WO2021139955A1 - Passivation composition suitable for inner surfaces of zinc coated steel tanks storing hydrocarbons - Google Patents

Abstract

The present invention is directed to 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) at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, wherein said polyphosphonic acid has the general formula (I): 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 ₁ -C ₂ alkylene); ii) at least one mineral acid; iii) at least one divalent metal cation (M²⁺); 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_pT_qF_rO_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, wherein said composition is characterized in that it comprises from 0 to 10 wt.% of film-forming organic polymer, wherein said film forming organic polymer consists of at least one polyurethane.

Description

Passivation composition suitable for inner surfaces of zinc coated steel tanks storing hydrocarbons"

The present invention is directed to aqueous, acidic passivation compositions comprising at least one water-soluble polyphosphonic acid. More particularly, the invention is directed to aqueous, acidic passivation compositions that are characterized as being free of hexavalent chromium and free of peroxide and persulphate compounds.

BACKGROUND OF THE INVENTION

Vehicular fuels tanks provide a number of essential functions including but not limited to: i) the storage of a given quantity of fuel without leakage and with limited and controlled evaporative emission; ii) the replenishment of fuel in a secured manner in the absence of sparks; and, iii) the controlled feeding of fuel to an engine via a pump. To achieve at least the first of these functions, fuel tanks must exhibit a durability against corrosion to prevent leakage as a consequence of that corrosion. In particular, the inner surface of a fuel tank must exhibit corrosion resistance within an environment containing formic acid and acetic acid, which acids form through the oxidation and degradation of olefinic hydrocarbons, the main constituents of gasoline. The outer surface of a fuel tank must exhibit resistance to salt damage and the material of the tank as a whole must demonstrate solderability, resistance weldability and press formability.

Historically, lead-antimony (Pb-Sb) alloy-coated steel sheets - known as terne sheets - were used as the main material for automobile fuel tanks as they could meet the aforementioned requirements. In recent years, however, environmental legislation such as EU Directive 2000/53/EC, has imposed restrictions on the use of lead and concomitantly mercury and cadmium in automobile components. As a consequence, the need arose for coated steel products for fuel tank usage which are free of lead and this lead to the development of inter alia· hot-dip aluminium- silicon (Al-Si)alloy-coated steel sheets; hot-dip tin-zinc (Sn-Zn) alloy-coated steel sheet; and, hot- dip zinc (Gl) plated nickel (Ni) double-layered steel sheet. The present invention is directed to steel substrates suitable for use as automobile fuel tanks and which are coated with zinc or zinc alloys. Having regard to protective coatings on steel based on zinc and zinc alloys, inadequate formation of a protective layer can lead to red rust, the red colored corrosion products of iron which are formed virtually instantaneously on exposure of iron to a moist atmosphere: a thin film of process water on a ferrous surface exposed by macroscopic defects in a coating layer is sufficient to initiate formation of red rust. A further problem with coatings based on zinc and zinc alloys is a surface condition known as “wet storage stain” which, whilst also unsightly, can present cells or depressive pits in an otherwise laminar coating. This stain, which is also known as “white rust” or “black rust” (for Galvalume® coatings) is attributable to the formation of zinc oxide and zinc hydroxide and develops upon exposure of the deposited zinc or zinc alloy to atmospheric oxygen and moisture.

Techniques to obviate wet storage stain on newly galvanized substrates are known and include inter alia· the application of duplex or powder coatings; the application of waxes and oil, particularly for base metal substrates in the forms of sheets, beams and wires; and, passivation treatments. The present invention is concerned with the treatment of zinc coatings or platings with passivation compositions which, in addition to providing corrosion resistance, can provide a variety of color coatings - including blue, yellow, olive or black - and an effective base for subsequent dyeing and coating operations.

Prior art passivation compositions have predominantly been based upon acidic aqueous solutions of chromate salts. Upon applying an acidic chromate passivation solution to a zinc coated or plated substrate, surface zinc atoms are oxidized to form, in effect, an interfacial layer of hydrated basic chromium chromate (CraChCrOs . XH2O) and hydrous oxides of both chromium and zinc. As the acid is consumed in the oxidation reaction, however, the pH at the surface-liquid interface increases: this diminishes the combining power of chromium in the aqueous phase and leads to the precipitation of a thin gelatinous film comprising chromium hydroxide and complexes of chromium ions and zinc. This film builds up until acid protons can no longer contact the zinc metal and the surface redox reactions are thereby stopped: the resulting gel-like film may then be permitted to harden.

Traditionally, hexavalent chromium (Cr⁶⁺ or chromium(VI)) was used in passivation compositions to supply the chromium present in the passivation film or conversion coating. However, the toxicological properties of chromium(VI) are problematic and the use of chromium(VI)-containing passivation treatments has also been strongly limited by inter alia EC directive 2000/53/EC. Consequently, there has been some focus in the art on the treatment of zinc surfaces with passivation compositions in which the chromium is at least partly in the trivalent state: mention in this regard may be made of the timeworn disclosures of: US Patent No. 2,559,878; US Patent No. 3,932,198; US Patent No. 3,647,569; US Patent No. 3,501 ,352; US Patent No. 4,359,345; US Patent No. 4,359,346; US Patent No. 4,359,347; US Patent No. 4,359,348; US Patent No. 4,349,392; US Patent No. 4,367,099; German Patent No. DE 2526832; and, UK Patent No. GB 1,461 ,244. The Cr(lll), as used in these citations, is not toxic and the concomitant waste removal of Cr(lll) is not as expensive as that of hexavalent chromium.

