Canadian Finishing & Coatings Manufacturing - July/August '24 - 23

PLATING AND ANODIZING: TESTING AND ANALYSIS
ANDCAUSE
M
ultiple factors contribute to the quality of
a finished anodized product. The choice of
aluminum alloy, the manufacturing process
of the alloy and any thermal treatments the
alloy undergoes, are a few of the preliminary factors that are
to be considered in determining the quality of anodic oxide.
Additionally, the interfacial surface where the oxide starts
growing is also non-trivial. Other major factors that play
a vital role come from the series of steps that the product
goes through during the anodizing process. These include
using single or multiple tanks, cross contamination from the
different steps during the process, the choice of cleaners, or
etchants and other pre-treatment solutions based on the alloy
composition.
With the current advancements and the knowledge that
we have, there are various characterization tools that can be
used to find out the root cause of the defects formed during
the manufacturing process. Characterization can be as simple
as a visual inspection (macro) and can go to the micron or
even nanometer level details based on the depth of analysis.
The first step would be to visually inspect the parts for any
defects that stand out such as discolouration, pitting, spalling,
crazing, burns etc. The most useful tool for a manufacturing
facility would be to invest in a high-resolution optical
microscope. This will come handy in examining the surface
level defects (micro scale) and studying the microstructure
of the substrate alloy. Not just examining the surface defects
and microstructure in a top-down orientation, but a cross
section view of the anodized product will reveal the thickness
of the oxide layer and give a global view of the precipitate
distribution. In most manufacturing units, consistently
DEFEC T
Basic defect characterization and preventive measures for anodized aluminum parts
BY MEDHA VELIGATLA
maintaining the oxide thickness between batches is important
to meet the product specifications. This is especially true for
products that require a decorative or cosmetic specification
including certain optical transparency or products that
undergo colour treatment/dying. Figure 1 illustrates a cross
sectional view of an anodized part. As seen in Figure 1a, the
anodic oxide is on the top and bottom surface of the aluminum
alloy. A high magnification of the top anodic layer is shown
in Figure 1b. The measured thickness of the oxide layer is
approximately ≈ 22 µm.
A systematic evaluation of the anodizing process is critical
to make sure the finished product meets the specifications.
A thorough inspection needs to be conducted at every stage
and the first stage being the examination of the substrate
alloy. This shall include the thorough understanding of the
aluminum alloy i.e. the foundation of the finished product.
Examination at this stage must include gathering information
or real data on the chemical composition of the aluminum
alloy and most importantly knowing the composition and
quantification of the secondary phases/particles. Example:
2xxx series alloys with Cu: As Cu is expected to dissolve during
the anodizing process, higher concentration of Cu causes
formation of a dull/gray finish on the surface. It is of utmost
importance to have an understanding of the base metal to
this level as it has been observed that aluminum alloys with
high concentration of alloying constituents form a heavy
gray smut during the etching step of anodizing process. Also,
the presence of excessive precipitates near the interfacial
surface (where the anodic oxide grows) can interfere with
the continuity of the columnar/lateral-cellular growth of
the oxide. As the oxide grows into the metal surface, during
Figure 1: Optical images
demonstrating a mechanically
polished cross-sectional view of an
anodized aluminum part. Both the top
and bottom surface of the aluminum
part have been anodized. (a) Low
magnification, both ends of the part
have a 20µm thick anodic oxide.
(b) High magnification, the anodic
oxide on the top has been measured.
The image also shows how the
precipitates are dispersed across the
cross section.
(a) Low magnification, Optical image (b) High magnification, Optical image
July/August 2024
23
Canadian Finishing & Coatings Manufacturing - July/August '24

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