Trivalent chromium conversion coating and method of application thereof

Abstract

The present invention is directed to a high protection, trivalent chromium coating composition that is particularly useful as a conversion coating on substrates in need of corrosion protection. The conversion coating composition comprises chromium (III) ions, cobalt (II) ions, nitrate ions, and sulfate ions. The invention further provides methods of applying a conversion coating to a plated article and articles with a conversion coating applied thereto.

1 . A conversion coating composition comprising: 
 (a) at least about 0.1 moles/L of chromium (III) ions;    (b) at least about 0.4 moles/L of nitrate ions;    (c) cobalt (II) ions; and    (d) sulfate ions.    
   
   
       2 . The conversion coating composition of  claim 1 , comprising at least about 0.2 moles/L of chromium (III) ions.  
   
   
       3 . The conversion coating composition of  claim 1 , comprising at least about 0.6 moles/L of nitrate ions.  
   
   
       4 . The conversion coating composition of  claim 1 , wherein the concentration of cobalt (II) ions is at least about 0.01 moles/L.  
   
   
       5 . The conversion coating composition of  claim 4 , wherein the concentration of cobalt (II) ions is at least about 0.02 moles/L.  
   
   
       6 . The conversion coating composition of  claim 1 , wherein the concentration of sulfate ions is at least about 0.1 moles/L.  
   
   
       7 . The conversion coating composition of  claim 6 , wherein the concentration of sulfate ions is at least about 0.2 moles/L.  
   
   
       8 . The conversion coating composition of  claim 1 , wherein said composition is free of fluoride ions.  
   
   
       9 . The conversion coating composition of  claim 1 , wherein said composition is substantially free of chelators.  
   
   
       10 . The conversion coating composition of  claim 1 , wherein the ratio of nitrate ions to the combination of chromium (III) ions and cobalt (II) ions is greater than 1.5:1.  
   
   
       11 . The conversion coating composition of  claim 1 , further comprising one or more additional components selected from the group consisting of silicate-containing compounds and halogenated carboxylic acids, or salts or esters thereof.  
   
   
       12 . The conversion coating composition of  claim 11 , wherein said silicate-containing compounds are selected from the group consisting of clays and magnesium hexafluorosilicate.  
   
   
       13 . The conversion coating composition of  claim 12 , wherein said clays comprise nanoparticulate clays.  
   
   
       14 . The conversion coating composition of  claim 11 , wherein said halogenated carboxylic acids are selected from the group consisting of halogenated acetic acids and salts or esters thereof.  
   
   
       15 . The conversion coating composition of  claim 14 , wherein said halogenated acetic acids comprise trifluoro acetic acid, trichloro acetic acid, or salts or esters thereof.  
   
   
       16 . The conversion coating composition of  claim 1 , further comprising up to about 1.0 moles/L of chloride ions.  
   
   
       17 . The conversion coating composition of  claim 1 , wherein the chromium (III) ions and the cobalt (II) ions are supplied as metal salts.  
   
   
       18 . The conversion coating composition of  claim 17 , wherein the metal salt comprises a sulfate metal salt.  
   
   
       19 . The conversion coating composition of  claim 1  comprising: 
 (a) about 0.1 moles/L to about 0.4 moles/L of chromium (III) ions;    (b) about 0.02 moles/L to about 0.09 moles/L of cobalt (II) ions;    (c) about 0.4 moles/L to about 1.5 moles/L of nitrate ions; and    (d) about 0.2 moles/L to about 1.0 moles/L of sulfate ions;    wherein the composition is free of fluoride ions and chelators.    
   
   
       20 . A method for applying a conversion coating to an article having an exposed surface, said method comprising contacting the exposed surface of the article with a conversion coating composition according to  claim 1 .  
   
   
       21 . The method of  claim 20 , wherein the exposed surface of the article is plated with zinc or a zinc alloy.  
   
   
       22 . The method of  claim 20 , wherein the pH of the conversion coating composition is between about 2.0 and about 3.0.  
   
   
       23 . The method of  claim 20 , wherein the temperature of the conversion coating composition is between about 20° C. and about 70° C.  
   
   
       24 . The method of  claim 20 , wherein said contacting step is carried out for a time of up to about 90 seconds.  
   
   
       25 . The method of  claim 20 , wherein said contacting step comprises immersing the article in the conversion coating composition.  
   
   
       26 . An article having an exposed surface with a conversion coating applied thereto, wherein said conversion coating is applied according to the method of  claim 20 .  
   
   
       27 . The article of  claim 26 , wherein the exposed surface of the article is plated with zinc or a zinc alloy.  
   
   
       28 . The article of  claim 26 , wherein the conversion coating applied to the exposed surface is effective for providing anti-corrosion protection such that the conversion coating, when subjected to a salt spray according to ASTM testing method B 117-03, is resistant to formation of white salts corrosion products for a time of at least about 200 hours.  
   
   
       29 . The article of  claim 28 , wherein the conversion coating applied to the exposed surface is resistant to formation of white salts corrosion products for a time of at least about 300 hours.