Apr
2017
I have an issue with aluminum parts, using the same parameters, minus the ultrasonics. The aluminum parts come out of the bath with dark, black streaks and spots. Should I not use the same bath to process aluminum and stainless steel? Can aqueous alkaline cleaner be used on aluminum?
A. Probably not. Cleaners that are formulated to clean metal like stainless steel usually cannot be used for aluminum. Since industrial processing of stainless steel (forming, bending, cutting, etc.) is typically more demanding than a similar operation for aluminum, the lubricants used can be difficult to remove. Therefore, most aqueous alkaline cleaners formulated for use on stainless steel have a high pH and will contain quite a bit of free caustic in the form of sodium or potassium hydroxide (in addition to ingredients like surfactants and buffers). If a cleaner like this is used on aluminum, it tends to etch the surface. This condition will be made worse with elevated temperature and longer periods of time. Additionally, the higher the concentration of the cleaner, the worse the etching and discoloration. Free caustic will quickly reduce the naturally-occurring oxide on aluminum and immediately start to etch and dissolve it. Most industrial aluminum is alloyed to attain specific properties (typically with copper, manganese, silicon, magnesium and zinc). The discoloration of the aluminum is the presence of the undissolved alloying elements and the secondary phases that they form that are not attacked in the high pH bath. They wipe off fairly easily, although chemical removal is generally only attained by use of an acid (like nitric) to dissolve these alloying elements.
Content shared on 3/1/2017 by PFonline
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