Multi-stage treatment for activated zinc phosphatizing of metallic components with zinc surfaces

Abstract

The present invention relates to a process for the anti-corrosion pretreatment of a multiplicity of components in series, wherein each component in the series at least partly has surfaces of zinc and undergoes successive process steps for the deposition of iron and for zinc phosphation. In the process step for the deposition of iron, the coating layer to be established is at least 10 milligrams of elemental iron per square meter of the zinc surfaces. The zinc phosphation subsequent to this deposition of iron takes place by means of an acidic aqueous composition which in addition to zinc ions, phosphate ions and free fluoride also comprises a particulate constituent dispersed in water which is at least partly composed of hopeite, phosphophyllite, scholzite and/or hureaulite, and is provided by means of an aqueous dispersion of these crystalline solids that is stabilized with at least one polymeric organic compound.

1 . A process for the anti-corrosion pretreatment of a plurality of components in series, wherein each component in the series at least partly has surfaces of zinc wherein each component first undergoes a wet-chemical process step (i) for the deposition of iron on the zinc surfaces and subsequently undergoes a process step (ii) for zinc phosphation,
 wherein in process step (i), a coating layer of at least 10 milligrams of elemental iron is deposited per square meter of the zinc surfaces of the component, and   wherein each component in process step (ii) is brought into contact with an acidic aqueous composition that has a free acid in points greater than zero, and contains
 (A) 5-50 g/kg of phosphates dissolved in water, calculated as PO 4 , 
 (B) 0.3-3 g/kg of zinc ions, 
 (C) free fluoride, and 
 (D) a water-dispersed particulate constituent comprising phosphates of polyvalent metal cations, wherein the phosphates are at least partly selected from hopeite, phosphophyllite, scholzite and/or hureaulite, 
   wherein the acidic aqueous composition is obtained by adding an amount of an aqueous dispersion to an acidic aqueous composition containing the components (A)-(C);   wherein the aqueous dispersion contains a water-dispersed particulate constituent (P) that comprises:
 at least one particulate inorganic compound (P1) composed of phosphates of polyvalent metal cations at least partly selected from hopeite, phosphophyllite, scholzite and/or hureaulite; and 
 at least one polymeric organic compound (P2). 
   
     
     
         2 . The process according to  claim 1 , wherein the aqueous dispersion to provide the acidic aqueous composition for process step (ii) is added in such an amount that a proportion by weight of phosphates from the particulate constituent (P) of the aqueous dispersion, based on the acidic aqueous composition, is at least 0.004 g/kg. 
     
     
         3 . The process according  claim 1 , wherein an amount of an activating aid containing a particulate constituent (P) in water-dispersed form is added continuously or discontinuously to the acidic aqueous composition in process step (ii) of the zinc phosphation, said particulate constituent (P) comprising:
 at least one particulate inorganic compound (P1) composed of phosphates of polyvalent metal cations at least partly selected from hopeite, phosphophyllite, scholzite and/or hureaulite,   and at least one polymeric organic compound (P2),   
       said amount being sufficient, under the conditions of process step (ii), to maintain a property of the acidic aqueous composition of depositing a zinc phosphate layer having a layer weight of less than 5.0 g/m 2  on a hot-dip galvanized steel surface (Z). 
     
     
         4 . The process according to  claim 1 , wherein, in process step (ii), the polymeric organic compound (P2) in the particulate constituent (P) of the aqueous dispersion or of the activating aid is at least partly composed of styrene and/or an alpha-olefin having no more than 5 carbon atoms; and the polymeric organic compound (P2) additionally comprises units of maleic acid, an anhydride of the maleic acid, an imide of the maleic acid or a combination of one or more thereof. 
     
     
         5 . The process according to  claim 1 , wherein, in process step (ii), the acidic aqueous composition for zinc phosphation has a pH value below 3.6 and free acid greater than 0.5 points. 
     
     
         6 . The process according to  claim 1 , wherein, in process step (ii), the acidic aqueous composition for zinc phosphation contains a source of free fluoride; and at least 10 mg/kg, but no more than 200 mg/kg of free fluoride. 
     
     
         7 . The process according to  claim 1 , wherein, in wet-chemical process step (i), a coating layer based on elemental iron of at least 20 milligrams, but less than 150 milligrams per square meter of the zinc surfaces of the component is deposited on the surfaces of the component formed by zinc. 
     
     
         8 . The process according to  claim 1 , wherein wet-chemical process step (i) is carried out by contacting at least the surfaces of zinc of the components in the series with an aqueous composition containing iron(II) and/or iron(III) ions, wherein the proportion of iron ions dissolved in water is at least 50 mg/L, but in total less than 10 mg/L of ionic compounds of the metals copper, nickel, cobalt, tin, manganese, molybdenum, chromium, and/or cerium are contained in the aqueous composition in each case relative to the metal element. 
     
     
         9 . The process according to  claim 8 , wherein wet-chemical process step (i) is carried out by contact with an alkaline aqueous composition, having a pH value not below 8.5, but not above 13.5; and which contains
 (a) at least 50 mg/L of iron(III) ions, and   (b) at least 100 mg/L of complexing agents selected from organic compounds (b1), which have at least one functional group selected from COOX, OPO 3 X and/or PO 3 X, wherein X is either an H atom or an alkali and/or alkaline earth metal atom, and/or condensed phosphates (b2) calculated as PO 4 ,   
       wherein the composition has a free alkalinity of at least 1 point, but less than 6 points. 
     
     
         10 . The process according to  claim 9 , wherein the alkaline aqueous composition additionally contains at least 100 mg/L, but no more than 10 g/L of phosphate ions, wherein a mass-based ratio of iron(III) ions to phosphate ions in the alkaline aqueous composition is in a range from 1:20 to 1:2. 
     
     
         11 . The process according to  claim 10 , wherein a molar ratio of all components (b) to iron(III) ions in the alkaline aqueous composition is greater than 1:1. 
     
     
         12 . The process according to  claim 1 , wherein prior to contact with the acidic aqueous composition in process step (ii) of the zinc phosphation, the components in the series are not brought into contact with a colloidal aqueous solution containing hopeite, phosphophyllite, scholzite and/or hureaulite or sparingly soluble salts of elemental Ti in the particulate constituent, and are not brought into contact with a colloidal aqueous solution for activating the surfaces of the components for zinc phosphation prior to being brought into contact with the acidic aqueous composition in process step (ii), and do not undergo an activation stage for activating the surfaces of the components for zinc phosphation. 
     
     
         13 . The process according to  claim 1 , wherein the components in the series are cleaned and optionally degreased in a cleaning stage prior to or together with process step (i), in particular by contact with an aqueous, alkaline cleaning agent, wherein process step (i) immediately follows the cleaning stage with or without an intermediate rinsing step. 
     
     
         14 . The process according to  claim 1 , wherein the components in the series additionally have surfaces of the metal aluminum, and optionally surfaces of iron. 
     
     
         15 . The process according to  claim 1 , wherein a zinc phosphate layer having a layer weight of at least 1.0 g/m 2  is deposited on the surfaces of zinc.