Patent Application
20250129504
This patent application claims benefit of European Patent Application No. 23204872.8, filed Oct. 20, 2023, which patent application is hereby incorporated herein by reference.
The present invention relates to a method for galvanically depositing a zinc coating on a steel substrate and a steel tube product.
Various methods are known for protecting metallic surfaces against corrosion, in particular surfaces of steel components.
Here, the galvanization of steel surfaces is nowadays one of the most effective methods for protecting steel components against general corrosion. Zinc coatings are used particularly widely in the mass production of tubes, chassis components and the like. The zinc coating, which is referred to as a zinc layer, serves in a corrosion medium as a sacrificial anode (in the case of good electrical contact with the steel surfaces) as a result of the less noble character of the zinc in comparison with iron. Thus, the zinc layer corrodes first before the iron is exposed to corrosion and forms red-brown corrosion products, which are also referred to as red rust.
To delay corrosion, it is known to deposit zinc alloys electrolytically. Here, usually only zinc-iron alloys, zinc-nickel alloys or zinc-cobalt alloys are used commercially. Such processes have economic and technical disadvantages, such as, for example, the relatively slow deposition rates, heavy metals which are harmful to health and the usually very high concentrations of highly complexing substances, which lead to a high level of pollution of the waste water and thus to cleaning and disposal problems.
It is an object of the present invention to provide a solution by means of which a steel product having a steel substrate can be given increased corrosion resistance.
According to a first aspect, this object is achieved by a method having the features of claim 1.
In particular, the invention relates to a method for the galvanic deposition of a zinc coating having at least three zinc tiers on a steel substrate in the form of a tube, wherein the method comprises at least the following step:
The galvanic deposition can also be referred to as electroplating. The steel substrate to be coated can be fed into the electrolytic bath by immersing it in the bath which is filled with the electrolyte and passing it through the bath. The electrolytic bath can also be referred to as an immersion basin. Applying current to the bath refers to conducting electrical current, in particular direct current, through the electrolyte accommodated in the bath. For this purpose, a corresponding electrical voltage is applied.
The zinc tier deposited by the method step of feeding the steel substrate into an electrolytic bath having an electrolyte which contains at least zinc ions, silicon compounds and a brightener, and applying current to the bath in order to deposit a silicon-containing zinc tier, can be a first, second, third and/or further zinc tier of the zinc coating. Here, the first zinc layer refers to the tier which is deposited directly on the steel substrate. The second, third and further zinc tiers refer to a tier which is deposited on an underlying zinc tier.
Here, all zinc tiers are preferably silicon-containing zinc tiers according to the present invention. However, it is also within the scope of the invention that one or a plurality of the zinc tiers constitutes conventional zinc tiers, in particular non-silicon-containing zinc tiers, as long as at least one of the tiers of the zinc coating constitutes a silicon-containing zinc tier according to the invention.
The zinc coating achieved by the method according to the invention can therefore consist of a silicon-containing zinc tier and at least one further silicon-containing zinc tier and/or at least one further non-silicon-containing zinc tier.
According to a preferred embodiment, a first silicon-containing zinc tier is deposited by inserting the steel substrate into a first electrolytic bath having an electrolyte which contains at least zinc ions, silicon compounds and a brightener, and applying current to the bath in order to deposit a first silicon-containing zinc tier, and the method comprises at least the following further method steps:
The steel substrate which is inserted into the second, third and optionally further electrolytic bath constitutes a steel substrate which is already provided with at least one zinc tier.
The electrolytic baths for the method according to the invention are preferably arranged one after the other, so that the steel substrate can preferably be guided successively through the first, second and third electrolytic bath and optionally one or more additional electrolytic baths.
According to the invention, the electrolyte contains at least zinc ions, silicon compounds and a brightener. The electrolyte is preferably an aqueous electrolyte on a chlorine or sulfate basis. The electrolyte preferably has a pH value in the range of 1.0 to 6.0, preferably in the range of 2.0 to 6.0, of 2.2 to 3.2, of 1.0 to 3.2, of 1.2 to 3.0, of 1.2 to 3.2 or of 1.2 to 2.9. The zinc ions can be in particular divalent zinc ions.
The silicon compounds which are contained in the electrolyte can be organic or inorganic silicon compounds. The brightener can also be referred to as brightening means or brightening agent. The brightener has a grain-refining effect during the deposition of the zinc on the steel substrate.
The number of baths into which the steel substrate is inserted is not restricted to three. For example, the zinc coating can be applied in three to twelve baths. The baths can be arranged successively with respect to one another. Alternatively, the zinc tiers can also be applied in three baths, for example, wherein, after passing through the third bath, the steel substrate is guided again through the first, then the second and then the third bath.
