and v) at least one divalent metal cation;
wherein said composition is substantially free of: nitrate and fluoride anions, and hexavalent chromium.

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) a source of trivalent chromium (Cr(III)) ions; ii) at least one -hydroxycarboxylic acid represented by general formula (I):
R.sub.1CH(OH)COOH(I) wherein: R.sub.1 represents a hydrogen atom, a C1-C4 alkyl group, a C2-C6 alkenyl group, a C1-C6 alkoxy group, a C.sub.3-C.sub.6 cycloalkyl group or a C6-C10 aryl group; iii) phosphoric acid; iv) at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, wherein said polyphosphonic acid corresponds to general formula (II): ##STR00004## in which: n is at least 2; and Z is a connecting organic moiety having an effective valency of n, said polyphosphonic acid comprising at least two phosphonic groups separated by an alkylene bridge having 1 or 2 carbon atoms (C.sub.1-C.sub.2 alkylene); and v) at least one divalent metal cation; wherein said composition is substantially free of nitrate and fluoride anions and is substantially free of hexavalent chromium (Cr(VI)).

2. The composition according to claim 1, wherein the source of trivalent chromium ions comprises a salt selected from the group consisting of: chromium sulfate (Cr.sub.2(SO.sub.4).sub.3); chromium alum (KCr(SO.sub.4).sub.2); chromium chloride (CrCl.sub.3); and chromium bromide (CrBr.sub.3).

3. The composition according to claim 1, wherein the source of trivalent chromium ions comprises: a) a source of hexavalent chromium ions (Cr(VI)); and b) at least one reducing agent present in an amount sufficient to ensure complete reduction of the hexavalent chromium ions (Cr(VI)) to trivalent chromium ions.

4. The composition according to claim 3, wherein: said source of hexavalent chromium ions (Cr(VI)) is selected from the group consisting of chromium (VI) oxide, alkali metal chromates, alkali metal dichromates and combinations thereof; and said at least one reducing agent is present in an amount up to a stoichiometric excess of 1 mol. % relative to the hexavalent chromium ions (Cr(VI)).

5. The composition according to claim 1, wherein the concentration of trivalent chromium ions (Cr(III)) is from 0.005 to 0.1 moles/liter.

6. The composition according to claim 1, wherein said at least one -hydroxycarboxylic acid is selected from the group consisting of: glycolic acid; lactic acid (2-hydroxypropanoic acid); 2-hydroxybutanoic acid; 2-hydroxypentanoic acid; 2-hydroxyhexanoic acid; and mixtures thereof.

7. The composition according to claim 1, wherein having regard to the carboxylic acid groups provided by said at least one -hydroxycarboxylic acid, the molar ratio of said carboxylic acid groups to chromium (Cr) is in a range from 1:10 to 1:2.

8. 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.

9. The composition according to claim 1, wherein said at least one polyphosphonic acid or the water soluble salt thereof is present in the composition in an amount such that a molar ratio of phosphonate groups to phosphoric acid (H.sub.3PO.sub.4) in the composition is in a range from 1:0.75 to 1:1.25.

10. The composition according to claim 1, wherein said least one divalent metal cation is selected from the group consisting of: Mg.sup.2+; Ca.sup.2+; Mn.sup.2+; Sr.sup.2+; Ba.sup.2+; Zn.sup.2+ and mixtures thereof.

11. The composition according to claim 1, wherein said least one divalent metal cation is present in the aqueous passivation composition at a molar concentration in a range from 0.01 to 1 moles/litre.

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

13. An aqueous passivation composition for the treatment of zinc or zinc alloy coatings comprising: i) a source of trivalent chromium (Cr(III)) ions, present in a concentration of from 0.005 to 0.1 moles/liter; ii) at least one -hydroxycarboxylic acid, wherein said at least one -hydroxycarboxylic acid is selected from the group consisting of glycolic acid, lactic acid, 2-hydroxybutanoic acid, 2-hydroxypentanoic acid and 2-hydroxyhexanoic acid, and wherein having regard to the carboxylic acid groups provided by said at least one -hydroxycarboxylic acid, a molar ratio of said carboxylic acid groups to chromium (Cr) is in a range from 1:10 to 1:2; iii) phosphoric acid; iv) at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, wherein 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; and wherein said at least one polyphosphonic acid or the water soluble salt thereof is present in the composition in an amount such that a molar ratio of phosphonate groups to phosphoric acid (H.sub.3PO.sub.4) in the composition is in a range from 1:0.75 to 1:1.25; and v) at least one divalent metal cation selected from the group consisting of Mg.sup.2+, Ca.sup.2+, Mn.sup.2+, Sr.sup.2+, Ba.sup.2+ and Zn.sup.2+, wherein a molar concentration of the divalent metal cations in the aqueous passivation composition is in a range from 0.01 to 1 moles/liter.

