Page 22 - CFCM May-June 2021_Neat

PLATING AND ANODIZING: ANODIZING PRETREATMENT

removed oil will begin splitting out of solution and the cleaner will no toxicity and their use has been outright banned in many regions.
longer be effective. The bath must be cut and topped up with fresh After the aluminum part is cleaned, the part will typically enter an
solution or dumped once this saturation point is reached. acid etch sequence or caustic etch sequence depending on the appli-
Non-emulsifying cleaners on the other hand, typically have an in- cation and the desired finish. Acid etches have become the standard
definite solution life. These cleaners are more labor-intensive since for matte finishes and can produce a consistent finish while boasting
the removed oil must be continuously skimmed to sustain the inte- economical results. An acid etch uses fluoride ions in solution to
grity of the cleaner. Both cleaners are typically run at elevated tem- remove minor defects like flow lines and die lines on aluminum
peratures to assist with the removal of oils. At higher temperatures, parts. The fluoride will attack the grain boundaries between
oils will be less viscous and detergents in the soak cleaner will be aluminum crystallites which effectively smoothens the surface and
more effective at removing the oil from the surface and promote provides a matte finish.
water shedding from the part. Grain boundary defects are often associated with corrosion sites
Three chemicals to avoid in soak cleaners include metal che- and can impede mechanical strength, so an acid etch has important
lators, silicates and cleaning agents like alkylphenols. Metal che- effects on the substrate in addition to appearance. Acid etches are
lators, such as ethylenediaminetetraacetic acid (EDTA), are also effective at maintaining tight material tolerances due to the rate
employed in some soak cleaners because they are so effective at re- at which they remove aluminum. Since an acid etch primarily
moving metal contaminants from the surface of a part. However, attacks grain boundaries, the etch will remove approximately 1 g/ft2
metal chelators are notoriously difficult to waste-treat and it is best of aluminum in 10 minutes, which is a tenth of the aluminum a tradi-
to avoid them whenever possible. tional caustic etch will remove. Using an acid etch will cut etching
Silicates are effective detergents and can inhibit attacks on alu- times, reduce the aluminum sludge produced and produce more
minum, however a residual siliceous film can form over that part and consistent parts making it an essential part of cleaning aluminum.
cause nonuniform appearance especially visible after electroless Following an acid etch, the parts will typically enter a caustic neu-
nickel plating. Again, unless there is a specific reason for using sil- tralization tank, or a low caustic step, to rinse any residual fluoride
icates, they should generally be avoided. Finally, alkylphenols are off the parts. While this step seems trivial in theory, it is important to
sometimes added to cleaners since they are effective cleaning com- consider the rate of activity in the caustic bath. An overly active
pounds. However, strict environmental restrictions have been caustic neutralization tank will lead to further etching of the part and
imposed on these compounds because of their persistence and often force shops to rework the aluminum.

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22 CANADIAN FINISHING & COATINGS MANUFACTURING MAY/JUNE 2021