Patent Publication Number: US-2023160068-A1

Title: Method for forming a black-passivation layer on a zinc-iron alloy and black- passivation composition

Description:
FIELD OF THE INVENTION 
     The present inventions refers to a method for forming a black-passivation layer on a zinc-iron alloy and a black-passivation composition for depositing a black-passivation layer on such, wherein the black-passivation composition comprises one or more than one blackening agent selected from the group consisting of formula (I) and formula (II) as described hereinafter. 
     BACKGROUND OF THE INVENTION 
     To protect metallic substrates from corrosive environmental influences, different methods are available according to the prior art. To apply a protective coating/layer of a metal or metal alloy on the metallic substrate is a widely used and established method. A well know principle is the deposition of a zinc or zinc-nickel coating/layer on metallic substrates, such as iron metal substrates. Such coating/layers are often called conversion coatings/conversion layers. Such conversion coatings/conversion layers typically comprise reaction products (which are insoluble in aqueous media over a wide pH range) of the metallic substrate with a respective conversion treatment solution. In order to further increase the corrosion resistance, such conversion coatings/conversion layers are additionally passivated with a passivation layer by contacting it with a passivation composition. Such passivation compositions and respective methods are known in the art. 
     Besides a zinc and zinc-nickel coating/layer, other alloys are becoming more prominent such as zinc-iron. Since nickel becomes more and more questionable in regard to environmental and health issues, less critical alternatives are desired. 
     In many cases the passivation composition furthermore modifies the color of the conversion coating/conversion layer, for example into a bluish or even dark black color. Depending on the application, such a color modification is often very much desired for optical reasons, in particular in the automotive field. 
     However, for certain conversion coatings/conversion layers a suitable color modification caused by means of passivation compositions is either not available at all, provides an insufficient color modification, deteriorates corrosion resistance upon color modification, or requires sophisticated passivation compositions, which are demanding to handle. This is particular true for zinc-iron conversion coatings/conversion layers, e.g. substrates protected with a zinc-iron layer. Blackening of same without compromising the corrosion resistance is still a demand. 
     EP 1 816 234 B1 refers to an aqueous passivating coating composition for zinc or zinc alloys and method for using same. 
     CN 104651823 A refers to a cobalt-free, environmentally friendly trivalent chromium black passivation liquid, comprising tungsten disulfide particles as blackening agent. CN′823 is silent with respect to zinc iron alloys. 
     WO 97/13888 A1 refers to a non-chromate containing, corrosion-inhibiting coating composition capable of protecting a wide variety of metal surfaces. 
     WO 02/49960 A2 refers to specific tripolyphosphates, mixtures thereof, and their use as anti-corrosion agents and as biocides (anti-microbial agents) and, where applicable, as encrustation-inhibiting agents. 
     EP 3 360 989 A1 refers to a method for electrolytically passivating an outermost chromium or outermost chromium alloy layer to increase corrosion resistance thereof. 
     US 2004/0170848 A1 refers to a corrosion inhibiting composition for coating an article or substrate such as a metal, metal coating, chromated metal coating, and the like comprises a film-forming compound such as a wax or a polymer, and a sulfide salt or thio compound or a derivative of a thio compound. US′848 is silent with respect to blackening. 
     Although black-passivation compositions are described in the art, there is an ongoing demand to improve blackening of zinc-iron conversion coatings/conversion layers. 
     Objective of the Present Invention 
     It was therefore the objective of the present invention to provide a method for forming a black-passivation layer particularly on a zinc-iron alloy with improved blackening quality and without compromising corrosion resistance. Furthermore, a respective black-passivation composition is needed too. 
     It is furthermore the objective to overcome the disadvantages mentioned above and to provide in particular a method (along with a respective composition), which is easy to handle and avoids sedimentation by being substantially free of particles. 
     SUMMARY OF THE INVENTION 
     The objectives mentioned above are solved by a method for forming a black-passivation layer on a zinc-iron alloy, the method comprising the steps:
     (a) providing a substrate comprising the zinc-iron alloy,   (b) providing a black-passivation composition for depositing the black-passivation layer on the zinc-iron alloy, the composition comprising
       (i) one or more than one blackening agent selected from the group consisting of   formula (I),   
       

     
       
         
         
             
             
         
       
         
         
           
             
               
                 wherein
               R 1  and R 2  are independently selected from the group consisting of hydrogen and C1 to C5 alkyl,   R 3  is selected from the group consisting of sulfonic acid, carboxylic acid, alkyl carboxylic acid, phosphonic acid, salts and esters thereof, and   n is 1, 2, 3, 4, or 5,   
             
               
             
             and formula (II), 
           
         
       
    
     
       
         
         
             
             
         
       
         
         
           
             
               
                 wherein
               R 4  is selected from the group consisting of sulfonic acid, carboxylic acid, alkyl carboxylic acid, phosphonic acid, salts and esters thereof, and   m is 1, 2, 3, 4, or 5, and   
             
               
             
             (ii) one or more than one species of metal ions selected from the group consisting of trivalent chromium, titanium, and zirconium, and 
           
         
         (c) contacting the substrate with said black-passivation composition such that the black-passivation layer is formed on the zinc-iron alloy. 
       
