MULTISTAGE PROCESS FOR COATING COMPONENTS HAVING STEEL SURFACES WITH CORROSION PROTECTION

Description

TECHNICAL FIELD

The present invention relates to a multistage process in which a series of components which each have surfaces of steel are initially provided with a conversion layer based on the elements Zr and/or Ti and are subsequently dip-coated, wherein the conversion treatment stage is followed by a rinsing stage in which at least the surfaces of steel of each component are brought into contact with an aqueous composition containing hydrogen peroxide. In the process according to the invention, excellent corrosion protection on the surfaces of steel is achieved even under unfavorable process conditions which usually promote corrosion defects on the steel surfaces and have a negative effect overall on the corrosion protection.

BACKGROUND

In the corrosion-protection pre-treatment of components having surfaces made of the materials steel, galvanized steel and/or aluminum, thin-film passivation based on amorphous conversion layers based on oxides and hydroxides of the elements Zr and/or Ti has become widely established as an alternative to phosphating, in the course of which crystalline coatings are formed. Efforts to further develop this type of conversion coating are substantially aimed at establishing resource-saving and chromium-free passivations that provide an excellent primed base for paint systems applied subsequently, in particular dip-coatings, wherein the aim is to achieve corrosion protection comparable to trication zinc phosphating. Especially in the case of amorphous thin films resulting from the conversion treatment of acidic aqueous solutions containing water-soluble compounds of the elements Zr and/or Ti, controlled film formation and growth of coatings with as few defects as possible is of great importance. To this end, the prior art, on the one hand, focuses on influencing the kinetics of layer formation, as set out in WO 2023/275270, and proposes, for example, a sequential formation of the conversion layer in a plurality of wet-chemical process steps in order to produce the layer deposits based on the hydroxides and oxides of the elements Zr and/or Ti, which bring about as complete conversion as possible of the conversion treatment based on fluoro complexes of the elements Zr and/or Ti. This is to prevent fluorides from remaining in the thin film, which can cause localized film defects in contact with corrosive media. In contrast, another procedure for the formation of the conversion layer, which is described as an example in EP 1 455 002 A1, aims at reducing the proportion of fluorides in the conversion coating and the associated improvement in corrosion behavior and paint adhesion to a subsequently applied electrocoating. To this end, EP 1 455 002 A1 proposes adding magnesium, calcium, an Si-containing compound, zinc, or copper to the conversion solution and alternatively, or in combination, drying the conversion coating or post-rinsing it with an alkaline aqueous composition. On the other hand, serial corrosion protection pre-treatment in industrial paint lines has shown that precisely the drying of the conversion coating on components which have surfaces of steel, whether this drying is brought about deliberately after the conversion treatment stage or is simply unavoidable due to the spatial conditions of the respective paint line when transferring the components to the dip-coating stage, is problematic for corrosion protection, paint adhesion and the appearance of the paint finish, with the latter disadvantage being particularly due to the formation of flash rust during drying and/or longer transfer times or temporary plant downtime.

SUMMARY

To address drawbacks of the prior art, an object of the present invention is to establish a process for the provision of conversion coatings on metal surfaces, in particular surfaces of steel, which are as free of defects as possible and which, during industrial pre-treatment and dip-coating of a large number of components, provides a high degree of robustness against corrosive impairment during the phase of transferring the components from the conversion treatment stage to the dip-coating stage. The aim of the process is especially the production of conversion coatings on steel that are not prone to formation of flash rust and can therefore be dip-coated even when dry without any loss of corrosion protection. Ideally, fluctuations in the corrosion protection performance and in the appearance of the dip-coated components during exceptionally long transfer times from the conversion treatment zone to the painting zone, e.g., during temporary plant downtime, are prevented thereby. The process must also be suitable for effectively protecting components consisting of a material mix of different metals, in particular of the metals steel, zinc and aluminum, from corrosion.

