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Abstract:
The effect of surface treatments on the surface characteristics of
aluininium alloy 2024-T3 before the appearance of filiform corrosion
(FFC) is investigated. The nature of the surface prior to coating and
initiation of FFC, with particular respect to the intermetallics is
investigated in this work.
The SKPFM (Scanning Kelvin Probe Force Microscopy), ToF-SIMS (Time of
Flight Secondary Ion Mass Spectroscopy), XPS (X-Ray Photo Electron
Spectroscopy) and SEM (Scanning Electron Microscopy) surface analysis
techniques were used to characterize polished AA2024-T3 before and after
etching or after etching with subsequent chromating-treatments. The
etching pretreatment is intended to remove surface intermetallics and
increase the oxide layer thickness. In these respects, the treatment was
partially successful: some, not all, of the particles were eliminated
from the surface and the oxide thickness increased by about 25%. In
addition, XPS depth profiling showed a copper and iron enrichment at the
oxide-metal interface from this treatment.
The oxide thickness is drastically increased following the application
of the chromate conversion coating. Furthermore, XPS and ToF-SIMS
analysis revealed that Cu and Fe were enriched on the oxide surface
indicating that either Cu is complexed into the Cr-Al layer following
chromating or that the chromating coating is insufficiently formed on
the Cu-containing intermetallics. SKPFM analysis provided further
information on the distribution of these complexes on seen by distinct
points of high potential on the treated alloy surface.