    
     By utilizing said one or more than one blackening agent, a very good blackening of a zinc-iron alloy is obtained along with good corrosion resistance. Moreover, the method of the present invention is simple and can be easily carried out. Our own experiments have furthermore shown that the blackening obtained by means of the method of the present invention and the respective black-passivation composition of the present invention is highly specific for zinc-iron alloys. Own experiments confirm that zinc alone and zinc nickel alloys are not blackened. Further details are given in the examples section below in the text. 
     The present invention also concerns a respective black-passivation composition as further described below in the text as well as a respective use of said one or more than one blackening agent for blackening a zinc-iron alloy. Generally, features described in regard to the method of the present invention, in particular features described as being preferred, apply likewise to the black-passivation composition of the present invention, most preferably to a black-passivation composition described as being preferred, and apply likewise to the use according to the present invention, most preferably to the use described as being preferred. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the context of the present invention, ions of trivalent chromium refers to chromium ions with the oxidation number+3 (also called trivalent chromium ions) including the free and complexed form, respectively. Thus applies mutatis mutandis to the metal ions of titanium and zirconium, respectively. 
     In the context of the present invention, the term “black-passivation layer” also denotes a black-conversion layer. 
     The Substrate 
     Preferably, the black-passivation layer has a darkness value L* of 40 or below, preferably of 33 or below, most preferably of 25 or below, based on the CIELAB color space definition. 
     As mentioned above, the method of the present invention is highly specific for a zinc-iron alloy. Preferred is a method of the present invention, wherein in the zinc-iron alloy the amount of iron ranges from 0.1 wt.-% to 30 wt.-%, based on the total weight of the zinc-iron alloy, preferably 0.6 wt.-% to 28 wt.-%, more preferably 2.1 wt.-% to 25 wt.-%, even more preferably 3.5 wt.-% to 22 wt.-%, most preferably 4.9 wt.-% to 18 wt.-%, even most preferably 6.1 wt.-% to 15 wt.-%. A very preferred amount of iron ranges from 4.9 wt.-% to 30 wt.-%. In this very preferred range, an excellent blackening is easily obtained. 
     The method of the present invention generally applies to a zinc-iron alloy. Preferred is a method of the present invention, wherein the zinc-iron alloy is present on the substrate as a layer, preferably as a layer resulting from a galvanization process, most preferably from a zinc-iron galvanization process. Thus, most preferred is a method of the present invention, wherein the zinc-iron alloy is distinct from the rest of the substrate. In such a way, the substrate is typically protected from corrosion. 
     Preferred is a method of the present invention, wherein the substrate comprises iron. This means that the substrate preferably comprises a base material, preferably a ferrous base material, more preferably steel, on which the zinc-iron alloy is deposited. Thus, the zinc-iron alloy is distinct from the rest of the substrate (i.e. is represented by the base material). 
     However, in a few cases, a method of the present invention is preferred, wherein the substrate comprises the zinc-iron alloy in a sense that the substrate itself is made of a zinc-iron alloy. In other words, preferably the base material is already the zinc-iron alloy and thus, the base material is the substrate. 
     Preferred is a method of the present invention, wherein the substrate is a metal or metal alloy substrate, preferably the substrate comprises iron, most preferably the substrate comprises iron and is different from the zinc-iron alloy. 
     Preferred is a method of the present invention, wherein the substrate is a work piece requiring anodic corrosion resistance, most preferably required due to environmentally caused corrosion. 
     Typically, a preferred substrate is selected from the group consisting of screws, bolts, nuts, and automotive parts. 
     Preferred is a method of the present invention, wherein more than one substrate is provided in step (a), preferably a plurality of substrates is provided in step (a). This in particular applies if the substrate refers to screws, bolts, and nuts. 
     Preferred is a method of the present invention, wherein the substrate is (preferably the substrates are) provided in a barrel or fixed on a rack. Thus, the method of the present invention is applicable to both kinds. 
     The Black-Passivation Composition 
     In the method of the present invention a black-passivation composition is utilized, preferably the black-passivation composition of the present invention (see text further below). 
     Preferably, the black-passivation composition is also called a conversion composition. 
     Preferred is a method of the present invention, wherein the black-passivation composition is aqueous (i.e. comprises water), wherein preferably water has a concentration of more than 50 vol.-% based on the total volume of the black-passivation composition, more preferably of 75 vol.-% or more, most preferably of 90 vol.-% or more. Very preferably, water is the only solvent. 
     Preferably, the black-passivation composition is a solution. Thus, preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, particles (including colloids). 
     Preferred is a method of the present invention, wherein the black-passivation composition is acidic, preferably having a pH from 1.0 to 4.5, preferably from 1.2 to 4.0, more preferably from 1.4 to 3.3, even more preferably from 1.5 to 2.8, most preferably from 1.6 to 2.2. 
     As mentioned above, the black-passivation composition comprises one or more than one blackening agent as defined above. 
     As indicated by formula (I) and (II), respectively, the one or more than one blackening agent utilized in the black-passivation composition is an organic blackening agent. Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, an inorganic blackening agent. An inorganic blackening agent is for example disclosed in CN 104651823 A. 
     Preferred is a method of the present invention, wherein the black-passivation composition comprises at least one or more than one (preferably one) blackening agent of formula (I). According to own experiments, a blackening agent of formula (I) provides excellent results (see examples below). 
     Only in some cases, a method of the present invention is preferred, wherein the black-passivation composition is substantially free of, preferably does not comprise, a blackening agent of formula (II). This most preferably applies if the black-passivation composition comprises already a blackening agent of formula (I). 
     Generally preferred is a method of the present invention, wherein in the black-passivation composition the one or more than one blackening agent selected from the group consisting of formula (I) and formula (II) are the only blackening agents in the black-passivation composition. 
     Preferred is a method of the present invention, wherein in the black-passivation composition the one or more than one blackening agent has a total concentration ranging from 0.2 mmol/L to 100 mmol/L, based on the total volume of the black-passivation composition, preferably from 0.3 mmol/L to 80 mmol/L, more preferably from 0.4 mmol/L to 60 mmol/L, even more preferably from 0.8 mmol/L to 45 mmol/L, most preferably from 1.6 mmol/L to 38 mmol/L. 
     In particular preferred is a method of the present invention, wherein the black-passivation composition comprises one or more than one (preferably one) blackening agent of formula (I) in a total concentration ranging from 0.4 mmol/L to 25.0 mmol/L, based on the total volume of the black-passivation composition, preferably from 0.6 mmol/L to 20.0 mmol/L, more preferably from 0.8 mmol/L to 12.0 mmol/L, even more preferably from 1.0 mmol/L to 10.0 mmol/L, most preferably from 1.2 mmol/L to 8.0 mmol/L. Most preferably, in combination with such a total concentration, compounds of formula (I) are the only blackening agents in the black-passivation composition utilized in the method of the present invention. Surprisingly, an excellent blackening was obtained even with a comparatively low total concentration of compounds of formula (I) including a total concentration of 0.4 mmol/L. It is very desired to maintain a comparatively low concentration of blackening agents such that the life-time of a respective black-passivation composition is as long as possible. A very preferred total concentration ranges from 0.4 mmol/L to 8.0 mmol/L. 
     Preferred is a method of the present invention, wherein R 1  and R 2  are independently selected from branched and unbranched C1 to C5 alkyl, preferably unbranched C1 to C5 alkyl. 
     Preferred is a method of the present invention, wherein in R 3  and R 4  the salts are independently selected from the group consisting of ammonium salts and alkaline salts, preferably from the group consisting of ammonium, sodium, and potassium. 
     Preferred is a method of the present invention, wherein R 1  and R 2  are independently selected from the group consisting of hydrogen, methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl, and tert-butyl, preferably hydrogen, methyl, ethyl, 1-propyl, and 2-propyl, most preferably hydrogen and methyl. 
     Preferred is a method of the present invention, wherein at least one of R 1  and R 2  is an alkyl group, preferably as described above as being preferred. 
     Preferred is a method of the present invention, wherein n is 2, 3, or 4, preferably 3. 
     In the method of the present invention, R 3  is selected from the group consisting of sulfonic acid, carboxylic acid, alkyl carboxylic acid, phosphonic acid, salts and esters thereof. 
     Sulfonic acid preferably means —SO 3 H, wherein “˜” denotes the covalent bond connecting this group with the rest of the compound of formula (I). Carboxylic acid preferably means ˜COOH, wherein “˜” denotes the covalent bond connecting this group with the rest of the compound of formula (I). Alkyl carboxylic acid preferably means a saturated, branched or unbranched alkyl comprising one or more than one (preferably two) carboxylic acid groups, more preferably means a saturated, branched or unbranched C1 to C6 alkyl (preferably C2 to C4 alkyl) comprising one or more than one (preferably two) carboxylic acid groups, even more preferably it comprises —C(COOH)(CH 2 ) k COOH, wherein “˜” denotes the covalent bond connecting this group with the rest of the compound of formula (I) and k is an integer ranging from 1 to 5, most preferably it comprises —C(COOH)CH 2 COOH. Phosphonic acid preferably means —PO 3 H 2 , wherein “˜” denotes the covalent bond connecting this group with the rest of the compound of formula (I). Preferably, the above mentioned regarding R 3  applies mutatis mutandis to R 4  in formula (II). 
     Preferred is a method of the present invention, wherein R 3  comprises at least sulfonic acid, salts and/or esters thereof, preferably R 3  comprises at least sulfonic acid, salts and/or esters thereof, and n is 3. 
     More preferred is a method of the present invention, wherein R 3  is selected from the group consisting of sulfonic acid, carboxylic acid, phosphonic acid, and salts thereof, most preferably R 3  comprises at least sulfonic acid and/or salts thereof, preferably R 3  comprises at least sulfonic acid and/or salts thereof, and n is 3. 
     Most preferred is a method of the present invention, wherein formula (I) comprises the compound 3-(N,N-Dimethylthiocarbamoyl)-thiopropanesulfonic acid, salts and/or esters thereof, preferably 3-(N,N-Dimethylthiocarbamoyl)-thiopropanesulfonic acid and/or salts thereof. 
     Preferred is a method of the present invention, wherein m is 2, 3, or 4, preferably 3. 
     Preferred is a method of the present invention, wherein R 4  comprises at least sulfonic acid, salts and/or esters thereof, preferably R 4  comprises at least sulfonic acid, salts and/or esters thereof, and m is 3. 
     More preferred is a method of the present invention, wherein R 4  is selected from the group consisting of sulfonic acid, carboxylic acid, phosphonic acid, and salts thereof, most preferably R 4  comprises at least sulfonic acid and/or salts thereof, preferably R 4  comprises at least sulfonic acid and/or salts thereof, and m is 3. 
     Most preferred is a method of the present invention, wherein formula (II) comprises the compound 3-(2-Benzthiazolylthio)-1-propanesulfonic acid, salts and/or esters thereof, preferably 3-(2-Benzthiazolylthio)-1-propanesulfonic acid and/or salts thereof. 
     The black-passivation composition utilized in the method of the present invention comprises one or more than one species of metal ions selected from the group consisting of trivalent chromium, titanium, and zirconium. Although a significant blackening of a zinc-iron alloy can be observed already even without the presence of these metal ions, i.e. in the total absence of these and other transition metal ions, said metal ions are in particular beneficial in order to obtain a significant corrosion resistance besides the blackening effect. Although a certain corrosion resistance is in some cases present, corrosion resistance can be thus significantly further increased. 
     Preferred is a method of the present invention, wherein the one or more than one species of metal ions is selected from the group consisting of trivalent chromium and titanium. More preferred is a method of the present invention, wherein the one or more than one species of metal ions at least comprises trivalent chromium, most preferred the one or more than one species of metal ions is trivalent chromium. Own experiments have shown that excellent corrosion resistance results are obtained with trivalent chromium ions. 
     Preferred is a method of the present invention, wherein the metal ions of trivalent chromium are from an organic and/or inorganic trivalent chromium ion source, preferably from an inorganic trivalent chromium ion source. A very preferred trivalent chromium ion source is an organic and/or inorganic trivalent chromium ion source. A preferred organic trivalent chromium ion source is trivalent chromium citrate. A preferred inorganic trivalent chromium ion source is trivalent chromium chloride hexahydrate. 
     Preferred is a method of the present invention, wherein said one or more than one species of metal ions has a total concentration ranging from 0.1 g/L to 30 g/L, based on the total volume of the black-passivation composition and based on the mass of the free ions, preferably from 0.2 g/L to 20 g/L, more preferably from 0.5 g/L to 14 g/L, even more preferably from 0.8 g/L to 10 g/L, most preferably from 1.3 g/L to 6.0 g/L. A very preferred total concentration is ranging from 0.1 g/L to 4.0 g/L. If the total concentration is significantly below 0.1 g/L, in many cases a particular desired corrosion resistance is not achieved. In contrast, if the total concentration is significantly above 30 g/L, no further benefit is identified and costs are typically inacceptable. 
     Since trivalent chromium ions are preferred, preferred is a method of the present invention, wherein the metal ions of trivalent chromium have a total concentration ranging from 0.1 g/L to 8.0 g/L, based on the total volume of the black-passivation composition, preferably from 0.2 g/L to 7.1 g/L, more preferably from 0.5 g/L to 6.1 g/L, even more preferably from 0.8 g/L to 5.0 g/L, most preferably from 1.0 g/L to 3.5 g/L. Most preferably, the above mentioned total concentration for metal ions of trivalent chromium applies with the proviso that these ions are the only species of transition metal ions in the black-passivation composition. 
     In the following a number of compounds and ions are listed that preferably are not contained in the black-passivation composition utilized in the method of the present invention. Such compounds/ions are either not helpful for solving the objectives mentioned in the outline above or even have a detrimental effect and are therefore avoided. In a few cases the presence of further compounds/ions is inevitable upon utilizing the black-passivation composition. Generally, it is preferred to utilize a comparatively simple black-passivation composition. 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, hexavalent chromium. This includes any compounds and ions comprising chromium with an oxidation number of (+VI). It is a very important objective that the method of the present invention avoids any utilization of hexavalent chromium due to its environmental and health issues. 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, thioglycolic acid and salts thereof. However, in some cases a method of the present invention is preferred, wherein the black-passivation composition comprises thioglycolic acid and/or salts thereof in addition to said one or more than one blackening agent selected from the group consisting of formula (I) and formula (II), as defined above. Although own experiments (data not shown) indicate that the mere presence of thioglycolic acid and/or salts thereof in many cases also cause a significant blackening, the obtained results are less good compared to the effect obtained with the blackening agents of formula (I) and (II). 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, nickel ions, preferably is substantially free of, preferably does not comprise, nickel. 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, intentionally added zinc ions. If zinc ions are present in the black-passivation composition they are released/dissolved from the zinc-iron alloy upon utilizing the black-passivation composition. Thus, preferred is a method of the present invention, with the proviso that, if zinc ions are present in the black-passivation composition, they are released from the zinc-iron alloy. In other word, if zinc ions are present, the source is the zinc-iron alloy. After setting up a respective black-passivation composition no zinc ions are typically present. After starting the method of the present invention, the total concentration is very low. Upon utilizing the black-passivation composition the total concentration typically increases. Thus, preferably, zinc ions are present in a total concentration of 10 g/L or below, based on the total volume of the black-passivation composition, preferably of 8 g/L or below, most preferably of 5 g/L or below, after step (C) is carried out multiple times. 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, silver ions, preferably is substantially free of, preferably does not comprise, silver. 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, nicotinic acid and salts thereof. 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, phytic acid and salts thereof. 
     In most cases, preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, intentionally added disulfides. This preferably means that the black-passivation composition is substantially free of, preferably does not comprise, intentionally added ionic disulfides and compounds comprising a covalent disulfide. In turn, only in very rare cases a method of the present invention is preferred, wherein the black-passivation composition comprises disulfides, preferably ionic disulfides and/or compounds comprising a covalent disulfide. However, in general, the absence of such compounds is generally preferred. 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, intentionally added iron ions. If iron ions are present in the black-passivation composition they are released/dissolved from the zinc-iron alloy upon utilizing the black-passivation composition. After setting up a respective black-passivation composition no iron ions are typically present. After starting the method of the present invention, the total concentration is very low. Upon utilizing the black-passivation composition the total concentration typically increases. Thus, preferably, iron ions are present in a total concentration of 1 g/L or below, based on the total volume of the black-passivation composition, preferably of 0.8 g/L or below, most preferably of 0.5 g/L or below, after step (C) is carried out multiple times. 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, tungsten ions. 
     More preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, tungsten. 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, tungsten disulfide particles, preferably is substantially free of, preferably does not comprise, tungsten disulfide. 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, thiodiglycol. 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, silicon dioxide, silicates, a silane, and a silane containing compound, preferably is substantially free of, preferably does not comprise, a silicon-containing compound. Only in very rare cases a method of the present invention is preferred, wherein the black-passivation composition comprises a silane and/or a silane containing compound. It is assumed that in some cases the presence of the silane and/or the silane containing compound positively affects the corrosion resistance of the black-passivation layer. However, in many cases an excellent corrosion resistance is already achieved even without the presence of a silane and/or a silane containing compound. 
     Preferred is a method of the present invention, wherein the black-passivation composition is substantially free of, preferably does not comprise, cobalt ions, preferably is substantially free of, preferably does not comprise, cobalt. The presence of cobalt becomes more and more an environmental issue. Only in very few cases, a method of the present invention is preferred, wherein the black-passivation composition comprises cobalt ions and/or cobalt containing compounds. The presence of cobalt typically increases corrosion resistance of a heat-treated substrate with the black-passivation layer obtained by the method of the present invention. 
     Preferred is a method of the present invention, wherein the black-passivation composition is further comprising
         (iii) one or more than one species of halogen ions, and/or   (iv) one or more than one carboxylic acid and/or salts thereof.       

