Process to mitigate grain texture differential growth rates in mirror-finish anodized aluminum

Abstract

Anodizing processes for providing durable and defect-free anodic oxide films, well suited for anodizing highly reflective surfaces, are described. In some embodiments, the anodizing electrolyte has a sulfuric acid concentration substantially less than conventional type II anodizing. In some embodiments, the electrolyte includes a mixture of sulfuric acid and one or more organic acids. In further embodiments, sulfuric acid is a relatively minor additive to an organic acid, primarily serving to minimize discoloration. The processes enables porous, optically clear, and colorless anodic films to be grown in a manner similar to conventional Type II sulfuric acid anodizing, but at lower current densities and/or higher temperatures, without compromising film surface hardness. The thickness uniformity of the resulting anodic oxide films can be within 5% between grains of {111}, {110} and {100} surface orientations. Furthermore, the anodic oxide films have minimal incorporated sulfates, thereby avoiding certain cosmetic and structural defects.

What is claimed is: 
     
         1 . A method of forming an anodic film, the method comprising:
 anodizing an aluminum alloy substrate in an electrolyte using a current density of no greater than 1 A/dm 2  and/or an electrolyte temperature of no less than 30 degrees C. such that the resultant anodic film has a hardness value of no less than 320 HV 0.05  and a thickness variation of less than 5% between the anodic film on grains having {111}, {110} and {100} crystallographic orientations.   
     
     
         2 . The method of  claim 1 , wherein the electrolyte has a sulfuric acid concentration no greater than 7% by weight. 
     
     
         3 . The method of  claim 1 , wherein the electrolyte comprises sulfuric acid and an organic acid. 
     
     
         4 . The method of  claim 3 , wherein after anodizing, the anodic film has an average concentration of sulfur of no greater than 4% by weight. 
     
     
         5 . The method of  claim 3 , wherein the organic acid comprises at least one of oxalic acid, glycolic acid, tartaric acid, malic acid, citric acid, and malonic acid. 
     
     
         6 . The method of  claim 3 , wherein the electrolyte has a higher concentration of organic acid compared to sulfuric acid. 
     
     
         7 . The method of  claim 6 , wherein the organic acid is oxalic acid, wherein the electrolyte has an oxalic acid concentration of at least 20 g/L and a sulfuric acid concentration between 5 g/L and 20 g/L, and wherein the resultant anodic film is grown to a thickness of at least 10 micrometers and is colorless with an a* of less than 1 and a b* of less than 1 as measured in accordance with CIE 1976 L*a*b* color space. 
     
     
         8 . The method of  claim 1 , wherein the aluminum alloy substrate comprises zinc, copper and/or magnesium. 
     
     
         9 . The method of  claim 1 , wherein the current density is no greater than 0.75 A/dm 2 . 
     
     
         10 . The method of  claim 1 , wherein the resultant anodic film is grown to a thickness of at least 10 micrometers and is colorless with an a* of less than 1 and a b* of less than 1 as measured in accordance with CIE 1976 L*a*b* color space. 
     
     
         11 . A method of forming an aluminum oxide coating, the method comprising:
 anodizing an aluminum or aluminum alloy substrate in an electrolyte with a sulfuric acid concentration ranging between 5 g/L and 70 g/L, wherein the electrolyte optionally includes one or more organic acids at an organic acid concentration ranging between 10 g/L and 100 g/L.   
     
     
         12 . The method of  claim 11 , wherein the electrolyte includes an organic acid, and wherein the electrolyte has a higher sulfuric acid concentration than organic acid concentration. 
     
     
         13 . The method of  claim 11 , wherein sulfuric acid concentration ranges between 5 g/L to 20 g/L and the organic acid concentration is at least 20 g/L, resulting in a colorless aluminum oxide coating with an a* of less than 1 and a b* of less than 1 as measured in accordance with CIE 1976 L*a*b* color space, and wherein the aluminum oxide coating is grown to a thickness of 10 micrometers or greater. 
     
     
         14 . The method of  claim 11 , wherein anodizing the aluminum or aluminum alloy substrate comprises anodizing using a current density no greater than 1 A/dm 2 . 
     
     
         15 . The method of  claim 11 , wherein anodizing the aluminum or aluminum alloy substrate comprises anodizing using a temperature ranging between 25° C. and 40° C. 
     
     
         16 . The method of  claim 11 , wherein after anodizing, the aluminum oxide coating has a hardness value of no less than 320 HV 0.05 . 
     
     
         17 . The method of  claim 11 , wherein after anodizing, the aluminum oxide coating has an average concentration of sulfur of no greater than 4% by weight. 
     
     
         18 . The method of  claim 11 , further comprising forming a highly reflective surface on the aluminum or aluminum alloy substrate, wherein after the anodizing the highly reflective surface is visible through the aluminum oxide coating and is uniform in thickness to within 5% on aluminum alloy substrate grains having {111}, {110} and {100} crystallographic orientations such that an interface between the aluminum or aluminum alloy substrate and the aluminum oxide coating is substantially free of indentations. 
     
     
         19 . A metal housing for an electronic device, the metal housing comprising:
 an aluminum alloy substrate comprising copper, zinc and/or magnesium; and   an anodic oxide comprising no greater than 4% by weight of sulfur, wherein the anodic oxide has a hardness value of no less than 320 HV 0.05 .   
     
     
         20 . The metal housing of  claim 19 , wherein the anodic oxide has a thickness variation of less than 5% on aluminum alloy substrate grains having {111}, {110} and {100} crystallographic orientations.