Chem Film Staining

Question:

I operate a clear and yellow chromating (chem film) Irridite line. When chromating our aluminum parts, which have two pieces spot-welded together or a piece of aluminum that has been bent and then squashed at the bend, I quite often get a yellowish staining, or leaching. How can this be avoided? Below is the process I am using.

Tank No. 1 Alkaline Cleaner (Non-Etch) 155F ( agitated )
Tank No. 2 Rinse, Ambient (city water, agitated)
Tank No. 3 Caustic, Ambient Etch (agitated)
Tank No. 2 Rinse, Ambient (city water, agitated)
Tank No. 4 Deoxidizer, Ambient (chrome based deox,agitated)
Tank No. 5 Rinse, Ambient (city water, non-agitated)
Tank No. 6 Clear Chrome, 80F (agitated)
Tank No. 9 Rinse, Ambient (DI water overflowed from final rinse, non-agitated)
Tank No. 8 Yellow Chrome, 90F (agitated)
Tank No. 9 Rinse, Ambient (DI water, same as No. 9)
Tank No. 10 Sealer, Ambient (low concentrate chromate, non-agitated)
Tank No. 11 Final Rinse, Ambient (DI water, agitated) M.H.

Answer:

Is this a “hand line” or a “hoist line”? Your process looks good except for the caustic etch. Do you have to etch the parts? It has been shown that better corrosion resistance is achieved if parts are not etched, or etched very lightly if they must be etched. Also, caustic soda (sodium hydroxide) is the most difficult chemical to rinse out of corners, crevices, crimps, etc.

Why is Tank No. 9 non-agitated? An agitated rinse would be of more benefit. Also, agitate Rinse No. 5, especially No. 5! I know you don’t want to “wash off” the newly formed chromate film in No. 9, but agitation in the rinse shouldn’t hurt. At least that has been my experience on aerospace parts. You might give it a try and see. As a matter of fact, I would agitate all the rinse tanks. They aren’t as effective if not agitated. An exception to agitating all the tanks might apply if this is a hand line and parts are rinsed vigorously by hand.

Sometimes it helps the rinsing effort if there are alternating hot and cold rinses that the part can be subjected to. Go back and forth between hot and cold until the part is rinsed thoroughly. This tends to cause the metal to expand and contract just enough to usually enable you to thoroughly rinse the parts and avoid staining or leaching. Use this technique after chromating. If you must etch the parts, this hot-cold rinsing procedure immediately after etching may also help avoid any trapped caustic solution.

Another possible solution to this problem might be to leave the crimped area open slightly until after chromate. Then close it up carefully after the parts have “aged” for 24 hours. There is not much you can do about the spot weld except as stated above.