Canadian Finishing & Coatings Manufacturing - July/August '24 - 25

PLATING AND ANODIZING: TESTING AND ANALYSIS
like) and are usually clustered.
The anodizing tanks should be
regulated to keep the chloride
concentration below 50-80 ppm to
avoid chloride pitting.
Comet tails: These pits are unique
in the sense that one end is streaked
i.e. appears as though there is a pull
out in the direction of extrusion.
The source of these defects is likely
to come from the pretreatment
step: chemical polish. Any dried
residue from the polishing media
or foreign debris on the polishing
pad can cause these defects. Also,
gas marks due to hydrogen gas
evolution could possibly form pits
that resemble comet tails. Effective
cleaners
or
a
degreasing
step
should be incorporated to make
sure polishing residue or foreign
debris are thoroughly removed.
Crazing
is another
type
of
commonly occurring defect that
is caused due to difference in
coefficient of expansion/elasticity
between the aluminum metal and
its anodic oxide. These are fine
microcracks that are formed in the
anodic layer and are well noticeable
on dyed products. These fine
cracks are also formed due to stress
build up from severe mechanical
forming. During the fabrication
process,
aluminum
undergoes
mechanical forming by extrusion,
bending, pounding, rolling etc.
Therefore, to avoid microcracking,
any type of mechanical alternation
on the products must be done
before the anodization process. In
addition to mechanical forming, the
oxide thickness also contributes to
residual stress build up. Due to the
difference in elasticity between the
aluminum and the anodic oxide,
the residual stress build up is much
higher when the oxides are grown
to greater thicknesses. Therefore,
an optimal oxide thickness is to
be determined to avoid crazing.
Extreme temperature changes
between the anodizing and post
anodizing treatments can also
leads to high residual stresses
from thermal shock. Incorporating
stabilised heating/cooling between
process steps will help reduce
thermal shocks.
Case Study: Dye Penetration in Anodic Oxide
The cosmetic surfaces of anodized parts especially sporting goods or consumer
electronics are coloured with one or more dye components to enhance the
appearance of the finished products. Post anodization, the dye penetrates through
the porous anodic oxide which is then locked-in with a sealant. One common issue
that these industries face with dyed products is the consistency of colour on the
cosmetic surface. Numerous factors can contribute to this such as anodic layer
thickness, uniformity of the oxide layer, grain structure or the concentration of
precipitates in the base alloy, etc. Here we compare the relative dye component
penetration in Al-6061 that was thermally treated at different temperatures.
The two heat treatment conditions that are reported here are T1 (Reference/UnAged):
Cooled from an elevated temperature shaping process and naturally aged
to a stable condition, and T4 (Aged): After the shaping process, the part is solution
treated and naturally aged to a stable condition. Post the thermal treatments, the
parts were anodized under similar conditions. Using Raman spectroscopy, we
demonstrate the effects of aging (T1 and T4) on the penetration of an organic
dye package that includes both Blue and Green dye components. We obtained
depth profiles of the anodic oxide (up to 14 um from the surface) to qualitatively
compare the two samples.
Blue Dye Component: Comparing the Raman results in Figure 2a & 2b, the Blue
dye component is more deeply absorbed in both the Un-Aged and Aged samples.
In the Un-Aged (reference) sample the blue dye concentration is plateaued
to about 6 µm deep into the anodic oxide (Figure 2a). Following which the dye
penetration monotonically decreases as it approaches the interfacial region (i.e.
aluminum and anodic oxide interface). In the Aged sample (Figure 2b), the Blue
dye component shows a slight increase in concentration from the surface to about
2 µm into the anodic oxide and then flowed by a decreasing trend.
Green Dye Component: In the Un-Aged sample (Figure 2a, Reference sample),
the green dye appears to monotonically decrease in concentration from the
surface towards the interfacial region with a plateau at 8 µm depth. However, in
the Aged sample, the dye penetration profile has changed. Figure 2b, shows an
overall reduction in Green dye absorption into the anodic structure.
Post heat treatment, the overall uptake of dye and the ratio of dye components,
blue vs green, has a significant change. This results in a colour shift between
Un-Aged and Aged parts that may not be acceptable to the end-client. To further
investigate the microstructural changes that led to this change between the UnAged
and Aged samples, we would have to deep dive into studying the anodic
structure using much more advanced techniques such as SEM and TEM imaging.
With this we conclude our case study showing how a simple Raman spectroscopy
signal can be beneficial in studying the dye penetration trends within the anodic
oxide layer.
Figure 2: Here we have Raman spectra of organic dyes (Blue and Green) obtained as a
function of anodic oxide depth starting at surface to the metal-oxide interface (0 µm to
14 µm). Figure 2a shows the penetration of the two dyes in " T1:reference sample " while
Figure 2b shows the penetration of the two dyes in " T4:aged sample " . The red dashed line
at 0 µm is to indicate the surface of the anodized part.
Medha Veligatla is a material scientist and project manager at California-based
Eurofins | EAG Laboratories. She can be reached at medhaveligatla@eurofinsEAG.com
July/August 2024
25

Canadian Finishing & Coatings Manufacturing - July/August '24

Table of Contents for the Digital Edition of Canadian Finishing & Coatings Manufacturing - July/August '24