Multi-stage method for anticorrosive coating of components with steel surfaces

Abstract

The present invention relates to a multi-stage process in which a series of components, each having a steel surface, first receive a conversion coating based on the elements Zr and / or Ti and are subsequently dip-coated. The conversion treatment stage is followed by a rinsing stage in which at least the steel surfaces of each component are brought into contact with an aqueous solution containing hydrogen peroxide. In the process according to the invention, excellent corrosion protection is achieved on the steel surfaces even under unfavorable process conditions, which typically promote corrosion defects on the steel surfaces and have an overall detrimental effect on corrosion protection.

Description

[0001] The present invention relates to a multi-stage process in which a series of components, each having a steel surface, first receive a conversion coating based on the elements Zr and / or Ti and are subsequently dip-coated. The conversion treatment stage is followed by a rinsing stage in which at least the steel surfaces of each component are brought into contact with an aqueous solution containing hydrogen peroxide. In the process according to the invention, excellent corrosion protection is achieved on the steel surfaces even under unfavorable process conditions, which typically promote corrosion defects on the steel surfaces and have an overall detrimental effect on corrosion protection.

[0002] In the corrosion-protective pretreatment of components with surfaces made of steel, galvanized steel, and / or aluminum, thin-film passivation based on amorphous conversion coatings derived from oxides and hydroxides of the elements Zr and / or Ti has become widely established as an alternative to phosphating, which forms crystalline coatings. Efforts to further develop this type of conversion coating primarily aim to establish resource-efficient and chromium-free passivation processes that provide an excellent base for subsequently applied coating systems, particularly dip coatings, with the goal of achieving corrosion protection comparable to that of trication zinc phosphating.Especially with amorphous thin films, such as those resulting from conversion treatments using acidic aqueous solutions containing water-soluble compounds of the elements Zr and / or Ti, controlled layer formation and the growth of virtually defect-free coatings are of great importance. Prior art techniques address this by influencing the kinetics of layer formation, as described in WO 2023 / 275270. For example, WO 2023 / 275270 proposes a sequential formation of the conversion layer in several wet-chemical process steps to produce layer coatings based on the hydroxides and oxides of Zr and / or Ti, thus achieving the most complete possible conversion of the Zr and / or Ti fluorocomplex conversion treatment. This approach aims to prevent fluorides from remaining in the thin film, which could cause local layer defects upon contact with corrosive media.Another method for conversion coating formation, described by way of example in EP 1 455 002 A1, aims at reducing the proportion of fluorides in the conversion coating and the associated improvement in corrosion resistance and paint adhesion to a subsequently applied electrocoating. EP 1 455 002 A1 proposes adding magnesium, calcium, a silicon-containing compound, zinc, or copper to the conversion solution and, alternatively or in combination, drying the conversion coating or rinsing it with an alkaline aqueous solution.On the other hand, serial corrosion-protective pretreatment in industrial paint lines shows that the drying of the conversion coating, especially on components with steel surfaces, is problematic for corrosion protection, paint adhesion and the appearance of the coating, whether it is deliberately brought about after the conversion treatment stage or simply unavoidable due to the spatial conditions of the respective paint line when transferring the components to the dip coating stage. The latter disadvantage is particularly due to the formation of flash rust during drying and / or longer transfer times or temporary plant shutdowns.

[0003] Based on this state of the art, the present task is to establish a process for providing conversion coatings on metal surfaces, particularly steel surfaces, that are as free of defects as possible. This process should ensure high robustness against corrosive effects during the transfer of components from the conversion treatment stage to the dip coating stage, following industrial pretreatment and dip coating of a large number of components. Specifically, the process aims to achieve conversion coatings on steel that are not prone to flash rust formation and can therefore be dip coated even when partially dried without loss of corrosion protection. Ideally, this will prevent fluctuations in corrosion protection performance and in the appearance of the dip-coated components during exceptionally long transfer times from the conversion treatment zone to the coating zone, for example.a temporary plant shutdown is prevented. The process must be suitable for effectively protecting components made of a mix of different metals, especially steel, zinc, and aluminum, from corrosion.

