Mister D

Topic: Aluminium anodizing - Chromate seal troubleshooting

The ph level in our chromate seal tank is higher than it should be (specs calls for 3,8) and seems out of control.  Chromic acid is used to drop it but something seems to interfere.  DI water is added due to evaporation at pH 5,6.

Any known contaminants? something to look for?

Regards.

DustinGebhardt

Re: Aluminium anodizing - Chromate seal troubleshooting

Are you using a good quality pH probe and meter?  Is the solution tested hot or allowed to cool to room temperature or does your pH probe have automatic temperature calibration?  Is the bath agitated? 


You say the bath is out of control.  Can you please elaborate so we might be able to better help you.

-Dustin Gebhardt, CEF

Advanced Manufacturing/Finishing Engineer

Moen

Sanford, NC

Mister D

Re: Aluminium anodizing - Chromate seal troubleshooting

Thanks for your reply.  Here is more information :

- pH probe is calibrated regularly and the process is supervised by the chemist

-everything is tested when the bath is hot (±195 F)

-bath is fully agitated

To lower the pH level, we add chromic acid.  However, according to our specs, we need to control hexavalent chrome in the bath.  The problem is that we cannot add chromic acid anymore since the concentration is at the maximum acceptable level.

We have not processed parts in the last few weeks, but the bath was heated.  DI water was added due to evaporation causing the ph level to reach the maximum limit.  No process, no chrome consumption...  We tried to process scrap parts to reduce chrome consumption, but we ran out of parts.

Any ideas?

DustinGebhardt

Re: Aluminium anodizing - Chromate seal troubleshooting

How do you know you are at the limit for chromic acid?  How are you analyzing for chromic acid?  Is this a bath that is supplied by an outside vendor?  If so, have you contacted them?


In my experience, you make up a chromate seal bath at 5% using sodium chromate or potassium chromate.  The pH is then adjusted using either NaOH/KOH or acetic acid (glacial) to a range of 5.5-6.0.  If you use NaOH, be sure to use rayon grade or lab grade (1.0N generally works for small adjustments).  Temp should be around 200 or more.  


If I recall correctly, we ended up using a bunch of the acetic acid to control the pH while the bath was not in use.  We had 2 x 4 gallon cases of the stuff on hand for a 80gal tank.

-Dustin Gebhardt, CEF

Advanced Manufacturing/Finishing Engineer

Moen

Sanford, NC

LeoH

Re: Aluminium anodizing - Chromate seal troubleshooting

Sometimes hard water will increase in pH when it evaporates.  Are you using DI or RO water?  Also I'd sugest sulfuric acid to lower the pH.