14. A process for imparting a chromate passivate film to a substrate to which a zinc or zinc alloy coating has been applied to at least one surface thereof, said process comprising contacting at least one zinc or zinc alloy coated surface of the substrate with an aqueous composition according to 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

The Passivation Composition

Component i)

[0059] The trivalent chromium ions can be directly introduced to the passivation composition in the form of a bath soluble and compatible compound: salts such as chromium sulfate (Cr.sub.2(SO.sub.4).sub.3), chromium alum (KCr(SO.sub.4).sub.2), chromium chloride (CrCl.sub.3) and chromium bromide (CrBr.sub.3) are particularly suitable for this purpose.

[0060] In an alternative, but not necessarily mutually exclusive embodiment, the trivalent chromium is introduced into the passivation composition through the reduction of an aqueous hexavalent chromium-containing solution. There is no particular intention to limit the sources of hexavalent chromium which may be used, save that the anions or cations introduced with the hexavalent chromium should not have a detrimental effect on either the composition itself or on the coated zinc obtained therefrom. Exemplary hexavalent chromium materials include but are not limited to: chromium (VI) oxide; alkali metal chromates, in particular sodium chromate and potassium chromate; and alkali metal dichromates, in particular sodium dichromate and potassium dichromate. These materials may be used alone or in combination.

[0061] Methods for reducing hexavalent chromium with inorganic reducing agents and organic reducing agentsincluding amine based compounds, such as hydrazine and hydroxylamineare known in the art and do not require substantial elucidation here. Particularly instructive methods are, by way of non-limiting examples, described in UK Patent No. GB 1,461,244 and U.S. Pat. No. 4,171,231, the disclosures of which are herein incorporated by reference. It will be recognized that those prior art redox methods may require modification in the amounts of reducing agent used to ensure complete reduction of the chromium (VI), as required in the present invention. For instance, U.S. Pat. Nos. 3,063,877 and 3,501,352 describe methods for the incomplete reduction of chromium (VI) oxide with aldehydes and alcohols, such as formaldehyde, sorbitol and butyl alcohol: the amount of said reducing agents needs to be increased to ensure complete reduction of the hexavalent chromium to trivalent chromium. In that regard, whilst the exact stoichiometric amount of reducing required for complete reduction of hexavalent to trivalent chromium can be used, it is preferred that a stoichiometric excess of up to 1 mol. % of the reducing agents be employed.

[0062] For completeness, it is noted that suitable inorganic reducing agents include but are not limited to: alkali metal iodides; tin (II) compounds, such as SnSO.sub.4 and SnCl.sub.2.2H.sub.2O; antimony (III) compounds; ferrous salts, such as ferrous sulphate heptahydrate (HH), ferrous sulphate monohydrate (MH) and ammonium ferrous sulphate; sulfur dioxide; and alkali metal sulfites, bisulfites and metabisulfites as well as -hydroxycarboxylic acids according to component ii). Of these, -hydroxycarboxylic acids according to component ii), especially glycolic acid, are preferred as will be outlined in the next section.

[0063] The concentration of the trivalent chromium ions in the passivation composition will conventionally be from 0.001 moles/liter up to saturation; concentrations of from 0.005 to 0.1 moles/liter, for example from 0.01 to 0.05 moles/liter are preferred.

Component ii)

[0064] The composition of the present invention comprises at least one -hydroxycarboxylic acid represented by the general formula (I):

R.sub.1CH(OH)COOH(I) [0065] wherein: R1 represents a hydrogen atom, a C1-C4 alkyl group, a C2-C6 alkenyl group, a C1-C6 alkoxy group, a C.sub.3-C.sub.6 cycloalkyl group or a C6-C10 aryl group.

[0066] 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); tartronic acid; tartaric acid; and malic acid.

[0067] In a preferred embodiment, said at least one -hydroxycarboxylic acid is selected from the group consisting of: glycolic 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 glycolic acid.

[0068] The -hydroxycarboxylic acids are preferably included in the compositions in an amount such that the molar ratio of carboxylic acid groups to chromium is in the range from 1:10 to 1:2. For example, -hydroxycarboxylic acids having one carboxylic acid group, such as glycolic acid and the other aforementioned preferred acids, may be included in the compositions such that the molar ratio of carboxylic acid groups to chromium is from 1:10 to 1:2, more preferably from 2:10 to 2:5.

[0069] Surprisingly, it was found that the -hydroxycarboxylic acids are effective reducing agents and readily convert a hexavalent chromium containing solution to a passivation solution of this invention. In this respect, it was observed that a passivation solution of this invention that is obtained from aqueous solutions comprising hexavalent chromium after reduction in the presence of a molar excess amount of -hydroxycarboxylic acid, especially glycolic acid, provide highly stable solutions which is a clear benefit when concentrated passivation solutions with a less amount of water are to be prepared.

