Patent Description:
Many consumer and industrial goods are formed from metal substrates and are exposed to the elements. As such, these metal goods are subject to corrosive environments; thus, they are often coated in protective coatings, including corrosion protection coatings and paints. Many such corrosion protective coatings are known as conversion coatings. Conversion coatings are understood in the art to be a type of metal pretreatment formed by contacting a metallic surface with a metal pretreatment composition, i.e., a conversion coating composition, which modifies the metallic surface and forms a conversion coating thereon. While these conversion coatings enhance the corrosion resistance of metal, the further improvement of conversion coatings is an ongoing market requirement for automotive and white goods applications.

A typical process for applying a conversion coating layer onto a metal substrate involves cleaning, rinsing, applying the conversion coating, post-rinsing or sealing, and painting (such as by electrophoretic coating, also known as E-coating). The bare metal surface is highly reactive after protective oils have been removed and the substrate can oxidize or flash-rust in the case of a steel substrate. The formation of flash-rust can be exacerbated during extended dwell times in the rinsing process. It is well known that visible oxidation product (rust) on the metal surface will result in a poor appearance of the E-coated surface, which is defined as a raised area of greater dry-film thickness than the nearby non-oxidized areas. These paint defects are costly to repair or rework and are to be avoided. In addition to such E-coat appearance defects, oxidation (flash-rust) of the substrate typically decreases the paint adhesion and corrosion resistance of the coated article. One current solution to the problem of oxidation during rinsing involves the use of sodium nitrite in the rinse stages. This solution is not applicable to all regions, however, due to environmental restrictions. Accordingly, the development of alternative methods of preventing or reducing the amount of oxidation or flash-rusting exhibited by a bare metal substrate surface would be highly desirable.

According to the present invention is provided a method as defined in claim <NUM>.

As used herein, the term "preformed" when referring to the reaction product means that the reaction product has been formed in advance of contacting an aqueous mixture comprised of the reaction product with a bare metal substrate surface, e.g., at least <NUM>, <NUM> or <NUM> minutes in advance of such contacting. Such preformed reaction products thus are to be contrasted with reaction products formed in situ by combining catechol compound and co-reactant compound in water in the presence of a bare metal substrate surface whereby the reaction product essentially simultaneously forms and deposits on the bare metal substrate surface. Thus, the present invention may comprise reacting at least one catechol compound and at least one co-reactant compound comprised of one or more functional groups reactive with the at least one catechol compound to obtain at least one preformed reaction product, storing the at least one preformed reaction product for a period of time (e.g., at least <NUM> day, or at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> days), using the preformed reaction product after being stored for a period of time to prepare a working bath, and contacting the working bath with a bare metal substrate surface to provide a pre-rinsed metal substrate surface (which thereafter may be conversion coated).

The aforementioned preformed catechol compound/co-reactant compound reaction products passivate the surfaces of metal substrates when used during the rinse stages after cleaning. The preformed reaction products are believed to deposit on the surface of the metal, forming a passivating barrier layer and preventing or inhibiting oxidation/flash-rust formation prior to the deposition of a conversion coating layer. In addition to the capacity to form barrier films, the preformed reaction products may have redox capabilities and as such are capable of acting as reducing agents to prevent or retard oxidation of the metal substrate surface. Without wishing to be bound by theory, the preformed catechol compound/co-reactant compound reaction products may function by reducing ions present on the metal substrate surface and/or by being oxidized preferentially as compared to the metal substrate surface.

The elimination or reduction of flash-rust using the preformed catechol compound/co-reactant compound reaction products in accordance with the present invention is evident even with long dwell times or elevated temperature. Further, rinsing with an aqueous mixture comprising such a preformed reaction product may help to reduce the risk of E-coat appearance defects (E-coat mapping).

It has also been found that the preformed catechol compound/co-reactant compound reaction products, when applied to a bare metal substrate surface, are capable of reducing the zirconium coating weight of a subsequently applied zirconium-based conversion coating system. This characteristic can be utilized to control the deposition of subsequent conversion coating compositions, i.e., to limit the amount of conversion coating deposited during long dwell times in such conversion coating stage (as may occur during line-stoppages, for example).

The preformed catechol compound/co-reactant compound reaction products, when used in rinse stages after cleaning, are also capable of modifying metal substrate surfaces to provide good paint adhesion and corrosion characteristics.

