Method for the galvanic deposition of a zinc coating on a steel substrate and steel tube product

Abstract

The present invention relates to a method for the electroplating of a zinc coating with at least three zinc layers onto a steel substrate in the form of a tube, wherein the method comprises at least the following step: immersing the steel substrate in an electrolytic bath containing an electrolyte that includes at least zinc ions, silicon compounds, and a brightening agent, and applying current to the bath to deposit a silicon-containing zinc layer. Furthermore, a steel tube product having a silicon-containing zinc coating is described.

Description

[0001] The present invention relates to a method for electroplating a zinc coating onto a steel substrate and a steel tube product.

[0002] Various methods are known for protecting metallic surfaces against corrosion, especially surfaces of steel components.

[0003] Today, galvanizing steel surfaces is one of the most effective methods for protecting steel components from general corrosion. Zinc coatings are particularly widely used in the mass production of pipes, body parts, and similar items. The zinc coating, also known as the zinc layer, acts as a sacrificial anode in a corrosive environment (given good electrical contact with the steel surfaces) due to zinc's less noble nature compared to iron. Thus, the zinc layer corrodes first, before the iron is exposed to corrosion and forms reddish-brown corrosion products, also known as red rust.

[0004] To slow down corrosion, zinc alloys are known to be deposited electrolytically. Commercially, only zinc-iron, zinc-nickel, or zinc-cobalt alloys are typically used. Such processes have economic and technical disadvantages, such as relatively slow deposition rates, the presence of harmful heavy metals, and the often very high concentrations of strong complexing agents, which lead to significant wastewater pollution and thus to treatment and disposal problems.

[0005] The object of the present invention is to provide a solution by which an increased corrosion resistance can be given to a steel product with a steel substrate.

[0006] According to a first aspect, this problem is solved by a method having the features of claim 1.

[0007] In particular, the invention relates to a method for the electroplating of a zinc coating with at least three zinc layers onto a steel substrate in the form of a tube, wherein the method comprises at least the following step: introducing the steel substrate into an electrolytic bath with an electrolyte containing at least zinc ions, silicon compounds and a brightening agent, and applying current to the bath to deposit a silicon-containing zinc layer.

[0008] Electroplating can also be called electroplating. The steel substrate to be coated is placed in 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 tank. Applying current to the bath refers to the process of conducting an electric current, especially direct current, through the electrolyte in the bath. This is achieved by applying a corresponding electrical voltage.

[0009] The zinc layer deposited by the process step of immersing the steel substrate in an electrolytic bath containing at least zinc ions, silicon compounds, and a brightening agent, and applying an electric current to the bath to deposit a silicon-containing zinc layer, can be a first, second, third, and / or subsequent zinc layer of the zinc coating. The first zinc layer is defined as the layer deposited directly onto the steel substrate. The second, third, and subsequent zinc layers are defined as layers deposited on top of an underlying zinc layer.

[0010] Preferably, all zinc layers are silicon-containing zinc layers according to the present invention. However, it is also within the scope of the invention that several of the zinc layers are conventional zinc layers, in particular non-silicon-containing zinc layers, as long as at least one of the layers of the zinc coating is a silicon-containing zinc layer according to the invention.

[0011] The zinc coating obtained by the method according to the invention can therefore consist of a silicon-containing zinc layer and at least one further zinc-containing zinc layer and / or at least one further non-silicon-containing zinc layer.

[0012] According to a preferred embodiment, a first silicon-containing zinc layer is deposited by introducing the steel substrate into a first electrolytic bath containing an electrolyte that includes at least zinc ions, silicon compounds and a brightening agent, and by applying current to the bath to deposit a first silicon-containing zinc layer, and the process comprises at least the following further process steps: Subsequent placement of the steel substrate in a second electrolytic bath containing an electrolyte that includes at least zinc ions, silicon compounds and a brightening agent, and application of current to the bath to deposit a second silicon-containing zinc layer, and subsequent placement of the steel substrate in a third electrolytic bath containing an electrolyte that includes at least zinc ions, silicon compounds and a brightening agent, and application of current to the bath to deposit a third silicon-containing zinc layer.

