Final Report - Strategic Environmental Research and Development ...

Al (including both Al 0 and Al 3+ components) was found to be 0.43 and 1.98 for the surfaces after
etching by Process I and II, respectively.
The second step in the coating preparation, acidic desmutting, has been proven to be critical
because it alters the morphology and chemistry of the AA2024-T3 alloy surface and can have a
significant effect on the formation of the conversion coating. After 5 min of acid desmutting
using Process I, the surface exhibited pits hundreds of nanometers in diameter or larger over the
entire surface. A similar morphology was observed after 3 min of the Process II desmutting step.
XPS analysis performed after desmutting shows that a small amount of Si was still on the
surface. The Si/Al atomic concentration ratio was 2x10 -4 and 3x10 -4 for the surfaces after
desmutting by Process I and II, respectively.
5.1.4.4 Surface Morphology of TCP Coating. Figure 1.8 shows an SEM image of TCP coating
formed on the substrate of AA2024-T3 after 10 min exposure to the TCP bath following Process
I. The sample surface was entirely covered with round particles of 500 nm diameter, which
merged together to form a compact film. Some larger agglomerations of the small particles were
observed on the surface. This surface morphology is similar to what has been found for CCCs.
Furthermore, the TCP coating exhibited cracking similar to the mud-cracking that has been
observed in SEM images of CCC. The TCP coating peeled off the substrate at certain locations
due to the cracks. It has been shown that exposure to the vacuum during SEM promotes
dehydration of the CCCs rather rapidly. Similar cracking was observed with the Process II
pretreatment.
Figure 1.8. SEM micrograph of 10 min TCP on AA2024-T3 after Process I pretreatment.
The drying of a 10 min TCP coating formed on AA2024-T3 after 10 min immersion was studied
in the ESEM. ESEM imaging was started on a freshly coated and still wet sample surface so that
the initiation and evolution of mud-crack morphology could be monitored in-situ. Two major
parameters in the ESEM chamber, pressure and temperature, were well controlled to maintain
water in the liquid phase before imaging. Then they were adjusted in a manner that water
evaporated gradually and the TCP dried slowly for imaging of film crack initiation. Under the
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