As previously mentioned, an aqueous mixture comprised of (in addition to water) at least one preformed reaction product of at least one catechol compound and at least one co-reactant compound comprised of one or more functional groups reactive with the at least one catechol compound ("preformed catechol compound/co-reactant compound reaction product") is utilized in the method of the present invention. The aqueous mixture may be in the form of a solution or a dispersion, preferably a storage-stable solution or dispersion; as used herein, the term "dispersion" includes mixtures in which none of the components of the mixture are dissolved in an aqueous medium as well as mixtures in which portions of one or more of the components of the mixture are dissolved in an aqueous medium. Generally, the catechol compound(s) and co-reactant compound(s) are selected and reacted to provide one or more organic reaction products in which multiple organic residues or moieties derived from these reactants are covalently bonded to each other. Typically, the reaction product(s) formed is/are polymeric. For example, the preformed reaction product may be a cross-linked polymer. According to advantageous embodiments of the invention, the preformed reaction products are soluble in water. For example, the preformed reaction products may have a solubility in water at <NUM> of at least <NUM>%, at least <NUM>%, at least <NUM>% or at least <NUM>% by weight. However, in other embodiments, the preformed reaction products may be dispersible in water, preferably providing storage-stable dispersions.

The term "catechol compound" as used herein means an organic compound with an aromatic ring system that includes at least two hydroxyl groups positioned on adjacent carbon atoms of the aromatic ring system. Suitable catechol compounds include compounds containing at least one <NUM>,<NUM>-dihydroxybenzene moiety, i.e., an aromatic ring with hydroxyl groups ortho to each other, wherein the aromatic ring may be substituted (at positions other than where the hydroxyl groups are located) with one or more substituents other than hydrogen. Combinations of two or more different catechol compounds may be used.

According to the present invention, the catechol compound contains at least one amine functional group, such as a primary or secondary amine group or a salt thereof (e.g., a hydrohalide salt).

According to certain embodiments, the catechol compound is soluble in water. For example, the catechol compound may have a solubility in water (e.g., pure neutral water) at <NUM> of at least <NUM>/L, at least <NUM>/L, at least <NUM>/L or even higher. In other embodiments, however, the catechol compound may be dispersible in water.

Illustrative, non-limiting examples of suitable catechol compounds include aminoalkyl-substituted catechols and salts thereof (such as dopamine, <NUM>,<NUM>-dihydroxy-L-phenylalanine, epinephrine, norepinedrine, α-methyldopamine, <NUM>-(<NUM>-(ethylamino)-<NUM>-hydroxyethyl)catechol, N-isopropyl dopamine, <NUM>-(<NUM>-aminopropyl)catechol, <NUM>,<NUM>-dihydroxybenzylamine, N-methyl dopamine, N,N-dimethyl dopamine, <NUM>-fluoro dopamine, dopexamine, <NUM>-aminoethylpyrogallol, and salts thereof, including hydrohalide salts such as hydrochloride and hydrobromide salts), amino-substituted catechols (e.g., <NUM>-amino catechol, <NUM>-amino dopamine and salts thereof, especially hydrohalide salts) and the like. Combinations of two or more different catechol compounds may be used.

One or more co-reactant compounds are reacted with one or more catechol compounds to form preformed reaction products useful in the present invention. Suitable co-reactant compounds (sometimes referred to herein as "functionalized co-reactant compounds") are compounds comprised of one or more (preferably two or more) functional groups per molecule reactive with the catechol compound(s) used.

In desirable embodiments of the invention, the co-reactant compound or combination of co-reactant compounds is soluble in water. For example, the co-reactant compound may have a solubility in water at <NUM> of at least <NUM>/L, at least <NUM>/L, at least <NUM>/L or even higher. However, in other embodiments, the co-reactant compound(s) may be dispersible in water.

According to the invention the at least one co-reactant compound includes at least one oligomeric or polymeric amine compound comprising a plurality of repeating units having structure-[CH<NUM>CH<NUM>NH]-. Such oligomeric and polymeric amine compounds may be linear or branched in structure. One or more polyethyleneimines, either linear or branched, may be used as the co-reactant compound(s), in accordance with desirable embodiments of the invention. The polyethyleneimine may have, for example, a number average molecular weight of <NUM> to <NUM>,<NUM>, <NUM> to <NUM>,<NUM> or <NUM> to <NUM>,<NUM> (as measured by gel permeation chromatography), although higher molecular weight polyethyleneimines (e.g., having number average molecular weights up to <NUM>,<NUM>,<NUM>) may also be utilized. Modified polyethyleneimines, such as ethoxylated polyethyleneimines, also are suitable for use. Polyethyleneimines may be prepared by ring-opening polymerization of aziridine, for example.

Other illustrative, non-limiting examples of suitable co-reactant compounds include amines corresponding to the structural formula H<NUM>N(CH<NUM>CH<NUM>NH)nCH<NUM>CH<NUM>NH<NUM>, where n is <NUM> or an integer of from <NUM> to <NUM>. The functionalized co-reactant compound(s) may be linear or branched in structure (including hyper-branched and dendritic structures).

