PROCESS SEQUENCE FOR THE PICKLING AND PASSIVATION OF STEEL

Description

[0001] The present invention relates to a method for the wet-chemical pretreatment of a plurality of iron components in series encompassing a pickling step and a subsequent conversion coating each based on aqueous sulfuric acid bath solutions wherein the conversion coating bath comprises water-soluble compounds of the elements Zr and/or Ti as well as at least one water-soluble compound of the element Mo.

[0002] For various industries, such as the parts engineering sector, iron components, in particular steel components, such as radiators, are processed and produced. During production, these components are pickled, and a conversion coating is applied thereafter. For this purpose, the components pass through established production lines having a particular sequence of various dipping baths, which are adapted to the chemical composition of the iron components. To avoid flash rust formation on steel components in a multi-step process sequence of pickling and conversion coating it has proven advantageous to apply an alkaline rinse directly after the acidic pickling step before applying the conversion coating. However, such a multi-step process sequence adds complexity, consumes additional chemicals, and requires more efforts in wastewater management also due to precipitates of iron. On the other hand, the formation of a homogeneous blueish iridescent conversion coating is oftentimes already impaired at the outset of flash rust formation. Therefore, there still exists a demand for a lean and resource-saving process sequence for the pickling and conversion coating of iron components that effectively reduces red rust formation and establishes a homogeneous conversion coating that promotes adhesion to subsequently applied paints such as powder paints or electrolytic or autophoretic dip coatings.

[0003] It has now surprisingly been found that a plurality of iron components in series, in which each of the iron components, in immediate succession, first passes through a pickle and then a reaction rinse can be successfully protected against red rust formation with a homogeneous blueish iridescent passivation layer provided that the reaction rinse takes place by contacting each iron component with an aqueous sulfuric acid bath solution having a pH of no less than 1.0 comprising water-soluble compounds of the elements Zr and/or Ti in an amount sufficient to form a conversion coating as well as water-soluble compounds of the element Mo.

[0004] In a first aspect, the invention therefore relates to a method for the wet-chemical pretreatment of a plurality of iron components in series, in which each of the iron components, in immediate succession, first passes through a pickle (1) and then a reaction rinse (2), characterized in that

i) the pickling (1) takes place by being brought into contact with an aqueous sulfuric acid bath solution (A) having a pH of less than 1.0; and

ii) the reaction rinse (2) takes place by being brought into contact with an aqueous sulfuric acid bath solution (B) having a pH of no less than 1.0, which contains

a) a total of at least 0.020 g/kg of water-soluble compounds of the elements Zr and/or Ti, in each case based on the respective elements,

b) a total of at least 0.020 g/kg of water-soluble compounds of the element Mo.

[0005] The pretreatment according to the invention is used to apply an anti-corrosive first coating as a primer for organic coatings. The pretreatment therefore ends in any case with the application of a first organic primer coating (electro dip-coating, powder paint) which itself is not a pretreatment step within the meaning of the present invention.

[0006] A "plurality," as used herein, refers to 2 or more, for example 2, 3, 4, 5, 6, 7, 8, 9, 10, 100, 200, 300, 400 or more.

[0007] The concentrations and amounts of the compounds indicated herein always refer to the respective bath solution, unless explicitly defined differently.

[0008] The method according to the invention relates to wet-chemical pretreatment of a plurality of iron components in series. Pretreatment in series is considered according to the invention to be when a plurality of components is brought into contact with bath solutions for the pickle (1) and/or reaction rinse (2) stored in the system tank, and/or additional wet-chemical treatment steps, the individual components being brought into contact successively and thus at different times.

[0009] A process step is "wet-chemical," if a heterogeneous chemical reaction at the surface of the treated component is induced by contact with a water-based liquid upon which reaction products are dissolved in the water-based liquid, such as a pickling reaction, or upon which reaction educts based on metal-element-based or semi-metal-element-based active compounds contained in the liquid are depleted, such as in the due course of a conversion coating based on oxides and hydroxides formed from dissolved precursor compounds of the elements Zr and/or Ti.

