Method for galvanically depositing a zinc coating on a steel substrate and steel tube product

Abstract

The present 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: 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. In addition, a steel tube product is described which has a silicon-containing zinc coating.

1 . 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:
 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.   
     
     
         2 . The method according to  claim 1 , wherein the silicon-containing zinc tier produced by the method step constitutes a first, second and/or further zinc tier of the zinc coating. 
     
     
         3 . The method according to  claim 1 , wherein the silicon-containing zinc tier produced by the method step constitutes a first zinc tier and the method comprises at least the following further method steps:
 subsequently feeding the steel substrate into a second 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 second silicon-containing zinc tier, and   subsequently feeding the steel substrate into a third 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 third silicon-containing zinc tier.   
     
     
         4 . The method according to  claim 1 , 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 10-100 nm. 
     
     
         6 . The method according to  claim 4 , wherein 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. 
     
     
         7 . The method according to  claim 1 , wherein the electrolyte is free of nitrogen compounds, nickel and/or ammonium. 
     
     
         8 . The method according to  claim 1 , wherein the brightener constitutes an organic brightener based on polysaccharide. 
     
     
         9 . The method according to  claim 1 , wherein the electrolyte has at least one non-ionic surfactant, in particular selected from the group of octylphenolethoxylates and/or alkylglucosides. 
     
     
         10 . The method according to  claim 1 , wherein the deposition takes place at a temperature in the range of 25 to 80° C., preferably of 50 to 60° C., and the deposition takes place at a current density of up to 200 A/dm 2 , preferably in the range of 10 to 140 A/dm 2 . 
     
     
         11 . The method according to  claim 1 , wherein 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. 
     
     
         12 . The method according to  claim 1 , wherein the method steps are carried out as a continuous method, in particular as a through-feed method, wherein the through-feed speed is in particular at least 2 m/min, preferably between 5-100 m/min. 
     
     
         13 . The method according to  claim 1 , wherein the electrolyte is formed on a chloride basis or sulfate basis. 
     
     
         14 . A steel tube product, which comprises a steel substrate and a zinc coating, characterized in that the zinc coating is provided on at least a part of the surface of the steel substrate,
 the zinc coating constitutes a silicon-containing zinc coating,   the zinc coating comprises at least three tiers,   at least one of the tiers 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 method.   
     
     
         15 . The steel tube product according to  claim 14 , wherein each of the tiers has zinc with dispersed silicon. 
     
     
         16 . The steel tube product according to  claim 14 , 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 . The steel tube product according to  claim 14 , wherein the layer thickness of each tier of the zinc coating is in the range of 1-5 μm. 
     
     
         18 . The steel tube product according to  claim 14 , 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 . The steel tube product according to  claim 18 , wherein the particle size is the same in each of the tiers. 
     
     
         20 . The steel tube product according to  claim 14 , 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 . The steel tube product according to  claim 20 , wherein the grain size is the same in each of the layers. 
     
     
         22 . The steel tube product according to  claim 14 , wherein the tube product is produced by a method according to  claim 1 . 
     
     
         23 . The steel tube product according to  claim 14 , characterized in that 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. 
     
     
         24 . The steel tube product according to  claim 14 , wherein 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.