Tiers refer to parts of the zinc coating which are applied separately from one another, in particular one after the other, and thus constitute closed tiers and are referred to jointly as zinc coating. The tiers of the zinc coating, which are also referred to as zinc tiers, thus constitute crystalline individual layers. The tiers of the zinc layer are recognizable in the cross-section polish of the zinc coating.
By applying at least three tiers to the steel substrate according to the invention, the tiers thickness of the individual tiers can be kept small and nevertheless a zinc coating formed from the zinc tiers can be created which has a sufficient layer thickness to prevent corrosion of the steel substrate. As a result of the small tier thickness of the individual zinc tiers which is possible due to the multitier nature of the electrolytic zinc coating, the forming capacity of the zinc coating is additionally increased without substantially impairing the corrosion protection. This is at least partially caused by the fact that the thinner zinc tiers have better ductility and a lower tendency to crack than thick electrolytic zinc layers. Zinc tiers with a small layer thickness can be formed better and cracks, in particular surface cracks, which would have to be feared in the case of a larger tier thickness, can be avoided. In particular, in the case of the small tier thicknesses, smaller crystals are present in the zinc tier and the sliding in the crystal lattice is smaller than in the case of a single-layer zinc layer.
With the method according to the invention, the silicon-containing coating can be applied to the steel substrate in a short time. In particular, the deposition rate is substantially better than the zinc alloy processes known in the prior art as a result of the use of higher current densities and can be achieved, for example, in a continuous process and a higher current efficiency can be achieved.
Furthermore, the method according to the invention eliminates disadvantages in the dispersion of silicic acid in zinc layers by providing a novel composition of the electrolyte which does not use nitric acid and complexing agents, and at the same time by the optimized brightener system which stabilizes the silicon compounds and in a targeted manner disperses the silicon during the fine-grain formation of the zinc crystals. As a result of the dispersion of the silicon in the zinc tier, the grain growth is additionally impeded and the silicon-containing zinc tier thereby has a fine-crystalline microstructure.
Thus, the corrosion resistance of the steel substrate provided with a zinc coating according to the invention is improved in particular on account of the multitier nature of the zinc coating, the fine-crystalline microstructure and the presence of silicon compounds.
According to one embodiment, the silicon compounds in the electrolyte comprise inorganic silicates. For example, silicic acid can be used as silicon compound.
Additionally or alternatively, the silicon compounds in the electrolyte can comprise organic silane compounds. Here, the following silanes or derivatives of silanes can be used:
According to one embodiment, the particle size of the inorganic silicates in the electrolyte is in the range of 10-100 nm.
According to one embodiment, the electrolyte has inorganic silicates in a range of 3-5 g/l and/or organic silane compounds in a range of 1-100 g/l. The electrolyte particularly preferably has silicic acid particles in a range of 3-5 g/l and/or organic silane compounds in a range of 1-100 g/l. In the case of a higher content of inorganic silicates and/or organic silane compounds, it can no longer be ensured that these can be brought into solution in the electrolyte. By contrast, in the case of a lower content of inorganic silicates and/or organic silane compounds, the incorporation of the silicon into the zinc tier is no longer reliably ensured and the corrosion resistance is therefore impaired.
The electrolyte is preferably free of nitrogen compounds, nickel and/or ammonium. The wastewater treatment can be simplified as a result. In particular, prescribed limit values can be adhered to in a simple manner if these harmful substances are not contained in the electrolyte.
According to one embodiment, the brightener constitutes an organic brightener based on polysaccharide. An organic brightener based on water-soluble polysaccharides is preferably used. Amylose, methylcellulose, laminarin in particular with glucose chains with a low degree of branching or salts of alginic acid, in particular sodium or potassium salts of alginic acid, can preferably be used as water-soluble polysaccharides.
In particular with this optimized brightener, in particular a brightener based on water-soluble polysaccharides, the silicon compounds can be stabilized and preferably bound in the electrolyte and the silicon, in particular the silicon compounds, can be dispersed in a targeted manner during the fine-grain formation of the zinc crystals. In particular, an uncontrolled precipitation of silicates and/or silanes in the electrolyte can be prevented.
With an organic brightener based on water-soluble polysaccharides, in particular amylose, methylcellulose, laminarin in particular with glucose chains with a low degree of branching or salts of alginic acid, in particular sodium or potassium salts of alginic acid, in particular organic silane compounds can be stabilized and preferably bound in the electrolyte and the silicon, in particular the silane compounds, can be dispersed in a targeted manner during the fine-grain formation of the zinc crystals. The silane compounds here can preferably constitute the abovementioned silanes or derivatives of silanes.
