Final Report - Strategic Environmental Research and Development ...

powder, slurried with ultrapure water, on a felt polishing pad. This was followed by another 20
min ultrasonic cleaning in ultrapure water. Polishing was performed until the surface exhibited a
mirror-like finish with no visible polishing striations.
A polished or unpolished plate was then degreased for 10 min. in Turco 6849 TM (Henkel Corp.,
20 % v/v) at 55 o C. This was followed by rinsing with tap water for 2 min. The sample was then
deoxidized for 2 min. in Turco Liquid Smut-Go NC TM (Henkel Corp., 20% v/v) at room
temperature (19-23 o C). This was again followed by rinsing with tap water for 2 min. Finally, the
cleaned and deoxidized sample was coated with TCP for 10 min. (Alodine TM T5900, Henkel
Corp., 100% RTU) at room temperature (immersion coating). While the actual Alodine TM T5900
composition is proprietary, it contains potassium hexafluorozirconate (K 2 ZrF 6 ), sulfate (SO -2 3 ),
fluoroboric acid (HBF 4 ), and a chrome (III) salt. The pH of this solution was 3.85. The coated
sample was again rinsed with tap water for 2 min., then with ultrapure water for 30 s, and
allowed to air dry. For reasons that remain unclear, the tap water rinses are critical for proper and
reproducible TCP film formation on this alloy. It could be that the more basic pH of the tap
water (~6.8), as compared to the ultrapure water (~5.8), aids in the formation of the hydrated
zirconia coating (see below). In most cases, the coated samples were dried overnight at room
temperature in a covered storage dish prior to any electrochemical or structural characterization.
Alodine TM 5200 (Henkel Corp.) was also coated (immersion) on some polished AA2024 samples,
for comparison. This is a hexafluorozirconate coating similar to the T5900, but it contains no
added chromium salt. Samples receiving this coating were prepared by degreasing and
deoxidation, as described above. The 5200 solution was diluted with ultrapure water to yield a
3% (v/v) solution. The pH of this solution was 2.74. The alloy sample was then immersed for 2
min. at room temperature (21-25 o C) to form the coating. The coated sample was then removed,
rinsed with tap water for 30 s and dried overnight at room temperature in a covered dish before
use.
In other experiments, AA2024-T3 samples were coated using two procedures. Process I
consisted of: (i) etching for 15 min at 55 o C in 15% (v/v) Turco 6849 TM , (ii) desmutting for 5 min
at room temperature in 20% (v/v) Liquid Turco Smut-go NC TM , (iii) coating with TCP in
Alodine TM 5900S bath for 5 min at room temperature. These solutions are products of Henkel
Corp., Madison Heights, MI. Turco 6849 contains nonylphenoxypoly ethanol, sodium xylene
sulfonate, ethanolamine and sodium tripolyphosphate and has a pH of 11.7~12.2. Smut-go NC
solution contains ferric sulfate, nitric acid and sodium bifluoride and has a pH ~ 0. Alodine TM
5900S is a fluorozirconate based bath. Deionized water rinsing was performed between each step.
Samples were given a final rinse and blown dry with compressed air at room temperature (RT) at
least 1 h prior to use if not mentioned otherwise. As a comparison, other samples were treated
using the second treatment procedure, Process II, consisting of: (i) etching at 65 o C for 2 min in a
solution of 32.4 g/l Na 2 SiO 3 + 48 g/l Na 2 CO 3 with pH 13.4, (ii) desmutting at 55 o C for 3 min in a
solution of 72 ml/l HNO 3 + 30 g/l Sanchem 1000 (sodium bromate based, Sanchem Inc.,
Chicago, IL) with pH 0.25 and (iii) conversion coating at RT for 5 min in Alodine TM 5900S bath.
Samples were rinsed with deionized (DI) water after each step and blown dry with compressed
air at RT at the end. The control samples in polarization experiments were prepared by
ultrasonic cleaning in ethyl alcohol to eliminate organic contamination. This condition is termed
“as received” hereafter.
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