Porous Anodic Film Formation on an Al-3.5 wt % Cu Alloy

Abstract

Anodic film growth has been undertaken on an electropolished Al-3.5 wt % Cu alloy to determine the influence of copper in solid solution on the anodizing behavior. At the commencement of anodizing of the electropolished alloy, in the presence of interfacial enrichment of copper, Al 3ϩ and Cu 2ϩ ions egress and O 2Ϫ ion ingress proceed; film growth occurs at the alloy/film interface though O 2Ϫ ion ingress, with outwardly mobile Al 3ϩ and Cu 2ϩ ions ejected at the film/electrolyte interface, and field-assisted dissolution proceeding at the bases of pores. Oxidation of copper, in the presence of the enriched layer, is also associated with O 2 gas generation, leading to development of oxygen-filled voids. As a result of significant pressures in the voids, film rupture proceeds, with electrolyte access to the alloy, dissolution of the enriched interfacial layer and re-anodizing. The consequence of such processes is the development of anodic films of increased porosity and reduced efficiency of film formation compared with anodizing of superpure aluminum under similar conditions.

Strategies for the replacement of chromic acid anodising for the structural bonding of aluminium alloys

“…The presence of the second phase particles including Cu-Al within the 2024-T3 alloy is the reason for the nodular structure. Work using an Al-3.5% Cu binary alloy has displayed the same nodular structure when anodised in sulphuric acid 88 .…”

“…In the 2xxx series alloys of interest in the present study, the copper-rich second phases affect adversely the film morphology and integrity, especially due to the formation of relatively large voids in the film 79,88 . Oxidation of copper and incorporation of Cu 2+ into the anodised alumina are also associated with O 2 gas evolution causing local film cracking and can lead to the nonuniform flow of ionic current through the film, due to the significant volume of O 2 bubbles within the anodic film 88,91 .…”

“…In the 2xxx series alloys of interest in the present study, the copper-rich second phases affect adversely the film morphology and integrity, especially due to the formation of relatively large voids in the film 79,88 . Oxidation of copper and incorporation of Cu 2+ into the anodised alumina are also associated with O 2 gas evolution causing local film cracking and can lead to the nonuniform flow of ionic current through the film, due to the significant volume of O 2 bubbles within the anodic film 88,91 . Voids can also be attributed to the high magnesium content of the Sphase particles in 2024 alloy as during film formation the magnesium species migrate up to three times faster than Al 3+ ions and are lost to the electrolyte on reaching the surface of the anodic film and in so forming voids at the alloy/film interface 92 .…”

Effects of potential and heat treatment on phase transition of alumina dielectric layer

“…At the central area of the anodic film the ratio is in good accord with the ideal as represented in Table 1, but at the upper and lower areas the values are deviated from the ideal. According to the growth mechanisms of anodic oxide layers [16,17], Al 3+ from aluminum substrate migrates outward to the direction of electrolyte/oxide interface through oxide layer because of strong electric field by an applied voltage, and, likewise, O 2− incorporated into the layer from electrolytes moves inward to the aluminum substrate. Al 3+ and O 2− ions transported from each side combine to form Al 2 O 3 in the mid area of the existing oxide, and, as further reaction takes place, the product layer becomes thicker.…”

The influence of substrate material on structure and properties of anodic oxide coatings from oxalic acid

“…Lokale Ausscheidungsanreicherungen bewirken die ungleichmäßige Rücklösung der Porenwände und eine räumlich inhomogene Schichtbildungsrate lenkt die Wachstumsrichtung der Poren aus der Schichtnormalen. Dabei bestimmt die Größe der Ausscheidungen signifikant die lokale Porenstruktur [4][5][6][7][8][9][10].…”