Patent Application
20190032222
The disclosure relates to sheet steel with a conversion coating, a method of producing conversion-coated sheet steel and a treatment agent for application of a conversion coating on sheet steel.
In the prior art, it is known to protect metal surfaces against corrosion by employing methods in which the metal surface is coated with a coating of a different, generally less noble, metal (e.g., zinc and chromium). Thus, it is known, e.g., to coat sheet steel with zinc or chromium or even tin (which is more noble than steel). In the production of packaging materials, particularly in the food sector, for example, extensively tin-coated blackplate (tinplate) is used. The distinguishing features of tinplate are excellent corrosion resistance, good formability properties and its weldability, which makes tinplate uniquely well suited for use in the production of packaging materials, e.g., beverage cans.
Tinplate has excellent properties as a packaging material for food products and has been produced and processed for this purpose for many decades. However, tin, which in tinplate constitutes the corrosion-inhibiting coating, has become a relatively high-value material because of the increasing worldwide shortage of this resource. As an alternative to tinplate, especially for use as a packaging material, it is known from the prior art to use sheet steel which is electrolytically coated with chromium and which is called “Tin-Free Steel (TFS)” or “Electrolytic Chromium Coated Steel (ECCS).” On the one hand, this tin-free sheet steel is distinguished by its excellent adhesion for lacquers or organic protective coatings (for example, PP or PET), on the other hand, however, the coating process entails considerable disadvantages due to the toxic and health-endangering properties of the chromium(VI)-containing materials used in the coating.
To protect sheet steel against corrosion and to create a good adhesion surface for lacquers and synthetic coatings, conversion coatings are frequently applied to the surface of sheet steel. The targeted creation of a conversion coating on sheet steel (blackplate) prevents the corrosion of sheet steel or at least slows it down considerably.
Conversion coatings are very thin non-metal coatings on a metal surface, which, as a rule, are created by chemical reaction of an aqueous treatment solution with the metal substrate.
Especially when applied to thin sheet steel (blackplate with thicknesses in the range of 0.1 to 0.5 mm), conversion coatings provide a highly effective protection against corrosion and a good adhesion surface for lacquers and synthetic materials, and they reduce surface friction and abrasion.
Electrolytic methods of applying conversion coatings to sheet steel are known from the prior art. Such conversion coatings were frequently created with chromium electrolytes based on carcinogenic chromium(VI) oxide. Due to statutory prohibitions, however, the use of chromium(VI)-containing conversion coatings is becoming increasingly rare. Alternatives to the classic chromium(VI) electrolyte are treatments based on chromium(III) oxide or complex fluorides (titanium compounds, zirconium compounds). Another possibility of forming a conversion coating is phosphating by means of aqueous phosphate solutions.
DE 101 61 383 A1 describes a method of coating metal surfaces, including steel surfaces, with an aqueous composition free of chromium(VI) compounds, with the aqueous composition, in addition to the solvent water, containing at least one organic film-forming substance with a water-soluble or water-dispersible polymer, a content of cations and/or hexa- and tetrafluoro complexes of cations, selected from the group of Ti, Zr, Hf, Si, Al and B, at least one inorganic compound in the form of particles with a particle diameter from 0.005 μm to 0.2 μm, and optionally a silane and/or siloxane, and optionally a corrosion inhibitor as well.
Conventional, commercially available chromium-free agents for forming conversion coatings on metal contain either film-forming substances or organic solvents. As a rule, film-forming substances are polymers which impart a number of valuable properties, such as adhesive capacity, to the conversion coating. However, the application of these polymer substances requires additional labor and equipment, which also makes the substances more costly. As a rule, the solvents contained in conventional chromium-free substances used to form conversion coatings are more expensive than water, they pose some risk to human health during the application of the conversion coating, and in most cases, they belong to the class of the so-called VOCs (volatile organic compounds) that should be avoided for environmental reasons.
Thus, one aspect of the disclosure relates to a chromium-free agent for forming conversion coatings on sheet steel. Furthermore, the disclosure also relates to a sheet steel with a chromium-free conversion coating and a method for its production, which sheet steel can be produced as inexpensively as possible, which can be used as a replacement for tin-free sheet steel (TFS or ECCS) and tinplate, and which, especially with respect to corrosion resistance and adhesion for lacquers or organic coatings, is comparable to tinplate or tin-free sheet steel.
Also disclosed are a method of producing conversion-coated sheet steel and a treatment agent for application of a conversion coating on sheet steel.
