White Spots on Aluminum Parts Clear Anodized on Titanium Racks
We are clear anodizing aluminum pieces, 1100 alloy, on titanium racks. We are experiencing white spots on the finished parts. Most of the white spots come off if we wipe the parts. Details of our processing:
- SOAP 5 MIN
- CAUSTIC ETCH 3 MIN (NEEDED FOR DULL FINISH)
- DESMUT 15 SECONDS
- ANODIZE 12 MIN AT CONSTANT VOLTAGE 15 VOLTS AT 12 AMPS / FT SQ AT 70 DEG F (H2SO4 = 200 G/L & ALUMINUM SULFATE = 8 G/L)
- SEAL = HOT WATER
- 2 MIN AT 190+ DEG F, PH = 6 - 7
- NICKEL ACETATE 2 MIN AT 190+ DEG F, PH = 4.5 - 5.0
We anodize tons of other parts on titanium racks & never see this problem. Any help or input on how to eliminate these spots would be greatly appreciated.David A. Kraft
- Long Island City, New York
David: What kind of thickness are you getting @ 15 V/12 A for 12 minutes, gotta be fairly thin, is there a reason for that? As far as the pitting goes, hrrrm, are any of the tanks you have listed in this setup stainless steel?
If so, are they unlined? Do your tanks have insulating materials to keep your work bar from touching the tank itself?
The thing I'm getting at is galvanic corrosion between the titanium racks and the stainless tank (if there is such a thing in that setup, if not, ignore my comments :-) ).
I had this problem in a dye tank that was made from stainless, either put a drop in liner in the tank, or line the areas where your work bars would make contact on the tanks and you solve the problem, or at least lessen its degree of destruction to the aluminum parts.
Have you had this problem with aluminum racks also? If not, then I'd attribute the problem to strictly a problem with the titanium and something along the lines that causing the pits. Are the rinses agitated? If so is there residue that is blown around in the tanks that could be causing the problems?Matthew Stiltner
- Toledo, Ohio
February 25, 2010
We have seemingly the same problem. We use Ti racks to clear anodise a 10 micron coating on a 1000 series Al. The white spots appear to have a central black pore from which the white comet like tail emanates. The tail seems to manifest itself hours after drying and inspection.
Thinking it is some form of galvanic pitting we have insulated the parts from the tanks and have even gone as far as setting up an electric circuit to negate the charge that could occur from the SS heating coils in our cold Ni seal.
In checking the Magnesium content we find it to be below 0.01% which should not create a problem as far as I am aware. If someone has additional pointer where we could go searching it would be greatly appreciated.
- Sydney Australia
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