Aqueous post-treatment compositions and methods for corrosion protection

Abstract

The present invention relates to an aqueous post-treatment composition for providing a post-treatment layer on at least a portion of a passivation layer covering at least a portion of a zinc layer on at least a portion of an iron-containing substrate, characterized in that the aqueous post-treatment composition comprises at least one source of chromium (III) ions and at least one compound containing the chemical element silicon, with the proviso that in said composition the molar ratio of silicon to chromium is in the range of 2600:1 to 1:1 for zinc layers obtained by electrolytic acidic zinc deposition, or in the range of 5200:1 to 1:1 for zinc layers obtained by electrolytic alkaline zinc deposition.

Description

[Technical field]

[0001] The present invention relates to an aqueous post-treatment composition for providing a post-treatment layer on at least a portion of a passivation layer that covers at least a portion of a zinc layer on at least a portion of an iron-containing substrate.

[0002] The present invention provides, in this order: i. providing an iron-containing substrate; ii. electrolytically depositing a zinc layer on at least a portion of said iron-containing substrate by using a zinc deposition bath; and iii. depositing a passivation layer on at least a portion of said zinc layer on at least a portion of said iron-containing substrate by using a passivation deposition bath. The present invention is further directed to a method for corrosion protection of an iron-containing substrate, comprising: [Background technology]

[0003] There are several methods known in the art for protecting the surface of metallic materials against the effects of corrosive environments. Coating of metallic workpieces protected by a coating of another metal is a commonly used and established method in the prior art. In a corrosive medium, the coating metal can be electrochemically more or less noble than the base metal of the material. If the coating metal is less noble, it acts in a corrosive medium as a sacrificial anode to the base metal (cathodic corrosion protection). Although the protective function resulting from the formation of corrosion products of the coating metal is desired, the corrosion products of the coating often result in undesirable decoration and sometimes in a deterioration of the functionality of the workpiece.

[0004] In order to reduce and prevent, if possible, corrosion of the coating metal, so-called conversion or passivation layers are often used, inter alia, for cathodically protecting less noble coating metals, such as zinc or aluminum and their alloys.

[0005] These conversion or passivation layers are the reaction product of a less noble coating metal with an aqueous post-treatment composition, the reaction product being insoluble in aqueous media over a wide pH range. Examples of these so-called conversion or passivation layers are the so-called phosphating and chromating treatments.

[0006] In the case of phosphating, the layer to be protected is immersed in an acidic solution containing phosphate ions (see, for example, WO 00 / 47799). The acidic medium leads to partial dissolution of zinc from the coating. The zinc cations thus released, together with the phosphate ions of the treatment solution, form a sparingly soluble zinc phosphate layer on the surface. Although the zinc phosphate layer itself is an excellent adhesive surface for varnishes and paints applied to it, it offers relatively poor protection against corrosion, so that its main area of ​​application is in its function as a base layer for the application of varnishes and paints.

[0007] In the case of chromate treatment, the surface to be treated is immersed in an acidic solution containing chromium (VI) ions (see EP 0 553 164 A1). For example, in the case of zinc surfaces, part of the zinc dissolves. Under reducing conditions that prevail, chromium (VI) is reduced to chromium (III) which is precipitated in a thin surface film, which becomes more alkaline due to the evolution of hydrogen, inter alia, as chromium (III) hydroxide or as a sparingly soluble chromium (III) complex. In parallel, sparingly soluble zinc chromate (VI) is formed. As a result, a tightly closed conversion or passivation layer is formed on the zinc surface, which provides good protection against corrosive attack by the electrolyte.

[0008] JPH0730457B2 shows a surface-treated steel sheet in which a thin chromate film is formed on a zinc or zinc alloy plated steel sheet. In order to improve corrosion resistance and fingerprint resistance, a first chromate layer is formed, followed by the formation of a second chromate layer containing a chromium compound and a silica compound.

[0009] JPH01177378(A) shows a steel sheet comprising a galvanized zinc or zinc alloy layer followed by two chromate layers, the first layer comprising chromium, phosphorus and optionally silicon, the second layer comprising an organic resin as the main component.

[0010] However, chromium(VI) compounds are acutely toxic and highly carcinogenic, so alternatives to the use of these compounds must be found.