[0013] The steel substrate that is introduced into the second, third and, if necessary, further electrolytic baths is a steel substrate already provided with at least one zinc layer.

[0014] The electrolytic baths for the process according to the invention are preferably arranged one after the other, so that the steel substrate can preferably be passed successively through the first, second and third electrolytic bath and optionally one or more additional electrolytic baths.

[0015] According to the invention, the electrolyte contains at least zinc ions, silicon compounds, and a brightening agent. The electrolyte is preferably an aqueous electrolyte based on chlorine or sulfate. Preferably, the electrolyte has a pH value in the range of 2 to 6. The zinc ions can, in particular, be divalent zinc ions. The silicon compounds contained in the electrolyte can be organic or inorganic silicon compounds. The brightening agent can also be referred to as a brightening agent or brightening agent. The brightening agent has a grain-refining effect during the deposition of zinc on the steel substrate.

[0016] The number of baths into which the steel substrate is immersed is not limited to three. For example, the zinc coating can be applied in three to twelve baths. The baths can be arranged sequentially. Alternatively, the zinc layers can be applied in three baths, with the steel substrate being passed through the first, then the second, and then the third bath after passing through the third.

[0017] Layers are parts of the zinc coating that are applied separately from one another, particularly sequentially, and thus constitute closed layers, collectively referred to as the zinc coating. These zinc coating layers, also called zinc layers, represent individual crystalline layers. The layers of the zinc coating are visible in a cross-section of the zinc coating.

[0018] According to the invention, by applying at least three layers to the steel substrate, the thickness of each individual layer can be kept low, yet a zinc coating formed from the zinc layers can be created that has a sufficient thickness to prevent corrosion of the steel substrate. The low thickness of the individual zinc layers, made possible by the multi-layered electrolytic zinc coating, also increases the formability of the zinc coating without significantly impairing corrosion protection. This is at least partly due to the fact that the thinner zinc layers exhibit better ductility and a lower tendency to crack than thick electrolytic zinc layers. Thin zinc layers are more formable, and cracks, especially surface cracks, which would be a concern with greater layer thickness, can be avoided.In particular, with the small layer thicknesses in the zinc layers, smaller crystals are present and the sliding in the crystal lattice is less than with a single-layer zinc layer.

[0019] The process according to the invention allows the silicon-containing coating to be applied to the steel substrate in a short time. In particular, the deposition rate is significantly better compared to zinc alloy processes known in the prior art due to the use of higher current densities and can, for example, be achieved in a continuous process, as well as a higher current efficiency.

[0020] Furthermore, the inventive method overcomes disadvantages in the dispersion of silica in zinc layers by providing a novel electrolyte composition that does not use nitric acid or complexing agents, and simultaneously by the optimized brightening agent system, which stabilizes the silicon compounds and selectively disperses the silicon during the fine grain formation of the zinc crystals. Dispersing the silicon in the zinc layer also inhibits grain growth, resulting in a fine-crystalline microstructure in the silicon-containing zinc layer.

[0021] Thus, the corrosion resistance of the steel substrate provided with a zinc coating according to the invention is improved, in particular due to the multi-layered nature of the zinc coating, the fine-crystalline microstructure and the presence of silicon compounds.

[0022] According to one embodiment, the silicon compounds in the electrolyte comprise inorganic silicates. For example, silicic acid can be used as the silicon compound.

[0023] Additionally or alternatively, the silicon compounds in the electrolyte can include organic silane compounds. The following silanes or silane derivatives can be used: Diethoxyddmethylsilane, demethoxyddemethylsilane, triethoxysilane, phenyltrimethoxysilane, aminoethylsilane, aminopropylsilane, glucidyloxyoropylsilane, aminosolane, aminoethyl-aminopropyltrimethoxysilane, aminopropyltrimethoxysilane and / or methyltriethoxysilane.

[0024] According to one embodiment, the particle size of the inorganic silicates in the electrolyte is in the range of 10-100nm.

[0025] According to one embodiment, the electrolyte contains inorganic silicates in the range of 3-5 g / l and / or organic silane compounds in the range of 1-100 g / l. Particularly preferably, the electrolyte contains silica particles in the range of 3-5 g / l and / or organic silane compounds in the range of 1-100 g / l. At higher concentrations of inorganic silicates and / or organic silane compounds, their dissolution in the electrolyte cannot be guaranteed. Conversely, at lower concentrations of inorganic silicates and / or organic silane compounds, the incorporation of silicon into the zinc layer is no longer reliably ensured, thus impairing corrosion resistance.