Chromium (III) passivate compositions as described in the aforementioned patents nearly invariably employ peroxide-type oxidizing agents, such as H2O2, a necessary bath constituents. These and like oxidizing agents, such as persulphates, can promote some conversion of trivalent chromium to hexavalent chromium during the formation of the conversion coating. A further problem associated therewith is the high rate of consumption and loss of the peroxide or persulphate oxidizing agent which necessitates their frequent replenishment and moreover a careful control of the pH of the composition to obviate concomitant rise in pH. The consumption of peroxide (and persulphate) compounds is due in part to the presence of various activating metal ions - present in the solution as additives or contaminants - which tend to catalyze decomposition of the oxidizing agent. The frequent replenishment of the peroxide and persulphate compounds represents an economic and energetic cost to the performance of the passivation or conversion process.

Certainly passivation compositions based on chromate (III) which do not employ peroxide or persulphate-type oxidizing agents are known in the art. For example, US Patent No. 4,578,122 A (Crotty) describes an aqueous acidic peroxide-free solution which is utilized in a process for treating receptive metal surfaces to impart a chromium passivate film thereon. The described aqueous solution contains: chromium ions, substantially all of which are present in the trivalent state; hydrogen ions to provide a pH of about 1.2 to about 2.5; at least one additional metal ion selected from the group consisting of iron, cobalt, nickel, molybdenum, manganese, lanthanum, cerium and lanthanide, said ion(s) being present in an amount effective to activate the formation of the chromate passivate film; and, nitrate ions as the essential oxidizing agent, said nitrate ions being present in an amount to provide a molar ratio of nitrate ions to the sum of chromium ions and activating metal ions of at least 4:1. The amount of nitrate ions should further be sufficient to activate the hydrated trivalent chromium to form a chromate film on the substrate. The aqueous acidic solution can optionally further contain controlled amounts of: sulfate ions; halide ions; organic carboxylic acids; a bath soluble and compatible silicate compound; and, at least one wetting agent.

The presence of nitrate salts in the composition of US Patent No. 4,578,122 is considered highly disadvantageous. Such salts are converted to NO_x during the spontaneous decomposition or the intended oxidation activity, and this NO_x diffuses into the atmosphere as a pollutant.

There would evidently be a benefit to developing passivation compositions that are free from either chromate (VI) or chromate (III) salts and any deleterious additive compounds: certain authors have indeed focused upon this. US Patent No. 6,203,854 (Affinito), for instance, describes a method for protecting a metal substrate from corrosion, said method comprising the steps of providing a metal substrate and applying a treatment solution to the surface of the metal substrate, wherein the treatment solution comprises a partially hydrolyzed aminosilane and a fluorine-containing inorganic compound. CN102317391 (Momentive Performance Materials Inc.) describes a passivation composition for the treatment of steel and zinc-coated steels, said composition being an aqueous solution of a silane compound and a silicon- based polyether copolymer. And W001/20058 A (Henkel Corporation et al) describes a chromium-free liquid, passivation composition that contains: (a) at least one resin selected from a group consisting of urethane resins, epoxy resins, and acrylic resins; (b) at least on silane coupling agent; and, (c) dispersed solid particles with a mean particle diameter of 1 micron or less.

Unfortunately, passivation compositions based on silicates and silanes are expensive. Moreover, such passivation compositions can exhibit inferior corrosion resistance - as demonstrated by neutral salt spray (NSS) tests - and can be destabilized by hydrolytic reactions.

There consequently remains a need in the art to develop passivation compositions which have particular applicability to steel surfaces coated with zinc or zinc alloys - and in which the levels of compounds such as chromate salts, peroxides, persulphates and nitrate salts can be minimized but wherein the reduction of such compounds in such developed compositions is not compensated by a decline in the performance of the compositions.

STATEMENT OF THE INVENTION In accordance with a first aspect of the present invention there is provided 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) at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, wherein said polyphosphonic acid has the general formula (I):

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 ( C1-C2 alkylene) ii) at least one mineral acid; iii) at least one divalent metal cation (M²⁺); 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):

HpT_qF_rOs (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, wherein said composition is characterized in that it comprises from 0 to 10 wt.% of film-forming organic polymer, wherein said film forming organic polymer consists of at least one polyurethane.

For completeness, the film forming organic polymer is an optional component of the passivation composition which, if present, is included only in a small amount. As such, the aqueous passivation composition may be regarded as being inorganic in character. In an important embodiment, the passivation composition comprises, based on the weight of the composition: from 50 to 90 wt.%, preferably from 60 to 80 wt.% of water; from 5 to 25 wt.%, preferably from 10 to 20 wt.% of i) said at least one water-soluble polyphosphonic acid or a water-soluble salt thereof; from 2 to 15 wt.%, preferably from 5 to 15 wt.% of ii) said at least one mineral acid; from 0.5 to 5 wt.%, preferably from 1 to 4 wt.% of iii) said at least one divalent metal (M²⁺) cation; from 1 to 15 wt.%, preferably from 5 to 15 wt.% of iv) said at least one water-soluble or water-dispersible fluoroacid or a salt thereof; and, from 0 to 10 wt.%, preferably from 1 to 8 wt.% of film-forming organic polymer, wherein said film forming organic polymer consists of at least one polyurethane.

In accordance with a second aspect of the present invention, there is provided a process for imparting a 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 said at least one coated surface of the substrate with an aqueous composition as defined herein above and in the appended claims at a temperature in the range from 20°C to 90°C for a period of time sufficient to form a passivate film thereon. In a particular embodiment, the substrate comprises or consists of steel.

In accordance with a third aspect of the invention, there is provided a passivated substrate obtained by the process defined herein above and in the appended claims.

DEFINITIONS

As used herein, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise.