[0010] The iron components used according to the method substantially comprise surfaces of steel and/or iron; preferably more than 60%, particularly preferably more than 80%, more particularly preferably 90% of all metal surfaces of the iron components are surfaces of steel and/or iron. In a particularly preferred embodiment of the method according to the invention, all metal surfaces of the iron components are surfaces of steel and/or iron. The iron component may optionally contain additives of other metals and/or non-metals. These additives may be selected from the group comprising carbon, silicon, chromium, nickel, manganese, molybdenum, tungsten and mixtures thereof; however, the group is not limited thereto.

[0011] In the pretreatment according to the invention, each of the iron components, in immediate succession, first passes through a pickle (1) and then a reaction rinse (2). Within the context of the pretreatment in series, process steps for a component are in "immediate" succession if they are not interrupted by something other than the subsequent wet-chemical pretreatment provided in each case or a rinsing step. In a preferred embodiment the iron components of the series are directly after the pickle (1) and without an intermediate rinsing or drying step transferred to the reaction rinse (2).

[0012] A "rinsing step" in the context of this invention means a process step which is used exclusively for the complete or partial removal of soluble residues, particles and active components that are carried over by adhering to the component from a previous wet-chemical treatment step, from the component to be treated, without metal-element-based or semi-metal-element-based active compounds, which are already consumed merely by bringing the iron surfaces of the component into contact with the rinsing liquid, being contained in the rinsing liquid itself. For example, the rinsing liquid can simply be city water or deionized water or, if necessary, can also be a rinsing liquid which contains surface-active compounds to improve the wettability by means of the rinsing liquid.

[0013] The pickling (1) takes place by being brought into contact with a sulfuric acid bath solution (A) having a pH of less than 1.0. The pH is preferably in the range of 0 to < 1. The bath solution (A) contains water as the main component and sulfuric acid to adjust the desired pH. It is especially preferred that the amount of other strong acids with a pKa value of less than 1.5 is smaller than 0.05 wt.-%, preferably less than 0,01 wt.-% in the bath solution (A).

[0014] The "pH" as used herein corresponds to the negative common logarithm of the hydronium ion activity at 20 °C and can be determined by means of pH-sensitive glass electrodes. The "pKa" as used herein corresponds to the negative common logarithm of the equilibrium constant at 20 °C for the first deprotonation step of the respective acid.

[0015] In the pickling step (1) oxide scale is removed from each iron components of the series as well as iron bulk material dissolved to an extent that ensures a homogeneously surface layer composition. For this purpose, in a preferred method according to the invention, the bath solution (A) of the pickle (1) has a free acid content in points of at least 50, particularly preferably of at least 130 to 200, but preferably of no more than 400.

[0016] The "free acid content" in points is determined according to the consumption of 0.1 N sodium hydroxide solution in milliliters until a pH of 4.0 is reached for a sampled volume of the particular bath solution of 10 ml and a dilution in a ratio of 1:5.

[0017] More specifically, it is preferred that the amount of sulfuric acid in the bath solution (A) of the pickle (1) is at least 3 wt.-%, more preferably at least 5 wt.-%, even more preferably at least 8 wt.-%, but preferably less than 15 wt.-%, more preferably less than 12 wt.-%.

[0018] In a preferred embodiment of the present invention, the pickle (1) contains no more than 80 g/kg, preferably no more than 60 g/kg of iron ions, but preferably at least 1.0 g/kg of iron ions, in each case based on the bath solution (A).

[0019] Since the formation of a passivating film in the pickle (1) would run counter to the formation of a homogenous conversion coating in the reaction rinse (2) it is preferred that the pickle (1) contains no more than 10 g/kg, more preferably no more than 2 g/kg, even more preferably no more than 0.5 g/kg of phosphate ions, in each case calculated as PO4 and based on the bath solution (A).

[0020] Similarly, and for the same reason, it is preferred that the pickle (1) contains less than in total 0.010 g/kg, more preferably less than 0.005 g/kg, of water-soluble compounds of the elements Zr and Ti calculated based on the amount of the respective elements.