[0026] Preferably, the electrolyte is free of nitrogen compounds, nickel, and / or ammonium. This simplifies wastewater treatment. In particular, prescribed limits can be easily met if these harmful substances are not present in the electrolyte.

[0027] According to one embodiment, the brightening agent is an organic brightener based on polysaccharide. In particular, this optimized brightener stabilizes the silicon compounds in the electrolyte and selectively disperses the silicon during the fine grain formation of the zinc crystals. Specifically, it prevents the uncontrolled precipitation of silicates and / or silanes in the electrolyte.

[0028] According to one embodiment, the electrolyte comprises at least one nonionic surfactant. The surfactant can also be referred to as a wetting agent. A nonionic surfactant is defined in particular as one that does not contain dissociable functional groups and therefore, although soluble in water upon contact, does not form ions. In the electrolyte, the nonionic surfactant ensures, among other things, good wetting of the steel substrate surface.

[0029] 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.

[0030] According to one embodiment, the electrolyte contains at least 90 - 200 g / l divalent zinc ions, 3 - 5 g / l silica and / or 1-100 g / l silane, 3 - 50 g / l polysaccharide, 1 - 5 g / l a non-ionic surfactant and unavoidable impurities.

[0031] The electrolyte can be chloride-based or sulfate-based.

[0032] Preferably, the zinc coating is deposited at a temperature in the range of 25 to 80 °C, preferably from 50 to 60 °C.

[0033] Preferably, the zinc coating is deposited at a current density of up to 200 A / dm², preferably in the range of 10 to 140 A / dm².

[0034] The inventive method can be used to coat various shapes of steel substrates. In particular, a steel substrate in the form of a tube can be coated. The inventive method allows for the deposition of adhesive and uniform coatings on these steel substrates, exhibiting very good opacity, a controlled silicon content, and various decorative and mechanical properties.

[0035] According to one embodiment, the process steps are carried out as a continuous process, in particular as a through-process.

[0036] According to one embodiment, the throughput speed is at least 2m / min, preferably at least 5-100m / min.

[0037] According to one embodiment, the method is a high-speed deposition process for zinc-silicon-containing layers, in which substrates to be coated are metallized by means of an acidic electrolyte containing at least zinc ions, silicon compounds, a brightening agent that simultaneously binds the silicon compounds, and a surfactant. Using such a method, adhesive coatings with controlled silicon contents and various decorative and mechanical properties can be deposited in a continuous process, and improved corrosion protection properties can be achieved.This is achieved in particular by adding silicon compounds, an organic glazing agent based on polysaccharides, and, if required, non-ionic surfactants from the group of octylphenol ethoxylates and / or alkyl glucosides to the chloride- or sulfate-based electrolyte, with which electrolytic zinc-silicon-containing layers with an adjustable incorporation rate of 0.01 to 1%, preferably 0.02 to 0.06% silicon are deposited.

[0038] According to one embodiment, the method is a high-speed deposition method for silicon-containing zinc layers, wherein the method comprises the following steps: Providing a steel substrate to be coated, immersing and passing the steel substrate into and through a first immersion tank filled with an acidic electrolyte containing at least zinc ions, silicon compounds, a brightening agent which simultaneously binds the silicon compounds and, if necessary, a surfactant, as well as unavoidable impurities, wherein the silicon compounds comprise silica with a particle size of 10-100 nm and / or silane compounds, immersing and passing the pre-coated steel substrate into and through a second immersion tank filled with an acidic electrolyte containing at least zinc ions, silicon compounds, a brightening agent which simultaneously binds the silicon compounds and, if necessary, a surfactant, as well as unavoidable impurities, wherein the silicon compounds comprise silica with a particle size of 10-100 nm and / or silane compounds.Immersion and passage of the pre-coated steel substrate into and through a third immersion tank filled with an acidic electrolyte containing at least zinc ions, silicon compounds, a brightening agent which simultaneously binds the silicon compounds and, if necessary, a surfactant, as well as unavoidable impurities, wherein the silicon compounds comprise silica with a particle size of 10-100 nm and / or silane compounds, wherein the silicon-containing zinc layers are deposited with a dispersion rate of 0.01 to 1% silicon and a layer thickness of 1 to 5 µm is established in each case.