     Said halogen ions are typically the counter ions of the one or more than one species of metal ions selected from the group consisting of trivalent chromium, titanium, and zirconium. 
     Preferred is a method of the present invention, wherein the one or more than one species of halogen ions have a total concentration from 1 g/L to 18 g/L, based on the total volume of the black-passivation composition, preferably from 2 g/L to 15 g/L, even more preferably from 3 g/L to 12 g/L, most preferably from 4 g/L to 10 g/L, even most preferably from 6 g/L to 9 g/L. 
     Preferred is a method of the present invention, wherein the one or more than one species of halogen ions comprises chloride ions and/or fluoride ions. In some cases, fluoride preferably also serves as complexing agent for the metal ions, most preferably if the one or more than one species of metal ions is selected from the group consisting of titanium and zirconium. Bromide ions are preferably not comprised in the black-passivation composition. 
     More preferably, the chloride ions have a total concentration from 1 g/L to 18 g/L, based on the total volume of the black-passivation composition, preferably from 2 g/L to 15 g/L, even more preferably from 3 g/L to 12 g/L, most preferably from 4 g/L to 10 g/L, even most preferably from 6 g/L to 9 g/L. Most preferably, in the black-passivation composition chloride ions are the only species of halogen ions. 
     Said one or more than one carboxylic acid and/or salts thereof typically serve as complexing agents for said one or more than one species of metal ions selected from the group consisting of trivalent chromium, titanium, and zirconium, most preferably for metal ions of trivalent chromium. 
     Preferred is a method of the present invention, wherein the one or more than one c comprises a dicarboxylic acid, a tricarboxylic acid, and/or salts thereof. 
     A preferred dicarboxylic acid and/or salts thereof comprises a C2 to C6 dicarboxylic acid and/or salts thereof, preferably oxalic acid, malonic acid, and/or salts thereof, most preferably oxalic acid and/or salts thereof. 
     A preferred tricarboxylic acid and/or salts thereof comprises citric acid and/or salts thereof. 
     Preferred is a method of the present invention, wherein the one or more than one carboxylic acid and salts thereof have a total concentration from 0.5 mmol/L to 120 mmol/L, based on the total volume of the black-passivation composition, preferably from 8 mmol/L to 105 mmol/L, even more preferably from 15 mmol/L to 90 mmol/L, most preferably from 30 mmol/L to 80 mmol/L, even most preferably from 45 mmol/L to 70 mmol/L. More preferably, the above total concentration applies with the proviso that the black-passivation composition comprises at least one or more than one dicarboxylic acid and/or salts thereof. 
     Even more preferably, the oxalic acid and salts thereof have a total concentration from 0.5 mmol/L to 120 mmol/L, based on the total volume of the black-passivation composition, preferably from 8 mmol/L to 105 mmol/L, even more preferably from 15 mmol/L to 90 mmol/L, most preferably from 30 mmol/L to 80 mmol/L, even most preferably from 45 mmol/L to 70 mmol/L. Most preferably, oxalic acid and salts thereof are the only dicarboxylic acids and salts thereof in the black-passivation composition, preferably the only carboxylic acids and salts thereof in the black-passivation composition. 
     In some cases a method of the present invention is preferred, wherein the black-passivation composition comprises citric acid and/or salts thereof, preferably in a total concentration from 0.5 mmol/L to 120 mmol/L, based on the total volume of the black-passivation composition, preferably from 8 mmol/L to 105 mmol/L, even more preferably from 15 mmol/L to 90 mmol/L, most preferably from 30 mmol/L to 80 mmol/L, even most preferably from 45 mmol/L to 70 mmol/L. In such cases, citric acid and salts thereof are preferably the only tricarboxylic acid and salts thereof in the black-passivation composition, most preferably the only carboxylic acid and salts thereof in the black-passivation composition. 
     Preferred is a method of the present invention, wherein the black-passivation composition is further comprising
         (v) nitrate ions.       