This object is achieved by a multistage process for the corrosion-protection treatment of components in series, which components comprise surfaces of steel, in which process each component in 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 fluoro complexes 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 rinsing steps immediately following one another, 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-coating by bringing into contact with an aqueous dispersion of an organic binder.

Corrosion-protection treatment of the components in series is when a plurality of components are brought into contact with treatment solution provided in the respective treatment stages i)-iii) of the process according to the invention and conventionally stored in system tanks, the individual components being brought into contact successively and thus at different times. 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 the series corrosion-protection treatment in accordance with the present invention.

In the context of the present invention, where reference is made to the treatment of a component composed of a metal material, in particular on the surfaces of the material steel to be treated in the process according to the invention, this thus includes all materials containing the respective element, i.e., iron in the case of steel, to an extent of more than 50 at. %. A corrosion-protection treatment always affects the component surfaces that are formed of metal materials. The material can be a uniform material or a coating. According to the invention, galvanized steel grades consist both of the material steel and of the material zinc, it being possible for surfaces of steel to be exposed at the cutting edges and cylindrical grinding points of, for example, an automobile body which is made of galvanized steel, in which case according to the invention there is pre-treatment of the material steel.

In the context of the present invention, insofar as the concentration of an active component or compound is referred to as the amount of substance (mol, mmol, g, mg) per kilogram, this is the amount of substance relative to the weight of the respective total composition.

The components treated in accordance with the present invention can be three-dimensional structures of any shape and design that originate from a manufacturing process, in particular also including semi-finished products such as strips, sheets, rods, pipes, etc., and composite structures assembled from said semi-finished products, in particular car bodies, the semi-finished products preferably being interconnected by means of adhesive bonding, welding and/or flanging to form a composite structure.

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. In connection with such cleaning/degreasing of the components to be treated for corrosion protection in the process according to the invention, it has been found that alkaline pre-treatment stages influence the sensitivity of the steel surfaces to flash rust. In particular when treating components that have surfaces of zinc in addition to surfaces of steel, passivation of the zinc surfaces is often carried out before the conversion treatment stage i) by means of an iron-containing alkaline degreasing or cleaning stage, in which the steel surfaces are also treated accordingly. The steel surfaces that undergo “ferrification” of this kind alongside the remaining surfaces of the component prove to be particularly susceptible to the formation of flash rust, but this can be effectively prevented by the present process according to the invention.

In a preferred embodiment of the process according to the invention for the corrosion-protection treatment of components in series, which components comprise surfaces of zinc in addition to surfaces of steel, each component of the series first undergoes an alkali treatment stage before the aforementioned successive treatment stages i)-iii) are carried out, wherein, within the alkali treatment stage in at least one treatment step, at least the surfaces of steel and zinc of the components of the series are brought into contact with an alkaline aqueous composition, which contains

• • (a) at least 50 mg/kg, preferably at least 100 mg/kg, of iron (III) ions,
  • (b) at least 100 mg/kg of phosphate ions,
  • (c) optionally at least one complexing agent which is preferably selected from organic compounds which have at least one functional group selected from —COOX, —OPO₃X and/or —PO₃X, wherein X is either an H atom or an alkali metal and/or alkaline-earth metal atom, and/or from condensed phosphates calculated as PO₄,
    wherein the alkaline aqueous composition has a free alkalinity of at least 1 point, but preferably of less than 6 points, and a pH of at least 10.5, preferably of at least 11.0. The free alkalinity is determined by titrating 2 ml of bath solution, preferably diluted to 50 ml, with a 0.1 n acid, for example hydrochloric acid or sulfuric acid, up to a pH value of 8.5. The consumption of acid solution in ml indicates the point number of the free alkalinity.

The treatment stages i)-iii) of the process 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 which is characteristic of the treatment stage and is more closely defined. For the purpose of the bringing into contact, these characteristic compositions are either stored or kept in system tanks, it being possible for the bringing into contact to take place in the system tank, for example by immersion in a composition kept therein, or outside the system tank, for example by spraying a composition stored in the system tank in a spray chamber.