[0004] This task is solved by a multi-stage process for the corrosion-protective treatment of components in series, comprising steel surfaces, in which each component in the series undergoes the successive treatment stages i)-iii): i) Conversion treatment stage comprising contact with an acidic aqueous composition containing a. at least 0.05 mmol / kg of fluorocomplexes of the elements Zr and / or Ti, calculated as the amount of the elements Zr and / or Ti, and b. an amount of free fluoride; ii) Rinsing stage comprising one or more immediately successive rinsing steps, wherein at least one rinsing step is by contact with an aqueous composition having a pH above 4.00 containing at least 20 mg / kg of hydrogen peroxide; iii) Coating stage comprising dip coating by contact with an aqueous dispersion of an organic binder.

[0005] A corrosion-protective treatment of components in series is achieved when a large number of components are brought into contact with the treatment solution provided in the respective treatment stages i)-iii) of the process according to the invention and usually held in system tanks, wherein the contacting of the individual components takes place sequentially and thus separately from one another. The system tank is the container in which the respective treatment solution, i.e., the acidic aqueous composition of the conversion treatment stage, the aqueous composition containing hydrogen peroxide of the rinsing stage, and the aqueous dispersion containing the organic binder of the coating stage, is located for the purpose of corrosion-protective treatment in series according to the present invention.

[0006] When, within the scope of the present invention, the treatment of a component composed of a metallic material is referred to, in particular to the surfaces of the steel material to be treated in the inventive method, this includes all materials that contain the respective element, i.e., iron in the case of steel, to more than 50 atm%. A corrosion-protective treatment always concerns the surfaces of the component formed by metallic materials. The material can be a single, solid material or a coating. For example, according to the invention, galvanized steel grades consist of both steel and zinc, whereby the steel surfaces may be exposed at the cut edges and ground-through points, for example, of an automobile body made of galvanized steel, and in this case, according to the invention, a pretreatment of the steel material is present.

[0007] Insofar as the concentration of an active component or compound is specified in the context of the present invention as an amount of substance (mol, mmol, g, mg) per kilogram, this refers to the amount of substance based on the weight of the respective total composition.

[0008] The components treated according to the present invention can be any three-dimensional structures of any shape and design that originate from a manufacturing process, in particular semi-finished products such as strips, sheets, bars, tubes, etc., and composite structures assembled from the aforementioned semi-finished products, especially automobile bodies, wherein the semi-finished products are preferably joined together to form the composite structure by gluing, welding, and / or flanging. Preferably, the components of the series are first cleaned and / or degreased before the conversion treatment stage i), particularly preferably by means of an alkaline aqueous, surfactant-containing composition.

[0009] The treatment stages (i)-(iii) of the method according to the invention each comprise at least one treatment step which provides for bringing the components of the series into contact with an aqueous composition characteristic of the treatment stage, as defined in more detail. For the purpose of bringing them into contact, these characteristic compositions are either stored or kept in system tanks, wherein the bringing into contact can take place either in the system tank, for example by immersion in a composition held there, or outside the system tank, for example by spraying a composition held in the system tank in a spray chamber.

[0010] For the success of the process according to the present invention, the rinsing stage ii) is of central importance. Within the rinsing stage, the components of the series are freed from the wet film adhering from the conversion treatment stage – firstly, to effectively complete the surface conversion, and secondly, to prevent the carryover of active components into the coating stage of the dip coating. For this purpose, the rinsing stage consists of one or more immediately successive rinsing steps. In the context of the present invention, rinsing steps are considered immediately successive if the components are not subjected to another wet chemical treatment step (not a rinsing step) in the interim, and if, after the complete removal of a component from the series, for example, immersion from a system tank containing a rinsing composition,or, after completion of contacting a surface of the component with an aqueous composition stored in the system tank of a rinsing step, no more than 120 seconds, preferably no more than 90 seconds, and particularly preferably no more than 60 seconds, elapse before a surface of the component is brought into contact with an aqueous composition of coating stage iii). For the actual function of a rinsing stage, which consists of preventing the carryover of active components into downstream wet-chemical treatment stages, it is advantageous and therefore also preferred within the scope of the present invention if rinsing stage ii) comprises several immediately successive rinsing steps for contacting the components of the series with each aqueous composition stored in the system tank of the respective rinsing step.wherein preferably at least a partial volume of the aqueous composition stored in the system tank of the last rinsing step is fed back into the system tank of the first rinsing step of rinsing stage ii) and is replaced in the system tank of the last rinsing step of rinsing stage ii) by a partial volume of an aqueous composition of at least the same size, wherein this aqueous composition to replace the partial volume fed back into the system tank of the first rinsing step preferably has a specific conductivity of less than 20 µScm.

