Page 23 - CFCM May-June 2021_Neat

A common method to suppress the ag- form a gray smut on the surface of the aluminum parts are exposed to the
gressiveness of the caustic bath is to main- parts. To remove this smut, a desmutting atmosphere. Typically, the solution will be
tain the dissolved aluminum. A low step is put in place which effectively composed of ferric sulfate and nitric acid.
dissolved aluminum concentration will dissolves the insoluble metal complexes. This type of oxidizing solution will oxidize
cause the bath to be overactive and further Inorganic mineral acids such as chromic the aluminum oxide layer, effectively
etching will occur. However, at high dis- acid and phosphoric acid were popular removing it from the bare aluminum
solved aluminum concentrations the bath desmutting baths in the past but with substrate, and remove any smut left on the
can become viscous and cause “tiger new environmental and health regulations, surface. The result is a highly active,
stripes” from high drag out and inadequate proprietary chemistries have become sparkling clean part ready for anodizing.
rinsing. Maintaining a dissolved alumi- more popular. Clean, residue-free surfaces are critical to
num concentration between 10 g/L and 50 Desmutting solutions are often paired achieving a blemish-free coating. n
g/L will maintain the integrity of the low with deoxidizing capabilities. A deoxidizing
caustic step and the desired effects from solution is designed to remove the alumi- Connor Stewart, B.Sc., is Technical Sales
the acid etch pretreatment. num oxide layer that naturally forms when and Service Representative, Dynamix.
While acid etching processes are becom-
ing more popular, alkaline etching is stan-
dard in the anodizing industry. Aluminum is
an amphoteric metal, meaning it reacts
with both acidic and alkaline solutions. An
alkaline etch consists of caustic soda which
effectively dissolves the aluminum at the
surface of the part:
2Al + 2NaOH + 2H O → 2NaAlO + 3H
(s) (l) 2 (l) 2(s) 2(g)
The etch will remove approximately 10
g/ft2 of aluminum in 10 minutes which
makes it effective at masking surface
defects. In a purely caustic bath, the sodium
aluminate would react with the water and
form insoluble aluminum hydroxide and
precipitates out of solution. Aluminum hy-
droxide forms hard scale at the bottom of
the tank which eventually impedes produc-
tion. Most alkaline etches will contain a
proprietary chelator to keep the removed
aluminum in solution and prevent it from
dropping out. These proprietary additives
allow the etch to keep up to 150 g/L of
dissolved aluminum in solution.
A consequence of keeping aluminum in
solution is an increase in the viscosity of
the solution. A more viscous etch increases
the drag-out from the tank. This is actually
an important consequence as it removes
dissolved aluminum from solution and
allows the etch to sequester fresh alumi-
num, keeping a type of equilibrium. That
being said, a high drag out operation does
require excellent rinsing to compensate.
As the caustic etch dissolves and scrubs
the aluminum surface, a smut will appear
on the part. Alloyed metals in aluminum
like copper, manganese and chromium are
insoluble in the alkaline etch solution and

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