The preformed reaction products of catechol compounds and functionalized co-reactant compounds used in the methods of the present invention may be prepared using any suitable technique known in the art. For example, the reaction may be carried out under oxidative conditions and/or conditions effective to achieve condensation of the catechol compound(s) and the functionalized co-reactant compound(s), thereby forming a polymeric reaction product. The precise reaction mechanisms are not well understood and the reaction products obtained are generally complex in structure. However, in at least some cases, it is believed that at least a portion of the reaction proceeds by way of Michael addition of a nucleophile (a Michael donor) in one of the reactants to an electrophilic site (a Michael acceptor) in another reactant.

Such Michael addition-type reactions typically result in the formation of covalent heteroatom-carbon bonds (e.g., nitrogen-carbon covalent bonds). However, other types of reactions resulting in the formation of covalent bonds between the reactants may also take place. Internal reaction of one or more of the reactants may also occur; for example, when the catechol compound is an aminoethyl-substituted catechol such as dopamine, cyclization of the aminoethyl group to form an indole group may be observed. Carbon-carbon and/or nitrogen-nitrogen coupling reactions may also take place.

For example, the catechol compound(s), the functionalized co-reactant compound(s) and the preformed reaction product(s) are all soluble in water. However, in other examples, one or more of the catechol compound(s), the functionalized co-reactant compound(s) and/or the preformed reaction product(s) are dispersible in water.

Exemplary methods of forming reaction products suitable for use in accordance with the present invention may comprise the following steps:.

Oxidative conditions may be provided by introducing molecular oxygen (O<NUM>) and/or other oxidants (oxidizing agents) into the reaction mixture. Suitable illustrative oxidants include, in addition to molecular oxygen, ozone, peroxide compounds (e.g., hydrogen peroxide), persulfates and the like.

Oxygen may be introduced into the reaction mixture by methods known to those of skill in the art, including by way of non-limiting example, bubbling or sparging air or oxygen into the reaction mixture, shaking or stirring the reaction mixture to introduce air bubbles and the like. Reaction conditions include maintaining a temperature in a range of about <NUM>° C to about <NUM>° C, desirably in a range of <NUM>° C to <NUM>° C, and preferably about <NUM> to <NUM>° C for a period of time sufficient to form the desired quantity of preformed reaction products of the catechol compound(s) and functionalized co-reactant compound(s). Higher reaction temperatures (e.g., temperatures above <NUM>) may also be employed, particularly where the reaction is carried out under pressure or in a sealed vessel. Reaction conditions generally are selected such that the reaction mixture remains liquid. Reaction time may range from <NUM> to <NUM> hours, desirably from about <NUM> to about <NUM> hours, and in one embodiment can be from <NUM> to <NUM> hours. The reaction time in other embodiments may be as little as <NUM> minutes, depending upon the reactivity of the catechol compound(s) and co-reactant compound(s), the reaction temperature and pressure, and oxidant (e.g., O<NUM>) availability, among other factors, provided such conditions do not negatively affect the performance of the resulting reaction product(s) to an unacceptable extent. The reaction product(s) may be produced in a continuous synthesis process, using any of the procedures known in the polymer art; in such a process, a residence time of as little as <NUM> to <NUM> minutes may be employed.

In one embodiment, a preformed reaction product suitable for using in the form of an aqueous mixture as a rinse after cleaning a metal substrate surface and/or before applying a conversion coating to a metal substrate surface may be prepared by a method comprising the following steps: a) providing an aqueous reactant mixture of at least one catechol compound (e.g., dopamine or a hydrohalide salt of dopamine) and at least one co-reactant compound (e.g., a polyethyleneimine); and b) stirring the aqueous reactant mixture with vigorous vortex inducing stirring at a temperature of <NUM> to <NUM>° C for a period of time from <NUM> to <NUM> hours to thereby form preformed reaction products of the catechol compound(s) and co-reactant compound(s).

The molar ratio of catechol compound(s) to reactive functional groups in the co-reactant compound(s) is not believed to be particularly critical. However, in certain embodiments, a molar ratio of catechol compound(s) to reactive functional groups in the co-reactant compound(s) is from <NUM>:<NUM> to <NUM>:<NUM> or <NUM>:<NUM> to <NUM>:<NUM>. In one embodiment, a molar excess of reactive functional groups relative to catechol compound is utilized. However, it will generally be desirable to select a molar ratio which is effective to provide preformed reaction products which are water-soluble, e.g., preformed reaction products which have a solubility in water at <NUM> of at least <NUM>, at least <NUM>, at least <NUM> or at least <NUM>% by weight. The amount by weight of catechol compound need not be particularly high; that is, preformed catechol compound/functionalized co-reactant compound reaction products that are effective in enhancing the oxidation resistance of a bare metal substrate surface may be prepared using relatively low weight amounts of catechol compound (e.g., <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>% by weight in total of catechol compound(s) based on the total weight of catechol compound and functionalized co-reactant compound).