[0011] Within the rinsing stage, the wet film of the conversion treatment stage should be removed as far as possible without incorporating any additional elements into the fresh conversion layer. In particular, it is important to avoid the incorporation of elements into the conversion layer that could adversely affect the corrosion protection performance. Therefore, it is preferred if the aqueous composition of the single or final rinsing step of rinsing stage ii), or preferably any aqueous composition of all rinsing steps of rinsing stage ii), (a) less than 10 mg / kg each of water-soluble compounds of the metals Bi, Ni, Co and / or Cu, calculated as the amount of each element in the aqueous composition, preferably less than 10 mg / kg each of water-soluble compounds of such metals having a standard reduction potential greater than -0.40 V (SHE), calculated as the amount of each element in the aqueous composition; (b) less than 1000 mg / kg in total, preferably less than 100 mg / kg, particularly preferably less than 50 mg / kg of surfactants, preferably of surface-active organic compounds, particularly preferably of organic compounds; (c) less than 100 mg / kg in total, preferably less than 10 mg / kg in total, of organosilanes and / or siloxanes, preferably of water-soluble compounds of the element silicon; (d) less than 100 mg / kg in total, preferably less than 20 mg / kg.(e) in total less than 5 mg / kg of water-soluble compounds of the elements Zr and / or Ti, (f) in total less than 50 mg / kg, preferably less than 10 mg / kg each, of sodium and / or potassium ions, (f) in total less than 50 mg / kg, preferably less than 10 mg / kg, of zinc ions, and / or (g) in total less than 100 mg / kg, preferably less than 10 mg / kg, of phosphates dissolved in water, preferably of phosphorus-containing compounds dissolved in water. contains.

[0012] The standard reduction potential is the reduction potential of the electrochemical half-cell Me / Me n+< determined against the normal hydrogen electrode H 2 / H+ (pH=0) at a metal ion activity of 1 mol / l and 20 °C.

[0013] The metallic surfaces of the components of the series, freshly converted to conventional coating in treatment stage i) according to the inventive method, are effectively protected from corrosive impairment during transfer to coating stage iii) in rinsing stage ii) due to at least one rinsing step in the presence of hydrogen peroxide, such that a homogeneous appearance of the components after dip coating and an additional improvement in corrosion protection properties, particularly on the surfaces of steel, is observed.

[0014] This is also the case if the wet film adhering to the components is already significantly reduced during the transfer from the rinsing stage to the dip coating and conditions exist that significantly promote corrosion, in particular the formation of flash rust on the conversion-coated surfaces. Accordingly, the inventive method is preferably used when the wet film adhering to the steel surfaces is reduced by at least 50%, particularly preferably by at least 80%, and especially preferably by at least 90%, in each case based on the mass of the wet film, when brought into contact with a first aqueous composition of treatment stage iii), compared to the wet film adhering to the steel surfaces immediately after rinsing stage ii).

[0015] From a plant engineering perspective, the aforementioned conditions, which are usually disadvantageous for corrosion-protective treatments including conversion coating and dip coating, are favored by longer transfer times of the components to the dip coating stage. Accordingly, the inventive method is preferably also used when the transfer of each component from rinsing stage ii) to treatment stage iii) takes at least as long as the time each component needs to pass through rinsing stage ii) (rinsing stage duration), particularly preferably at least twice, and more preferably at least three times this duration (rinsing stage duration), and most preferably when the transfer of each component from rinsing stage ii) to treatment stage iii) takes more than 120 seconds, preferably more than 150 seconds, and particularly preferably more than 180 seconds.