[0070] Consequently, the present invention also encompasses a passivating solution having a pH of less than 3 comprising: [0071] i) a source of trivalent chromium (Cr(III)) ions; [0072] ii) at least one -hydroxycarboxylic acid represented by the general formula (I):

R1CH(OH)COOH(I)

wherein: R1 represents a hydrogen atom, a C1-C4 alkyl group, a C2-C6 alkenyl group, a C1-C6 alkoxy group, a C3-C6 cycloalkyl group or a C6-C10 aryl group; [0073] iii) phosphoric acid; [0074] iv) at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, wherein said polyphosphonic acid has the general formula (II):

##STR00002## [0075] in which: [0076] n is at least 2; and [0077] 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 (C1-C2 alkylene); and [0078] v) at least one divalent metal cation,
wherein said composition is characterized in that it is substantially free of nitrate and fluoride anions and is substantially free of hexavalent chromium (Cr(VI)); wherein said composition is obtainable through mixing a portion comprising hexavalent chromium dissolved in water with an amount of -hydroxycarboxylic acids according to component ii), preferably comprising or consisting of glycolic acid, in molar excess preferably to such extend that a molar ratio of carboxylic acid groups to chromium in the range from 1.1:1 to 1.5:1, more preferably from 1.2:1 to 1.4:1 is established and thereafter adding the components iii) to v) to said mixture.
Component iii)

[0079] 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.

Component iv)

[0080] A further essential 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 (II):

##STR00003##

in which:

[0081] n is at least 2; and

[0082] 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).

[0083] 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.

[0084] Suitable water soluble salts of the aforementioned polyphosphonic acids include the sodium, potassium, calcium, magnesium, ammonium, triethanolammonium, diethanolammonium and monoethanolammonium salts.

[0085] 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 1:0.75 to 1:1.25, more preferably in the range from 1:0.8 to 1:1.2 and most preferably from 1:0.9 to 1:1.1. It is noted that these preferred ranges each encompass a molar ratio of phosphonate groups to phosphoric acid (H.sub.3PO.sub.4) of 1:1: compositions at or near a molar ratio of 1:1 have been found to be stable without promoting substantial etching of the coated substrates to which they are applied.

Component v)

[0086] The passivation composition further contains at least one divalent metal cation. In preferred embodiments, said least one divalent metal cation is selected from the group consisting of: Mg.sup.2+; Ca.sup.2+; Mn.sup.2+; Sr.sup.2+; Ba.sup.2+; and Zn.sup.2+, preferably from the group consisting of Mg.sup.2+, Mn.sup.2+ and Zn.sup.2+, more preferably from Mn.sup.2+ and/or Zn.sup.2+, most preferably from Mn.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 of course precluded.

[0087] The molar concentration of the divalent metal cations 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.

Adjunct Ingredients

[0088] In the addition to the aforementioned -hydroxycarboxylic acids, the composition of the present invention may optionally contain at least one further carboxylic acid (III), wherein said further carboxylic acid is an alkyl, aryl, alkenyl or alkynyl carboxylic acid which is characterized in that it does not contain polar, in particular protic, groups with exception of the carboxylic group(s). In particular, said further carboxylic acid should not contain any of the following groups: OH, SO.sub.3H, NH.sub.2, NHR.sup.3, N(R.sup.3).sub.2 or N(R.sup.3).sub.3.sup.+, wherein each R.sup.3 independently represents a C.sub.1-C.sub.6 alkyl group. Said carboxylic acid (II) may, however, contain the following groups: halogen; alkyl; aryl; vinyl; alkoxy; and nitro groups.

[0089] Examples of acids which are suitable as said carboxylic acid (II) include, but are not limited to: formic acid; acidic acid; propionic acid; butyric acid; iso-butyric acid; valeric acid; hexanecarboxylic acid; cyclopentanecarboxylic acid; acetylsalicylic acid; benzoic acid; nitrobenzoic acid; 3,5-dinitrobenzoic acid; sorbic acid; trifluoracetic acid; 2-ethylhexanoic acid; acrylic acid; chloroacetic acid; 2-chlorobenzoic acid; 2-chloro-4-nitrobenzoic acid; cyclopropanecarboxylic acid; methacrylic acid; 3-nitrobenzoic acid; phenoxyacetic acid; isovaleric acid; pivelinic acid; 2-ethylbutyric acid; furan-2-carboxylic acid; bromoacetic acid; crotonic acid; 2-chloropropionic acid; dichloroacetic acid; glyoxylic acid; 4-methoxybenzoic acid; 3,4-dimethoxybenzoic acid; levulinic acid; pentenoic acid; phenylacetic acid; tiglic acid; and vinylacetic acid.

