Method for Coating a Substrate Containing Cobalt, Nickel and/or Iron with a Corrosion-Resistant Layer

Abstract

A method is provided for corrosion-proofing substrates made of materials containing cobalt, nickel and/or iron, in which cracking in the material and in a corrosion-proofing layer can be avoided. For this purpose, a slurry containing aluminium is applied to a substrate and the substrate, together with the applied slurry, is then subjected to a brief annealing of the surface layer at reduced temperature. Due to the heating of the surface layer of the substrate and of the slurry during the annealing of the surface layer, an exothermic reaction between the aluminium and the substrate metal takes place on the surface of the substrate, so that aluminide phases are formed which produce an aluminide diffusion layer on the substrate. Above this layer remains a residual layer of unused slurry.

1 . A method for coating a substrate that contains cobalt, nickel and/or iron with a corrosion-resistant layer, wherein a slurry containing aluminum is applied to the substrate, and the substrate together with the slurry is then subjected to a brief annealing of the surface layer at a reduced temperature. 
     
     
         2 . The method according to  claim 1 , wherein during the annealing of the surface layer, the substrate is heated to a temperature that is above the melting point of the aluminum in the slurry. 
     
     
         3 . The method according to  claim 1 , wherein the slurry contains the aluminum as a metal powder having a particle size of 5-100 μm. 
     
     
         4 . The method according to  claim 1 , wherein the surface of the substrate is cleaned before the slurry is applied. 
     
     
         5 . The method according to  claim 1 , wherein the surface of the substrate is precoated galvanically or by thermal spraying before the slurry is applied. 
     
     
         6 . The method according to  claim 1 , wherein the surface annealing is carried out in a vacuum, in an inert gas atmosphere or in the open air. 
     
     
         7 . The method according to  claim 1 , wherein the slurry contains an organic solvent and a binder system which can be thermally decomposed. 
     
     
         8 . The method according to  claim 7 , wherein the binder system is a synthetic, water-soluble or dispersible polymer that contains no inorganic chromates, phosphates, molybdates or wolframates. 
     
     
         9 . The method according to  claim 7 , wherein the binder system contains amphiphiles, low-molecular phenols, aromatic acids, surface-active alkyl phosphates and/or high-molecular polyelectrolytes. 
     
     
         10 . The method according to  claim 1 , wherein the particles of a metal powder in the slurry are coated with a silicon-containing compound. 
     
     
         11 . The method according to  claim 1 , wherein prior to the surface annealing, in which the aluminide phases that form the coating are formed, the substrate is heated to 400° C. 
     
     
         12 . The method according to  claim 1 , wherein the metallic powder contains at least one of the following elements in the indicated maximum concentrations: Si (10 wt %), Ge (20 wt %), Cr (25 wt %), Ti (2 wt %), Ta, V or Mo (5 wt % each), B (2 wt %), Fe (10 wt %), Co (20 wt %) and Ni (30 wt %). 
     
     
         13 . The method according to  claim 1 , wherein the substrate or the slurry contains tin (up to 30 wt %), Si (up to 10 wt %), Pt (up to 10 wt %), Mg (up to 20 wt %), Ca (up to 20 wt %) and additionally one or more elements from the group comprising: lanthanum, cerium, zirconium, hafnium and yttrium or oxides thereof (<1 wt %). 
     
     
         14 . The method according to  claim 1 , wherein the substrate is a steel or a nickel-based alloy, or in that the substrate is a nickel-plated surface of a component. 
     
     
         15 . The method according to  claim 2 , wherein the slurry contains the aluminum as a metal powder having a particle size of 5-100 μm. 
     
     
         16 . The method according to  claim 8 , wherein the binder system contains amphiphiles, low-molecular phenols, aromatic acids, surface-active alkyl phosphates and/or high-molecular polyelectrolytes. 
     
     
         17 . The method according to  claim 10 , wherein the silicon-containing compound is an alkoxide. 
     
     
         18 . The method according to  claim 17 , wherein the alkoxide is tetraethoxysilane as a silicon-containing precursor.