The ratio between the silicon compounds and an organic brightener based on polysaccharides is preferably set in a targeted manner. For this purpose, the ratio of the concentration of silicon compounds to polysaccharides in the electrolyte can preferably be set in a range of 3:1-1:50, preferably in a ratio of 2:1 to 1:30. The incorporation rate of silicon in the deposited zinc coating can be controlled via this.
According to one embodiment, the electrolyte has at least one non-ionic surfactant. The surfactant can also be referred to as a tenside or a wetting agent. Non-ionic surfactant refers in particular to a surfactant which contains no dissociable functional groups and therefore, although soluble in water on contact with water, does not form ions. In the electrolyte, the non-ionic surfactant ensures inter alia good wetting of the surface of the steel substrate.
According to one embodiment, the surfactant is selected from the group of octylphenol ethoxylates and/or alkyl glucosides. This selection is particularly low-foaming and stable in the electrolyte.
According to one embodiment, the electrolyte has at least 90-200 g/l of divalent zinc ions, 3-5 g/l of silicic acid and/or 1-100 g/l of silane, 3-50 g/l of polysaccharide, 1-5 g/l of a non-ionic surfactant and unavoidable impurities.
The electrolyte can be formed on a chloride basis or sulfate basis.
Preferably, the deposition of the zinc coating takes place at a temperature in the range of 25 to 80° C., preferably of 50 to 60° C.
Preferably, the deposition of the zinc coating takes place at a current density of up to 200 A/dm2, preferably in the range of 10 to 140 A/dm2. The silicon incorporation rate can be linear, in particular linearly increasing, with the present invention, in particular with the electrolyte used, over the entire current density range of 10 to 140 A/dm2. Thus, the silicon incorporation rate has no dependence on the current density in the preferred range. In particular, as the silicon concentration in the electrolyte increases, the silicon content that is incorporated in the coating also increases, and this dependence does not change over the current density range.
The incorporation rate of silicon in the deposited zinc coating can be controlled according to the present invention by setting the ratio of the concentration of silicon compounds and brightener. Silicon compounds and brightener, in particular of polysaccharides, are preferably used in a ratio of 3:1-1:50 in the electrolyte.
Different forms of steel substrates can be coated with the method according to the invention. In particular, a steel substrate in the form of a tube can be coated. With the method according to the invention, adherent and uniform coatings with a very good cover ability and a controlled silicon content and different decorative and mechanical properties can be deposited on these steel substrates.
According to one embodiment, the method steps are carried out as a continuous method, in particular as a through-feed method. In the through-feed method, an electrolyte is preferably used which has a pH value in the range of 1.0 to 3.2, preferably in the range of 2.0 to 3.2 or of 1.2 to 3.0 and more preferably in a range of 1.2 to 2.9. The electrolyte used in the through-feed method is therefore preferably a strongly acidic electrolyte.
According to one embodiment, the through-feed speed is at least 2 m/min, preferably at least 5-100 m/min.
According to one embodiment, the method constitutes a method for the high-speed deposition of zinc-silicon-containing tiers, with which substrates to be coated are metallized by means of an acidic electrolyte which contains at least zinc ions, silicon compounds, a brightener which simultaneously binds the silicon compounds and has a surfactant. With the aid of such a method, adherent coatings with controlled silicon contents and different decorative and mechanical properties can be deposited in a through-feed method, and higher corrosion protection properties can be achieved. This is achieved in particular by adding silicon compounds, an organic brightener based on polysaccharides and, if required, non-ionic surfactants from the group of octylphenol ethoxylates and/or alkyl glucosides to the electrolyte formed on a chloride or sulfate basis, whereby electrolytic zinc-silicon-containing tiers with an adjustable incorporation rate of 0.01 to 1%, preferably 0.02 to 0.06%, of silicon are deposited.
According to one embodiment, the method constitutes a method for the high-speed deposition of silicon-containing zinc layers, wherein the method has the following steps:
According to a further aspect, the present invention relates to a steel tube product, which comprises a steel substrate in the form of a tube and a zinc coating. The steel tube product is characterized in that the zinc coating is provided on at least a part of the surface of the steel substrate,
Advantages and features which have been described with respect to the method according to the invention also apply—where applicable—to the steel tube product according to the invention and vice versa and are therefore optionally described only once.
The zinc coating is provided, in particular deposited, on at least a part of the surface of the steel tube substrate. However, it is also within the scope of the invention that the entire surface of the steel tube substrate has the zinc coating.
The zinc coating has at least three tiers. However, the number of tiers can also be higher. For example, the zinc coating can have up to 12 tiers.
Preferably, the silicon of the silicon-containing zinc coating is present in each of the tiers in the dispersed state, which are applied by galvanic deposition. However, it is also within the scope of the invention that at least one of the tiers of the zinc coating constitutes a non-silicon-containing zinc tier.