The terms “comprising", “ comprises ” and “comprised of” as used herein are synonymous with “including”, “includes", “ containing ” or “contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. If used, the phrase "consisting of’ is closed, and excludes all additional elements. Further, the phrase "consisting essentially of excludes additional material elements, but allows the inclusions of non-material elements that do not substantially change the nature of the invention. When amounts, concentrations, dimensions and other parameters are expressed in the form of a range, a preferable range, an upper limit value, a lower limit value or preferable upper and limit values, it should be understood that any ranges obtainable by combining any upper limit or preferable value with any lower limit or preferable value are also specifically disclosed, irrespective of whether the obtained ranges are clearly mentioned in the context.

The words "preferred", "preferably", “particularly’ and “desirably’ are used frequently herein to refer to embodiments of the disclosure that may afford particular benefits, under certain circumstances. However, the recitation of one or more preferable, preferred, particular or desirable embodiments does not imply that other embodiments are not useful and is not intended to exclude those other embodiments from the scope of the disclosure.

As used throughout this application, the word “may” is used in a permissive sense - that is meaning to have the potential to - rather than in the mandatory sense.

The present compositions are defined herein as being “substantially free’’ of certain compounds, elements, ions or other like components. The term “substantially free’’ is intended to mean that the compound, element, ion or other like component is not deliberately added to the composition and is present, at most, in only trace amounts which will have no (adverse) affect on the desired properties of the coating. The term “substantially free’’ encompasses those embodiments where the specified compound, element, ion, or other like component is completely absent from the composition or is not present in any amount measurable by techniques generally used in the art.

As used herein, room temperature is 23°C plus or minus 2°C.

As defined herein, the term “conversion coating” or “conversion treatment,” refers to a treatment of the surface of a substrate which causes the surface material to be chemically converted to a different material. The term “passivation” refers to a treatment of the surface of a substrate to form a barrier layer to corrosive conditions on said surface but without a cohesive film forming a chemical bond between the surface and the passivation layer.

The term “passivation composition” as used herein refers to that composition which actually contacts the zinc-coated or zinc-alloy coated substrate. As is known in the art, such contacting typically occurs in a so-called “bath” which is shaped, sized and disposed to enable at least part of the substrate to be immersed therein. The passivation bath should moreover be sized to allow for movement of the composition around and throughout the loaded substrate, which movement can be further enhanced with recirculation and / or ultrasonics. The pH of the composition within the bath, the temperature of the bath, and contact time of the substrate are result effective variables which should be monitored either manually or automatically, whenever possible.

Viscosities of the passivation compositions may be determined using the Brookfield Viscometer, Model RVT at standard conditions of 20°C. and 50% Relative Humidity (RH). The viscometer is calibrated using silicone oils of known viscosities, which vary from 5,000 cps to 50,000 cps. A set of RV spindles that attach to the viscometer are used for the calibration. Measurements of the passivation compositions are done using the No. 6 spindle at a speed of 20 revolutions per minute for 1 minute until the viscometer equilibrates. The viscosity corresponding to the equilibrium reading is then calculated using the calibration.

Unless otherwise stated, where a molar ratio is given herein with respect “to metal”, this refers to the total content of metal in the composition, independent of the oxidation state(s) of that metal.

As used herein, the term “alio refers to a substance composed of two or more metals or of a metal and a non-metal which have been intimately united, usually by being fused together and dissolved in each other when molten. The term “zinc alio y” therefore denotes an alloy of which zinc metal is a constituent component, which zinc will generally comprise at least 40 wt.% - more typically at least 50 wt.% or at least 60 wt.% - of the alloy, on a metals basis. Metals which may be alloyed with zinc include, but are not limited to, aluminium, tin, nickel, titanium and cobalt.

Herein, for a zinc / aluminum alloy, it is preferred that zinc constitutes, on a metals basis, at least 40 wt.% of the alloy and conversely that aluminum constitutes, on a metals basis, up to 60 wt.% of the alloy. For a zinc / tin alloy, it is preferred that zinc constitutes, on a metals basis, at least 70 wt.% and more particularly at least 80 wt.% of the alloy and conversely that tin constitutes, on a metals basis, up to 30 wt.% and more particularly up to 20 wt.% of the alloy.

Herein, for a zinc / titanium alloy, it is preferred that zinc constitutes, on a metals basis, at least 85 wt.% and more particularly at least 90 wt.% of the alloy and conversely that titanium constitutes, on a metals basis, up to 15 wt.% and more particularly up to 10 wt.% of the alloy. For a zinc / nickel alloy, it is similarly preferred that zinc constitutes, on a metals basis, at least 85 wt.% and more particularly at least 90 wt.% of the alloy and conversely that nickel constitutes, on a metals basis, up to 15 wt.% and more particularly up to 10 wt.% of the alloy. For a zinc / cobalt alloy, it is preferred that zinc constitutes, on a metals basis, at least 95 wt.% of the alloy and conversely that cobalt constitutes, on a metals basis, up to 5 wt.% of the alloy.

As used herein, "mineral acicf' refers to an acid derived from one or more inorganic compounds. A mineral acid is not organic and all mineral acids release hydrogen ions when dissolved in water.

As used herein, "phosphoric acid' refers to ortho-phosphoric acid having the formula H₃PO₄, which acid is typically available as an aqueous solution having a concentration up to 75 wt.% H₃PO₄. As used herein “phosphonic acid’ refers to the phosphorus oxoacid having the formula H₃PO₃ that consists of a single pentavalent phosphorus covalently bound via single bonds to a single hydrogen and two hydroxy groups and via a double bond to an oxygen.

As used herein, the term “a-hydroxycarboxyiic acid’ means a carboxylic acid having at least one hydroxyl functional group occupying an a-position on said acid (carbon adjacent to a carboxylic acid functional group). The presence of hydroxyl groups occupying positions in the molecule other than the a-position on said acid is not precluded. This a-hydroxycarboxylic acid is included in the present composition in the form of the free acid.

