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

Abstract

Methods of forming anodic oxide coatings on 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 . A part, comprising:
 an aluminum alloy substrate including zinc as an alloying element; and   an anodic oxide coating formed on the aluminum alloy substrate, wherein the zinc is enriched at an interface between the anodic oxide coating and the aluminum alloy substrate, wherein the aluminum alloy substrate includes a sulfur-scavenging species at a sufficient concentration to bind with sulfur species within the anodic oxide coating preventing at least some of the enriched zinc from forming a zinc-sulfur compound at the interface, the zinc-sulfur compound associated with reducing an adhesion strength between the anodic oxide coating and the aluminum alloy substrate.   
     
     
         2 . The part of  claim 1 , wherein the aluminum alloy substrate includes additional elements other than zinc, magnesium, and the sulfur-scavenging species, 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 part of  claim 1 , wherein the anodic oxide coating has a thickness of at least about 10 micrometers and is characterized as having b* color space parameter value between −1 and 1, optionally between −0.5 and 0.5, as defined by CIE Standard Illuminant D65 white spot standard. 
     
     
         4 . The part of  claim 1 , wherein an adhesion strength of the anodic oxide coating 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 coating. 
     
     
         5 . The part of  claim 1 , wherein the aluminum alloy substrate has a yield strength of at least 330 MPa in a T6 temper. 
     
     
         6 . The part of  claim 1 , wherein the sulfur-scavenging species is selected from the group consisting of lithium, magnesium, calcium, strontium, barium, scandium, and yttrium. 
     
     
         7 . The part of  claim 1 , wherein a concentration of the sulfur-scavenging species ranges from about 0.5 weight % and about 3 weight %. 
     
     
         8 . The part of  claim 1 , wherein magnesium is added in excess over a balanced ratio for magnesium-zinc precipitate formation so as to eliminate or reduce a concentration of non-precipitated zinc in the aluminum alloy substrate in a T6 or T7 temper, thereby reducing a discrepancy between growth rates of different portions of the anodic oxide coating on grains of distinct surface orientations, resulting in the anodic oxide coating having a thickness uniformity of within 5% among grains of {111} surface orientation and other surface orientations. 
     
     
         9 . The part of  claim 8 , wherein a concentration of the magnesium is in excess of a stoichiometric amount required to combine with the zinc to form η-MgZn 2  precipitates, wherein at least some of the excess magnesium binds with the sulfur species within the anodic oxide coating preventing at least some of the zinc from forming a zinc-sulfur compound at an interface between the anodic oxide coating and the aluminum alloy substrate. 
     
     
         10 . The part of  claim 1 , wherein the aluminum alloy substrate comprises magnesium, wherein an atomic concentration of the magnesium is at least half an atomic concentration of the zinc. 
     
     
         11 . The part of  claim 1 , wherein the part is an enclosure for an electronic device. 
     
     
         12 . The part of  claim 1 , wherein the aluminum alloy substrate is comprised of a 7000-series aluminum alloy. 
     
     
         13 . A method of anodizing an aluminum alloy substrate comprising zinc, the method comprising:
 anodizing the aluminum alloy substrate in a sulfuric acid-based solution, wherein a sulfur species from the sulfuric acid-based solution becomes incorporated within a resultant anodic oxide coating, wherein some of the zinc becomes enriched at an interface between the anodic oxide coating and aluminum alloy substrate during the anodizing,   wherein the aluminum alloy substrate includes a sulfur-scavenging species that binds with the sulfur species preventing at least some of the enriched zinc from forming a zinc-sulfur compound at the interface, the zinc-sulfur compound associated with reducing an adhesion strength between the anodic oxide coating and the aluminum alloy substrate.   
     
     
         14 . The method of  claim 13 , wherein the sulfur-scavenging species is selected from the group consisting of lithium, magnesium, calcium, strontium, barium, scandium, and yttrium. 
     
     
         15 . The method of  claim 14 , wherein a concentration of the sulfur-scavenging species ranges from about 0.5 weight % and about 3 weight %. 
     
     
         16 . The method of  claim 13 , wherein a concentration of the zinc within the aluminum alloy substrate is at least 4 weight %. 
     
     
         17 . The method of  claim 13 , wherein the aluminum alloy substrate includes magnesium as another alloying element, wherein an atomic concentration of the magnesium is at least half an atomic concentration of the zinc. 
     
     
         18 . An enclosure for an electronic device, the enclosure comprising:
 an aluminum alloy substrate including zinc and magnesium; and   an anodic oxide coating formed on the aluminum alloy substrate, the anodic oxide coating including a sulfur species incorporated therein, wherein some of the sulfur species is bonded with a sulfur-scavenging species that prevents the sulfur species from binding with the zinc.   
     
     
         19 . The enclosure of  claim 18 , wherein the aluminum alloy substrate with the anodic oxide coating is characterized as having a CIELAB b* color space parameter value between −1 and 1. 
     
     
         20 . The enclosure of  claim 18 , wherein the sulfur-scavenging species is selected from the group consisting of lithium, magnesium, calcium, strontium, barium, scandium, and yttrium.