[0021] From an environmental health perspective, it is further preferred that the total fluorine content of the pickle (1) is less than 0.050 g/kg, more preferably less than 0.020 g/kg. The total fluorine is determined in a TISAB-buffered aliquot portion of the pickle (1) using a fluoride-sensitive electrode at 20 °C (TISAB: "Total Ionic Strength Adjustment Buffer"), the mixture ratio of buffer to the aliquot portion of the pickle liquor by volume being 1:1. The TISAB buffer is prepared by dissolving 58 g NaCl, 1 g sodium citrate and 50 ml glacial acetic acid in 500 ml deionized water (κ < 1µScm^-1), setting a pH of 5.3 using 5 N NaOH and filling to a total volume of 1000 ml, again with deionized water (κ < 1µSCM^-1).

[0022] Generally, it is preferred that the amount of other strong acids with a pKa value of below 1.5 but different from watersoluble compounds of the elements Zr and/or Ti is below 0.05 wt.-%, preferably below 0,01 wt.-%.

[0023] The reaction rinse (2) takes place by being brought into contact with a sulfuric acid bath solution (B). The bath solution (B) has a pH which is no lower than 1.0. The bath solution (B) preferably has a pH above 2.0, more preferably above 2.5, even more preferably above 3.0, but preferably a pH of less than 6.0, more preferably of less than 5.0, even more preferably of less than 4.5. The main component of the bath solution (B) is water. The desired pH of the bath solution (B) can be adjusted using acids, in particular a mixture of sulfuric acid and hydrofluoric acid. In various embodiments, this mixture can comprise at least 0.5 wt.%, particularly preferably at least 1 wt.%, sulfuric acid and 1 to 200 ppm, preferably 5 to 50 ppm hydrofluoric acid. It is especially preferred that the amount of other strong acids with a pKa value of below 1.5 but different from watersoluble compounds of the elements Zr and/or Ti is below 0.05 wt.-%, preferably below 0,01 wt.-% in the bath solution (B).

[0024] However, it is also possible to adjust or maintain the pH of the bath solution (B) both by adding sulfuric acid or mixtures comprising sulfuric acid and hydrofluoric acid, as described above, and by means of the entrained bath solution (A). The bath solution (B) of the reaction rinse (2) preferably contains a free acid content in points of at least 3, particularly preferably of at least 10, but preferably of no more than 30.

[0025] For a rapid conversion of the metal surfaces of the components, it is generally preferable for the bath solution (B) to have a minimum content of free fluoride of 5 ppm.

[0026] The amount of free fluoride can be determined potentiometrically by means of a fluoride-sensitive measuring electrode at 20 °C in the relevant acidic aqueous composition after calibration with fluoride-containing buffer solutions without pH buffering. Suitable sources of free fluoride are hydrofluoric acid and the water-soluble salts thereof, such as ammonium bifluoride and sodium fluoride, as well as complex fluorides of the elements Zr, Ti and/or Si, in particular complex fluorides of the element Si.

[0027] The bath solution (B) in the reaction rinse (2) of the method according to the present invention comprises a total of at least 0.020 g/kg of water-soluble compounds of the elements zirconium and/or titanium, in each case based on the respective elements. In various embodiments, the reaction rinse (2) comprises a total of less than 5.0 g/kg, preferably of less than 2.0 g/kg, particularly preferably of less than 1.5 g/kg, but preferably a total of at least 0.100 g/kg, particularly preferably of at least 0.200 g/kg, even more preferably of at least 0.300 g/kg of water-soluble compounds of the elements zirconium and/or titanium, in each case based on the respective elements while the presence of water-soluble compounds of Zr is generally preferred for the formation of a passivating thin conversion coating.

[0028] The water-soluble compounds can all be water-soluble compounds of these metals that are known in the prior art and suitable for this purpose. Within the meaning of the present invention, compounds are "water-soluble" when the solubility thereof in deionized water having a conductivity of no more than 1 µScm^-1 at a temperature of 20 °C is at least 1 g/l.

[0029] Suitable and preferred water-soluble compounds of the elements Zr and/or Ti are selected from fluoroacids, preferably selected from hexafluorozirconic acid and/or hexafluorotitanic acid, carbonates, preferably selected from ammonium zirconium carbonate, and/or zirconyl or titanyl compounds, preferably selected from nitrates and/or acetylacetonates.