     Nitrate ions preferably act as oxidizing agent in the black-passivation composition. 
     Preferred is a method of the present invention, wherein the nitrate ions have a total concentration ranging from 0.1 g/L to 20 g/L, based on the total volume of the black-passivation composition, preferably from 0.4 g/L to 15 g/L, even more preferably from 0.8 g/L to 11 g/L, most preferably from 1.2 g/L to 7 g/L, even most preferably from 1.7 g/L to 4.5 g/L. 
     The Contacting with the Black-Passivation Composition (Step (c)) 
     In step (c) the substrate is contacted with the black-passivation composition, preferably as described above, more preferably as described above as being preferred. 
     Preferred is a method of the present invention, wherein in step (c) the black-passivation composition has a temperature in a range from 10° C. to 80° C., preferably from 15° C. to 65° C., even more preferably from 19° C. to 45° C., most preferably from 22° C. to 38° C. If the temperature is significantly exceeding 80° C., in many cases an undesired rapid dissolution (stripping) of the zinc-iron alloy is observed impairing the corrosion resistance. However, if the temperature is too low, in many cases the contacting in step (c) is undesirably long. 
     Preferred is a method of the present invention, wherein in step (c) the contacting is performed for a time period from 10 seconds to 200 seconds, preferably from 20 seconds to 160 seconds, even more preferably from 40 seconds to 130 seconds, most preferably from 60 seconds to 100 seconds. If the time period is significantly exceeding 200 seconds, in many cases an undesired rapid dissolution (stripping) of the zinc-iron alloy is observed impairing the corrosion resistance. However, if the time period is too low, typically the blackening is insufficient and thus, the optical appearance is negatively impaired. 
     Preferred is a method of the present invention, wherein step (c) is performed without applying an electrical current. Preferably, in step (c) the substrate is dipped into the black-passivation composition. 
     Post-Treatment 
     As shown in the examples below, an indeed very good corrosion resistance is obtained if after step (c) the substrate is further treated in order to increase corrosion resistance. 
     Generally preferred is a method of the present invention, wherein the substrate obtained after step (c) of the method of the present invention is additionally treated with a post-dip composition and/or a sealer composition, preferably as outlined below, either in this order or in reversed order. 
     Preferred is a method of the present invention, wherein step (c) is followed by step
     (d) contacting the substrate obtained after step (c) with a post-dip composition such that a post-dipped substrate is obtained.   