The preformed reaction products obtained may be subjected to one or more purification steps prior to being used in a pre-rinse solution in accordance with the present invention. Such methods include, by way of illustration, filtration, dialysis, membrane treatment, ion exchange, chromatography and the like and combinations thereof. For example, halide salts may be formed as by-products, depending upon the reactants used to prepare the preformed reaction product. If the presence of such halide salts (chloride salts, in particular) is determined to be detrimental to the performance of the pre-rinse solution, they may be removed or reduced by any suitable method, such as treatment with an ion exchange resin capable of exchanging a less harmful anion for the halide. If unreacted catechol compound and/or unreacted co-reactant compound is present, together with preformed reaction product, such unreacted materials may, if so desired, be removed before using the preformed reaction product in a rinse step. In certain embodiments of the invention, however, the aqueous mixture when used as a pre-rinse additionally is comprised of unreacted catechol compound(s), unreacted co-reactant compound(s), or both unreacted catechol compound(s) and unreacted co-reactant compound(s) in addition to preformed reaction product. An advantage of the present invention is that aqueous mixtures (e.g., aqueous solutions or aqueous dispersions, which preferably are storage-stable) of preformed catechol compound/functionalized co-reactant compound reaction products may be prepared in advance and conveniently stored in stable solution form until such time as it is desired to contact the preformed catechol compound/functionalized co-reactant compound reaction products with a bare metal substrate surface. Thus, forming the reaction products in situ during a pre-rinse operation, which would likely lead to significant delays in processing time, is not necessary.

As used herein, the term "storage-stable" when referring to a mixture (whether a solution or a dispersion) means that the mixture after being stored in a sealed container over a period of observation of at least <NUM> months at <NUM>, during which the mixture is mechanically undisturbed, exhibits no phase separation and no precipitation or settling out of any material that is visible to the unaided human eye.

For example, an aqueous mixture of at least one preformed reaction product of at least one catechol compound and at least one co-reactant compound comprised of one or more functional groups reactive with the at least one catechol compound is brought into contact with a bare metal substrate surface. Such an aqueous mixture (which may be in the form of a solution or a dispersion and preferably is a storage-stable mixture) may be formed by any suitable method. For example, where the at least one preformed reaction product is obtained as an aqueous dispersion or solution (as a result of carrying out the reaction of catechol compound and functionalized co-reactant compound while the reactants are dispersed or dissolved in water, for example), such an aqueous mixture may be used directly or after dilution of the aqueous mixture to a particular desired end-use concentration. Water alone may be used for such dilution, but in other examples it is contemplated that one or more other types of components may be included in the aqueous mixture. For example, an acid, base or buffer may be combined into the aqueous solution to modify its pH characteristics. The aqueous mixture in certain embodiments of the invention is basic, but in other embodiments may be acidic or neutral. In certain embodiments of the invention, the pH of the aqueous mixture, when contacted with a bare metal substrate surface (i.e., when used in a working pre-rinse bath) may be from <NUM> to <NUM>, from <NUM> to <NUM>, from <NUM> to <NUM>, from <NUM> to <NUM>, from <NUM> to <NUM>, from <NUM> to <NUM>, or from <NUM> to <NUM>, for example.

As used (i.e., when contacted with a bare metal substrate surface), the aqueous mixture may have a concentration of preformed catechol compound/co-reactant compound reaction product of, for example, <NUM> to <NUM>,<NUM> ppm, <NUM> to <NUM> ppm, <NUM> to <NUM> ppm, <NUM> to <NUM> ppm, <NUM> to <NUM> ppm, <NUM> to <NUM> ppm, <NUM> to <NUM> ppm, <NUM> to <NUM> ppm or <NUM> to <NUM> ppm.

Also contemplated by the present invention is the utilization of concentrates comprising water and preformed catechol compound/co-reactant compound reaction product(s), wherein the concentration of preformed reaction product is higher than the desired concentration of preformed reaction product in the aqueous mixture to be contacted with a bare metal substrate surface. The concentrate may be combined with an amount of water effective to achieve such desired end-use concentration, prior to treatment of a bare metal substrate surface in accordance with the present invention. The concentration of preformed catechol compound/co-reactant compound reaction product in such a concentrate may be, for example, <NUM> to <NUM>% by weight or <NUM> to <NUM>% by weight.