The term "hydrocarbyl group" is used herein in its ordinary sense, which is well-known to those skilled in the art.

As used herein, the term “Ce-Cw aryl group’’ refers to an aromatic monocyclic or multicyclic ring system of 6 to 10 carbon atoms. The “aryl group” may optionally be substituted with one or more C1-C12 alkyl, alkylene, alkoxy, or haloalkyl groups. Exemplary aryl groups include phenyl or naphthyl, or substituted phenyl or substituted naphthyl.

Unless otherwise indicated, the term “alkyf’, as used herein, includes straight chain moieties, and where the number of carbon atoms suffices, branched moieties. The alkyl group may optionally be substituted. As such, the term “C₁-C₄ alkyf’ includes saturated straight chain and branched alkyl groups having from 1 to 4 carbon atoms. Examples of C₁-C₄ alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, isobutyl and tert-butyl. The terms “alkylene group" refers to a group that are radicals of a linear, branched or cyclic alkane, which group may be substituted or unsubstituted and may optionally be interrupted by at least one heteroatom.

As used herein, “C2-C6 alkenyf’ group refers to an aliphatic carbon group that contains 2 to 6 carbon atoms and at least one double bond disposed in any position. Like the aforementioned alkyl group, an alkenyl group can be straight or branched, and may optionally be substituted. The term “aikeny\” also encompasses radicals having “cis” and “trans” configurations, or alternatively, Έ” and “Z” configurations, as appreciated by those of ordinary skill in the art. In general, however, a preference for unsubstituted alkenyl groups containing from 2 to 6 (C₂-C₆) or from 2 to 4 (C₂-C₄) carbon atoms should be noted. And Examples of C₂-Ce alkenyl groups include, but are not limited to: ethenyl; 1-propenyl; 2-propenyl; 1-methyl-ethenyl; 1-butenyl; 2-butenyl; 4-methylbutenyl; 1- pentenyl; 2-pentenyl; 3-pentenyl; 4-pentenyl; 4-methyl-3-pentenyl; 1-hexenyl; 3-hexenyl; and, 5- hexenyl.

The term “C3-C6 cycloalkyf’ as used herein means an optionally substituted, saturated cyclic hydrocarbon having 3-6 carbon atoms. Exemplary cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl or cyclohexyl groups.

The term “ alkox y”, as used herein, means “ — O-alkyF’ or ‘‘alkyl-0 — wherein “alkyF’ is defined as above.

The term “substituted’ refers to substitution with at least one suitable substituent. For completeness: the substituents may connect to the specified group or moiety at one or more positions; and, multiple degrees of substitution are allowed unless otherwise stated. Further, the terms “ substitution ” or “substituted with” include the implicit proviso that such substitution is in accordance with permitted valence of the substituted atom and the substituent, and that the substitution results in a stable compound that does not spontaneously undergo transformation by, for instance, rearrangement, cyclization or elimination.

Having regard to the a-hydroxycarboxylic acid defined above and hereinbelow, substitution(s) of the group Ri will conventionally be selected from the group consisting of: halogen; oxo; — OH; and, — COOH. Where mentioned, the expression " interrupted by at least one heteroatom" means that the main chain of a residue comprises, as a chain member, at least one atom that differs from carbon atom. More particularly the term “heteroatom” refers to nitrogen, oxygen, halogens, phosphorus or sulfur. Oxygen (O) and nitrogen (N) may be mentioned as typical heteroatoms in the context of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The composition comprises by necessity acidic components. In toto the added amount of the acidic components 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 (i)

A first 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):

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 (C1-C2 alkylene).

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.

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

The polyphosphonic acids or the water soluble salts thereof are preferably included in the compositions in an amount of from 5 to 25 wt.%, for example from 10 to 20 wt.%, based on the weight of the composition.

Component (ii)

The passivation compositions of the present invention comprise at least one mineral acid. The use of nitric acid is not precluded but is not preferred; conversely, the addition of at least one of phosphoric acid, phosphonic acid, sulphurous acid, sulphuric acid, hydrochloric acid and hydrobromic acid is considered to be particularly suitable. A particular preference for the use of at least one of phosphoric acid, phosphonic acid, sulphurous acid and sulphuric acid may be mentioned. And in an important embodiment, the mineral acid of the composition in constituted by phosphoric acid.

The above recited pH of the passivation composition is somewhat determinative of the added amount of such acid(s). Within that pH constraint, the passivation composition may comprise from 2 to 15 wt.%, for example from 5 to 15 wt.% of mineral acid.

In that embodiment where the mineral acid comprises or consists of phosphoric acid, it is preferred that the molar ratio of phosphonate groups to H₃PO₄ in the passivation 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 zinc or zinc alloy coated substrates to which they are applied. Component (iii)

The passivation composition further contains at least one divalent metal cation (M²⁺). In preferred embodiments, said at least one divalent metal cation (M) is selected from the group consisting of: Mg²⁺; Ca²⁺; Mn²⁺; Co²⁺; Ni²⁺; Sr²⁺; Ba²⁺; and, Zn²⁺. 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.

In a preferred embodiment of the present invention, the passivation composition comprises magnesium (Mg²⁺) and / or manganese (Mn²⁺). This magnesium and manganese are desirably introduced into the aqueous passivation composition as one or more of: manganese chloride; manganese sulphate; magnesium oxide, magnesium hydroxide; magnesium sulphate; and, magnesium chloride. A particular preference for magnesium oxide or magnesium hydroxide may be noted.

