Processes to avoid anodic oxide delamination of anodized high strength aluminum alloys

Abstract

Methods of forming anodic oxide coatings on certain high strength aluminum alloys are described. Methods involve preventing or reducing the formation of interface-weakening species, such as zinc-sulfur compounds, at an interface between an anodic oxide coating and underlying aluminum alloy substrate during anodizing. In some embodiments, a micro-alloying element is added in very small amounts to an aluminum alloy substrate to prevent enrichment of zinc at the anodic oxide and substrate interface, thereby reducing or preventing formation of the zinc-sulfur interface-weakening species. In some embodiments, a sulfur-scavenging species is added to an aluminum alloy substrate to prevent sulfur from a sulfuric acid anodizing bath from binding with zinc and forming the zinc-sulfur interface-weakening species at the anodic oxide and substrate interface. In some embodiments, a micro-alloying element and a sulfur-scavenging species are added to an aluminum alloy substrate. Resultant anodic oxide coatings have minimal or no discoloration.

What is claimed is: 
     
         1 . An enclosure for an electronic device, the enclosure comprising:
 an aluminum alloy substrate including zinc, magnesium, and a micro-alloying element, wherein the micro-alloying element is added to a target concentration, wherein the target concentration is no more than 0.1 weight %; and   an anodic oxide formed on the aluminum alloy substrate, wherein the micro-alloying element is enriched at an interface between the aluminum alloy substrate and the anodic oxide.   
     
     
         2 . The enclosure of  claim 1 , wherein the aluminum alloy substrate includes additional elements other than zinc, magnesium, and the micro-alloying element, wherein the additional elements comprise:
 chromium at no more than 0.01 weight % concentration,   copper at no more than 0.01 weight % concentration,   manganese at no more than 0.01 weight % concentration,   zirconium at no more than 0.01 weight % concentration,   titanium at no more than 0.02 weight % concentration,   silicon at no more than 0.05 weight % concentration,   iron at no more than 0.08 weight % concentration, and   any other element at no more than 0.01 weight % concentration, to a total maximum of 0.1 weight % concentration of the additional elements.   
     
     
         3 . The enclosure of  claim 1 , wherein the anodic oxide has a thicknesses of at least 10 micrometers, wherein a color of the anodic oxide in a non-dyed state is within 1 b* value, and optionally within 0.5 b* value, based on D65 white spot color measurement convention. 
     
     
         4 . The enclosure of  claim 1 , wherein the aluminum alloy substrate has substantially balanced concentrations of zinc and magnesium for achieving an optimal strength through precipitation upon ageing. 
     
     
         5 . The enclosure of  claim 4 , wherein the aluminum alloy substrate comprises an atomic % of zinc that is about double the atomic % of magnesium so as to form MgZn 2  precipitates. 
     
     
         6 . The enclosure of  claim 4 , wherein the aluminum alloy substrate comprises a zinc concentration of about 5.5 weight % and a magnesium concentration of about 1 weight %. 
     
     
         7 . The enclosure of  claim 1 , wherein the micro-alloying element has a higher Gibbs free energy of oxide formation than the zinc. 
     
     
         8 . The enclosure of  claim 1 , wherein the micro-alloying element is selected from the group consisting of vanadium, germanium, cobalt, antimony, copper, tellurium, osmium, selenium, iridium, rhodium, palladium, silver, and gold. 
     
     
         9 . The enclosure of  claim 1 , wherein the micro-alloying element is selected from the group consisting of copper, silver, and antimony. 
     
     
         10 . The enclosure of  claim 1 , wherein the anodic oxide is grown to a thickness of at least about 10 micrometers using a Type II anodizing process, wherein the aluminum alloy substrate with the anodic oxide has a b* value of less than 1 based on CIE 1976 L*a*b* color measurement convention. 
     
     
         11 . The enclosure of  claim 1 , wherein the micro-alloying element includes more than one type of element. 
     
     
         12 . The enclosure of  claim 11 , wherein a first type of element reduces enrichment of the zinc at the interface and a second type of element offsets discoloration of the anodic oxide due to the presence of the first type of element. 
     
     
         13 . A method of forming an enclosure for an electronic device, the method comprising:
 anodizing an aluminum alloy substrate comprising zinc, magnesium, and a micro-alloying element, wherein the micro-alloying element is added to a target concentration, wherein the target concentration is no more than 0.1 weight %, wherein the micro-alloying element reduces enrichment of the zinc at an interface between the aluminum alloy substrate and a resultant anodic oxide, enrichment of the zinc at the interface associated with reducing an adhesion of the anodic oxide to the aluminum alloy substrate.   
     
     
         14 . The method of  claim 13 , wherein an adhesion strength of the resultant anodic oxide as measured by 5-by-5 array 10 kg Vickers indentations spaced 350 micrometers apart and as viewed by scanning electron microscope imaging is less than 10 detached regions of the anodic oxide. 
     
     
         15 . The method of  claim 13 , wherein the aluminum alloy substrate includes additional elements other than zinc, magnesium, and the micro-alloying element, wherein the additional elements comprise:
 chromium at no more than 0.01 weight % concentration,   copper at no more than 0.01 weight % concentration,   manganese at no more than 0.01 weight % concentration,   zirconium at no more than 0.01 weight % concentration,   titanium at no more than 0.02 weight % concentration,   silicon at no more than 0.05 weight % concentration,   iron at no more than 0.08 weight % concentration, and   any other element at no more than 0.01 weight % concentration, to a total maximum of 0.1 weight % concentration of the additional elements.   
     
     
         16 . The method of  claim 13 , wherein the anodic oxide is grown to a thicknesses of at least 10 micrometers, wherein a color of the anodic oxide in a non-dyed state is within 1 b* value, and optionally within 0.5 b* value, based on D65 white spot color measurement convention. 
     
     
         17 . The method of  claim 13 , wherein the aluminum alloy substrate has substantially balanced concentrations of zinc and magnesium for achieving an optimal strength through precipitation upon ageing. 
     
     
         18 . A method of forming an enclosure for an electronic device, the method comprising:
 anodizing an aluminum alloy substrate comprising zinc, magnesium, and a micro-alloying element, wherein the micro-alloying element is added to a target concentration, wherein the target concentration is no more than 0.1 weight %,   wherein the micro-alloying element reduces a discrepancy between growth rates of different portions of an anodic oxide on grains of distinct surface orientations, resulting in an anodic oxide having a thickness uniformity of within 5% between grains of {111} surface orientation and other surface orientations.   
     
     
         19 . The method of  claim 18 , wherein the aluminum alloy substrate includes additional elements other than zinc, magnesium, and the micro-alloying element, wherein the additional elements comprise:
 chromium at no more than 0.01 weight % concentration,   copper at no more than 0.01 weight % concentration,   manganese at no more than 0.01 weight % concentration,   zirconium at no more than 0.01 weight % concentration,   titanium at no more than 0.02 weight % concentration,   silicon at no more than 0.05 weight % concentration,   iron at no more than 0.08 weight % concentration, and   any other element at no more than 0.01 weight % concentration, to a total maximum of 0.1 weight % concentration of the additional elements.   
     
     
         20 . The method of  claim 16 , wherein the micro-alloying element is selected from the group consisting of vanadium, germanium, cobalt, antimony, copper, tellurium, osmium, selenium, iridium, rhodium, palladium, silver, and gold.