An aqueous mixture (working bath) repeatedly contacted with bare metal substrate surfaces can, over time, become depleted with respect to the concentration of preformed catechol compound/co-reactant compound reaction product. Should this happen, the aqueous mixture in the working bath may be replenished by addition of an amount of preformed catechol compound/co-reactant compound reaction product (in concentrate form, for example) effective to restore the desired concentration. Further, it is understood that a repeatedly used working bath containing the aqueous mixture may accumulate some amount of various components carried over from a cleaning stage, such as alkaline builders (sodium hydroxide, potassium hydroxide, alkali metal carbonates, alkali metal bicarbonates, phosphates, silicates), surfactants and oil/grease/dirt contaminants. When the levels of such components reach a point where the performance of the working bath or the quality of the conversion coated metal substrates being processed becomes adversely affected, the contents of the working bath may be discarded and replaced or treated to remove or reduce such components or otherwise counteract their effect (by pH adjustment and/or ion exchange, for example).

An aqueous mixture comprised of preformed catechol compound/co-reactant compound reaction product(s) is contacted with a cleaned surface of a metal substrate, in accordance with the present invention. Such contacting may be accomplished by any suitable method, such as, for example, spraying, immersion, dipping, brushing, roll-coating or the like. Typically, the aqueous mixture during such contacting is maintained at a temperature of from ambient temperature (e.g., room temperature) to a temperature moderately above ambient temperature. For instance, the temperature of the aqueous mixture in a working bath may be from <NUM> to <NUM>, from <NUM> to <NUM> or from <NUM> to <NUM>.

The contact time should be selected to be a time sufficient to deposit an effective amount of preformed catechol compound/co-reactant compound on the bare metal substrate surface, which may generally be regarded as an amount effective to reduce the amount of oxidation (e.g., flash rusting) on the surface of the metal substrate once the surface is exposed to air, as compared to a control where the bare metal substrate surface is contacted with water alone under the same conditions. Typically, contact times of from <NUM> to <NUM> minutes (e.g., <NUM> seconds to <NUM> minutes, or <NUM> seconds to <NUM> minutes, or <NUM> seconds to <NUM> minutes) will be suitable.

Once the desired contact time has been reached, contacting is discontinued and the pre-rinsed metal substrate may be taken on to further processing steps. For example, spraying may be stopped or the article comprising the pre-rinsed metal substrate may be removed from an immersion bath. Residual or surplus aqueous mixture may be permitted to drain from the rinsed surface of the metal substrate. Removal of residual or surplus aqueous solution can be accomplished by any suitable method or combination of methods, such as drip-drying, squeegeeing, wiping, draining or rinsing with water. According to certain embodiments, the pre-rinsed metal substrate surface may be dried (e.g., air-dried, heat or oven dried). In other embodiments, the pre-rinsed metal substrate is not dried before proceeding with further processing steps such as conversion coating.

The present invention is particularly useful in connection with the treatment of metal substrate surfaces that are susceptible to oxidation once cleaned, especially bare metal surfaces that exhibit flash-rusting when exposed to molecular oxygen (e.g., air). Ferrous (iron-containing) metal substrates may be treated in accordance with the present invention, for example. Exemplary metal substrates include, without limitation, iron; steel substrates such as cold rolled steel, hot rolled steel, and stainless steel; steel coated with zinc metal, zinc alloys such as electrogalvanized steel, galvalume, galvanneal, and hot-dipped galvanized steel; magnesium alloys; aluminum alloys and aluminum plated steel substrates. A component or article containing more than one type of metal substrate can be processed in accordance with the procedures set forth herein. The present invention may also be practiced using metal substrates in which an iron-containing component or layer is covered with a metal coating that does not contain iron (e.g., a zinc coating), wherein the iron-containing component or layer becomes exposed as a result of cutting, forming, fitting, sanding, grinding, polishing, scoring or other such operations.

