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The Anodizing Tank

  • Temperature 68F (20C)

  • Material = PVC

  • Agitation = Yes, vigorous agitation is needed to keep the parts at a constant temperature. Air will create more acid mess but does work.

  • Cathode Material 1100 best, 6061 ok, lead ok but takes more volts. Use 1-2mm thick and bend over the top of the tank and make the electrical connection on the outside.

  • Thermometer = Fish tank floating, non contact IR

  • ASF = Amps per Square Foot (I use 12 which is common)

  • mils = 1 mil = 0.001" (anodizing max is 1 mil)

  • Time = 60 minutes

Start Up

  1. Setup agitation and cooling then test using water.

    • Acid ratio should be 15% - 20% by weight. Battery acid is 35% sulfuric acid and 65% distilled water by VOLUME. A 50-50 mix of battery acid to distilled water ends up 19.6% sulfuric acid by weight. The industry standard is 15% sulfuric acid by weight.

  2. Add distilled water to tank then add battery acid for a 50-50 mix.

  3. Place the aluminum cathodes into the tank for a couple of days.

  4. Prepare a thin 6061 plate 6" x 12" and place into the tank.

  5. Apply 12 amps to the plate (raising slowly to 12) and if the volts reads 12 then your tank is ready for anodizing.

Anodizing Calculations

  • Time = (720 / ASF) * mils

  • Current = ASF * surface area in sq. ft.

  • Peak Ohms = 0.95 ohms per sq. ft. / surface area in sq. ft.

  • Peak Voltage = Peak Ohms x Current

Sulfuric Acid Concentration

This is not working out right as 1 gal water to 1 gal of battery acid should result in 19.6% acid by weight….

  • One gallon of sulphuric acid 95% concentration at 68F weighs 15.35 lbs.

  • One gallon of pure water at 39F weighs 8.345 lbs

  • Battery acid is 35% sulphuric acid and 65% water by volume. .35 x 15.35 = 5.3725 lbs sulphuric acid = 49.76% by weight .65 x 8.345 = 5.4242 lbs water = 50.23% by weight One gallon of battery acid weighs 10.7967 lbs

  • Mix one gallon of distilled water to one gallon of battery acid and we have: 5.3725 lbs sulphuric acid (5.3725 / 19.1417) * 100 = 28.07% by weight 13.7692 lbs of distilled water (13.7692 / 19.1417) * 100 = 71.93% by weight 19.1417 lbs total

An electrolyte is not typically checked with a ph meter… it’s titrated for both acid concentration, and dissolved aluminum. I got the following directly from a commercial anodizer:

Here is what you will need to titrate your anodize tank and keep it within operating parameters.

  1. 0-50ml burette (recommend self zeroing burette)

  2. 400ml beaker

  3. magnetic stir station

  4. stir bar

  5. bottle of lab grade 1.0 N NaOH reagent (sodium hydroxide)

  6. hand held ph meter

  7. 0-10ml Pipette

  8. PH Buffer Solutions to calibrate PH probe. Get a 3.01, 7.01 and 10.01

TITRATION INSTRUCTIONS

  1. FILL BURETTE WITH NaOH

  2. PIPETTE 5 ML OF anodize BATH INTO 400ML BEAKER

  3. ADD 200ML OF DI WATER TO BEAKER

  4. PLACE STIR BAR IN BEAKER AND SET ON MAGNETIC STIR STATION

  5. PLACE TIP OF PH PROBE INTO BEAKER CAREFULLY AVOIDING CONTACT WITH STIR BAR

  6. SLOWLY ADD NaOH TO BEAKER FROM BURETTE. ADD UNTIL PH METER READS 3.6 THEN RECORD READING FROM BURETTE. THIS IS READING A

  7. CONTINUE TO ADD NaOH UNTIL A PH OF 8.5 IS REACHED. RECORD NEXT READING FROM BURETTE. DO NOT TOP OFF BURETTE BETWEEN READINGS. THIS IS READING B

CALCULATION

G/L H2SO4 (SULFURIC ACID) = A x 9.8 G/L ALUMINUM: = (B-A) x 1.8

You want your target of g/l of H2SO4 to be between 180-200 g/l You want your target aluminum to be between 10-15 %

If you’re over 15% remove half of your bath and top off with DI water then re-titrate. If your aluminum content is below 10% calculate how many pounds of aluminum sulfate to add to the bath to bring it back up to 10%. I have an excel spreadsheet that I can give you to help with all the calculations.

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