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

PLATING AND ANODIZING: TESTING AND ANALYSIS
anodization, these particles get lifted
to the surface and become part of the
transparent film or may even dissolve
during the process to leave voids. In
such cases, the continuity of the oxide
network is disrupted causing patchy
regions. This can be quite concerning
especially in case of dyed products
where such open regions with no oxide
growth will not undergo dyeing. These
open sites appear different from the
surrounding dyed area where the dye
is penetrated through the columnar
structure causing non-uniformity in
the coloured products.
To get a complete understanding
of the base alloy, it is critical to know
how the part was made and what
metallurgical forming processes
the part has undergone. Aluminum
components can be made using a
variety of methods including casting,
extrusion, machining, rolling, 3D
printing, etc. These forming methods
are each unique and occur at different
temperatures, thus having different
effects on the grain structure of the
material. Some methods can result in
elongated grains while others can result
in equiaxed coarse or fine grains, etc.
With such huge variations caused from
the different forming techniques, it is
critical to understand how the part is
made and select the pre-treatment/
anodizing steps accordingly. In addition
to the different forming techniques,
it is also worthwhile to know if the
alloy has undergone any thermal
treatments. This also plays a significant
role in altering the microstructure or
inducing or relieving internal stresses.
As mentioned, grain structures and
internal stresses can play a significant
part in inhibiting the growth of the
anodic oxide. Example: In higher grain
density alloys, due to misaligned atomic
ordering at the grain boundaries, these
sites will adversely interfere with the
quality of the oxide layer and disrupt
the continuity of the anodic oxide.
In addition to these intrinsic factors,
the surface finish of the interfacial
region also plays a significant part
in determining the quality of anodic
oxide. Machining/cutting/extrusion/
rolling defects, etc. or any mechanical
abrasions intervene in the growth of
the anodic layer as the integrity of the
anodic cell structure is compromised.
Lastly, to avoid any unforeseen
damages/defects on finished parts,
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keeping the aluminum parts pristine
and being mindful of where the parts
are being stored and how the parts are
being transported before or during
the anodizing process is of utmost
importance. During shelf time, before
anodizing, the metal surface is likely to
come in contact with various medium.
Often, these metal parts are stored in
wooden or cardboard boxes. The metal
surface that undergoes anodization
must be protected from any contact
with such medium. Protective measures
must be taken to guard the metal surface
from any inherent fumes generated
during the anodizing, humidity in the
air that can alter the native oxide in
select regions, oils from handling with
bare hands or cross contamination
from machining (oil or grease) etc. All of
these factors can either lead to surface
oxide alteration or leave stubborn
residues that can cause blemishes post
anodizing. Proper handling guidelines
such as handling parts with gloves,
making sure the facility has good air
circulation, etc. are to be maintained in
manufacturing sites to minimise such
cross contamination.
Common Occurring Defects
Observed on Post Anodized
Parts
Burn marks: Due to the nature of
these
defects,
they can be spotted
with visual inspection. These marks
literally show up as burn marks or
heavily grayed regions. Defects like
these are indicators of poor electrical
contact or a faster rate of anodizing.
This results in a non-uniform growth of
the oxide layer. Good electrical contact
is critical for a uniform growth of the
oxide layer. The contact must endure
the force from the growing oxide on
the surface which in turn starts to act
as an electrical insulator. Aluminum
metal is conductive while the oxide is
highly resistive. Too much current flow
will lead to immediate burning. With
this knowledge, the current must be
increased as the oxide grows thicker.
If the anodizing fixture is making poor
contact, because of the controlled
increase in voltage, there may be no
current flowing in the beginning of
the process. As the voltage increases,
the aluminum experiences a sudden
voltage breakthrough. At this point, due
to no oxide growth, the breakthrough
voltage is excessive on the conductive
aluminum thus leading to burn marks.
In addition to contact, excessive
current densities or insufficient
agitation in the electrolyte bath may
also cause anodizing burns. Checking
the temperature and the concentration
of the electrolyte bath may also be a
good way to assess the cause for burn
marks.
White spots: White spots on post
anodized and dyed parts is a common
issue faced in the industry. White
spots that can be wiped off are likely
to be precipitated salts from the post
anodizing steps i.e. sealants. Whereas
the ones that cannot be wiped off could
have been initiated by a variety of
sources. To start with, these white spots
can be perfectly rounded or randomly
shaped. An optical microscope will
come handy to check the shape of
these white spots. The rounded white
spots are likely to occur due to foreign
residue on the surface that did not go
through adequate cleaning before the
anodization step or due to tiny blisters
that popped during the anodizing
process. It is also worthwhile to check
for cleanliness of the pretreatment or
the anodizing tanks as their walls can
be a source for mold nucleation. If not
regularly maintained, fragments of
such mold can transfer and adhere to
the metal surface thereby inhibiting
the growth of the oxide layer. Another
factor that can contribute to white
spot defects is the vapourized etchant
or evolution of alkaline gases from the
etchant tanks. Good air
circulation
is important to make sure that these
alkaline gases exit the facility without
interacting with the pristine parts.
Pitting: A small hole or a cluster
of small holes may appear on the
finished parts due to corrosion from
alkali sources, acid sources, or from
excessive chlorides in the process.
Corrosion pits are typically surrounded
by discolouration. If any such deep
pits are observed before the etching
or anodizing steps, it is likely that the
aluminum has encountered some sort
of acid or caustic chemicals from cross
contamination. Better housekeeping
and storage need to be incorporated
to avoid such cross contamination.
Corrosion pits formed due to
excessive chloride contamination
have a distinctive appearance (star
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Canadian Finishing & Coatings Manufacturing - July/August '24

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