[0016] Similarly, a temporary plant shutdown for components located in the intermediate zone between the conversion treatment stage and the dip coating stage, or complete drying of the wet film initially adhering to the components, often leads to significant corrosive impairment of the freshly formed conversion coating. Accordingly, the inventive method is preferably also used where a drying step is performed before treatment stage iii) and after rinsing stage ii). A drying step in this sense is the drying of the wet film adhering to the component by technical means, for example, by supplying thermal energy, an airflow, and / or extending transfer times between two wet-chemical treatment steps.

[0017] The positive effect of rinsing stage ii) on corrosion damage to the conversion coating and the metal substrate, particularly the formation of flash rust on steel, requires the presence of hydrogen peroxide in at least one rinsing step of rinsing stage ii), which leads to a corresponding conditioning of the conversion coating. It has also proven advantageous in this context if a wet film remains on the surfaces of the component after rinsing stage ii), which still contains a quantity of hydrogen peroxide. In a preferred embodiment of the process according to the invention, the wet film adhering to the surfaces of steel immediately after rinsing stage ii) should therefore still contain a quantity of hydrogen peroxide that is preferably at least 10 mg / kg, more preferably at least 50 mg / kg, and most preferably at least 100 mg / kg of hydrogen peroxide based on the mass of the adhering wet film.The concentration of hydrogen peroxide in the adhering wet film can be quantitatively determined after blowing off a portion of the wet film from the component with nitrogen via a redox tritration with potassium permanganate solution as a titrant. Adjusting the amount of hydrogen peroxide in the wet film after removal from the rinsing stage can be achieved, for example, by setting the maximum hydrogen peroxide concentration in the peroxide-containing rinsing step of rinsing stage ii) and / or, in the case of multiple rinsing steps, by positioning the peroxide-containing rinsing step within the specific rinsing sequence, such as being the last rinsing step of rinsing stage ii).In this context, and also in general for sufficient resistance to corrosive damage to the conversion coating and the metal substrate, in particular to flash rust formation on steel, it is advantageous and therefore preferred in the inventive process if the aqueous composition of that rinsing step of rinsing stage ii) with the highest concentration of hydrogen peroxide contains at least 100 mg / kg, preferably at least 400 mg / kg, particularly preferably at least 1000 mg / kg, but preferably not more than 5000 mg / kg of hydrogen peroxide.

[0018] The pH value of the aqueous composition of the rinsing step of rinsing stage ii) with the highest concentration of hydrogen peroxide is preferably above 4.50, particularly preferably above 5.00, very preferably above 5.50, and particularly preferably above 6.00, but the pH value is preferably not above 8.00, and particularly preferably not above 7.50. It is generally advantageous if all other compositions used for rinsing (bringing into contact) the components in all rinsing steps of rinsing stage ii) are aqueous and have a pH value in the range above 6.00 to preferably 8.00, and particularly preferably 7.50.

[0019] The contact of the components with the aqueous composition(s) of rinsing step ii) can be carried out by immersion in the system tank containing the respective aqueous composition of the respective rinsing step or by spraying the respective aqueous composition stored in the system tank. For effective and resource-efficient conditioning of the conversion coating in rinsing step ii), it is preferred that the contact of the components in the rinsing step of rinsing step ii) that is carried out with the aqueous composition with the highest concentration of hydrogen peroxide is carried out by spraying, preferably by misting, the aqueous composition stored in the system tank of this rinsing step.

[0020] Rinsing stage ii) of the process according to the invention is ideally suited for conditioning conversion coatings based on Zr and / or Ti, which are deposited from acidic aqueous, fluoride-containing compositions. The conversion layer formation in treatment stage i) yields an amorphous oxide / hydroxide coating based on the elements Zr and / or Ti containing fluoride.