[0039] According to a further aspect, the present invention relates to a steel tube product comprising 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 portion of the surface of the steel substrate. the zinc coating is a silicon-containing zinc coating, the zinc coating comprises at least three layers, at least one of the layers has zinc with dispersed silicon, and the zinc coating consists predominantly of zinc and has a silicon content of 0.01-1 wt.%, and wherein the zinc coating is deposited by an electrolytic process.

[0040] Advantages and features 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 only described once if necessary.

[0041] The zinc coating is provided, in particular deposited, on at least a portion of the surface of the steel pipe substrate. However, it is also within the scope of the invention for the entire surface of the steel pipe substrate to have the zinc coating.

[0042] The zinc coating has at least three layers. However, the number of layers can also be higher. For example, the zinc coating can have up to 12 layers.

[0043] Preferably, the silicon in the silicon-containing zinc coating is present in a dispersed state in each of the layers applied by electroplating. However, it is also within the scope of the invention that at least one of the layers of the zinc coating is a non-silicon-containing zinc layer.

[0044] The silicon content in the zinc coating is in the 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 another 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 zinc layers are also included in the zinc coating, the specified silicon content is preferably present in the silicon-containing zinc layer(s).

[0045] The dispersed silicon can be determined in the zinc layer using Inductively Coupled Plasma (ICP) analysis.

[0046] Preferably, the silicon content is the same in each of the silicon-containing layers of the zinc coating.

[0047] Preferably, the thickness of each layer of the zinc coating is in the range of 1–5 µm. With these small layer thicknesses, improved formability, particularly ductility, and the prevention of cracks, especially surface cracks, can be achieved. In particular, at this small layer thickness, crystal growth is limited to the individual layers. According to the invention, for example, four zinc layers can be applied successively to the steel substrate. However, it is also within the scope of the invention to have a higher number of zinc layers, for example, 12. The higher the number of zinc layers, the smaller the layer thickness of the respective zinc layers. Preferably, all zinc layers of the zinc coating have the same layer thickness.

[0048] According to one embodiment, the particle size of the silicon particles in the zinc coating is in a range of 10nm to 100nm, preferably 30 - 50nm.

[0049] Preferably, the particle size of the silicon particles is the same in each of the silicon-containing layers of the zinc coating.

[0050] According to one, the grain size of the zinc crystals in the zinc coating is in a range of 10nm to 500nm, preferably 100-400nm.

[0051] According to one embodiment, the grain size is the same in each of the silicon-containing layers. The grain size can be influenced, for example, by the proportion of glazing agent in the respective electrolyte.

[0052] According to a preferred embodiment, the steel tube product is manufactured by the method according to the invention.

[0053] 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 several layers, the formability of the zinc coating and thus improved corrosion protection can be ensured despite this relatively large total layer thickness.

[0054] According to one embodiment, the steel product is a pipe product comprising a base pipe made of a steel alloy with an inner circumferential surface and an outer circumferential surface, wherein the base pipe has the silicon-containing zinc coating on at least a portion 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 to both the outer and inner circumferential surfaces.

[0055] According to one embodiment, a formed pipe bend sample of the pipe product with a bending angle of 180° and a bending radius of at least 2.5x pipe outer diameter shows no base metal corrosion in the neutral salt spray test according to DIN EN ISO 9227 after 480 hours.

[0056] A passivation layer, sealant, and / or paint can be applied to the zinc coating of the steel pipe product. The silicon dispersed in the zinc coating does not impede the adhesion of these additional layers and further improves corrosion protection.

[0057] The corrosion protection improved according to the invention is illustrated by example in the accompanying figures. They show: Figure 1 : Comparison of an embodiment of a steel tube product according to the invention as a straight tube with a conventional steel product; Figure 2: Comparison of an embodiment of a steel tube product according to the invention as a bent tube with a conventional steel product after a first testing period; Figure 3 : Comparison of an embodiment of a steel tube product according to the invention as a bent tube with a conventional steel tube product after a second testing period; and Figure 4 : a schematic sectional view of an embodiment of the steel tube product according to the invention through the tube wall of the steel tube product.