The sheet steel disclosed, which can be, specifically, strip-shaped blackplate or sheet steel coated with an anticorrosive metal film coating, has a conversion coating on at least one surface, which conversion coating contains at least one of the following components
i) hexafluorotitanate
ii) zinc phosphate and/or iron phosphate,
iii) phosphoric acid,
iv) or a mixture of components i) to iii), with the conversion coating not containing any organic substances.
The conversion coating preferably consists of one of the components i) to iii) or of a mixture of these components, most preferably of a mixture of components i) and ii) or of a mixture of all three components i), ii) and iii).
To produce the sheet steel disclosed, metal-coated or uncoated sheet steel (blackplate) is used, the surface of which is degreased in a first processing step and subsequently rinsed with water or another rinsing liquid and finally, in the next step, a wet film of a conversion coating is applied by applying a wet film of a chromium-free treatment solution, which consists exclusively of inorganic components, to at least one degreased surface of the sheet steel, with the wet film of the conversion coating being prepared from components that were dissolved in water and with the components being selected from the group comprising hexafluorotitanate, zinc phosphate and/or iron phosphate, phosphoric acid and/or a mixture of these components, provided that the components do not contain any organic substances and any inorganic particles with an average particle diameter greater than 0.005 μm. The wet coating volume is preferably in the range of 1 mL/m2 to 10 mL/m2. In a last step of the method disclosed, the wet film of the conversion coating is finally dried.
The chromium-free treatment solution preferably consists exclusively of water and one of the components hexafluorotitanate, zinc phosphate and/or iron phosphate or phosphoric acid, or a mixture of these components, most preferably of a mixture of the components hexafluorotitanate and zinc phosphate or a mixture of the components hexafluorotitanate, zinc phosphate and phosphoric acid. While the wet film of the conversion coating prepared from the aqueous treatment solution is drying, the solvent (water) evaporates, so that the dried conversion coating in these preferred embodiment examples consists of only the active components of the chromium-free treatment solution, i.e., of hexafluorotitanate, zinc phosphate and/or iron phosphate and phosphoric acid, or a mixture of these active components.
Preferred embodiments are also disclosed. The disclosure will be explained in greater detail below:
The starting material of the sheet steel disclosed is preferably cold-rolled, annealed and temper-rolled or temper-passed sheet steel made of steel with a carbon content of 20-1000 wt ppm. The sheet steel (blackplate) preferably has the following properties:
The steel of the sheet steel can be, for example, ferritic steel or a multiphase steel which comprises a plurality of structural constituents, in particular ferrite, martensite, bainite and/or retained austenite. Such multiphase steels are distinguished by a high tensile strength of more than 500 MPa and, at the same time, by a high elongation at break of more than 10%. With respect to the intended use of blackplate, which is treated as disclosed, as packaging steel, preferably the grades of sheet steel defined in DIN EN 10202:2001: “Cold reduced tin mill products—Electrolytic tinplate and electrolytic chromium/chromium oxide coated steel” are used. This standard also defines, inter alia, the analysis and mechanical properties of steel. The grades range especially between TS230 (mild steel grade, batch-annealed, yield strength 230 MPa) to TH620 (continuous annealing furnace, 620 MPa). The sheet steel can also be metal-coated sheet steel, for example, electrolytically tin-plated sheet steel.
The method disclosed is carried out by moving the sheet steel, preferably in the form of a strip, at a strip speed of more than 200 m/min and up to 750 m/min and by subjecting it to an electrochemical pretreatment. In the course of this pretreatment, the moving sheet steel is first cleaned and degreased. Cleaning and degreasing preferably takes place by passing the sheet steel, as a cathode, through an electrolyte. Degreasing is vital since, after recrystallization annealing, the cold-rolled and recrystallization-annealed sheet steel is, as a rule, temper-rolled or temper-passed, which, for example, during wet temper rolling, causes the surfaces of the sheet steel to be contaminated with a water-oil suspension and, during dry temper rolling, with oil, abraded iron particles, soaps and other contaminants. These contaminants are removed in the cleaning step.
To clean and degrease the sheet steel, the sheet steel can be passed, for example, through a cleaning tank containing an alkaline sodium or potassium hydroxide solution. The concentration of the alkaline degreasing agent is preferably in the range of 20 to 100 g/L at bath temperatures from 20-70° C. Degreasing blackplate preferably takes place in two steps, where the first step involves a dipping process and the second step an electrolytic process with current densities from 2 to 30 A/dm2. After degreasing, each surface of the blackplate strip is rinsed, for example, through a triple cascade rinse with 10-30 m3/h water. If necessary, oxide residues can be removed by passing the blackplate strip through additional cleaning tanks containing hydrochloric acid pickle or sulfuric acid pickle with a concentration of, for example, 10 to 120 g/L in two consecutive dipping steps, followed by a dipping rinse in one dipping step. The temperatures of the pickling solution and the rinsing water are typically in the range of 20° C. to 60° C.
