Stainless steel electrolytic coating

Abstract

Stainless steel coatings are formed on metallic substrates by employing the metallic substrates as a cathode in an aqueous electrolyte containing chromium, nickel and iron in specified concentrations or ratios. A coumarin leveling agent is preferably used. A product is obtained having a coating comprising, in weight ratios, weight ratios chromium 6-15:nickel 3-5:iron 32-39; the product has the appearance and many of the properties of monolithic stainless steel.

1 . Method of forming a coating on a metallic substrate comprising employing said metallic substrate as a cathode in an aqueous electrolytic bath comprising 35-65 grams per liter chromium chloride, 9-15 grams per liter nickel chloride, and 4-8 grams per liter ferrous chloride, and a leveling agent in an amount effective to enhance brightness in said coating, and subjecting said bath to an electric current effective to deposit chromium, nickel and iron as a coating on said substrate while maintaining said metallic substrate in turbulent contact with said bath.  
     
     
         2 . Method of  claim 1  wherein said chromium in said bath is maintained substantially entirely in the trivalent state.  
     
     
         3 . Method of  claim 1  wherein said current is 50-500 amperes per square foot of surface on said substrate.  
     
     
         4 . Method of  claim 1  wherein said bath is maintained at a pH of 0.8 to 2.0.  
     
     
         5 . Method of  claim 1  wherein said leveling agent comprises a coumarin leveling agent.  
     
     
         6 . Method of  claim 1  wherein said bath includes a stress reliever.  
     
     
         7 . Method of  claim 1  wherein said metallic substrate comprises carbon steel.  
     
     
         8 . Method of  claim 7  wherein said carbon steel has a precoat comprising nickel.  
     
     
         9 . Method of  claim 7  conducted continuously, wherein said carbon steel is in the form of steel strip, said steel strip is passed continuously through said aqueous electrolytic bath.  
     
     
         10 . Method of  claim 9  wherein said chromium chloride, nickel chloride, and ferrous chloride are replenished continuously or intermittently to maintain said grams per liter.  
     
     
         11 . Method of  claim 9  including continuously or intermittently removing at least one breakdown product of said leveling agent and continuously or intermittently replenishing said leveling agent.  
     
     
         12 . Method of  claim 7  wherein said carbon steel is formed into an incipient article prior to employing it as the cathode in said bath.  
     
     
         13 . Method of  claim 1  wherein said coating comprises 12-30% chromium, 6-10% nickel, and 64-78% iron.  
     
     
         14 . Method of  claim 13  wherein said coating comprises 17-20% chromium, 7-9% nickel, and 71-76% iron.  
     
     
         15 . Method of  claim 1  followed by removing said metallic substrate from said bath and heat treating it whereby said metallic substrate has a corrosion resistance similar to that of stainless steel.  
     
     
         16 . Method of  claim 9  wherein said bath includes a stress reliever in an amount effective to inhibit the formation of microcracks in said coating, followed by forming a three-dimensional steel product from a portion of said steel strip, said three-dimensional steel product being substantially free of microcracks.  
     
     
         17 . Method of  claim 1  wherein the anode in said aqueous electrolytic bath comprises stainless steel and wherein at least a portion of said chromium, said nickel, and said iron deposited from said bath is replenished sacrificially from said stainless steel anode.  
     
     
         18 . A product made by the method of  claim 15 .  
     
     
         19 . Steel having a coating comprising 12-30% chromium, 6-10% nickel, and 64-78% iron, said coating being substantially free of microcracks, having a brightness similar to monolithic stainless steel, superior adherence as demonstrated in fold and drop tests, and a substantially consistent distribution of chromium, nickel and iron throughout said coating.  
     
     
         20 . Steel of  claim 19  wherein said coating comprises 16-22% chromium, 7-9% nickel, and 71-76% iron.  
     
     
         21 . Steel of  claim 19  wherein said coating is 1-100 micrometers thick.  
     
     
         22 . Steel of  claim 19  having an undercoat of nickel.  
     
     
         23 . An article made of the steel of  claim 19 .  
     
     
         24 . An aqueous solution useful as an electrolytic bath for depositing a coating of stainless steel on a metallic substrate comprising 35-65 grams per liter chromium chloride, 9-15 grams per liter nickel chloride, and 4-8 grams per liter ferrous chloride, said solution having a pH of 0.8 to 2.0 and including a small amount of hydrofluosilicic acid and a small amount of a coumarin leveling agent.  
     
     
         25 . An aqueous solution of  claim 24  including at least one of (a) ammonium chloride in an amount up to 100 g/L, (b) potassium chloride in an amount up to 60 g/L, and (c) citric acid in an amount up to 100 g/L.  
     
     
         26 . Method of forming a stainless steel coating on a steel substrate, said stainless steel coating comprising chromium, nickel and iron within the range of weight ratios chromium 6-15:nickel 3-5:iron 32-39 comprising electrolytically depositing said metals from an aqueous electrolyte containing chromium, nickel and iron dissolved in weight ratios of chromium 53.4-79.12:nickel 12.9-31.8:iron 6.4-19.9.  
     
     
         27 . Method of  claim 26  wherein said aqueous electrolyte also includes a coumarin leveling agent in an amount effective to improve brightness of said coating.  
     
     
         28 . Method of  claim 27  which is continuous and wherein a breakdown product of said coumarin leveling agent is continuously or intermittently removed from said aqueous electrolyte.  
     
     
         29 . Method of  claim 26  wherein said chromium, said nickel, and said iron are present in said aqueous electrolyte substantially entirely in the form of chlorides.  
     
     
         30 . Method of  claim 26  wherein said chromium in said aqueous electrolyte is substantially entirely trivalent.