316SS reaction with sulphuric acid during aluminium pickling

[Dee2e]

Hello

I am experiencing a smut issue when pickling aluminium for powder-coating. The acid etching is being conducted in a steel bath (316 grade) with approx 1.6% sulphuric acid solution.
The bath is turning green over time and producing smut.

The aluminium goes through a cleaning inspection with isoproponol used to wipe off any residual machine oils prior to pickling.

What could be causing the excessive smut level, discoloration in the acid bath.


Suggestions welcome.

Thank you for the help.

 

[EdStainless]

How good is your bath control?
Are you measuring the acid concentration (not pH), metals content, and so on?

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P.E. Metallurgy, consulting work welcomed

 

[Dee2e]

For Bath control, we check the concentration every morning before work begins, the bath is heated to 30c and gurgitation is run before doing so.

We have a third party that comes once a month to check the metals content, I am waiting on the most recent results (yesterday) to come back to know what concentrations we are dealing with now. Previous reports have indicated Nickle and Copper in the bath.

Our Aluminium grade is 1050 and our precleaning of the aluminium before pickling is fairly thorough, sanding imperfections followed by full wipe down with solvents.

We have had to drain and reset our acid concentration twice so far this year, which is not ideal and not typical.

 

[EdStainless]

Is the acid filtered? How often do you drain part of it and refresh the bath?
What method are you using for measuring the acid content?
How is the bath agitated?

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P.E. Metallurgy, consulting work welcomed

 

[Dee2e]

We check the concentration by titration daily.
Bath is skimmed with a sieve for surface contaminants daily
Agitated by air pump
We only drain and refresh when pointage is out of line, which was recommended by our chemist who helps manage the system and checks monthly.

 

[EdStainless]

I would suggest that you change the tank out in steps. If you usually go 4 months between drain why not replace 25% each month?
This will keep your bath chemistry much more constant.
You should have upper and lower control limits set for acid and metals with action limits.
Is your air that you agitate with very pure? As in totally free from any measurable oil?
Even very small traces of oil can cause issues. Especially if this tank is where debris in the air can settle into it.

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P.E. Metallurgy, consulting work welcomed

 

[Dee2e]

We have a cover for the tank when not in use, but the whole process is in an open warehouse with all stations exposed including the prep area, and powder coating area. There definitely is some debris noticeable on our rinse tanks at the end of a day.

I have not considered the aerator a source of oil contamination I will look into it.

We do conduct etch tests on aluminium samples daily to ensure the correct limits are being met.

I am currently looking into the steel grades of the tank, dipping cage, support bars, while also looking at the precleaning stage as we do it manual and not by immersion and whether it is sufficient to remove the fabrication oils.