[0011] At present, a number of methods have been established which use various complexes of trivalent chromium compounds as alternatives to chromate treatment methods using hexavalent chromium compounds (see DE-A-19638176).

[0012] The protection against corrosion that can be achieved therewith is usually inferior to that achieved by methods using hexavalent chromium, and in many cases an additional organic sealing layer is applied to the workpiece surface, usually by precipitation from an aqueous polymer dispersion. In particular, when using the so-called black conversion or passivation, i.e. the method in which a black layer is formed on zinc-containing surfaces by using trivalent chromium compounds, according to the prior art, a post-treatment of this first conversion or passivation layer is indispensable to improve the protection against corrosion (see WO 02 / 07902 A2).

[0013] The disadvantage of this additional process step using polymer dispersions is the formation of drain wires with the rack-coated workpieces and / or adhesion of bulk-coated pieces. Furthermore, there are problems with the dimensional accuracy of threads etc. resulting from the thickness of such organic sealants. If such sealants provide strong protection against corrosion, their adhesion to the coating surface is also usually very strong. This means that they also adhere very well to the parts of the coating equipment, making their cleaning difficult. Moreover, any items with coating defects that are to be recycled through the entire coating process must be de-coated with considerable effort, which usually requires an additional process step. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] WO 00 / 47799 [Patent Document 2] European Patent Application Publication No. 0553164 [Patent Document 3] JPH0730457B2 [Patent Document 4] JPH01177378(A) [Patent Document 5] German Patent Application Publication No. 19638176 [Patent Document 6] International Publication No. 02 / 07902A2 [Non-patent literature]

[0015] [Non-Patent Document 1] DIN EN ISO10289 [Non-Patent Document 2] DIN EN ISO9227 Summary of the Invention [Problem to be solved by the invention]

[0016] In view of the prior art, it was therefore an object of the present invention to provide an aqueous post-treatment composition for providing a post-treatment layer to improve the protection against corrosion of metallic surfaces, in particular zinc-containing surfaces, provided with a conversion or passivation layer, which does not exhibit the aforementioned disadvantages of the known prior art compositions.

[0017] Furthermore, an object was to provide a method for the corrosion protection of iron-containing substrates, which ultimately results in enhanced corrosion protection of the substrate, achieved by a post-treatment layer.

[0018] At the same time, the decorative and functional properties of the surface are maintained or even improved.

[0019] Furthermore, the problems mentioned above resulting from the use of chromium(VI)-containing compounds or from post-treatment with polymer dispersions are to be avoided.

[0020] In particular, it was an object of the present invention to provide an aqueous post-treatment composition and method for corrosion protection which makes it possible to enhance corrosion protection while at the same time minimizing, or ideally completely avoiding, the occurrence of white or red rust corrosion of the substrate. [Means for solving the problem]

[0021] These objects, and also further objects not explicitly stated but which can be readily derived or recognised from the context discussed herein by the introduction, are achieved by an aqueous post-treatment composition having all the features of claim 1. Appropriate modifications to the aqueous post-treatment composition of the invention are protected in dependent claims 2 to 8. Furthermore, claim 9 comprises a method for the corrosion protection of iron-containing substrates. Appropriate modifications to the method of the invention are protected in dependent claims 10 to 15.

[0022] Accordingly, the present invention provides an aqueous post-treatment composition for providing a post-treatment layer on at least a portion of a passivation layer covering at least a portion of a zinc layer on at least a portion of an iron-containing substrate, the composition comprising: Provided is an aqueous post-treatment composition, characterized in that it comprises at least one source of chromium (III) ions and at least one compound containing the chemical element silicon, in which the molar ratio of silicon to chromium is provided with the proviso that for zinc layers obtained by an electrolytic acidic zinc deposition process, said molar ratio is in the range of 2600:1 to 1:1, or for zinc layers obtained by an electrolytic alkaline zinc deposition process, said molar ratio is in the range of 5200:1 to 1:1.

[0023] It is therefore possible, in an unexpected way, to provide an aqueous post-treatment composition for providing a post-treatment layer to improve the protection against corrosion of metallic surfaces, in particular zinc-containing surfaces, provided with a conversion or passivation layer, which does not exhibit the aforementioned disadvantages of the known prior art compositions.