     Preferred is a method of the present invention, wherein the post-dip composition is acidic, preferably has a pH ranging from 3.0 to 6.8, more preferably from 3.5 to 6.5, even more preferably from 4.0 to 6.3, most preferably from 4.3 to 6.0. 
     Preferred is a method of the present invention, wherein the post-dip composition comprises trivalent chromium ions. 
     Preferred is a method of the present invention, wherein the sealer composition is substantially free of, preferably does not comprise, compounds and ions comprising hexavalent chromium. 
     Preferred is a method of the present invention, wherein the post-dip composition comprises phosphate ions. 
     Preferred is a method of the present invention, wherein the post-dip composition comprises one or more than one wetting agent. 
     Preferred is a method of the present invention, wherein the post-dip composition comprises one or more than one complexing agent, preferably for the trivalent chromium ions. 
     Preferred is a method of the present invention, wherein in step (d) the post-dip composition has a temperature in a range from 18° C. to 60° C., preferably 20° C. to 58° C., more preferably from 28° C. to 56° C., even more preferably from 33° C. to 54° C., most preferably from 38° C. to 50° C. 
     Preferred is a method of the present invention, wherein in step (d) the contacting is performed for a time period from 5 seconds to 200 seconds, preferably from 10 seconds to 140 seconds, even more preferably from 20 seconds to 100 seconds, most preferably from 30 seconds to 70 seconds. 
     Preferred is a method of the present invention, wherein step (d) is followed by step
     (e) contacting the substrate obtained after step (d) with a sealer composition such that a sealed substrate is obtained.   

