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Having some issues with dark spots, need help

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  • Having some issues with dark spots, need help

    Pictures here:

    I cut and machined some pieces of some 6061 and tried anodizing them but I keep getting these odd dark spots on the flat areas. My process is as follows:

    1.) prep the parts, these 4 were prepped in different ways. One was sandblasted with fine glass, one was polished with Mothers Mag & aluminum polish, one was polished with a buffing wheel, and one was smoothed with a fine scotchbrite pad. after which all were put into an ultrasonic cleaner in a cup of acetone for 3 minutes, then put into a cup of simple green + distilled water for 3 minutes.

    2.) Parts were then laid down on a clean microfiber where I attached a 6-32 aluminum bolt to each of them with a 14 guage aluminum wire wrapped around the top with an aluminum nut securing it to the head of the bolt. This was all done with fresh nitrile gloves.

    3.) the parts were submerged in a strong lye+distilled water solution for 20 seconds, one of the parts were left for 2 minutes (one of the two standing straight up in the photos). After this they were submerged in distilled water and rinsed again with distilled water.

    4.) The parts are now transferred to the anodizing bath which consists of a 5 gallon bucket with 1.5 gallons of distilled water and 1.5 gallons of battery acid from Oreilly's (cant find the concentration anywhere on the box). a 24x12 sheet of lead is wrapped around the wall of the bucket and will act as the cathode. At the bottom of the tank is a large fine aerating pad. the parts hang from a fiberglass rod that sits in a groove on the top of the bucket. The anode and cathode are then connected to an adjustable power supply set to .8 amps, the 720 rule suggested .21 amps for the part so I set it to .84 since all 4 parts were in at the same time. Parts were all spread in a way that theyd have the large side facing the lead sheet without blocking another part. All were linked by the same 12 guage aluminum wire.

    5.) After an hour the parts were removed and placed in a Baking soda/distilled water bath to neutralize the acid, then were transferred to a cup full of distilled water. A bucket with red rit powder dye and distilled water is currently sitting in the ultrasonic cleaner and is heated to 45c. At no point in the entire process since the wires were added have the parts been dry, they have always been submerged in distilled water between processes. They spend 15 minutes in the dye before being transferred to an electric kettle filled with deionized water and boiled for an hour.

    After all this I still have these dark spots that appear on the flat surfaces of almost every part I try anodizing, what is causing this? Also I was wondering if anyone here thinks a vacuum chamber or an ultrasonic would help dye absorption? They seem like they should but I havent found anything exploring this. Any help is greatly appreciated!

  • #2

    I'm having similar issues.

    Click image for larger version

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    Top one was sandblasted with glass beads ans has glass beads. Second one was left untouched and is fine.

    Anodizing solution is 3:1

    Feels like perhaps the glass beads are penetrating the aluminum and causing this black discoloration.

    Hopefully someone can help explain what we are doing wrong.

    In the meantime, I am #120 sand brown aluminum oxide and will try again,


    • #3
      the lye should release anything that you embed from the blasting operation. it looks like you just need to improve on your blasting technique. you are not creating a uniform surface when you blast.