As used herein, the term "bare metal substrate surface" refers to the metallic surface of a metal substrate which is essentially free of any contaminants and which is not conversion coated or coated with some other substance. For example, the bare metal substrate surface to be treated with preformed catechol compound/functionalized co-reactant compound reaction products is obtained by cleaning a contaminated metal substrate surface (in particular, a metal substrate surface that would be a bare metal substrate surface but for the presence of surface contaminants such as dirt, grease, oil and the like). For example, prior to contacting a metal substrate surface with an aqueous mixture comprised of a preformed catechol compound/co-reactant compound reaction product in accordance with the present invention, the surface may be cleaned to remove grease, oil, dirt or other extraneous materials and contaminants using any of the cleaning procedures and materials known or conventionally used in the art, including for example mild or strong alkaline cleaners, neutral cleaners and acidic cleaners. Methods of cleaning metal surfaces are described, for example, in Murphy, "Metal Surface Treatments, Cleaning", Kirk-Othmer Encyclopedia of Chemical Technology, <NUM>. Aqueous as well as non-aqueous (i.e., organic solvent-based) cleaners may be employed. Components of suitable cleaners may include, for example, inorganic bases (alkali metal hydroxides, alkali metal carbonates, alkali metal bicarbonates, for example), builders (e.g., phosphates, silicates), surfactants, water, organic solvents and the like. Examples of alkaline cleaners include Parco® cleaner ZX-<NUM>, Parco® cleaner <NUM>, and Bonderite® C-AK T51, each of which is available from Henkel Corporation, Madison Heights, Michigan. The cleaner may be applied to and contacted with the metal substrate surface using any method known to be suitable for removing contaminants, such as spraying, immersion, wiping and so forth. The temperature during such contacting may be, for example, from about room temperature to a temperature somewhat above room temperature (e.g., <NUM> to <NUM>). The duration of the contacting between the cleaner and the metal substrate may be any time effective to achieve the desired extent of contaminant removal (for example, from <NUM> seconds to <NUM> minutes). Mechanical action may be utilized to assist in contaminant removal. While typically the cleaners used for such purpose are in liquid or solution form, it is also possible to clean metal substrate surfaces using mechanical means alone, such as sanding, sand blasting or blasting with other dry media. The metal substrate, following a cleaning step, may optionally be subjected to one or more further steps prior to being contacted with a solution or dispersion comprising one or more preformed catechol compound/co-reactant compound reaction products. For example, the metal substrate surface may be rinsed one or more times with water and/or an aqueous acidic solution, after cleaning.

A bare metal substrate surface may also be prepared by methods of forming or finishing metal articles which result in bare metal surfaces being generated, such as cutting, scoring, filing, grinding, abrasion, sanding and the like.

Subsequent to a step comprising contacting a bare metallic surface of a metal substrate with an aqueous mixture comprised of preformed catechol compound/co-reactant compound reaction product(s), the metal substrate surface is subjected to a conversion coating step. The conversion coating step may be carried out immediately after the contacting step or after a further rinsing step as described above, or after an extended period of time following the contacting step. Exposing the bare metal substrate surface to such an aqueous mixture, whereby some amount of preformed catechol compound/co-reactant compound reaction product is deposited on the bare metal substrate surface, helps to protect the metal substrate surface from oxidation. The present invention thus is quite useful in coating operations in which a coating line may be interrupted for a period of time or where bare metal substrates are to be stored before being conversion coated.

Conversion coatings are coatings for metals in which the surface of a metal is converted into the coating with a chemical or electro-chemical process. Examples include chromate conversion coatings, phosphate conversion coatings (e.g., iron phosphate coatings, zinc phosphate coatings), phosphate-free conversion coatings, Group IV metal oxide coatings (e.g., zirconium oxide coatings), bluing, black oxide coatings on steel, and anodizing. In typical chemical conversion coating processes, a metal substrate surface (which may have been previously cleaned and/or rinsed) is contacted with a conversion coating composition for a time and at a temperature effective to form a conversion coating layer on the metal substrate surface, the optimum or suitable conditions being determined by the nature of the metal substrate surface and the components present in the conversion coating composition, with such conditions being familiar to or readily ascertained by those skilled in the art. Conversion coatings may be used for corrosion protection, to add decorative color or appearance to a metal substrate and as paint primers.

According to the present invention, the conversion coating step involves the use of an acidic aqueous conversion coating composition comprised of one or more Periodic Table Group IV metals such as Zr, Ti and Hf, typically containing other components as well (such as a metal etchant (e.g., fluoride), optionally also copper and/or nitrate and/or zinc and/or Si-based substances). Such conversion coating compositions are sometimes referred to as Group IV metal oxide-depositing conversion coating compositions (e.g., zirconium oxide-depositing conversion coating compositions). One such conversion coating composition is known as Bonderite® M-NT <NUM>, sold by Henkel, which is based on Zr as the Group IV metal. The aqueous acidic conversion coating may, for example, have a pH of <NUM> or less and comprise: <NUM> to <NUM> ppm of at least one Group IV metal; <NUM> to <NUM>, <NUM> to <NUM> or <NUM> to <NUM> ppm of copper; <NUM> to <NUM> ppm of free fluoride; optionally, greater than <NUM> ppm of nitrate; and, optionally, Si-based substances such as silanes, SiO<NUM>, silicates and the like.