[0024] Additionally, a method is provided for corrosion protection of iron-containing substrates, ultimately resulting in enhanced corrosion protection of the substrate through a post-treatment layer.

[0025] At the same time, the decorative and functional properties of the surface are maintained or improved.

[0026] Furthermore, the problems mentioned above resulting from the use of chromium(VI)-containing compounds or post-treatment with polymer dispersions are avoided.

[0027] In particular, aqueous post-treatment compositions and methods for corrosion protection have been provided that are capable of enhancing corrosion protection while at the same time minimizing, or ideally completely avoiding, the occurrence of white or red rust corrosion of a substrate.

[0028] Brief explanation of the table Objects, features, and advantages of the present invention will also become apparent from the following description taken in conjunction with the tables.

[0029] Table 1 presents the aqueous post-treatment compositions prepared and used in the experiments according to inventive and comparative embodiments of the present invention.

[0030] Table 2 shows the corrosion resistance result evaluation scheme used to evaluate the corrosion resistance results of the experiments according to the inventive and comparative embodiments of the present invention.

[0031] Table 3 shows the corrosion resistance results of the final obtained substrates, in which the zinc layer was obtained by electrolytic alkaline zinc deposition using an alkaline zinc deposition bath, according to the inventive and comparative embodiments of the present invention.

[0032] Table 4 shows the corrosion resistance results of the finally obtained substrates, in which the zinc layer was obtained by electrolytic acid zinc deposition using an acid zinc deposition bath, according to the inventive and comparative embodiments of the present invention.

[0033] Table 5 presents the corrosion resistance results of the finally obtained substrates, in which the zinc layer is obtained by electrolytic alkaline zinc deposition using an alkaline zinc deposition bath according to an inventive embodiment and a comparative embodiment of the present invention, and in which a heat treatment of the substrate is carried out as an additional method step v.

[0034] Table 6 presents the corrosion resistance results of the finally obtained substrates, in which the zinc layer is obtained by electrolytic acid zinc deposition using an acid zinc deposition bath according to an inventive embodiment and a comparative embodiment of the present invention, and in which a heat treatment of the substrate is carried out as an additional method step v. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] The aqueous post-treatment composition according to the present invention is an aqueous bath, the water content being greater than 65% by volume of all the solvents used, preferably greater than 75% by volume, more preferably greater than 85% by volume.

[0036] As used herein, in particular when said aqueous post-treatment composition comprises at least one colloidal silicic acid, colloidal silica and / or colloidal silicate, said aqueous post-treatment composition according to the present invention can be a solution or a suspension, preferably a suspension.

[0037] The at least one source of chromium (III) ions can be added to the aqueous post-treatment composition either in the form of an inorganic chromium (III) salt, such as basic chromium (III) sulfate, chromium (III) hydroxide, chromium (III) dihydrogen phosphate, chromium (III) chloride, chromium (III) nitrate, potassium chromium (III) sulfate, or chromium (III) salts of organic acids, such as chromium (III) methanesulfonate, chromium (III) citrate, or can be formed by reduction of a suitable chromium (VI) compound in the presence of a suitable reducing agent.

[0038] Suitable chromium(VI) compounds are, for example, chromium(VI) oxide, chromates, such as potassium chromate or sodium chromate, dichromates, such as potassium dichromate or sodium dichromate.

[0039] Suitable reducing agents for the in situ formation of chromium(III) ions are, for example, sulfites, such as sodium sulfite, sulfur dioxide, phosphites, such as sodium hypophosphite, phosphorous acid, hydrogen peroxide, and methanol.

[0040] As already mentioned, the present invention avoids the problems mentioned above by using chromium (VI)-containing compounds or by post-treatment with polymer dispersions. Thus, preference is given to aqueous post-treatment compositions that are free of hexavalent chromium (i.e., free of hexavalent chromium). This means that the aqueous post-treatment composition does not contain any chromium compounds and / or chromium ions, including chromium in the oxidation state (VI).

[0041] Preferred are aqueous post-treatment compositions of the present invention wherein the at least one source of chromium (III) ions is the only source of chromium ions, most preferably the only source of chromium.