     Preferred is a method of the present invention, wherein the sealer composition is alkaline, preferably has a pH of 9 or higher, more preferably in a range from 9.1 to 12, even more preferably from 9.3 to 11, most preferably from 9.5 to 10.5. 
     Preferred is a method of the present invention, wherein the sealer composition comprises two or more than two organic compounds. 
     Preferred is a method of the present invention, wherein the sealer composition is substantially free of, preferably does not comprise, trivalent chromium ions. 
     Preferred is a method of the present invention, wherein the sealer composition comprises at least one organic polymer, preferably comprising a polyurethane, a polyalkylene (preferably polyethylene), a polyfluoroalkylene (preferably polytetrafluoroethylene) and/or a polyacrylate. 
     Preferred is a method of the present invention, wherein the sealer composition comprises one or more than one wax. 
     Preferred is a method of the present invention, wherein the sealer composition comprises one or more than one silicon-containing compound, preferably at least one silane and/or at least one inorganic silicate. Preferably, the at least one inorganic silicate is a colloid. 
     Preferred is a method of the present invention, wherein in step (e) the sealer composition has a temperature in a range from 15° C. to 35° C., preferably 17° C. to 30° C., more preferably from 19° C. to 27° C., most preferably from 21° C. to 25° C. 
     Preferred is a method of the present invention, wherein in step (e) the contacting is performed for a time period from 5 seconds to 200 seconds, preferably from 10 seconds to 140 seconds, even more preferably from 20 seconds to 100 seconds, most preferably from 30 seconds to 70 seconds. 
     When performing steps (d) and (e) as outlined above in this order, not only a very good black-passivation layer is obtained but additionally also an excellent corrosion resistance, most preferably up to 480 hours, based on ISO 9227, 5% white rust limit. 
     Preferred is a method of the present invention, wherein step (e) is followed by step
     (f) drying the substrate obtained after step (e).   