In one embodiment, a conversion coating composition (in particular, a Group IV metal oxide-depositing conversion coating composition) may be applied to a surface of a pre-rinsed reactive metal substrates by contacting the metal substrate with the conversion coating composition for approximately <NUM> minutes at a temperature of <NUM>-<NUM>° C. Contacting may be accomplished by any suitable means including but not limited to dipping, spraying, roll-coating and the like. Contact times and temperatures may be varied, but are typically less than <NUM>, preferably less than <NUM> minutes. Desirably contact time is at least about <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> seconds and is no more than about <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> minutes. Desirably temperature ranges from at least about <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>° C and no more than about <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>° C, Higher or lower temperatures, for example at least greater than the freezing point of the bath and up to <NUM>° C, may be employed provided that they do not interfere with deposition of the conversion coating or negatively affect the metal pretreatment working bath or performance of the conversion coating.

The present invention may also be practiced wherein the conversion coating step involves phosphating, whereby metal phosphate (e.g., zinc phosphate) conversion coatings are formed on metal substrate surfaces. Zinc phosphating is a type of conversion coating well known in the art, wherein a metal substrate is contacted with a zinc phosphating composition. Present day zinc phosphate coating solutions are dilute aqueous solutions of phosphoric acid, zinc and other chemicals (e.g., other metal cations such as nickel and/or manganese as well as other types of ions such as nitrate, nitrite, chlorate, fluoroborate and/or silicofluoride) which, when applied to the surface of a metal react with the metal surface forming an integral layer on the surface of the metal of a substantially insoluble zinc phosphate coating, which may be amorphous or crystalline. The zinc phosphating compositions sold by Henkel Corporation under the brand name "Bonderite" may be utilized such as, for example, Bonderite® M-ZN <NUM>.

After contacting a metal substrate surface with a conversion coating composition, the conversion coated metal substrate may optionally be rinsed, for example with water and/or with a post-rinse solution or dispersion (sometimes referred to in the art as a "sealer") which further enhances the corrosion resistance of the conversion coated metal substrate surface.

Following conversion coating and optionally, one or more post-rinsing (or "sealing") steps, the metal substrate may be subjected to one or more further processing steps, including in particular the application of a paint or other decorative and/or protective coating. In such applications, the conversion coating may function as a primer or anti-corrosion layer. Any such coating known in the art may be employed, including for example, electrophoretic coatings (E-coatings), solvent-borne paints, aqueous-borne paints, powder coating and the like.

Accordingly, the present invention may be practiced in accordance with the following exemplary multi-step process:.

<NUM> grams of dopamine hydrochloride and <NUM> grams of Lupasol® FG (BASF polyethyleneimine, molecular weight about <NUM>) were dissolved in <NUM> grams of deionized water and allowed to react with vigorous agitation for <NUM> hours at ambient temperature (<NUM>-<NUM>). Vigorous agitation is used to introduce oxygen to the reaction mixture, which participates in/promotes the desired reaction between the dopamine hydrochloride and the polyethyleneimine. The product thereby obtained is referred to hereafter as "Dopamine/PEI Reaction Product A.

Dopamine/PEI Reaction Product A was incorporated into a rinse solution at a concentration of 200ppm (<NUM> grams/L). The solution was evaluated as follows: ACT CRS panels were processed as outlined and examined for visual oxidation (red-rust). For comparison, DI water and sodium nitrite solutions were evaluated as controls.

The <NUM> and <NUM> minute application times were utilized to simulate long dwell times or line stoppage in the rinse stage. The percentage of flash-rust indicates the area of surface covered with visible red-rust.

FTIR and GDOES depth profiles were obtained on the samples prepared using a <NUM> minute application time. These results indicated that Dopamine/PEI Reaction Product A deposits on the surface of the metal and effectively protected the surface from oxidation.

Dopamine/PEI Reaction Product A was incorporated into a rinse solution at a concentration of 200ppm. The solution was evaluated as follows: ACT CRS panels were processed as outlined and examined for visual oxidation (red-rust). For comparison, DI water and sodium nitrite were evaluated as controls.

In the test involving Dopamine/PEI Reaction Product A, some corrosion is present in localized areas, which is believed to be possibly due to the presence of <NUM> ppm chloride ion in the solution.

The <NUM> minute application times were utilized to simulate long dwell times or line stoppage in the rinse stage. The percentage of flash-rust indicates the area of surface covered with visible red-rust.

Dopamine/PEI Reaction Product A was purified with Dow Amberlite® IRN78 ion exchange resin to remove the chloride ion. The resulting product, Dopamine/PEI Reaction Product B, was incorporated into a rinse solution at a concentration of <NUM> ppm. The solution (which contained <<NUM> ppm chloride ion) was evaluated as follows: ACT CRS panels were processed as outlined and examined for visual oxidation (red-rust). For comparison, DI water and sodium nitrite solutions were evaluated as controls.

This example demonstrates the effect of a pre-rinse using a solution of Dopamine/PEI Reaction Product B on subsequent zirconium deposition from a conversion coating composition.

Dopamine/PEI Reaction Product B was used to prepare rinse stages at two conditions.