[0021] For the formation of homogeneous, closed conversion coatings, it is preferred that the proportion of free fluoride is above 5 mg / kg, particularly preferably above 10 mg / kg, and most preferably above 20 mg / kg, but preferably does not exceed 100 mg / kg, particularly preferably 80 mg / kg, and most preferably 60 mg / kg. The amount of free fluoride is to be determined potentiometrically at 20 °C in the respective solution provided, after calibration with fluoride-containing buffer solutions without pH buffering, using a fluoride-sensitive measuring electrode.

[0022] For the same reason, it is preferred if the proportion of fluorocomplexes of the elements Zr and / or Ti, calculated as the amount of the elements Zr and / or Ti, is above 0.10 mmol / kg, particularly preferably above 0.20 mmol / kg, but preferably does not exceed 5.0 mmol / kg, particularly preferably not exceeding 2.0 mmol / kg. Typical representatives of these compounds are hexafluorotitanic acid (H₂TiF₆) and its water-soluble salts and / or hexafluorozirconic acid (H₂ZrF₆) and its water-soluble salts.

[0023] Furthermore, for the formation of a homogeneous and compact amorphous conversion layer, it is advantageous if the pH of the acidic aqueous composition in treatment stage i) is not too acidic. It is therefore preferred if the pH of the acidic aqueous composition in treatment stage i) is above 3.0, particularly preferably above 3.5, and especially preferably above 4.0, but preferably below 4.5, since otherwise the precipitation of sparingly soluble hydroxides of the elements Zr and / or Ti in the system tank of the conversion treatment becomes problematic and can only be controlled within a narrow process window.

[0024] The corrosion-protective coating in the method according to the invention comprises, in coating stage iii), the dip coating of the components of the series. Such dip coating is carried out by deposition from an aqueous dispersion of an organic binder and can be performed autophoretically or electrophoretically. Within the scope of the present invention, the electrophoretic coating variant is preferred, which in turn is preferably carried out as cathodic electrocoating.

[0025] In the process according to the invention, components can be coated with a corrosion-resistant finish, i.e., conversion-treated and dip-coated, which, in addition to steel, also include other metallic materials, preferably components that are assembled in composite structures, for example, automobile bodies, and which, in addition to the aforementioned steel surfaces, also have surfaces of aluminum and / or zinc, particularly preferably aluminum and zinc. Suitable metallic materials whose surfaces can be pretreated with a corrosion-resistant finish in the process according to the invention are, besides steel: zinc, electrolytically (ZE), hot-dip galvanized (Z), and alloy-galvanized (ZA), (ZF), and (ZM), and aluminum-coated (AZ), (AS) strip steel, as well as the light metals aluminum and magnesium and their alloys.

Claims

1. A process for the corrosion-protective treatment of components in series, which comprise steel surfaces, in which each component of the series undergoes the successive treatment stages i) - iii): i) conversion treatment stage comprising bringing into contact with an acidic aqueous composition containing a. at least 0.05 mmol / kg of fluorocomplexes of the elements Zr and / or Ti calculated as the amount of the elements Zr and / or Ti, and b. an amount of free fluoride; ii) rinsing stage comprising one or more immediately successive rinsing steps, wherein at least one rinsing step is carried out by bringing into contact with an aqueous composition having a pH above 4.00 containing at least 20 mg / kg of hydrogen peroxide; iii) coating stage comprising dip painting by bringing into contact with an aqueous dispersion of an organic binder.

2. Method according to claim 1, characterized in thatthe wet film adhering to the surfaces of steel immediately after the rinsing stage ii) contains an amount of hydrogen peroxide which is preferably at least 10 mg / kg, more preferably at least 50 mg / kg and especially preferably at least 100 mg / kg of hydrogen peroxide based on the mass of the adhering wet film.

3. Method according to one or both of the preceding claims, characterized in thatthe rinsing stage ii) comprises several immediately successive rinsing steps for bringing the components of the series into contact with an aqueous composition stored in the system tank of the respective rinsing step, wherein preferably at least a partial volume of the aqueous composition stored in the system tank of the last rinsing step is fed back into the system tank of the first rinsing step of the rinsing stage ii) and is replaced in the system tank of the last rinsing step of the rinsing stage ii) by an at least equally large partial volume of an aqueous composition, wherein this aqueous composition for replacing the partial volume fed back into the system tank of the first rinsing step preferably has a specific conductivity of less than 20 µScm -1 has.