The total amount of the divalent metal cations (M²⁺) in the aqueous composition is conventionally in the range from 0.5 to 5 wt.%, preferably from 1 to 4 wt.%, based on the weight of the composition.

Component (iv)

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):

HpT_qF_rOs (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. Exemplary fluoroacids used in the process of the invention may be selected from the group consisting of: fluorotitanic acid (H2T 6); fluorozirconic acid (FhZrFe); fluorosilicic acid (H2S 6); fluoroboric acid (HBF4); fluorostannic acid (FhSnFe); fluorogermanic acid (FhGeFe); fluorohafnic acid (FhHfFe); and, fluoroaluminic acid (H₃AIF₆). Preferred fluoroacids are: fluorotitanic acid (H2T 6) and fluorozirconic acid (FhZrFe).

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: (NH₄)₂ZrF₆; H(NH₄)ZrF₆; (NH₄)2TiF_6; H(NH₄)₂TiF_6; Na₂ZrF_6; K₂ZrF₆; U₂ZrF_6; Na₂TiF₆; K₂TiF₆; and, Li₂TiF₆.

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.

The fluoroacid or salt thereof is typically included in the composition such that the molar ratio of mineral acid to the metal (T) of said fluoroacid is in the range from 10:1 to 2:1, preferably from 9:1 to 3:1 and more preferably 8:1 to 4:1. When the level of mineral acid is outside the above ranges, the stability of the formulation is diminished: at lower levels of mineral 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.

In an alternative but not mutually exclusive expression, the fluoroacid or salt thereof should be included in the passivation composition in an amount of from 1 to 15 wt.%, for example from 5 to 15 wt.%, based on the weight of the composition.

Adjunct Ingredients

As noted above, the passivation composition of the present invention may optionally comprise film forming organic polymer in an amount up to 10 wt.%, based on the weight of the composition, wherein said film forming organic polymer consists of at least one polyurethane polymer. When present, the composition may comprise from 1 to 8 wt.%, for example from 2 to 8 wt.% or from 2 to 5 wt.% of said film forming organic polymer, based on the weight of the composition. The film forming polyurethane should be non-functional by which is meant the polyurethane contains substantially no un-reacted isocyanate or isocyanate-reactive groups. To ensure sufficient stability of an aqueous passivation composition according to the invention that comprises a polyurethane polymer as a film forming organic polymer said polyurethane polymer is preferably cationically stabilized.

In the alternative, and as is known in the art, suitable polyurethanes may be obtained from the reaction of: i) at least one polyol; and, ii) at least one polyisocyanate compound. The OH:NCO equivalence ratio of the reactants should be at least 1 : 1 and preferably from 1 : 1 to 1.2: 1 to ensure that no free NCO groups are present in the product.

As used herein, "po/yo/" refers to any compound comprising two or more hydroxyl groups: the term is thus intended to encompass diols, triols and compounds containing four or more -OH groups. Moreover, the at least one reactant polyol may herein preferably be selected from the group consisting of: polyester polyols; polyether polyols; and, polycarbonate polyols, more preferably aliphatic polycarbonate polyols being preferably based on 2,2-dimethyl-propandiol.

As used herein " olyisocyanate" means a compound comprising at least two -N=C=0 functional groups, for example from 2 to 5 or from 2 to 4 -N=C=0 functional groups. Suitable polyisocyanates include aliphatic, cycloaliphatic, aromatic and heterocyclic isocyanates, dimers and trimers thereof, and mixtures thereof.

Aliphatic and cycloaliphatic polyisocyanates can comprise from 6 to 100 carbon atoms linked in a straight chain or cyclized and having at least two isocyanate reactive groups. Examples of suitable aliphatic isocyanates include but are not limited to straight chain isocyanates such as ethylene diisocyanate, trimethylene diisocyanate, tetramethylene diisocyanate, 1,6- hexamethylene diisocyanate (HDI), octamethylene diisocyanate, nonamethylene diisocyanate, decamethylene diisocyanate, 1,6,11-undecanetriisocyanate, 1,3,6- hexamethylene triisocyanate, bis(isocyanatoethyl)-carbonate, and bis (isocyanatoethyl) ether. Exemplary cycloaliphatic polyisocyanates include, but are not limited to, dicyclohexylmethane 4,4'-diisocyanate (H12MDI), 1-isocyanatomethyl-3-isocyanato-1, 5, 5-trimethyl-cyclohexane (isophorone diisocyanate, IPDI), cyclohexane 1,4-diisocyanate, hydrogenated xylylene diisocyanate (HbCϋI), 1-methyl-2,4- diisocyanato-cyclohexane, m- or p-tetramethylxylene diisocyanate (m-TMXDI, p-TMXDI) and dimer fatty acid diisocyanate.

The term “aromatic polyisocyanate ” is used herein to describe organic isocyanates in which the isocyanate groups are directly attached to the ring(s) of a mono- or polynuclear aromatic hydrocarbon group. In turn the mono- or polynuclear aromatic hydrocarbon group means an essentially planar cyclic hydrocarbon moiety of conjugated double bonds, which may be a single ring or may include multiple condensed (fused) or covalently linked rings. The term aromatic also includes alkylaryl. Typically, the hydrocarbon (main) chain includes 5, 6, 7 or 8 main chain atoms in one cycle. Examples of such planar cyclic hydrocarbon moieties include, but are not limited to, cyclopentadienyl, phenyl, napthalenyl-, [10]annulenyl-(1,3,5,7,9-cyclodecapentaenyl-), [12]annulenyl-, [8]annulenyl-, phenalene (perinaphthene), 1,9-dihydropyrene, chrysene (1,2- benzophenanthrene). Examples of alkylaryl moieties are benzyl, phenethyl, 1-phenylpropyl, 2- phenylpropyl, 3-phenylpropyl, 1-naphthylpropyl, 2-naphthylpropyl, 3-naphthylpropyl and 3- naphthylbutyl.