The pre-rinses using a solution of Dopamine/PEI Reaction Product B were found to slightly reduce the amount of zirconium deposited by the conversion coating stage.

Dopamine/PEI Reaction Product B was used in a pre-rinse application followed by a zirconium oxide conversion coating system. ACT CRS panels were coated as follows.

Panels were tested for corrosion resistance using GMW14872 (<NUM> cycles).

Based on these results, it was concluded that rinsing with a solution of Dopamine/PEI Reaction Product B prevents flash-rust and provides equivalent corrosion performance (as compared to rinsing with DI water).

The following tests were prepared to examine the E-coat paint appearance and the effect of using a solution of a preformed dopamine/PEI reaction product as a pre-rinse to prevent flash-rust. ACT CRS panels were tested without surface modification (control), sanded (<NUM> grit, <NUM> circular rotations by hand), and stone abrasion (GM silicate stone, <NUM> double-rubs). These surface modifications were performed to simulate body-shop metal finishing activities. These surface activities are understood to activate the metal surface and increase the risk of oxidation. Samples were then treated in accordance with the following procedure.

Pre-rinsing with the solution of the Dopamine/PEI Reaction Product B was found to be effective in preventing flash-rust and E-coat mapping.

The following example demonstrates the performance of a Dopamine/PEI reaction product pre-rinse using two materials. Dopamine/PEI Reaction Product C is a replicate of Dopamine/PEI Reaction Product A and Dopamine/PEI Reaction Product D is a replicate of Dopamine/PEI Reaction Product B (purified to remove Cl). The example also demonstrates the performance of the inventive pre-rinse step in conjunction with conversion coating solutions at [Cu]=20ppm and [Cu]=30ppm. ACT CRS panels were treated as follows.

Paint adhesion was tested in accordance with GMW14829/<NUM>. Results are reported as percentage of paint remaining.

Based on these results, it was concluded that pre-rinsing with solutions of preformed dopamine/PEI reaction product is not detrimental towards paint adhesion.

Corrosion resistance was evaluated using a Hot Salt Water Soak procedure (<NUM> days).

Corrosion resistance was evaluated in accordance with GMW14872 (<NUM> cycles).

The following example demonstrates the effectiveness of pre-rinsing with a solution of a preformed dopamine/PEI reaction product to prevent flash-rust at elevated temperatures.

The preformed dopamine/PEI reaction product ("Dopamine/PEI Reaction Product E") was incorporated into a rinse solution at a concentration of 200ppm. Dopamine/PEI Reaction Product E was prepared in accordance with the procedures of Example <NUM>, but purified using Dow Amberlite® IRN78 to remove chloride ion. The solution was evaluated as follows: ACT CRS panels were processed as outlined and examined for visual oxidation (red-rust). For comparison, DI water was evaluated as a control.

In this example, a dopamine solution was evaluated as a pre-rinse for comparison to the solution of preformed dopamine/PEI reaction product.

A 200ppm solution of dopamine was prepared using DI water, dopamine hydrochloride, and ammonium bicarbonate to adjust the pH to <NUM>.

The solution was evaluated as follows: ACT CRS panels were processed as outlined and examined for visual oxidation (red-rust). For comparison, DI water was evaluated as a control.

The dopamine solution changed with increasing temperature. It was a clear brown solution at <NUM>, a dark brown solution at <NUM>, and a black solution at <NUM> (the solution was not stable). At <NUM>, the dopamine solution formed a black precipitate, which most likely was solid polydopamine.

In this example, a polyethyleneimine solution was evaluated as a pre-rinse for comparison to the preformed dopamine/PEI reaction product.

A 200ppm solution of polyethyleneimine was prepared using DI water and polyethyleneimine at pH=<NUM>.

ACT CRS panels were processed as outlined and examined for visual oxidation (red-rust). For comparison, DI water was evaluated as a control.

These tests showed that a polyethyleneimine solution did not prevent flash-rust.

The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature.

Claim 1:
A method, comprising a step a) comprising contacting a bare metal substrate surface with an aqueous mixture comprised of at least one preformed reaction product of at least one catechol compound that includes at least one amine-functionalized catechol compound or salt thereof and at least one co-reactant compound comprised of one or more functional groups reactive with the at least one catechol compound to provide a pre-rinsed metal substrate surface, wherein the at least one co-reactant compound includes at least one oligomeric or polymeric amine compound comprising a plurality of repeating units having structure -[CH<NUM>CH<NUM>NH]-, and a step b) comprising conversion coating the pre-rinsed metal substrate surface to provide a conversion-coated metal substrate, wherein the pre-rinsed metal substrate surface is contacted with an acidic aqueous conversion coating composition comprised of at least one Group IV metal.