[0042] The aqueous post-treatment composition has a pH between pH 2.5 and pH 7, preferably between pH 3 and pH 6, and more preferably between pH 3.5 and pH 5.

[0043] Optionally, the aqueous post-treatment composition may additionally contain one or more complexing agents. Suitable complexing agents are in particular organic chelating ligands. Examples of suitable complexing agents are polycarboxylic acids, hydroxycarboxylic acids, hydroxypolycarboxylic acids, aminocarboxylic acids, or hydroxyphosphonic acids. Examples of suitable carboxylic acids are citric acid, tartaric acid, malic acid, lactic acid, gluconic acid, glucuronic acid, ascorbic acid, isocitric acid, gallic acid, glycolic acid, 3-hydroxypropionic acid, 4-hydroxybutyric acid, salicylic acid, nicotinic acid, alanine, glycine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, and lysine. Suitable hydroxyphosphonic acids are, for example, Dequest 2010™ (available from Solutia Inc.), and suitable aminophosphonic acids are, for example, Dequest 2000™ (available from Solutia Inc.).

[0044] Typically, at least one metal or metalloid is added to the aqueous post-treatment composition to increase protection against corrosion, such as, for example, Sc, Y, Ti, Zr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Si, P, etc. These elements can be added in the form of their salts or in the form of complex anions or the corresponding acids of these anions, such as hexafluoroboric acid, hexafluorosilicic acid, hexafluorotitanic acid, or hexafluorozirconic acid, tetrafluoroboric acid, or hexafluorophosphoric acid, or salts thereof.

[0045] Particularly preferably, zinc is added, which may be added in the form of a zinc(II) salt, such as, for example, zinc sulfate, zinc chloride, zinc phosphate, zinc oxide or zinc hydroxide.

[0046] Preferably, between 0.5 g / l and 25 g / l, particularly preferably between 1 g / l and 15 g / l, of zinc ions are added to the aqueous post-treatment composition.

[0047] The recitation of zinc compounds only details examples of compounds suitable for the present invention, but the group of suitable zinc compounds is not limited to the materials specifically listed.

[0048] Additionally (optionally), to improve film formation on the treated surface and to increase the hydrophobicity of the surface, the aqueous post-treatment composition may contain one or more water-soluble or water-dispersible polymers selected from the group consisting of polyethylene glycols, polyvinylpyrrolidones, polyvinyl alcohols, polyitaconic acids, polyacrylates, and copolymers of the respective constituent monomers.

[0049] The concentration of the at least one polymer is preferably in the range between 50 mg / l and 20 g / l. The addition of the aforementioned polymers to the aqueous post-treatment composition leads to a significant improvement in the properties of the corrosion protection layer thus deposited.

[0050] Additionally (optionally), the aqueous post-treatment composition may contain one or more surfactants, which ensures a uniform composition of the coating layer and improved drainage behavior, especially in the case of complex moieties or difficult-to-wet surfaces.

[0051] Preferably, the aqueous post-treatment composition additionally comprises one or more phosphate compounds, most preferably oxo compounds derived from phosphorus in the oxidation state +V, and their esters containing organic residues having up to 12 carbon atoms, and preferably salts of mono- and diesters having 2 to 10 carbon atoms.

[0052] Suitable phosphate compounds are in particular alkyl esters of phosphoric acid, containing alkyl groups having up to 12 carbon atoms, preferably having 2 to 10 carbon atoms.

[0053] An example of a suitable phosphate compound is orthophosphoric acid (H 3 P0 4) and their salts, polyphosphoric acid and its salts, metaphosphoric acid and its salts, methyl phosphate (mono-, di-, and triester), ethyl phosphate (mono-, di-, and triester), n-propyl phosphate (mono-, di-, and triester), i-propyl phosphate (mono-, di-, and triester), n-butyl phosphate (mono-, di-, and triester), 2-butyl phosphate (mono-, di-, and triester), tert.-butyl phosphate (mono-, di-, and triester), the salts of the aforementioned mono- and diesters, and diphosphorus pentoxide, and mixtures thereof.

[0054] The term "salts" includes not only salts of fully deprotonated acids, but also salts of all possible degrees of deprotonation, such as hydrogen phosphate and dihydrogen phosphate.