Exemplary aromatic polyisocyanates include, but are not limited to: all isomers of toluene diisocyanate (TDI), either in the isomerically pure form or as a mixture of several isomers; naphthalene 1 ,5-diisocyanate; diphenylmethane 4,4'-diisocyanate (MDI); diphenylmethane 2,4'- diisocyanate and mixtures of diphenylmethane 4,4'-diisocyanate with the 2,4' isomer or mixtures thereof with oligomers of higher functionality (so-called crude MDI); xylylene diisocyanate (XDI); diphenyl-dimethylmethane 4,4'-diisocyanate; di- and tetraalkyl-diphenylmethane diisocyanates; dibenzyl 4,4'-diisocyanate; phenylene 1 ,3-diisocyanate; and, phenylene 1,4-diisocyanate.

It is noted that the term “ polyisocyanate ” is intended to encompass pre-polymers formed by the partial reaction of the aforementioned aliphatic, cycloaliphatic, aromatic and heterocyclic isocyanates with polyols to give isocyanate functional oligomers, which oligomers may be used alone or in combination with free isocyanate(s).

To facilitate their inclusion in the compositions of the present invention, the at least one polyurethane may be initially provided as an aqueous dispersion, the particles of which dispersion may desirably be characterized by a mean particle size of less than 1 micron, for instance less from 20 to 500 nm, as measured by dynamic liqht scattering. The composition of the present invention may, in certain embodiments, comprise at least one finely divided wax. When present, the composition may comprise up to 7 wt.% of said wax, for example from 1 to 5 wt.%, based on the weight of the composition.

The term "wax" is known to the person skilled in the art and reference may be made to the definition in Ullmann's Encyclopedia of Industrial Chemistry, 6th Edition, Electronic Release (1998). However, without intention to limit the present invention, exemplary waxes include: paraffin wax [CAS No. 8002-74-2]; polyethylene wax [CAS No. 9002-88-4]; polyethylene- polypropylene waxes; co-polymeric polyethylene waxes, for example copolymers of ethylene with at least one monomer selected from (meth)acrylic acid, maleic anhydride, vinyl acetate and vinyl alcohol, which copolymers are available under, for instance CAS Nos. 38531-18-9, 104912-80-3 and 219843-86-4; polybutene waxes; Fischer-Tropsch waxes; oxidized waxes, for example oxidized polyethylene wax [CAS No. 68441-17-8]; polar modified polypropylene waxes; microcrystalline waxes, for example microcrystalline paraffin waxes [CAS No. 63231-60-7]; montan wax and montan wax raffinates; montanic acids and salts and esters thereof; fatty acid amides such as erucamide [CAS No. 112-84-5], oleamide [CAS No. 301-02-0] and 1,2- ethylenebis(stearamide) [CAS No. 110-30-5]; and, carnauba wax.

It is preferred that any waxes included in the present composition meet at least one of the following conditions: i) an acid number of less than 200 mg KOH/g, preferably less than 100 mg KOH/g; ii) a melting point of from 40 to 200°C, preferably from 60 to 180°C; and, iii) a number average molecular weight (Mn) of at least 200 g/mol, preferably at least 400 g/mol. For completeness, these conditions are not intended to be mutually exclusive: waxes may meet one, two or three of these conditions.

A particular preference may be noted for the use of at least one wax selected from polyethylene waxes, oxidized polyethylene waxes, polypropylene waxes, oxidized polypropylene waxes and co-polymeric waxes based on ethylene or propylene as the main monomers, wherein said at least one wax is further characterized by a number average molecular weight (Mn) of from 400 to 30 000 g/mol, preferably from 1000 to 25 000 g/mol.

To facilitate their inclusion in the compositions of the present invention, waxes may be provided: i) in finely divided powder form, in particular in a micronized form characterized by a mean particle size of less than 20 microns, as measured by laser diffraction; and / or; ii) as an aqueous dispersion, the particles of which dispersion may desirably be characterized by a mean particle size of less than 1 micron, for instance less from 20 to 500 nm, as measured by dynamic liqht scattering.

In addition to component iv) above, 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. lnF4 ¹); fluorophosphates (e.g. PF₆ ¹); fluoroarsenates (e.g. AsF₆ ¹); fluoroantimonates (e.g. SbF₆ ¹); fluorobismuthates (e.g. BiF₆ ¹); fluoro sulfates (e.g. SF₆ ²); fluoroselenates (e.g. SeF₆ ²); fluorotellurates (e.g. TeF₆ ² orTeOFs ¹); fluorocuprates (e.g. CUF3 ¹); fluoroargentates; fluorozincates (e.g., ZnF4 ²); fluorovanadates (e.g. VF ²); fluoroniobates (e.g. NbF ²); fluorotantalates (e.g. TaF ²); fluoromolybdates (e.g. M0F6 ³); fluorotungstates (e.g. WF₆ ¹); fluoroyttrates (e.g. YF₆ ³); fluorolanthanates (e.g. LaF₆ ³); fluorocerates (e.g. CeF₆ ³ or CeF₆ ²); fluoromanganates (e.g. MnF₆ ²); fluoroferrates (e.g. FeF₆ 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.

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.

The presence in the passivation composition of free fluoride ions - not bound in complex form - is 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.

This aside, the presence of free fluoride ions - not bound in complex form - is 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.

The passivation composition may include up to 5 wt.%, for example from 1 to 3 wt.%, of non-ionic surfactants, based on the weight of the composition. Whilst other non-ionic surfactants may be have utility in the present invention, a particular preference for the use of fatty alcohol ethoxylates may be mentioned, of which examples include ethoxylated lauryl alcohol, stearyl alcohol, behenyl alcohol and oleyl cetyl alcohol.