The rinsing stage ii) is of critical importance to the success of the process according to the present invention. Within the rinsing stage, the components of the series are freed from any adhering wet film from the conversion treatment stage—firstly in order to effectively complete the conversion of the surface, and also to prevent the carryover of active components into the coating stage of the dip-coating. To this end, the rinsing stage consists of one or more rinsing steps immediately following one another. In the context of the present invention, rinsing steps immediately follow one another if the components are not subjected to another wet-chemical treatment step which is not a rinsing step in the interim, and if, after the complete removal of a component from the series, for example from a system tank containing a composition for rinsing, or after completion of bringing a surface of the component into contact with an aqueous composition stored in the system tank of a rinsing step, not more than 120 seconds, preferably not more than 90 seconds, particularly preferably not more than 60 seconds elapse before a surface of the component is brought into contact with an aqueous composition of the coating stage iii). For the proper functioning of a rinsing stage which consists in preventing the carryover of active components into downstream wet-chemical treatment stages, it is required and thus also preferred in the context of the present invention if the rinsing stage ii) comprises a plurality of rinsing steps immediately following one another for bringing each of 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.

Within the rinsing stage, the wet film of the conversion treatment stage should be removed as much as possible without incorporating additional elements into the fresh conversion layer. In particular, the absorption of elements into the conversion layer that have a detrimental effect on the corrosion protection performance should be prevented. It is therefore preferred if the aqueous composition of the only, or of the last, rinsing step of the rinsing stage ii), preferably each aqueous composition of all of the rinsing steps of the rinsing stage ii), contains

• • (a) in each case less than 10 mg/kg of compounds of the metals Bi, Ni, Co and/or Cu dissolved in water, calculated as the amount of the respective element in the aqueous composition, preferably in each case less than 10 mg/kg of compounds of such metals dissolved in water which have a standard reduction potential of greater than −0.40 V (SHE), calculated as the amount of the respective element in the aqueous composition,
  • (b) a total of less than 1000 mg/kg, 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) a total of less than 100 mg/kg, preferably a total of 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.

The standard reduction potential is the reduction potential of the electrochemical half-cell Me/Meⁿ⁺, determined against the normal hydrogen electrode H₂/H+(pH=0), at a metal ion activity of 1 mol/l and at 20° C.

The metal surfaces of the components of the series, freshly conversion-coated in the treatment stage i) by the process according to the invention, are effectively protected in the rinsing stage ii) against corrosive impairment during transfer to the coating stage iii) due to the at least one rinsing step in the presence of hydrogen peroxide, such that a homogeneous appearance of the components after the dip-coating, and also an improvement in the corrosion protection properties, in particular on the surfaces of steel, is observed.

This is still the case even if the wet film adhering to the components has already been significantly degraded during the transfer from the rinsing stage to the dip-coating and conditions that significantly promote corrosive impairment, in particular the formation of flash rust on the conversion-coated components, are present. Accordingly, the process according to the invention is preferably used when the wet film adhering to the surfaces of steel during the bringing into contact with a first aqueous composition of the treatment stage iii) is reduced by at least 50%, particularly preferably by at least 80%, especially preferably by at least 90%, in each case relative to the mass of the wet film, compared to the wet film adhering to the surfaces of steel immediately after the rinsing stage ii).

In terms of plant technology, the aforementioned conditions, which are usually detrimental to a corrosion-protection treatment comprising formation of the conversion layer and dip-coating, are favored by longer transfer times of the components to the dip-coating stage. Accordingly, the process according to the invention is preferably also used when the transfer of each component from the rinsing stage ii) to the treatment stage iii) takes at least the time required by each component to pass through the rinsing stage ii) (rinsing stage duration), particularly preferably at least twice, especially preferably at least three times this time (rinsing stage duration); and most particularly preferably when the transfer of each component from the rinsing stage ii) to the treatment stage iii) takes more than 120 seconds, preferably more than 150 seconds, particularly preferably more than 180 seconds.

