Q. I want to ask about aluminum pretreatment chemicals. Which chemicals can be used for an aluminum degreasing bath? Tri-sodium phosphate or other chemicals?
A. The choice of cleaning and pretreatment chemicals is generally based on the type of removal intended (lubricants and/or oxide) as well as subsequent processing. If the goal is a simple degreasing with no base metal attack, then choice of alkaline builder will focus on chemistries that are slightly basic. The trisodium phosphate may have too high a pH alone. Trisodium polyphosphate has a lower pH and a lower tendency to attack the aluminum. The tetrasodium pyrophosphate has a slightly higher pH and will initiate a light attack on the base aluminum itself. Most other builders have a higher pH and require the use of sodium metasilicate to inhibit the attack of the base aluminum.
If the intent is to etch the aluminum surface, then the use of higher pH builders such as sodium carbonate and free caustic like sodium hydroxide will heavily attack the aluminum. With elevated temperatures and longer times, it would be possible to produce a matte surface finish. Most grades of aluminum will require a follow-up with nitric acid to “desmut” the surface, removing some of the remaining alloying elements and secondary phases that are left behind following the dissolution of the aluminum.
Originally published in the August 2015 issue.
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