[0055] The term "iron-containing substrate" in the sense of the present invention denotes a substrate made of an iron-containing alloy, for example cast iron (an alloy of iron and ferrous iron, preferably with carbon and / or silicon as the main alloying elements). Typical substrates are, for example, brake calipers and fasteners. The substrate is cleaned before any plating procedure, according to standard methods known in the art. For example, cleaners including surfactants, acidic cleaners, etc., and the application of ultrasonic radiation or electric current during cleaning can be applied to the substrate to be plated by the method according to the present invention.

[0056] In this specification, the term "passivation layer" denotes, within the meaning of the specification, a conversion or passivation layer, preferably a so-called black conversion or passivation layer, which is widely applied to zinc layers, which blacken the surface.

[0057] The conversion or passivation layers applied to the zinc layer are common in the art and comprise a basic chromium (III) complex and an oxidizing agent in an acidic solution. These formulations form a chromium (III)-based conversion or passivation layer, preferably a black conversion or passivation layer that contains in situ generated black pigment particles. The chromium (III) complex-based layer enhances the corrosion protection already provided by the zinc layer. The additional corrosion protection provided by said layer occurs due to a barrier function that retards the access of any corrosive solutions to the zinc layer.

[0058] In one embodiment, for a zinc layer obtained by electrolytic acidic zinc deposition, said molar ratio is in the range of 2550:1 to 1:1, preferably 270:1 to 1:1, more preferably 200:1 to 1:1.

[0059] In an alternative embodiment thereof, for a zinc layer obtained by electrolytic acidic zinc deposition, said molar ratio is in the range of 1700:1 to 1:1, preferably 700:1 to 1:1, more preferably 450:1 to 1:1.

[0060] In one embodiment, for a zinc layer obtained by electrolytic alkaline zinc deposition, said molar ratio is in the range of 5150:1 to 1:1, preferably 2600:1 to 1:1, more preferably 450:1 to 1:1.

[0061] In an alternative embodiment thereof, for a zinc layer obtained by electrolytic alkaline zinc deposition, said molar ratio is in the range of 1200:1 to 1:1, preferably 450:1 to 1:1, more preferably 200:1 to 1:1.

[0062] In one embodiment, the concentration of the at least one source of chromium (III) ions ranges from 0.005 g / l to 30 g / l, preferably from 0.008 g / l to 20 g / l, more preferably from 0.01 g / l to 10 g / l.

[0063] In one embodiment, the aqueous post-treatment composition comprises at least one colloidal silicic acid, colloidal silica, and / or colloidal silicate.

[0064] In one preferred embodiment thereof, the aqueous post-treatment composition comprises at least one anionic colloidal silica, which is preferably pH stable and has an acidic pH value.

[0065] Such colloidal silicic acids can be provided in the form of a sol or gel, in particular in the form of a sol, and are commercially available, for example, under the trade names "Ludox" or "Levasil". Colloidal silicic acids are silicates, preferably lithium salts, known as "lithium polysilicates". The SiO 2 / Li 2 The O molar ratio is in the range of 4.5 to 5.5, and preferably in the range of 4.8 to 5.2.

[0066] Colloidal materials can be used to adjust the pH and reach the desired pH by appropriate mixing. The use of lithium polysilicate or other anionic polysilicates, sols or gels can result in an alkaline pH, for example, in the range of 9-11, while colloidal cationic SiO 2 An acidic pH is produced by using lithium polysilicate or other anionic polysilicate, sol or gel and colloidal cationic silica gel to produce any desired intermediate pH value.

[0067] In one embodiment, the aqueous post-treatment composition comprises at least one silane.

[0068] Preferably, the silane has the general formula:

[0069] [ka]

[0070] where A is the following residue:

[0071] [ka]

[0072] In the formula, x is an integer from 1 to 6, y is 0 or an integer from 1 to 6, and R, which may be the same or different, is hydrogen or C. 1 ~C 6 It is an alkyl group, and z is an integer of 1 to 4.

[0073] More preferably, in the above formula, x and y are independently integers from 1 to 4, and most preferably, x is 1 and y is 2 to 4, especially 3. More preferably, R is a hydrogen atom or a methyl group. More preferably, A is CH 2 It is an OH-CHOH- group.