[0030] Water-soluble titanium compounds which can be used according to the invention include in particular salts and esters of titanic acid, referred to as titanates. For example, H₄TiO₄ and the corresponding alkoxides such as tetraethyl titanate can be used therefore. In particular, the hexafluoro acids of the titanium H₂TiF₆ and the water-soluble salts thereof may be used, such as (NH4)2TiF6, Li₂TiF₆, K₂TiF₆ and Na₂TiF₆.

[0031] Water-soluble zirconium compounds which can be used according to the invention include (NH₄)₂Zr(CO₃)₂(OH)₂, zirconium acetates, zirconium acetylacetonates and hexafluorozirconic acid and the salts thereof such as (NH₄)₂ZrF₆, Li₂ZrF₆, K₂ZrF₆ and Na₂ZrF₆. H₂ZrF₆ and the water-soluble salts thereof are particularly suitable.

[0032] The bath solution (B) in the reaction rinse (2) of the method according to the present invention also comprises a total of at least 0.020 g/kg of water-soluble compounds of the element Mo. Such amount being required to safeguard a uniform formation of a blueish iridescent conversion coating in the reaction rinse (2). In various embodiments, the reaction rinse (2) comprises a total of less than 5.0 g/kg, preferably of less than 2.0 g/kg, particularly preferably of less than 1.5 g/kg, but preferably a total of at least 0.100 g/kg, particularly preferably of at least 0.200 g/kg, even more preferably of at least 0.300 g/kg of water-soluble compounds of the element Mo.

[0033] Suitable water-soluble compounds of the elements Mo are selected from molybdates, preferably selected from ammonium, sodium and/or potassium molybdates such as (NH₄)₆Mo₇O₂₄·4H₂O, (NH₄)₂MoO4, Na₂MoO₄·2H₂O, or Na₂MoO₄·10H₂O.

[0034] In various embodiments, the reaction rinse (2) may additionally comprise iron ions to accelerate the formation of the conversion coating. In this regard, an amount of at least 0.100 g/kg, in particular at least 1.0 g/kg of iron ions in the bath solution (B) is preferred. However, an amount of iron ions above 10 g/kg does further positively influence the conversion layer formation and is thus not preferred.

[0035] Again, it is also in the context of the conversion coating formation in the reaction rinse (2) advantageous to prevent the deposition of a phosphate layer so that in a preferred embodiment in the reaction rinse (2), a total of less than 1.0 g/kg, more preferably less than 0.5 g/kg, particularly preferably less than 0.1 g/kg of phosphate ions, in each case calculated as PO4 and based on the bath solution (B), is contained.

[0036] The "contact" of the components with the respective bath solutions in the method according to the invention comprises any such time-limited technical measure due to which the components are wetted with the particular bath solution. These measures include dipping, spraying, printing and/or coating with the particular bath solution, but are not limited to these options.

[0037] Bringing the plurality of iron components into contact with the bath solutions in the pickle (1) and/or the reaction rinse (2) can take place over a period of 1 second to 10 minutes, preferably of 5 seconds to 5 minutes, particularly preferably of 10 seconds to 3 minutes.

[0038] In addition, the contact can take place at an elevated temperature of the particular bath solution(s). In various embodiments, the contact takes place at an elevated temperature of the bath solutions (A) and (B). For example, the contact in the pickle (1) and/or in the reaction rinse (2) can take place at a temperature of 5°C to 65°C. In a preferred embodiment, the contact can be carried out at a temperature of 10°C to 60 °C, more preferably 15°C to 55°C, particularly preferably 20°C to 40°C. In this case, it is possible for the pickle (1) and the reaction rinse (2) to be carried out at different temperatures over different periods of time depending on the particular component and on the particular bath solutions (A) and (B).

[0039] In various embodiments, the reaction rinse (2) is followed by a further process step or sequence that establishes an organic primer and/or top coating on the as pretreated plurality of iron components, more preferably without an intermediate phosphatizing step or even more preferably without any additional intermediate wet-chemical treatment step.

[0040] Thus, in a preferred embodiment of this invention after the reaction rinse (2), the iron components undergo a paint deposition, preferably by means of an electrodeposition, more preferably by means of a cathodic electrodeposition.