Likewise, a temporary plant downtime, or a complete drying of the wet film initially adhering to the components, often leads to a significant corrosive impairment of the freshly formed conversion coating for components that are in the intermediate zone between the conversion treatment stage and the dip-coating. Accordingly, the process according to the invention is preferably also used where a drying step is carried out before the treatment stage iii) and after the 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 air stream and/or extended transfer times between two wet-chemical treatment steps.

The positive effect of the rinsing stage ii) against corrosive damage to the conversion coating and the metal substrate, in particular the formation of flash rust on steel, requires the presence of hydrogen peroxide in at least one rinsing step of the rinsing stage ii), which leads to corresponding conditioning of the conversion coating. In this context, it has also proven advantageous if, after the rinsing stage ii), a wet film which still contains an amount of hydrogen peroxide remains on the surfaces of the component. In a preferred embodiment of the process according to the invention, the wet film adhering to the surfaces of steel immediately after the rinsing stage ii) should therefore still contain an amount of hydrogen peroxide which is preferably at least 10 mg/kg, particularly preferably at least 50 mg/kg and especially preferably at least 100 mg/kg of hydrogen peroxide relative to 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 partial volume of the wet film from the component with nitrogen, using a redox titration with potassium permanganate solution as the titrant. The amount of hydrogen peroxide in the wet film after removal from the rinsing stage can be adjusted, for example, by means of the maximum concentration of hydrogen peroxide in the rinsing step of the rinsing stage ii) which contains the peroxide and/or, in the case of a plurality of rinsing steps, by the placement of the peroxide-containing rinsing step in the specific rinsing sequence, for example as the last rinsing step of the rinsing stage ii). In this context and also overall for sufficient resistance to corrosive damage to the conversion coating and the metal substrate, in particular the formation of flash rust in steel, it is advantageous and thus preferred in the process according to the invention if the aqueous composition of that rinsing step of the rinsing stage ii) which has 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.

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

The components can be brought into contact with the aqueous composition(s) of the rinsing step ii) 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. For effective and resource-saving conditioning of the conversion layer in the rinsing stage ii), it is preferred if the components are brought into contact in that rinsing step of the rinsing stage ii) which is carried out with the aqueous composition having the highest concentration of hydrogen peroxide by spraying, preferably by atomizing, the aqueous composition stored in the system tank of this rinsing step.

The rinsing stage ii) of the process according to the invention is exceptionally well-suited to the conditioning of conversion coatings based on Zr and/or Ti which are deposited from acidic aqueous, fluoride-containing compositions. The formation of the conversion layer in the treatment stage i) provides amorphous oxide/hydroxide coating based on the elements Zr and/or Ti containing fluoride.

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

For the same reason, it is preferred if the proportion of fluoro complexes 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 2.0 mmol/kg. Typical representatives of these compounds are hexafluorotitanic acid (H₂TiF₆) and the water-soluble salts thereof, and/or hexafluorozirconic acid (H₂ZrF₆) and the water-soluble salts thereof.

Furthermore, for the formation of an amorphous conversion layer that is as homogeneous and compact as possible, it is advantageous if the pH of the acidic aqueous composition in the treatment stage i) is not set to be too acidic. It is thus preferred if the pH of the acidic aqueous composition in the treatment stage i) is above 3.0, particularly preferably above 3.5, especially preferably above 4.0, but preferably below 4.5, because 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 kept under control in a narrow process window.

The corrosion-protection coating in the process according to the invention comprises, in coating stage iii), the dip-coating of the components of the series. Such a dip-coating is carried out by deposition from an aqueous dispersion of an organic binder and can be carried out autophoretically or electrophoretically. In the context of the present invention, the electrophoretic coating variant is preferred, which in turn is preferably carried out as cathodic electrocoating.