The composition of the present invention may optionally comprise at least one a- hydroxycarboxylic acid represented by the general formula (III):

RiCH(OH)COOH (III) wherein: Ri represents a hydrogen atom, a C1-C4 alkyl group, a C2-C6 alkenyl group, a Ci-

C₆ alkoxy group, a C3-C6 cycloalkyl group or a C6-C10 aryl group.

Suitable a-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.

In an embodiment, said at least one a-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 a-hydroxycarboxylic acid(s) of the coating composition should comprise or consist of gluconic acid.

For completeness, it is again noted that the above recited pH of the passivation composition is somewhat determinative of the added amount of such a-hydroxycarboxylic acid(s). When added within that pH constraint, the a-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.

It is considered that the corrosion-protection performance of the disclosed passivation compositions - and resulting passivate films - can 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.

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).

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; 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.

Exemplary Formulation of the Passivation Compositions

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, based on the weight of the composition: from 60 to 80 wt.% of water; from 10 to 20 wt.% of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP); from 5 to 15 wt.% of phosphoric acid; from 1 to 4 wt.% of divalent metal cations, wherein said cations comprise Mg²⁺ and optionally at least one further divalent metal cation (M²⁺) selected from the group consisting of Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Sr²⁺, Ba²⁺, and Zn²⁺; from 5 to 15 wt.% of at least one fluoroacid selected from fluorotitanic acid (H2T1F6) and fluorozirconic acid (H₂ZrF₆); and, from 1 to 8 wt.% of film forming polymer, wherein said film forming polymer consists of at least one polyurethane preferably cationically stabilized and based on polycarbonate polyols. Preparation of the Passivation Compositions

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 concentrates - that can be converted by dilution with water only - or 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.

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.%, preferably from 50 to 90 wt.% and more preferably from 60 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.s (50 cps to 1000 cps), as measured using a Brookfield viscometer at 25°C.

METHODS AND APPLICATIONS

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: substrates comprising or consisting of steel may be mentioned in particular. Such metal and alloy substrates may be provided in various forms, including sheets, plates, cuboids, spheres, annuli, solid cylinders, tubes and wires: the provision of substrates in more complex, shaped forms - obtained by conventional techniques such as bending, blanking, casting, forging, rolling and welding - is of course not precluded. 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: galvanized and galvanneal steel meeting the requirements of ASTM Designation A653; 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.

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.

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.

After said cleaning, degreasing and / or pre-treatment steps, the passivation composition is applied to the substrate. The passivation composition may be applied at ambient temperature or the temperature of the passivation compositions may be elevated prior to application to, for instance, a temperature in the range from 30°C to 90°C, for instance from 30°C to 70°C.

To produce a double-face plated sheet, it is conventional commercially that 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.

In certain circumstances it will only be necessary to form a passivate film on a single surface of the substrate. In the context of the present invention, the passivate film might only be applied to that plated surface of a steel substrate which is to form the inner side of the fuel tank, the side which contacts the fuel stored therein: forming passivate films on both the inner and outer surfaces of such a plated steel substrate may be deleterious to the subsequent weldability of that substrate. Techniques for applying the passivate composition to only a singular surface include but are not limited to: painting; brushing; roll coating; wiping; air-atomized spraying; air-assisted spraying; airless spraying; high-volume low-pressure spraying; and, air-assisted airless spraying.

At the conclusion of the application step, the article is dried using, for instance, ambient air drying, circulating warm air, forced air drying or infrared heating. The surface temperature of the substrate is controlled during drying: the peak metal temperature (PMT) need not exceed 100°C and should, more particularly be in the range from 20 to 90°C, for example 50 to 75°C.

Subsequent to drying, 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. The rinsed substrate may be dried after completion of the rinsing step(s) or, if applicable, after each rinse solution.

The above described treatment should desirably yield a protective passivate monolayer over the zinc or zinc alloy, which monolayer has a film weight of from 50 to 600 mg/m², preferably from 100 to 500 mg/m². If the film weight is less than 50 mg/m², the passivate film may impart insufficient corrosion resistance. If the film weight is larger than 600 mg/m², the adhesion of the passivate film to the surface will be insufficient, such that exfoliation of the coating may occur during further processing of the substrate. 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 for at least 250 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 for at least 250 hours before the observed onset of white rust corrosion (as defined by ASTM B-201) when treated with neutral salt spray (NSS, 5 wt.% NaCI, 95 wt.% H2O) under steady state conditions in accordance with the procedure of ASTM B-117.

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 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 US Patent No. 5,743,971 (Inoue) and US Patent 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, roll-coating, electro-coating or powder coating.

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

EXAMPLES

The following commercial products are used in the Reference Compositions and the Composition 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 (H2T 6) available S.B. Chemicals.

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

Standard Test Panel Preparation: Specimens of Advanced Coating Technology (ACT) G-90 hot dipped galvanized steel were mechanically cut into squares of 4cm x 4 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. Each passivation composition selected for evaluation was applied to one surface of the panels by roller coater: nine panels were prepared from the aqueous passivation composition according to the present invention; and, quadruplicate panels were prepared for each reference passivation composition. The resultant coated test panels were then baked to the peak metal temperature (PMT) given in Table 2 herein below. The obtained coating weight of the test panels was determined on a metals basis and is also given in Table 2.

Example 1

Based on these tabulated aqueous compositions, the following tests were performed utilizing both the aqueous passivation compositions of the present invention and the reference compositions.