The silicon content in the zinc coating is in a range of 0.01-1 wt.-%. According to one embodiment, the silicon content is in the range of 0.02-0.6 wt.-%. According to a further embodiment, the silicon content is in the range of 0.01-0.09 wt.-%, preferably in the range of 0.02-0.08 wt.-%. In an embodiment in which non-silicon-containing zinc tiers are also contained in the zinc coating, the specified silicon content is preferably present in the silicon-containing zinc tier or tiers provided.
The dispersed silicon can be determined by means of inductively coupled plasma (ICP) analysis in the zinc layer.
Preferably, the silicon content is the same in each of the silicon-containing tiers of the zinc coating.
Preferably, the layer thickness of each tier of the zinc coating is in the range of 1-5 μm. With these small layer thicknesses, or tier thicknesses, the effect of better formability and in particular ductility and the avoidance of cracks, in particular surface cracks, can be achieved. In particular, with this small layer thickness, the crystal growth is restricted to the individual tiers. According to the invention, for example, four zinc tiers can be applied one after the other to the steel substrate. However, it is also within the scope of the invention that the number of zinc tiers is higher, for example 12. The higher the number of zinc tiers, the lower the tier thickness or layer thickness of the respective zinc tiers is set. Preferably, all zinc tiers of the zinc coating have the same tier thickness.
According to one embodiment, the particle size of the silicon particles in the zinc coating is in a range of 10 nm to 100 nm, preferably 30-50 nm.
Preferably, the particle size of the silicon particles in each of the silicon-containing tiers of the zinc coating is the same.
According to one embodiment, the grain size of the zinc crystals in the zinc coating is in a range of 10 nm to 500 nm, preferably 100-400 nm.
According to one embodiment, the grain size is the same in each of the silicon-containing tiers. The grain size can be influenced, for example, by the proportion of brightener in the respective electrolyte.
According to a preferred embodiment, the steel tube product is produced by the method according to the invention.
According to one embodiment, the zinc coating has a total layer thickness in the range of 3 to 40 μm, preferably in the range of 4 to 25 μm. Since the zinc coating is formed from a plurality of tiers, the formability of the zinc coating and therefore an improved corrosion protection can be ensured in spite of this relatively large total layer thickness.
According to one embodiment, the steel product constitutes a tube product which comprises a base tube made of a steel alloy having an inner circumferential surface and an outer circumferential surface, wherein the base tube has the silicon-containing zinc coating at least on a part of the circumferential surfaces. Preferably, only the outer circumferential surface is coated with the zinc coating. However, it is also possible to apply the zinc coating both on the outer circumferential surface and on the inner circumferential surface.
According to one embodiment, a formed tube bend sample of the tube product with a bending angle of 180° and a bending radius of at least 2.5×tube outer diameter shows no base metal corrosion after 480 hours in the neutral salt spray mist test according to DIN EN ISO 9227. The tube bend sample can be taken from a tube product which has no bend, that is to say constitutes a non-formed tube product or a rectilinear partial length of a tube bent at another point, and can subsequently be brought into the form of the tube bend sample. Alternatively, the tube bend sample can be taken from a formed tube product which has already been subjected to the forming required for the test, in particular bending through 180° with a bending radius of at least 2.5×tube outer diameter.
A passivation layer, a seal and/or a lacquer can be applied to the zinc coating of the steel tube product. As a result of the silicon dispersed in the zinc coating, the adhesive strength of these further layers is not impeded and the corrosion protection is further improved.
The corrosion protection improved according to the invention is illustrated by way of example in the accompanying figures. In the figures:
As can be derived from
As can be derived from
The present invention has a number of advantages.
As a result of the targeted incorporation of silicon compounds during the deposition of electrolytically produced zinc tiers, both the appearance and the corrosion protection can be improved over pure zinc tiers.
In addition, a method for the high-speed deposition of zinc-silicon-containing tiers is specified with the method according to the invention, which method achieves substantially higher deposition rates, in particular in a through-feed method, as compared with the zinc alloy processes known in the prior art and achieves a higher efficiency.
With the method according to the invention and in particular the composition of the electrolyte according to the invention, a current efficiency of >60%, preferably >75%, can be achieved. The remainder of the current intensity or energy is converted into heat and is lost as a result of undesired side reaction with hydrogen.
Finally, the invention and in particular the zinc-silicon-containing tires produced with the method according to the invention are distinguished over zinc layers by a substantially better corrosion resistance in spite of only low silicon contents in the zinc coating.
| Number | Date | Country | Kind |
|---|---|---|---|
| 23204872.8 | Oct 2023 | EP | regional |