In the process according to the invention, components can be coated to protect against corrosion, i.e., can be conversion treated and dip-coated, which components, in addition to steel, also comprise other metal materials, preferably components which are joined together in particular in a composite construction, for example car bodies, and which, in addition to the aforementioned surfaces of steel, also have surfaces of aluminum and/or zinc, particularly preferably aluminum and zinc. In addition to steel and iron, suitable metal materials, the surfaces of which can be corrosion protection pretreated in the process according to the invention, are zinc, electrolytic (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.

EXEMPLARY EMBODIMENT

In a series of tests on the corrosion-protection treatment of steel substrates, steel sheets (CRS) were wet-chemically treated as follows and the sheets were assessed for corrosive infiltration of the paint layer structure, after a salt spray test in accordance with VW PV 1210 after 30 cycles, and stone chipping in accordance with DINI EN ISO 20567-1.

(A) Cleaning and Degreasing Stage

• • a. Spray degreasing at 55° C. for 60 seconds
  • b. Dip degreasing at 55° C. for 120 seconds
  • each with cleaner from Henkel AG & Co. KGaA (pH 11.5; total alkalinity 12 points) based on Bonderite® C-AK 2011 and prepared with deionized water (k<1 μScm⁻¹) containing 2.50 g/L of PO₄.

(B) Dip Ferrification Stage at 60° C. For 90 Seconds

• • based on Bonderite® C-AK 2020 (Henkel AG & Co. KGaA) and prepared with demineralized water,
  • adjusted to pH 12.1 (free alkalinity 1.6 points) containing

	PO4	2.00 g/L
	P2O7	1.80 g/L
	HEDP	7.35 g/L
	Iron (III)	0.35 g/L
	Zinc	1.00 g/L

(C) Rinsing Stage with Deionized Water (k<1 μScm⁻¹)

• • a. spraying at 20° C. for 30 seconds
  • b. dipping at 20° C. for 30 seconds

(D) Conversion Treatment Stage at 35° C. For 120 Seconds

• • based on Bonderite® M-NT 1850 (Henkel AG & Co. KGaA) and prepared with demineralized water,
  • adjusted to pH 4.7 containing

	Zr	150 mg/L
	Cu	10 mg/L
	Zn	0.6 g/L
	F-free	50 mg/kg determined using potentiometry
		by means of a fluoride-sensitive electrode
	NO3	6 g/L

(E) Rinsing Stage 1

• • a. with deionized water (k<1 μScm⁻¹); or
  • b. 1.00 g/L H₂O₂ in deionized water (k<1 μScm⁻¹)

(F) Rinsing Stage 2

• • a. with deionized water (k<1 μScm⁻¹); or
  • b. 1.00 g/L H₂O₂ in deionized water (k<1 μScm⁻¹)

(G) Drying in Air

(H) Cathodic Dip-Coating with Cathoguard® 800 (BASF SE) with Dry Layer Thickness of 20 μm.

The steel sheets were treated, after the conversion coating which gave a layer deposit measured by X-ray fluorescence analysis of zirconium in the range of 45-55 mg/m², in the subsequent rinsing stages 1 and 2 either with deionized water (k<1 μScm⁻¹) or a solution containing hydrogen peroxide.

Table 1 summarizes the corrosion results after salt spray testing. It is clear that a hydrogen peroxide rinse is able to significantly improve the corrosive infiltration at the scratch and the paint adhesion after stone chipping (CE vs. I1-I3) and, if the hydrogen peroxide rinse is carried out immediately after the conversion treatment, the paint adhesion can be further improved (I1 vs. I3).

TABLE 1
	Rinse after	Salt spray
	conversion treatment	test (30 cycles)

	Rinsing stage 1	Rinsing stage 2	U/2
Test no.	H2O2	H2O2	(mm)	K-value
CE	−	−	1.5	4.5
I1	+	−	1.0	3.0
I2	+	+	0.9	3.0
I3	−	+	1.1	3.5