Friction Coefficient Testing: Where applicable this parameter was tested for (coated) panels in accordance with ASTM G115 Standard Guide for Measuring and Reporting Friction Coefficients. The uncoated steel G-90 hot dipped galvanized steel (m) panels had a friction coefficient of 0.35 to 0.4.

Neutral salt spray (NSS): This test was carried out according to ASTM B117 with a 5% NaCI 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. A visual inspection of the coated panels was undertaken after 500 hours:

1^st Gasoline Dip Tests: In accordance with SAE J1681-2000 Gasoline, Alcohol and Diesel Fuel Surrogates for Materials Testing, panels coated with each of the selected passivation compositions were independently immersed in three liquids as follows:

Condition DT1 : gasoline for a duration of 168 hours;

Condition DT2: a mixture of gasoline (90 vol.%) and ethanol (10 vol.%) for 2500 hours; and,

Condition DT3: a mixture of gasoline (90 vol.%) and water (10 vol.%) for 100 hours, this third mixture containing 100 ppm formic acid and 100 ppm chloride. The test panels were not allowed to contact other surfaces and condensed or corrosion products on their surfaces were not permitted to cross-contaminate each other. Photographic recording of the test panels was performed after the afore-stated exposure time. After said exposure, test panels were rinsed in deionised water to remove salt deposits from their surface and then immediately dried. The residual coating weight of the test panels was determined on a metals basis, thereby enabling calculation of the weight loss of coating.

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

Table 2

Example 2

Based on these tabulated aqueous compositions, the following gasoline dip tests were performed utilizing the aqueous passivation compositions of the present invention.

2^nd Gasoline Dip Tests: In accordance with SAE J1681-2000 Gasoline, Alcohol and Diesel Fuel Surrogates for Materials Testing, panels coated with each of the selected passivation compositions were independently immersed for 2500 hours in five liquids as follows: Condition DS1: a mixture of gasoline (90 vol.%) and ethanol (10 vol.%);

Condition DS2: a mixture of gasoline (75 vol.%) and ethanol (25 vol.%);

Condition DS3: a mixture of gasoline (50 vol.%) and ethanol (50 vol.%);

Condition DS4: a mixture of gasoline (25 vol.%) and ethanol (75 vol.%); and,

Condition DS5: ethanol (100 vol.%).

The test panels were not allowed to contact other surfaces and condensed or corrosion products on their surfaces were not permitted to cross-contaminate each other. Photographic recording of the test panels was performed after the afore-stated exposure time. After said exposure, test panels were rinsed in deionised water to remove salt deposits from their surface and then immediately dried. The residual coating weight of the test panels was determined on a metals basis, thereby enabling calculation of the weight loss of coating.

The results of these tests are illustrated in Table 3 herein below.

Table 3

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.

Claims

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) at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, wherein said polyphosphonic acid has the general formula (I):

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 (C1-C2 alkylene ); ii) at least one mineral acid; iii) at least one divalent metal cation (M²⁺); 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):

HpT_qF_rOs (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, wherein said composition is characterized in that it comprises from 0 to 10 wt.% of film forming organic polymer, wherein said film forming organic polymer consists of at least one polyurethane.

2. The composition according to claim 1 comprising, based on the weight of the composition: from 50 to 90 wt.% of water; from 5 to 25 wt.% of i) said at least one water-soluble polyphosphonic acid or a water- soluble salt thereof; from 2 to 15 wt.% of ii) said at least one mineral acid; from 0.5 to 5 wt.% of iii) said at least one divalent metal (M²⁺) cation; and, from 1 to 15 wt.% of iv) said at least one water-soluble or water-dispersible fluoroacid or a salt thereof.

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

4. The composition according to any one of claims 1 to 3, 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.

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

6. The composition according to any one of claims 1 to 5, wherein said at least one mineral acid is selected from the group consisting of phosphoric acid, phosphonic acid, sulphurous acid and sulphuric acid.

7. The composition according to claim 6, wherein said mineral acid comprises or consists of phosphoric acid.

8. The composition according to any one of claims 1 to 7, wherein said at least one divalent metal cation (M²⁺) is selected from the group consisting of: Mg²⁺; Ca²⁺; Mn²⁺; Co²⁺; Ni²⁺; Sr²⁺; Ba²⁺; and, Zn²⁺.

9. The composition according to claim 8 comprising Mg²⁺ and / or Mn²⁺.

10. The composition according to any one of claims 1 to 9, wherein in formula (II):

T is selected from Ti or Zr; p is 1 or 2; q is 1; r is 2, 3, 4, 5 or 6; and, s is 0, 1, or 2.

11. The composition according to any one of claims 1 to 9, wherein said at least one fluoroacid is selected from the group consisting of: fluorotitanic acid (H2T 6); fluorozirconic acid (HhZrFe); fluorosilicic acid (H2S 6); fluoroboric acid (HBF₄); fluorostannic acid (FhSnFe); fluorogermanic acid (FhGeFe); fluorohafnic acid (FhHfFe); and, fluoroaluminic acid (HsAIFe);

12. The composition according to claim 11, wherein said at least one fluoroacid is selected from the group consisting of: fluorotitanic acid (H2T 6) and fluorozirconic acid (FhZrFe).

13. The composition according to any one of claims 1 to 12, wherein said composition comprises from 1 to 8 wt.% of said film forming polymer.

14. The composition according to any one of claims 1 to 13 being substantially free of chromium compounds, peroxide compounds and persulphate compounds.

15. A process for imparting a 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 said at least one coated surface of the substrate with an aqueous composition as defined in any one of claims 1 to 14 at a temperature ranging from 20°C to 90°C for a period of time sufficient to form a passivate film thereon.

16. The process according to claim 15, wherein the substrate comprises or consists of steel.

17. A passivated substrate obtained by the process defined in claim 15.