[0074] Most preferably, the silane is the hydrolysis product of 3-glycidyloxypropyltrialkoxysilane, glycidyloxytrimethoxysilane, or a mixture thereof.

[0075] Furthermore, the object of the present invention is also to provide, in this order: i. providing an iron-containing substrate; ii. electrolytically depositing a zinc layer on at least a portion of said iron-containing substrate by using a zinc deposition bath. iii. depositing a passivation layer on at least a portion of said zinc layer on at least a portion of said iron-containing substrate by using a passivation deposition bath. A method for corrosion protection of an iron-containing substrate comprising: an additional method step iv, which is carried out subsequently after method step iii, iv. depositing a post-treatment layer on at least a portion of said passivation layer of method step iii. by using an aqueous post-treatment composition comprising at least one source of chromium (III) ions and at least one compound containing the chemical element silicon, with the proviso that the molar ratio of silicon to chromium is in the range of 2600:1 to 1:1 for the use of an acidic zinc deposition bath of method step ii., or in the range of 5200:1 to 1:1 for the use of an alkaline zinc deposition bath of method step ii., The problem is also solved by a method characterized by:

[0076] What has been said above with respect to the aqueous post-treatment composition of the present invention preferably applies equally to the method of the present invention.

[0077] Preference is given to a method according to the invention, wherein in step iii., the passivation layer is a black passivation layer, which preferably means that in step iii., the passivation deposition bath is a black passivation deposition bath.

[0078] In the method according to the invention, the deposition of the aqueous after-treatment layer on at least part of the passivation layer can be carried out by methods known as such, in particular by immersion.

[0079] The temperature of the aqueous post-treatment composition is preferably between 10°C and 80°C, more preferably between 20°C and 60°C, and most preferably between 30°C and 50°C.

[0080] The duration of the above precipitation method step iv is preferably between 0.5 and 180 seconds, more preferably between 20 and 100 seconds, most preferably between 40 and 80 seconds.

[0081] Prior to carrying out the method according to the invention, the aqueous post-treatment composition can be prepared by diluting the corresponding more concentrated composition concentrate.

[0082] In one embodiment, after carrying out the inventive method, the finally obtained substrate is not washed but is preferably dried, for example by centrifugation at 80° C. for 10 minutes, more preferably for such time and temperature, most preferably at 750 revolutions per minute (rpm).

[0083] In one embodiment, in process step iv, for using the acidic zinc deposition bath of process step ii., said molar ratio is in the range of 2550:1 to 1:1, preferably 270:1 to 1:1, more preferably 200:1 to 1:1.

[0084] In one embodiment, in process step iv, for the use of the alkaline zinc deposition bath of process step ii., said molar ratio is in the range of 1200:1 to 1:1, preferably 450:1 to 1:1, more preferably 200:1 to 1:1.

[0085] In one embodiment, after performing method step iv, no subsequent heat treatment step is performed.

[0086] Thereby, the present invention clearly distinguishes between the general drying step described above as an optional method step and the so-called heat treatment step, in which the finally obtained substrate is annealed for a defined longer period of time to affect the final substrate properties. A drying step, if foreseen, is carried out in contrast to a heat treatment step to obtain a dry thin film layer on the substrate surface from a liquid film present on said substrate surface.

[0087] In one alternative embodiment thereof, further additional method steps v., carried out subsequently after method step iv. v. performing a heat treatment of the substrate received after method step iv. Includes:

[0088] Surprisingly, it has been found that these additional, subsequently performed process steps, the surface of which is covered with a zinc layer, a passivation layer and a post-treatment layer, are advantageous for the corrosion protection of the iron-containing substrate, and the achieved corrosion resistance of the final substrate gives better salt spray test results than conventional processes that do not use such aqueous post-treatment compositions as claimed herein.

[0089] In a preferred embodiment of the present invention, in process step iv, said molar ratio is in the range of 1700:1 to 1:1, preferably 700:1 to 1:1, more preferably 450:1 to 1:1 for the use of an acidic zinc deposition bath in process step ii.

