To obtain consistent result with any process on aluminum and especially for those involving an immersion treatment, it is essential that the various cleaning and conditioning treatments provide a surface of uniform activity. First the surface should be free of oil, grease, or other foreign material. With heavy oil or grease contamination, vapor degreasing or solvent cleaning may be necessary. This is ordinarily followed by a mild etching type of alkaline cleaner. Such a cleaner can be conveniently made with 23 g/l sodium carbonat (Na2CO3) and 23 g/l trisodium phosphate (Na3PO4. 12 H2O) and is used at 71 to 83 oC for 1 to 3 min. Another solution frequently used is 5 % sodium hydroxide (NaOH), operated at 60 to 65 oC for 30 to 60 sec.
After appropriate cleaning, a further conditioning treatment is generally necessary in the zinc immersion processes to replace the original oxide film with a thinner and more uniform film, and to remove any microconstituents which may interfere with the formation of a continuous adherent zinc layer.
For commercially pure aluminum and aluminum-manganese wrought alloys, good conditioning may be obtained by following the mild alkaline cleaner with dip in 50% (vol) nitric acid (HNO3) solution. These alloys do not contain interfering constituents, and this method of conditioning is ample.
An effective conditioning treatment for removing undesirable micro constituents from aluminum-magnesium and aluminum-magnesium-silicon alloys consists of a 2 to 5 min etch in 25 % (wt) sulfuric acid and (H2SO4) used at 83 oC, followed by the dip in 50 % nitric acid solution.
For casting alloys containing high percentage of silicon, a 3 to 5 sec dip in three parts commercial nitric acid and one part 48 % hydrofluoric acid (HF) may sometimes be desirable before applying the zinc immersion film.
Perhaps the most useful conditioning treatment for many cast and wrought alloys consists of a double zinc immersion treatment. In this procedure, the first immersion dip removes the oxide film and replace it with a zinc layer. This zinc layer is then dissolved in a 50 % nitric acid solution, which leaves the surface in good condition for deposition of the final film of zinc or other metal.