After cleaning and degreasing, an additional electrochemical treatment of the sheet steel can be carried out so as to create a homogeneous steel surface which provides a good adhesion surface for a conversion coating. In this additional electrochemical treatment, the sheet steel is used as the anode and passed through an alkaline electrolyte. The alkaline electrolyte can be, for example, a sodium hydroxide solution or a sodium carbonate solution (Na2CO3).
After the anodic treatment in the alkaline electrolyte, the sheet steel is rinsed with water or another rising liquid and subsequently dried. Drying can take place, for example, in a continuous drying furnace or by means of a blowing unit which blows a laminar stream of hot air stream onto the surface of the moving sheet steel.
After rinsing and drying, the conversion coating is applied to at least one surface of the sheet steel. For this purpose, a wet film of an aqueous, chromium-free treatment solution which is based exclusively on inorganic components is applied to the electrolytically pretreated and dried surface of the sheet steel. This is preferably performed in a no-rinse process in which the rinsing step after application of the wet film is omitted. The aqueous treatment solution which forms the conversion coating can be applied to the surface of the sheet steel, for example, by means of a roll coater or it can be sprayed onto the surface with spray nozzles, e.g., with a rotation sprayer.
After application of the wet film of the treatment solution, the conversion coating thus formed is dried. For this purpose, the sheet steel is passed, for example, through a belt dryer in order to dry the wet film of the conversion coating. Drying preferably takes place at temperatures of 50° C. to 250° C. After the conversion coating has dried, (on each side) a dry film of the thus formed conversion coating having a surface weight of 1 to 1000 mg/m2, preferably of 10 mg/m2 to 400 mg/m2, remains on the surface of the sheet steel. The desired dry film thickness of the conversion coating can be controlled by the quantity of the aqueous treatment solution delivered per unit time in the application step. If necessary, the surface weight of the conversion coating applied can also be adjusted prior to the drying step by squeezing off any excess treatment solution.
As a final step, the surface of the dry conversion coating can optionally be treated with dioctyl sebacate (DOS), acetyl tributyl citrate (ATBC), butyl stearate (BSO) or polyalkylene glycol, especially polyethylene glycol (PEG, preferably with a molecular weight of 6000 g/mol), or combinations thereof.
In addition to the solvent water, the aqueous treatment solution used to apply the conversion coating to the sheet steel contains at least one of the following components:
or a mixture of components i) to iii), provided that the components i) to iii) do not contain any organic substances.
In preferred embodiment examples, the aqueous treatment solution, in addition to the solvent water, consists of the following components or mixtures thereof (unless expressly noted otherwise, all parts and percentages refer to parts and percentages by weight):
The effect of the treatment solutions and the properties of the conversion coatings depend on the concentration of the components used in the aqueous treatment solution and on the wet film volume of the aqueous treatment solution on the surface of the sheet steel.
In the aqueous treatment solution, the above-mentioned components of the conversion coating can be used, for example, in the following concentrations, with these concentrations equally applying to any mixtures of the components among each other:
hexafluorotitanic acid: 1%, 3%, 5%, 7% and 10%, noting that these values can also be considered as boundaries for any conceivable concentration ranges;
zinc phosphate (Zn3(PO4)2): 1%, 3%, 5%, 7% and 10% and the concentration ranges correspondingly derivable therefrom;
iron phosphate (FePO4)): 1%, 3%, 5%, 7% and 10% and the concentration ranges correspondingly derivable therefrom;
zinc phosphate (Zn3(PO4)2) and/or iron phosphate (FePO4)) in a mixture with phosphoric acid: concentrations same as given above for zinc phosphate and iron phosphate) plus 1.4%, 2.3%, 3.2% 4.2% and 5.5% phosphoric acid and the concentration ranges correspondingly derivable therefrom;
TiPO4, i.e., a mixture of hexafluorotitanic acid and zinc phosphate with an addition of phosphoric acid in a ratio of 1:2.5:1 to 4:10:4 at a starting concentration of a component as specified above for hexafluorotitanic acid and zinc phosphate.