[0090] In another preferred embodiment of the present invention, in process step iv, said molar ratio is in the range of 5150:1 to 1:1, preferably 2600:1 to 1:1, more preferably 450:1 to 1:1 for using an alkaline zinc deposition bath of process step ii.

[0091] In another embodiment of the present invention, the method can include a further subsequent method step in which a topcoat layer is deposited on at least a portion of the post-treatment layer by using a topcoat composition comprising a dispersed wax and / or a lubricant.

[0092] The present invention therefore addresses the problem of improving the corrosion protection of such substrates by providing a post-treatment layer on at least a portion of the passivation layer that covers at least a portion of the zinc layer on at least a portion of such iron-containing substrates.The post-treatment compositions of the present invention as described above have been found to be suitable for rack and barrel applications.

[0093] The following non-limiting examples are provided to illustrate embodiments of the present invention and to facilitate an understanding of the present invention, but are not intended to limit the scope of the present invention, as defined in the claims appended hereto. EXAMPLES

[0094] All experiments herein were performed using M8 * The experiments were carried out with iron-containing substrates, which were 70 8.8 steel screws, all containing a 10 micrometer zinc layer electrolytically deposited by an alkaline process step using Protolux3000S (commercially available alkaline zinc deposition bath from Atotech Deutschland GmbH) (experiments listed in Tables 3 and 5) or by an acidic process step using Zylite HT Plus (commercially available acidic zinc deposition bath from Atotech Deutschland GmbH) (experiments listed in Tables 4 and 6).

[0095] A black passivation layer was then deposited on the surface of each zinc layer by an electroless process step. For zinc layers previously deposited by an alkaline deposition bath, Tridur Zn H11 (commercial black passivation deposition bath from Atotech Deutschland GmbH) was used (experiments listed in Tables 3 and 5). For zinc layers previously deposited by an acidic deposition bath, Tridur Zn H2 (commercial black passivation deposition bath from Atotech Deutschland GmbH) was used (experiments listed in Tables 4 and 6).

[0096] Finally, a post-treatment layer was deposited on the surface of each black passivation layer by an electroless process step, using each aqueous post-treatment composition listed in Table 1. In each experiment, the pH value of the aqueous post-treatment composition was 4, the temperature was 40° C., and the immersion time was 60 seconds.

[0097] The post-treatment layer was dried by centrifugation at 80° C. and 750 revolutions per minute (rpm) for 10 minutes.

[0098] Tables 5 and 6 herein represent experiments that were performed in which an additional heat treatment step (annealing at 210° C. for 6 hours) was added as an additional process step.

[0099] Table 1 presents 20 different prepared aqueous post-treatment compositions used in the experiments shown in Tables 3 to 6. The respective concentrations of silicon and chromium (calculated as chemical elements) in grams per liter and the respective molar amounts of silicon and chromium are given herein for each prepared aqueous post-treatment composition.

[0100] The respective resulting molar ratios of silicon to chromium chemical elements are given as bold highlighted values ​​to facilitate comparison between the various experiments in Tables 3-6.

[0101] [Table 1]

[0102] Table 2 represents the corrosion resistance result evaluation scheme used by the applicant of the present invention to evaluate the quality of the corrosion resistance of the finally obtained substrate. The above evaluation scheme is based on DIN EN ISO10289 for the scale of the area of ​​the surface of the finally obtained substrate indicating corrosion defects. The scale of white rust and red rust corrosion itself was applied by the applicant as a measure of the respective corrosion resistance based on the percentage of the area of ​​defects. The salt spray test was carried out according to DIN EN ISO9227 itself.

[0103] White rust corrosion resistance of at least w4 (meaning less than 5% white rust) is generally required by customers, for example in the automotive industry. Red rust is generally completely prohibited in the respective specifications of customer companies. Therefore, to facilitate the analysis of which silicon to chromium molar ratios meet the above industry requirements, the examples of the present invention in Tables 3 to 6 are highlighted in bold.

[0104] It is clearly seen that this inventive choice is clearly differentiated from aqueous post-treatment compositions in which either at least one source of chromium (III) ions or at least one compound containing the chemical element silicon is provided alone. Moreover, even when at least one source of chromium (III) ions and at least one compound containing the chemical element silicon are provided, it is easily seen that the molar ratios found are selective to meet the objectives of the present invention.

