Alkaline oxide conversion coatings for aluminum alloys Page: 4 of 14
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A MODIFIED PROCESS FOR ALUMINUM ALLOYS CONTAINING COPPER
In many aqueous surface finishing procedures including degreasing, deoxidizing and conversion
coating, the presence of copper in the alloy substrate is problematic. During these processes, the surfaces
of work pieces become enriched with a variety of copper compounds, occasionally including metallic
Cu, that are collectively referred to as "smut". Cu smut forms in etching alkaline degreasing solutions
where Cu solubility is low 18. Less frequently, it is observed to accumulate on surfaces during
deoxidation where aggressive chemical action intended to remove surface oxides attacks the underlying
alloy substrate7. Metallic copper deposits form because the open circuit potential for the alloy substrate
is negative to the reduction potential for Cu. Cu smut interferes with conversion coating formation19,
anodization20 and with bonding of subsequently applied adhesives and paints 19,21,22. It is also suspected
to contribute to increased susceptibility to corrosion during service due to galvanic coupling with Cu-
rich surface regions.
A possible remedy to this situation has been identified for conversion coating processes using
alkaline Li salt solutions. A modification to the basic process enables simultaneous coating formation
and Cu removal. This process modification is based on the fact that it is thermodynamically possible to
use complexing reactions to extract and retain Cu in aqueous solution, and that Cu solubility increases
appreciably in very alkaline solutions. Coatings formed using the modified coating process offer good
corrosion resistance on 2024-T3 (Al-4.4Cu-2.5Mg-0.6Mn) in both electrochemical and exposure
corrosion testing (salt spray).
High corrosion resistance is due in part to removal of Cu compounds from the coating. The
removal of Cu from the surface film can be explained by copper solubility in alkaline solutions and by
copper complex formation by carbonate. The solubility minimum for Cu occurs at pH 9.8. At pH 11.5,
the pH of the first stage coating bath, Cu solubility is less than 10-8 M. Cu enrichment in the surface film
during exposure to this solution is therefore expected. At pH 13.5, Cu solubility increases to
approximately 5 x 10-4 M. Since hydrotalcite formation is still possible at this pH, a Cu-free film forms.
Removal of Cu is assisted by Cu complex formation which further increases the total solubility for Cu2+.
At pH 13.5 the following complex formation process is possible:
Cu2+ + 2C032- -+ Cu(C03)22- (aq) (eq. 1)
Kform= 9.83 +0.04.
To illustrate how this modification to the process works, it is helpful to compare it to the more
mature variant that offers good corrosion resistance on Al, Al-Mg and Al-Mg-Si alloys, but does not
perform well on Al-Cu-Mg alloys. This process, which will be referred to as the "basic" process, is
formed in a solution containing 7.4 g/L Li2CO3 plus 200 to 400 ppm Na2A1204 with a pH of 11.2 to 11.8
at 50 50 C for 15 minutes. In the modified process, the basic process is carried out followed
immediately by a second immersion step carried out in a solution containing 7.4 g/L Li2CO3, 7.2 g/L
LiOH and 11.1 g/L Na2A1204 with a pH of 13.5. This solution is also operated at a temperature of 50t50
C and immersion times range from 15 to 180 minutes. The modified process has been used to generate
highly corrosion resistant coatings on 2024-T3.
Figure 6 is a scanning electron micrograph (SEM) of a surface of a 2024-T3 sample after
hydrotalcite coating with the basic process. The surface is featureless except for cracks that formed by
shrinkage as the film dried. Figure 7 shows the surface morphology after coating according to the
modified process. In this case, the distinctive surface morphology associated with hydrotalcite coatings
is observed23. Grazing incidence angle x-ray diffraction of the coating formed by the basic process
shows that the primary compound in the coating is bayerite, Al(OH)3. However, coating formed using
the modified process contains hydrotalcite with no detectable amounts of bayerite.
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Buchheit, R.G. Alkaline oxide conversion coatings for aluminum alloys, article, February 1, 1996; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc664546/m1/4/: accessed October 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.