For the conversion coatings disclosed which contain titanium, i.e., especially for the conversion coatings containing hexafluorotitanate or a conversion coating containing a mixture of hexafluorotitanate with a phosphate, the preferred dry coating weight relative to titanium was found to be in the range of 1 mg/m2 to 50 mg/m2, preferably in the range of 10 mg/m2 to 40 mg/m2. For the conversion coatings disclosed which contain phosphate, i.e., especially for the conversion coatings made from or with zinc phosphate or iron phosphate, the preferred dry coating weight relative to phosphate ions was found to be in the range of 10 mg/m2 to 1000 mg/m2 and preferably in the range of 100 mg/m2 to 400 mg/m2.
In the following, embodiment examples of the disclosure will be explained in detail. The specified concentrations of the components of the treatment solutions which are applied to create a conversion coating on a surface of a sheet steel refer to the treatment solution as such and not to possibly used starting solutions with a higher concentration. All concentrations given refer to parts by weight of the active components in the aqueous treatment solution, regardless of whether the used raw materials as such had already been diluted, e.g., as aqueous solutions.
For the tests in the embodiment examples, test sheets in the form of blackplate sheets (uncoated, cold-rolled sheet steel) with a thickness of 0.27 mm were used. The surface of the test sheets was first subjected to cathodic degreasing in 5% sodium carbonate solution Na2CO3 (time=30 s, temperature=38° C., current=5 A/dm3) and subsequently rinsed with water and deionized water. Using a roll coater (LARA), a wet film of the aqueous treatment solutions as listed in Table 1 was applied to the degreased surface and subsequently dried in a drying chamber (time=50 s, temperature 98° C.). The resulting surface weight of the conversion coating (dry weight of the treatment solution) is listed in Table 1. The surfaces of the conversion-coated test sheets were subsequently inspected. The dry weight of the conversion coating was determined by means of XRF, and the test sheets were subjected to cyclic voltammetry to determine the electron transfer barrier. The current density was measured at a potential of −770 mV. This is characteristic of the oxidation of iron into its divalent form. The higher the measured value, the greater the oxidizability. The results of the determination of the dry coating weight and of the cyclic voltammetry are listed in Table 1.
In conversion coatings containing hexafluorotitanate, the values measured by cyclic voltammetry and listed in Table 1 are relatively high, which indicates that the coating is permeable. The values of the conversion coatings containing the phosphates (zinc phosphate, iron phosphate) are invariably lower, which is indicative of an opaque and denser coating. With respect to oxidizability, the phosphate-containing conversion coatings therefore have better properties and specifically a higher corrosion resistance. Measurements by cyclic voltammetry indicate that the combination of hexafluorotitanate or hexafluorotitanic acid with zinc phosphate leads to surprisingly favorable results (10 μA/cm2 to 50 μA/cm2). These values are in the range of commercially available agents with complex structures for use in the production of conversion coatings on metals. The positive properties of the titanium-containing conversion coatings and the phosphate-containing conversion coatings can be combined by preparing the aqueous treatment solution from a mixture of hexafluorotitanate or hexafluorotitanic acid and zinc phosphate and/or iron phosphate with water as a solvent, in particular with percentages by weight of 1 to 10% for the hexafluorotitanic acid and (a total of) 1 to 10% for zinc phosphate and/or iron phosphate. In addition, phosphoric acid can be added to this preferred mixture, for example, in a percentage by weight of 1 to 10%. The addition of phosphoric acid offers the advantage that it is able to dissolve the zinc phosphate.
Subsequently, four different lacquers (Gold lacquer AN 101.597 with a coating weight of 5 g/m2; Gold lacquer BPA NI Metlac 816714 with a coating weight of 5 g/m2; Gold lacquer GL 300 MF with a coating weight of 5 g/m2; White lacquer BPA NI Valspar R 1016 with a coating weight of 15 g/m2) were applied to the dried conversion coatings, and the lacquered test sheets were subjected to loading tests (deformation and sterilization) and tested using the cross-cut test and the Erichsen scale to assess the lacquer adhesion. For this purpose, a point scoring system was used, in which points from 0 (no adhesion) to 7 (optimum adhesion) were assigned in accordance with the quality of lacquer adhesion. The points given to a conversion coating for the different lacquers were added together, and the sum of this addition is listed in Table 2. The higher the total value, the better the adhesiveness of the conversion coating for organic lacquers.