[0105] [Table 2]

[0106] [Table 3]

[0107] [Table 4]

[0108] [Table 5]

[0109] [Table 6]

[0110] The principles of the invention have been described in relation to certain specific embodiments, which are provided for illustrative purposes, but it should be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification. It should therefore be understood that the invention disclosed herein is intended to include such modifications as fall within the scope of the appended claims. The scope of the invention is limited only by the scope of the appended claims.

Claims

1. 1. An aqueous post-treatment composition for providing a post-treatment layer on at least a portion of a passivation layer covering at least a portion of a zinc layer on at least a portion of an iron-containing substrate, comprising: the aqueous post-treatment composition comprises at least one source of chromium (III) ions and at least one compound containing the chemical element silicon, and is free of hexavalent chromium, with the proviso that in the composition the molar ratio of silicon to chromium is in the range of 1700:1 to 1:1 for the zinc layer obtained by electrolytic acidic zinc deposition, or in the range of 450:1 to 1:1 for the zinc layer obtained by electrolytic alkaline zinc deposition, the compound containing the chemical element silicon is colloidal silica, colloidal silicate and / or silane, 10. An aqueous post-treatment composition comprising: a chromium(III) ion in a concentration of from 0.005 g / l to 30 g / l;

2. 2. The aqueous post-treatment composition according to claim 1, characterized in that for the zinc layer obtained by electrolytic acidic zinc deposition, said molar ratio is in the range of 270:1 to 1:1, preferably 200:1 to 1:

1.

3. 2. The aqueous post-treatment composition according to claim 1, characterized in that for the zinc layer obtained by electrolytic acidic zinc deposition, said molar ratio is in the range of 700:1 to 1:1, preferably 450:1 to 1:

1.

4. 2. The aqueous post-treatment composition of claim 1, characterized in that for the zinc layer obtained by electrolytic alkaline zinc deposition, said molar ratio is in the range of 200:1 to 1:

1.

5. 5. The aqueous post-treatment composition according to claim 1, wherein the concentration of the at least one chromium(III) ion is in the range of 0.008 g / l to 20 g / l, preferably 0.01 g / l to 10 g / l.

6. In this order: i. providing an iron-containing substrate; ii. electrolytically depositing a zinc layer on at least a portion of said iron-containing substrate by using a zinc deposition bath. iii. depositing a passivation layer on at least a portion of the zinc layer on at least a portion of the iron-containing substrate by using a passivation deposition bath. A method for corrosion protection of an iron-containing substrate comprising: an additional method step iv, which is carried out subsequently after method step iii, iv. depositing a post-treatment layer on at least a portion of said passivation layer of method step iii. by using an aqueous post-treatment composition comprising at least one source of chromium (III) ions and at least one compound containing the chemical element silicon, and free of hexavalent chromium, with the proviso that in said composition, for the use of an acidic zinc deposition bath of method step ii., said molar ratio of silicon to chromium is in the range of 1700:1 to 1:1, or for the use of an alkaline zinc deposition bath of method step ii., said molar ratio is in the range of 450:1 to 1:1; the compound containing the chemical element silicon is colloidal silica, colloidal silicate and / or silane, 4. The method of claim 3, wherein the concentration of the at least one chromium (III) ion is from 0.005 g / l to 30 g / l.

7. 7. A method for corrosion protection according to claim 6, characterized in that in method step iv, for the use of the acidic zinc deposition bath of method step ii., the molar ratio is in the range from 270:1 to 1:1, preferably from 200:1 to 1:

1.

8. 7. The method for corrosion protection according to claim 6, characterized in that in method step iv, for using the alkaline zinc deposition bath of method step ii., the molar ratio is in the range from 200:1 to 1:

1.

9. A further additional method step v, which is carried out subsequently after method step iv, v. performing a heat treatment of the substrate received after method step iv, said heat treatment being annealing at 210° C. for 6 hours; 7. The method for corrosion protection according to claim 6, characterized by:

10. 10. A method for corrosion protection according to claim 9, characterized in that in method step iv, for the use of the acidic zinc deposition bath of method step ii., the molar ratio is in the range from 700:1 to 1:1, preferably from 450:1 to 1:1.

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