[0041] In an even more preferred embodiment of the method of this invention after the reaction rinse (2), the iron components do not undergo any additional wet-chemical pretreatment that is a phosphating treatment, preferably do not undergo any additional wet-chemical pretreatment other than a paint deposition, but preferably pass through at least one, particularly preferably at least two rinsing steps prior to the paint deposition.

Claims

1. A method for the wet-chemical pretreatment of a plurality of iron components in series, in which each of the iron components, in immediate succession, first passes through a pickle (1) and then a reaction rinse (2), characterized in that

i) the pickling (1) takes place by being brought into contact with an aqueous sulfuric acid bath solution (A) having a pH of less than 1.0; and

ii) the reaction rinse (2) takes place by being brought into contact with an aqueous sulfuric acid bath solution (B) having a pH of no less than 1.0, which contains

a) a total of at least 0.020 g/kg of water-soluble compounds of the elements Zr and/or Ti, in each case based on the respective elements,

b) a total of at least 0.020 g/kg of water-soluble compounds of the element Mo.

2. The method according to claim 1, characterized in that the bath solution (A) of the pickle (1) has a free acid content in points of at least 50, particularly preferably of at least 130-200, but preferably of no more than 400.

3. The method according to one or more of the preceding claims, characterized in that the pickle (1) contains no more than 10 g/kg, preferably no more than 2 g/kg, more preferably no more than 0.5 g/kg of phosphate ions, in each case calculated as PO4 and based on the bath solution (A).

5. The method according to one or more of the preceding claims, characterized in that the bath solution (B) of the reaction rinse (2) has a free acid content in points of at least 3, particularly preferably of at least 10, but preferably of no more than 30.

6. The method according to one or more of the preceding claims, characterized in that the bath solution (B) of the reaction rinse (2) has a pH above 2.0, preferably above 2.5, more preferably above 3.0, but preferably a pH of less than 6.0, more preferably of less than 5.0, even more preferably of less than 4.5.

7. The method according to one or more of the preceding claims, characterized in that, in the reaction rinse (2), a total of less than 5.0 g/kg, preferably less than 2.0 g/kg, particularly preferably less than 1.5 g/kg, but preferably at least 0.100 g/kg, particularly preferably at least 0.200 g/kg, even more preferably at least 0.300 g/kg of water-soluble compounds of the elements Zr and/or Ti, in each case based on the respective elements and the bath solution (B), are contained.

8. The method according to one or more of the preceding claims, characterized in that, the water-soluble compounds of the elements Zr and/or Ti are selected from fluoroacids, preferably selected from hexafluorozirconic acid and/or hexafluorotitanic acid, carbonates, preferably selected from ammonium zirconium carbonate, and/or zirconyl or titanyl compounds, preferably selected from nitrates and/or acetylacetonates.

9. The method according to one or more of the preceding claims, characterized in that, in the reaction rinse (2), a total of less than 5.0 g/kg, preferably less than 2.0 g/kg, particularly preferably less than 1.5 g/kg, but preferably at least 0.10 g/kg, particularly preferably at least 0.20 g/kg, even more preferably at least 0.30 g/kg of water-soluble compounds of the element Mo, in each case based on the respective element and the bath solution (B), are contained.

10. The method according to one or more of the preceding claims, characterized in that, the water-soluble compounds of the element Mo are selected from molybdates, preferably selected from ammonium, sodium and/or potassium molybdates.

11. The method according to one or more of the preceding claims, characterized in that, in the reaction rinse (2), a total of less than 1.0 g/kg, preferably less than 0.5 g/kg, particularly preferably less than 0.1 g/kg of phosphate ions, in each case calculated as PO4 and based on the bath solution (B), is contained.

12. The method according to one or more of the preceding claims, characterized in that, after the reaction rinse (2), the iron components undergo a paint deposition, preferably by means of an electrodeposition, more preferably by means of a cathodic electrodeposition.

13. The method according to claim 12, characterized in that, after the reaction rinse (2), the iron components do not undergo any additional wet-chemical pretreatment that is a phosphating treatment, preferably do not undergo any additional wet-chemical pretreatment other than a paint deposition, but preferably pass through at least one, particularly preferably at least two rinsing steps.