Table 2 indicates that the conversion coatings made with hexafluorotitanate yield the best results with respect to lacquer adhesion and that the adhesiveness increases as the coating weight increases. For the conversion coatings with phosphates (zinc phosphate, iron phosphate), it was found that lower coating weights tend to improve the lacquer adhesion and that, given the same coating weight, the adhesiveness of zinc phosphate is better than that of iron phosphate. Thus, zinc has a positive effect on lacquer adhesion. Zinc can be present in a coating weight that corresponds to any of the above-mentioned coating weight ranges if the conversion coating contains the component zinc phosphate. Conversion coatings with a percentage by weight of 1% to 5% of zinc (relative to the total weight of the conversion coating), preferably with 2 to 4 wt % of zinc, proved to be especially advantageous.
The results of the lacquer adhesion tests were compared to the results of comparison samples made of conventional tin-free sheet steel (ECCS or TFS) and to sheet steel that had been coated with the commercially available material “Bonderite®” by Henkel. The conventional tin-free sheet steel (ECCS or TFS) was given a total score of 115 and, depending on the coating weight, the sheet steel coated with “Bonderite®” was given a total score of 88 to 118.
Based on comparison tests with different coating weights (surface weight) of the conversion coatings, it was possible to demonstrate that the conversion coating with hexafluorotitanate up to a coating weight of approximately 50 mg/m2 (relative to titanium) yields good results. Thus, a preferred range of the surface weight of the conversion coating (relative to titanium) is in the range of 1 to 50 mg/m2, more preferably 3 to 40 mg/m2, especially 10 to 40 mg/m2 or even 20 to 40 mg/m2 or 15 to 30 mg/m2.
With the phosphate coating, good results are obtained up to a coating weight of approximately 500 mg/m2 (relative to the phosphate ion PO4). Thus, a preferred range of the surface weight of the conversion layer relative to phosphate (PO4) is in the range of 10 to 500 mg/m2, more preferably 20 to 400 mg/m2, especially 50 to 300 mg/m2 or even 100 to 250 mg/m2 or 150 to 300 mg/m2.
In another test, the formability of the lacquered test sheets was investigated. For this purpose, the lacquered test sheets were formed into 13-2 cups by means of deep drawing. On the one hand, it was found that the conversion coatings with hexafluorotitanate (hexafluorotitanic acid) perform best, even when subjected to extreme deformations. On the other hand, from the results of formability tests, it was found that the titanium-containing conversion coatings with coating requirements >40 mg/m2 are inferior, which is the reason that coating requirements of less than 40 mg/m2 are preferred for the titanium-containing conversion coatings.
In addition, tests were conducted in which the conversion-coated test sheets were laminated with films (instead of with a lacquer coating). After deep drawing the film-laminated test sheets to form standard cups, investigations to test the detachment of the film laminate were carried out.
For each of the treatment solutions according to the present disclosure, the test results obtained show that after a short thermal post-treatment, the adhesion values are very good and equivalent to those of the commercially available complex conversion agents with organic substances.
In general, the results indicate that to create conversion coatings on sheet steel which contain organic substances (such as polymers and organic film-forming agents) and organic compounds in the form of particles with particle sizes greater than 50 nm, it is not necessary to use complex treatment solutions in order to achieve good corrosion resistance and good adhesion properties for lacquers and plastic laminates. As disclosed in the present disclosure, comparable results are obtained with purely inorganic treatment solutions of a simpler composition that is limited to the active ingredients and therefore less costly, which are comparable to the conventional chromium-containing conversion coatings on sheet steel.
Thus, based on the present disclosure, it can be concluded that a coating with the active component titanium in the range of titanium coating weights of 1 mg/m2 to 50 mg/m2, preferably in the range of 10 mg/m2 to 40 mg/m2, yields good results. In the phosphate-containing conversion coatings disclosed, the element zinc has a positive effect on the properties and leads to good results up to a zinc concentration of 5%.
The method disclosed can be integrated without considerable installation expense and effort into an existing coating line, e.g., into a strip coating line used to produce ECCS (or TFS). In such strip coating lines, the strip speed is typically 80-600 m/min.
The method disclosed has the advantage that sheet steel can be coated with a chromium-free conversion coating, which is based exclusively on inorganic components and which is therefore environmentally friendly, health-compatible and highly cost-effective. Sheet steel treated as described in the present disclosure has excellent suitability for producing packaging materials, especially cans, and can therefore replace the tinplate and tin-free steel (TFS or ECCS) conventionally used as packaging steel. Blackplate (cold-rolled sheet steel without metal coating) that is coated with a conversion coating is comparable to tinplate with respect to its corrosion resistance and, similarly, has good adhesion properties for organic lacquers and plastic coatings (for example, of PP or PET) as tin-free sheet steel (TFS or ECCS).
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2017 117 080.4 | Jul 2017 | DE | national |