     Preferred is a method of the present invention, wherein step (f) is carried out at a temperature ranging from 55° C. to 95° C., preferably 58° C. to 90° C., more preferably from 58° C. to 85° C., most preferably from 60° C. to 80° C. 
     Preferred is a method of the present invention, wherein step (f) is carried out for a time period from 2 minutes to 20 minutes, preferably from 3 minutes to 16 minutes, even more preferably from 4 minutes to 13 minutes, most preferably from 6 minutes to 10 minutes. 
     Preferably, a drying step, preferably as defined above, is also carried out after one or more than one of the previous steps, e.g. after step (c), step (d), etc. Very preferred is a method of the present invention, wherein a drying step, preferably as defined in step (f), is carried out after step (d) and prior to step (e). This is very preferred because in step (d) the post-dip composition is acidic, wherein in step (e) the sealer composition is alkaline. 
     The present invention furthermore refers to a black-passivation composition for depositing a black-passivation layer on a zinc-iron alloy, the composition comprising
         (i) one or more than one blackening agent selected from the group consisting of   formula (I),       

     
       
         
         
             
             
         
       
         
         
           
             
               
                 wherein 
                 R 1  and R 2  are independently selected from the group consisting of hydrogen and C1 to C5 alkyl, 
                 R 3  is selected from the group consisting of sulfonic acid, carboxylic acid, alkyl carboxylic acid, phosphonic acid, salts and esters thereof, and 
                 n is 1, 2, 3, 4, or 5, 
               
             
             and formula (II), 
           
         
       
    
     
       
         
         
             
             
         
       
         
         
           
             
               
                 wherein 
                 R 4  is selected from the group consisting of sulfonic acid, carboxylic acid, alkyl carboxylic acid, phosphonic acid, salts and esters thereof, and 
                 m is 1, 2, 3, 4, or 5, and 
               
             
             (ii) one or more than one species of metal ions selected from the group consisting of trivalent chromium, titanium, and zirconium, 
             with the proviso that
           the black-passivation composition is substantially free of, preferably does not comprise, nickel ions, cobalt ions, and tungsten ions.   
         
           
         
       
    
     In particular preferred is a black-passivation composition of the present invention, wherein the black-passivation composition is substantially free of intentionally added zinc ions, preferably does not comprise intentionally added zinc ions. Most preferably, the aforementioned regarding zinc ions in view of the method of the present invention in particular applies likewise to the black-passivating composition of the present invention. 
     Preferably, the aforementioned regarding the black-passivation composition utilized in the method of the present invention (in particular what is defined as being preferred) applies likewise to the black-passivation composition of the present invention. This applies in particular to compounds and ions not contained in the black-passivation composition utilized in the method of the present invention. 
     The present invention furthermore refers to the use of one or more than one blackening agent selected from the group consisting of
         formula (I),       

     
       
         
         
             
             
         
       
         
         
           
             
               
                 wherein 
                 R 1  and R 2  are independently selected from the group consisting of hydrogen and C1 to C5 alkyl, 
                 R 3  is selected from the group consisting of sulfonic acid, carboxylic acid, alkyl carboxylic acid, phosphonic acid, salts and esters thereof, and 
                 n is 1, 2, 3, 4, or 5, 
               
             
             and formula (II), 
           
         
       
    
     
       
         
         
             
             
         
       
         
         
           
             
               
                 wherein 
                 R 4  is selected from the group consisting of sulfonic acid, carboxylic acid, alkyl carboxylic acid, phosphonic acid, salts and esters thereof, and 
                 m is 1, 2, 3, 4, or 5, 
               
             
             for blackening a zinc-iron alloy. 
           
         
       