4. Method according to one or more of the preceding claims, characterized in thatthe aqueous composition of the only or last rinsing step of rinsing stage ii), preferably each aqueous composition of all rinsing steps of rinsing stage ii) (a) in each case less than 10 mg / kg of water-dissolved compounds of the metals Bi, Ni, Co and / or Cu, calculated as the amount of the respective element in the aqueous composition, preferably in each case less than 10 mg / kg of water-dissolved compounds of such metals that have a standard reduction potential greater than -0.40 V (SHE), calculated as the amount of the respective element in the aqueous composition, (b) in total less than 1000 mg / kg, preferably less than 100 mg / kg, particularly preferably less than 50 mg / kg of surfactants, preferably surface-active organic compounds, particularly preferably organic compounds, (c) in total less than 100 mg / kg, preferably in total less than 10 mg / kg of organosilanes and / or siloxanes,preferably of compounds of the element silicon dissolved in water, (d) a total of less than 100 mg / kg, preferably less than 20 mg / kg, particularly preferably less than 5 mg / kg of compounds of the elements Zr and / or Ti dissolved in water, (e) a total of less than 50 mg / kg, preferably less than 10 mg / kg each, of sodium and / or potassium ions, (f) a total of less than 50 mg / kg, preferably less than 10 mg / kg, of zinc ions, and / or (g) a total of less than 100 mg / kg, preferably less than 10 mg / kg of phosphates dissolved in water, preferably of phosphorus-containing compounds dissolved in water.

5. Method according to one or more of the preceding claims, characterized in thatthe wet film adhering to the surfaces of steel when brought into contact with a first aqueous composition of treatment stage iii) is reduced by at least 50%, preferably by at least 80%, particularly preferably by at least 90%, in each case based on the mass of the wet film, compared to the wet film adhering to the surfaces of steel immediately after rinsing stage ii).

6. Method according to one or more of the preceding claims, characterized in thatthe transfer of each component from rinsing stage ii) to treatment stage iii) takes at least the time required by each component to pass through rinsing stage ii) (rinsing stage duration), preferably at least twice, particularly preferably at least three times this time (rinsing stage duration), particularly preferably the transfer of each component from rinsing stage ii) to treatment stage iii) takes more than 120 seconds, preferably more than 150 seconds, particularly preferably more than 180 seconds.

7. Method according to one or more of the preceding claims, characterized in that a drying step is carried out before treatment step iii) and after rinsing step ii).

8. Method according to one or more of the preceding claims, characterized in thatthe aqueous composition of the rinsing step of rinsing stage ii) with the highest concentration of hydrogen peroxide contains at least 100 mg / kg, preferably at least 400 mg / kg, particularly preferably at least 1000 mg / kg, but preferably not more than 5000 mg / kg of hydrogen peroxide.

9. Method according to one or more of the preceding claims, characterized in that the aqueous composition of the rinsing step of rinsing stage ii) with the highest concentration of hydrogen peroxide has a pH above 4.50, preferably above 5.00, more preferably above 5.50, especially preferably above 6.00, but preferably not above 8.00, more preferably not above 7.

50.

10. Method according to one or more of the preceding claims, characterized in thatthe bringing into contact of the components with the aqueous composition(s) of the rinsing step ii) is carried out by immersion into the system tank of the respective rinsing step containing the respective aqueous composition or by spraying the respective aqueous composition stored in the system tank.

11. Method according to claim 10, characterized in that bringing the components into contact in the rinsing step of rinsing stage ii) which is carried out with the aqueous composition having the highest concentration of hydrogen peroxide is carried out by spraying, preferably by atomising, the aqueous composition stored in the system tank of this rinsing step.

12. Method according to one or more of the preceding claims, characterized in that the components of the series are first cleaned and / or degreased before the conversion treatment stage i).

13. Method according to one or more of the preceding claims, characterized in thatthe components of the series have surfaces made of aluminum and / or zinc, preferably aluminum and zinc, in addition to steel surfaces.

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