[0090] When added to the present compositions, said further carboxylic acid(s) (II) should in toto only be present in an amount up to 10 mol. %, preferably up to 5 mol. %, based on the total number of moles of the -hydroxycarboxylic acid.

[0091] In addition to the aforementioned phosphoric acid, the passivation compositions may comprise one or more further mineral acids: the use of nitric acid is precluded but, 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.

[0092] 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. 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.

Preparation of the Passivation compositions

[0093] The aqueous passivation compositions are formulated by simple mixing of the various components i) to v) as well as any adjunct ingredients. In an alternative route, the passivation compositions of this invention can be obtained through mixing a portion comprising hexavalent chromium (Cr(VI)) dissolved in water with an amount of -hydroxycarboxylic acids according to component ii), preferably comprising or consisting of glycolic acid, in molar excess preferably to such extend that a molar ratio of carboxylic acid groups to chromium in the range from 1.1:1 to 1.5:1, more preferably from 1.2:1 to 1.4:1 is established and thereafter adding the components iii) to v) to said mixture.

[0094] 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 affected 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.

[0095] 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.

Methods and Applications

[0096] 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, aluminum 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.

[0097] 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.

[0098] 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.

[0099] After said cleaning, degreasing and/or pre-treatment steps, a trivalent chromium operating bath as hereinbefore described 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.

[0100] 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/I, for example from 5 to 15 g/I, calculated as SiO2. The rinsed substrate may be dried after completion of the rinsing step(s) or, if applicable, after each rinse solution.

[0101] The composition according to the present invention yields a passivate film that is yellow, olive or black 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 chromated 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 chromated 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.

[0102] 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.

[0103] 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.

[0104] Various features and embodiments of the disclosure are described in the following examples, which are intended to be representative and not limiting.

EXAMPLES

[0105] The following commercial products are used in the Reference Examples and Examples according to the invention: TD-1355-HM: Polymer resin available from Henkel Surface Technologies PVT Ltd.

[0106] Aqueous passivation compositions were prepared by mixing the ingredients given in Table 1 herein below:

TABLE-US-00001 TABLE 1 Composition (g) Reference Reference Ex- Ex- Example Example ample ample Ingredient 1 2 1 2 Water 36.1 48.9 67.8 66.3 Chromium(6+)oxide 6.8 9.6 7.5 7.5 Phosphoric acid (85%) 15.6 17.0 5.2 5.2 Gluconic acid 4.0 0 0 0 (50%, Technical Grade) Chromium nitrate 25.0 0 0 0 1-Hydroxyethylidene-1,1- 0 0 7.5 7.5 Diphosphonic Acid (60%) Aminotrimethylene phosphonic 0 0 0 1.5 acid Urea 0.5 0 0 0 TD-1355-HM * 12 0 0 0 Manganese (II) Oxide (MnO) 0 0 2 2 Hydrofluoric Acid (49%) 0 2.0 0 0 Hydrofluosilicic Acid (25%) 0 15 0 0 Sorbitol (70%) 0 3.3 0 0 Glycolic Acid (70%) 0 4 10 10 Zinc oxide (ZnO) 0 0.031 0 0 * polymer resin available from Henkel Surface Technologies Pvt. Ltd., India
Based on these tabulated aqueous compositions, the following tests were performed.

[0107] Stability at pH 8.5: The pH of the aqueous compositions was raised to 8.5 through the addition of 0.1 M NaOH and visual observations made of any sedimentation and precipitation within the compositions after 10 min without stirring.

[0108] 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 coated by a chemcoater/roll coater with each passivation composition selected for evaluation: duplicate panels were prepared for each passivation composition. The resultant coated test panels were then removed from the bath and baked to a peak metal temperature (PMT) of from 55-60 C. The obtained coating weight of the test panels was from 35-40 mg/m.sup.2 of chromium.

[0109] 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 optical emission spectroscopy (ICP-OES) was applied.

[0110] 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 deionized 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.

[0111] The results of these tests are illustrated in Table 2 herein below.

TABLE-US-00002 TABLE 2 Reference Reference Test Parameters Example 1 Example 2 Example 1 Example 2 pH (25% v/v bath) 1.04 1.60 2.62 2.38 Total Acidity* of 27-31 21-23 13-16 13-16 Composition (mg KOH/g) Composition Precip- Precip- Stable, no Stable, no Stability at pH 8.5 itation itation precipitation precipitation Bath Dissolved Zn 4587 6902 2106 2200 (ppm) Salt Spray Tests Fail Fail Pass Pass (ASTM B117) *titration end point at pH 8.2

[0112] 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.