Aluminum bright dipped surface is cloudy

"The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys"
by Wernick, Pinner
& Sheasby

(note: this book is two volumes)

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A. Hello M.Saqib. I believe that you have correctly identified the problem of the color differences as being caused by the cloudy surface appearing in the bright dipping process. The next step is to determine what, if anything, is wrong with the bright dipping chemistry or operation. We probably need details of that.

Meanwhile, have you found any correlation between the dull areas and their position in the bright dip tank? Good luck.

A. Actually I don't have analytical process for this bath and we are adding randomly. I am following book of Mr. Arthur Grace: The Technology of Anodizing Aluminum [on AbeBooks, on Amazon, on eBay" (affil links)affil links]. We are taking trails in small bath of 2 litres making addition till the required finish is achieved and then applying the same equation to my big bath of 3000 litres. We used to add water but we found results very dull and less shiny, so right now we have ceased adding water. I am always stressed out whenever I need to start my brightening bath, because the results are dicey. On a good day I get the best quality which is very rare. And many times we need to go for trial and error.

Getting back to your pic, I feel that problem of yours is not only of cloudy but also of uneven pickling in bright bath. It may be because of poor degreasing or bad rinse. Some times the carry over from the bright bath contaminate the rinse badly giving scope for another reaction. So check your rinse bath, then degreasing as the oil transferred in process of extrusion or polishing may not have been removed correctly. In many factories degreasing bath is been ignored or absent. I am trying to get a pic of my problem , but my camera fails in capturing the cloudy thing properly. It makes the section lil dull. Some small small white spots comes. It's not pitting but some thing else.

A. For density analysis, you just need to introduce 100 mL of your bright solution at 20 °C in a volumetric flask,then weigh it.

For nitric acid, it's a bit harder.
First you need to prepare the titrant solution. Add 50 mL of H2SO4 96-98% to 700 mL of distilled water (approx.). Then add 250 g of FeSO4.7H2O. Make sure all of it is dissolved.
Then, we are going to standardize this solution. We need to prepare a phosphoric-nitric standard. Add 25 mL of nitric acid 60% in a 500 mL volumetric flask, make up to volume with phosphoric acid 85%. Take 100 mL of phosphoric acid and 5 mL of our standard solution and put them in a beaker. Heat it to 45-50 °C, and titrate the solution with FeSO4. It will turn Golden-Brown. After adding the first 1-2 mL wait some seconds. It will turn Brown and then transparent again. When the titration is coming to an end, you will see bubbles and fumes. The volume used will be "A".
Now you have to analyze your bright sample, in the same way as above. That volume will be "B".

%HNO3= (B/A)*5

If you want to control your sulfuric and phosphoric acid, you will need a pH meter.
Put 5 mL of your sample in a volumetric flask, and make up to volume with distilled water.
Then take 20 mL of that solution, add 5 mL of EtOH and titrate it with NaOH 1N [1N NaOH on Amazon [affil links] , to pH=4,9. M=mL of NaOH consumed
Don't fill the burette now, and keep titrating to pH=9,4. N=mL of NaOH consumed.

%w/w H3PO4= (98*(N-M)/1342)*100
%w/w H2SO4= (49*(2*M-N)/1756)*100

I hope that this help you. Sorry if something is not clear, English is not my native language. I'll clarify anything that you don't understand.

Regards.

Ethanol

A. Hi Sumit. Yes, EtOH does mean ethyl alcohol. Also, from curating this site I have learned that some Europeans use a comma where you & I would use a decimal point: so yes, pH 4,9 means the same thing as pH 4.9. Good luck.

Regards,

A. Hi Sumit, the volumetric flask volume is 100 mL.

Q. Thanks a lot Victor. I tried your procedure and I tell it's really great and results are satisfying.

Would like to know one more thing. Do you measure aluminium content in the bath? Can you please help me to measure aluminium content in the tank.

Once again, thanks a lot to you and also to finishing.com

Regards