[0058] In Figure 1 Samples of a steel tube product according to the present invention and of a conventional steel tube product are shown. The steel tube product is a straight tube. Neither the steel tube product according to the invention nor the steel tube product with a conventional zinc coating without silicon was subjected to passivation. Figure 1 shows the samples after 240h of a neutral salt spray test (NSS test) according to ISO 9227.

[0059] In Figure 1a The steel tube provided with a silicon-containing zinc coating according to the invention is shown. The steel tube has an outer diameter of 8 mm and a wall thickness of 1 mm and is made of steel DIN / EN 235. The zinc coating thickness is 5 µm and was deposited at a current density of < 30 A / dm².

[0060] In Figure 1b A steel tube coated with a conventional zinc layer without silicon is shown. The steel tube has an outer diameter of 8 mm and a wall thickness of 1 mm and is made of DIN / EN 235 steel. The zinc layer is 5 µm thick and was deposited at a current density of < 30 A / dm².

[0061] As can be seen from Figure 1 The result is that the steel tube coated according to the invention has at least 30% higher corrosion protection against base metal corrosion.

[0062] In Figures 2 and 3Samples of a steel tube product according to the present invention and a conventional steel tube product are shown. The steel tube product is a bent tube. Both the steel tube product according to the invention and the steel tube product with a conventional zinc coating without silicon were subjected to Cr-VI-free passivation. Figure 2 shows the samples after 192h of a neutral salt spray test (NSS test) according to ISO 9227. Figure 3 Shows the samples after 480h of a neutral salt spray test (NSS test) according to ISO 9227. The pipe samples were bent by 180° with a bending radius of 2.5x pipe outer diameter.

[0063] In Figures 2a and 3aThe steel tube provided with a silicon-containing zinc coating according to the invention is shown. The steel tube has an outer diameter of 8 mm and a wall thickness of 1 mm and is made of steel DIN / EN 235. The zinc coating thickness is 5 µm and was deposited at a current density of < 30 A / dm².

[0064] In Figures 2b and 3b A steel tube coated with a conventional zinc layer without silicon is shown. The steel tube has an outer diameter of 8 mm and a wall thickness of 1 mm and is made of DIN / EN 235 steel. The zinc layer is 5 µm thick and was deposited at a current density of < 30 A / dm².

[0065] As can be seen from the Figures 2 and 3 The result is that the steel tube coated according to the invention shows no base metal corrosion after 480 hours of NSS testing.

[0066] In Figure 4Figure 1 shows a schematic sectional view of an embodiment of the steel product 1 according to the invention. In the embodiment shown, a zinc coating 11 is applied to one side of the steel substrate 10. The zinc coating 11 consists of four zinc layers 110. Silicon 12, which is dispersed in the zinc layers, is schematically indicated in each of the zinc layers 110.

[0067] The present invention has a number of advantages.

[0068] By selectively incorporating silicon compounds during the deposition of electrolytically produced zinc layers, both the appearance and the corrosion protection can be improved compared to pure zinc layers.

[0069] Furthermore, the inventive method provides a method for high-speed deposition of zinc-silicon-containing layers, which achieves significantly higher deposition rates, particularly in a continuous process, compared to zinc alloy processes known in the prior art, and also achieves a higher efficiency.

[0070] With the method according to the invention, and in particular with the electrolyte composition according to the invention, a current efficiency of > 60%, preferably > 75%, can be achieved. The remaining current or energy is converted into heat and is lost through an undesired side reaction with hydrogen.

[0071] Finally, the invention, and in particular the zinc-silicon-containing layers produced using the inventive method, are characterized by significantly better corrosion resistance compared to zinc layers, despite only low silicon content in the zinc coating. Reference symbol list

[0072] 1 Steel pipe product 10 Steel substrate 11 Zinc coating 110 Layer 12 Silicon

Claims

1. A method for the electroplating of a zinc coating having at least three zinc layers on a steel substrate in the form of a tube, the method comprising at least the following step: introducing the steel substrate into an electrolytic bath containing an electrolyte which contains at least zinc ions, silicon compounds and a brightener, and applying current to the bath to deposit a silicon-containing zinc layer.