    
     Preferably, the aforementioned regarding the one or more than one blackening agent of formula (I) and (II) (in particular what is defined as being preferred) used in the black-passivation composition which is utilized in the method of the present invention applies likewise to the use of the present invention. 
     The present invention is described in more detail by the following non-limiting examples. 
     Examples 
     In the examples, various test passivation compositions were prepared with the numbering as introduced in Table 1 below, each composition is aqueous and generally comprises a species of metal ions; 6 g/L to 8 g/L chloride ions if chromium ions were utilized; 50 mmol/L to 70 mmol/L oxalic acid if chromium ions were utilized or 10 mmol/L to 300 mmol/L fluoride ions if titanium ions and zirconium ions were utilized, respectively, as complexing agents; approximately 1 g/L to 7 g/L nitrate ions; and one of the following compounds abbreviated as below: 
     DPS: 3-(N,N-Dimethylthiocarbamoyl)-thiopropanesulfonate, sodium salt; also known as Raluplate DPS (CAS 18880-36-9; Raschig company); a compound of formula (I), wherein R 1  and R 2  are methyl, R 3  is the sodium salt of sulfonic acid, and n is 3; 
     ZPS: 3-(2-Benzthiazolylthio)-1-propanesulfonate, sodium salt; also known as Raluplate ZPS (CAS 49625-94-7; Raschig company); a compound of formula (II), wherein R 4  is the sodium salt of sulfonic acid, and m is 3; 
     SPS: Bis-(3-sulfopropyl)-disulfide, disodium salt; also known as Raluplate SPS (CAS 27206-35-5; Raschig company); comparative example; 
     SPV: 1-(3-Sulfopropyl)-2-vinylpyridinium betaine; also known as Raluplate SPV (CAS 90552-35-5; Raschig company); comparative example; 
     MPS: 3-Mercaptopropanesulfonate, sodium salt; also known as Raluplate MPS (CAS 17636-10-1; Raschig company); comparative example; 
     DTO: Dithiooxamide; comparative example. 
     The specific compound and its respective total concentration is outlined in Table 1 below. Each test passivation composition has a pH of approximately 2. 
     In each example, as substrates a plurality of u-shaped iron plate specimens (base material) galvanized with a silver-like colored zinc-iron layer (Hiron-Zn/Fe for high iron content and Protedur Plus for low iron content, respectively, each is a product of Atotech; for iron content see Table 1 below) was dipped for approximately 90 seconds in the respective test passivation composition, the compositions having a temperature of approximately 22° C. In examples according to the invention, a blackening was immediately observed. 
     Afterwards the treated specimens were dried, and a post-dip composition (acidic, comprising trivalent chromium phosphate) as well as a sealer composition (alkaline, comprising a wax and a silicon-containing compound) was applied. Subsequently, the blackening quality was visually evaluated; the corrosion resistance according to ISO 9227. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 %  
                 blackening 
                   
                 Species of 
                   
                 Blackening 
                 corrosion* 
               
               
                 No. 
                 Fe 
                 agent 
                 [mmol/L] 
                 metal ions 
                 [g/L] 
                 quality 
                 [h] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 I-1 
                 1.6 
                 DPS 
                 1.9 
                 Cr(III) 
                 3-4 ##   
                 + 
                 &lt;72 
               
               
                 I-2 
                 15 
                 DPS 
                 3.8 
                 Zr(IV) 
                 0.5 
                 ++ 
                 &lt;24 
               
               
                 I-3 
                 15 
                 DPS 
                 1.9 
                 Ti(IV) 
                 0.3 
                 +++ 
                 120 
               
               
                 I-4 
                 15 
                 DPS #   
                 1.9-8 ##   
                 Cr(III) 
                 3-4 
                 +++ 
                 480 
               
               
                 II-1 
                 15 
                 ZPS 
                 32.0 
                 Cr(III) 
                 3-4 
                 ++ 
                 240 
               
               
                 C1 
                 15 
                 SPS 
                 14.1 
                 Cr(III) 
                 3-4 
                 − 
                 n.d. 
               
               
                 C2 
                 15 
                 SPV 
                 22.0 
                 Cr(III) 
                 3-4 
                 − 
                 n.d. 
               
               
                 C3 
                 15 
                 MPS 
                 28.1 
                 Cr(III) 
                 3-4 
                 − 
                 n.d. 
               
               
                 C4 
                 15 
                 DTO 
                 41.6 
                 Cr(III) 
                 3-4 
                 n.d. 
                 n.d. 
               
               
                   
               
               
                 “*” denotes: time until max. 5% white rust was observed, 
               
               
                 “ # ” also tested on screws in barrel applications, 
               
               
                 “ ## ” tested in compositions with various concentrations, 
               
               
                 “n.d.” denotes not determined, typically because of no (sufficient) blackening, 
               
               
                 “+++” denotes an excellent blackening; no visual defects are noticeable all over the specimen, 
               
               
                 “++” denotes still an acceptable blackening with only minor but still acceptable visual defects; the whole specimen is sufficiently black, 
               
               
                 “+” denotes not anymore acceptable blackening with significant visual defects and areas of the zinc-iron layer, 
               
               
                 “−” denotes no or almost no blackening, visual defects and large areas of the zinc-iron layer 
               
            
           
         
       
     
     In all examples according to the invention (i.e. examples I-1, I-2, I-3, I-4, and II-1) a blackening of a zinc-iron alloy was obtained. Very good results were obtained in examples I-3 and I-4, wherein example I-4 provided these very good results even over a comparatively wide concentration range for DPS and chromium ions. 
     In contrast, alternative compounds (comparative examples C1-C3) provide no or almost no blackening of the zinc-iron alloy. Example C4 revealed that DTO was not soluble at all and, thus, could not be used for testing and is found unsuitable. If no blackening was obtained, corrosion resistance was not further tested because blackening was a basic requirement. 
     In further comparative examples, above test passivation compositions (in particular according to the present invention) were tested with said substrates but having a zinc (no zinc alloy) or a zinc-nickel alloy layer thereon. In each further example no blackening was obtained (i.e. evaluated as “˜”). As a conclusion, the test passivation compositions utilized in the method of the present invention very specifically blacken zinc-iron alloys.