2. The method according to claim 1, wherein the silicon-containing zinc layer produced by the method step represents a first, second and / or further zinc layer of the zinc coating.

3. The method according to claim 1, wherein the silicon-containing zinc layer produced by the method step represents a first zinc layer and the method comprises at least the following further method steps: subsequently introducing the steel substrate into a second electrolytic bath with an electrolyte which contains at least zinc ions, silicon compounds and a brightener, and applying current to the bath to deposit a second silicon-containing zinc layer, and subsequently introducing the steel substrate into a third electrolytic bath with an electrolyte which contains at least zinc ions, silicon compounds and a brightener, and applying current to the bath to deposit a third silicon-containing zinc layer.

4. A process according to any one of claims 1 to 3, wherein the silicon compounds comprise inorganic silicates and / or organic silane compounds.

5. The method according to claim 4, wherein the particle size of the inorganic silicates in the electrolyte is in the range of 10-100nm.

6. A process according to any one of claims 4 or 5, wherein the electrolyte comprises inorganic silicates in a range of 3-5 g / l and / or organic silane compounds in a range of 1-100 g / l.

7. A method according to any one of claims 1 to 6, wherein the electrolyte is free of nitrogen compounds, nickel and / or ammonium.

8. A process according to any one of claims 1 to 7, wherein the brightener is an organic brightener based on polysaccharide.

9. The method according to any one of claims 1 to 8, wherein the electrolyte comprises at least one non-ionic surfactant, in particular selected from the group of octylphenol ethoxylates and / or alkyl glucosides.

10. A method according to any one of claims 1 to 9, wherein the deposition takes place at a temperature in the range of 25 to 80 °C, preferably 50 to 60 °C and the deposition at a current density of up to 200 A / dm 2 , preferably in the range of 10 to 140 A / dm 2 occurs.

11. The method according to any one of claims 1 to 10, wherein the electrolyte comprises 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.

12. The method according to any one of claims 1 to 11, wherein the method steps are carried out as a continuous method, in particular as a continuous method, wherein the throughput speed is in particular at least 2 m / min, preferably between 5-100 m / min.

13. A method according to any one of claims 1 to 12, wherein the electrolyte is chloride-based or sulfate-based.

14. A steel tube product comprising a steel substrate and a zinc coating, characterized in that the zinc coating is provided on at least part of the surface of the steel substrate, the zinc coating is a silicon-containing zinc coating, the zinc coating comprises at least three layers, at least one of the layers comprises zinc with dispersed silicon and the zinc coating consists predominantly of zinc and has a silicon content of 0.01-1 wt.%, and wherein the zinc coating is deposited by an electrolytic process.

15. A steel pipe product according to claim 14, wherein each of the layers comprises zinc with dispersed silicon.

16. A steel pipe product according to claim 14 or 15, wherein the silicon content is in the range of 0.02-0.6 wt%, or in the range of 0.01-0.09 wt%.

17. Steel pipe product according to one of claims 14 to 16, wherein the layer thickness of each layer of the zinc coating is in the range of 1-5µm.

18. Steel pipe product according to one of claims 14 to 17, wherein 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.

19. A steel pipe product according to claim 18, wherein the particle size is the same in each of the layers.

20. Steel pipe product according to one of claims 14 to 19, wherein 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.

21. A steel pipe product according to claim 20, wherein the grain size is the same in each of the layers.

22. A steel pipe product according to any one of claims 14 to 21, wherein the pipe product is manufactured by a method according to any one of claims 1 to 13.

23. Steel pipe product according to one of claims 14 to 22, characterized in that the zinc coating has a total layer thickness in the range of 3 to 40µm, preferably in the range of 4 to 25µm.

24. Steel pipe product according to one of claims 14 to 23, wherein a formed pipe bend sample of the pipe product with a bending angle of 180° and a bending radius of at least 2.5x pipe outer diameter shows no base metal corrosion after 480 hours in the neutral salt spray test according to DIN EN ISO 9227.

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