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Aluminum Bright Dip out of balance and pitting!

Q. I am a very new quality manager in a new industry (for me) - metal finishing. Last week our bright dip went "crazy" and pitted some machined parts costing us some big dollars. I am three months new in the industry. I know the following: bright dip solution can be extremely damaging to some aluminum if it gets off balanced. Specific gravity represents the phosphoric content, my FAS titration measures the nitric content and I know the nominal levels of both. I made my first batch of FAS last week and I could not get the ferrous ammonium phosphate ^ sulphate hydroxide to "dissolve" in the sulfuric. I tried to normalize today and finally succeeded.

So we have added both nitric and bright dip solution in extreme amounts but BOTH the buoyancy and the nitric content are not increasing. Please help!

David Reece
Plating shop employee - Portland, Oregon
May 5, 2010


A. Hi David,

You are right that specific gravity is a measure of phosphoric acid content. However, I think that it is more helpful to think of it a measure of water content. The water content is critical in getting good results. Too high (low SG) and the brightening is poor. Too low (high SG) and aluminium phosphate will precipitate on the surface of the aluminium. When the aluminium is rinsed in water the phosphate dissolves but leaves behind small bumps on the surface. These bumps are easily mistaken for pits. Could this be your bright dip going "crazy"?

Adding more bright dip to your bath will not greatly change the bath SG. It will marginally lower it if the top-up has a lower SG than the bath.

The nitric acid content will increase if you add more nitric acid the bath. Are you now happy will the analysis? You talk of "ferrous ammonium phosphate hydroxide". Is this a slip and did you mean ferrous ammonium sulphate? Can you quote figures for nitric acid added to the bath and the before and after analyses?

Harry Parkes
- Birmingham, UK

I am not experienced in aluminium chem polishing so will not comment on the pitting
Re.: nitric analysis by FAS it is Ferrous Ammonium Sulphate which is the reagent not phosphate.
The analysis requires some "technique" to be done accurately and suggest you get your supplier to assist.
Otherwise make up a standard (with phosphoric/sulphuric) with an accurate amount of nitric to standardize your FAS
Re.: the SG you are also adding water when adding nitric etc so to increase SG you will need to boil off water.
Your pitting may be from another reason e.g., contamination

Geoffrey Whitelaw
Geoffrey Whitelaw
- Port Melbourne, Australia

thumbs up sign You are all absolutely correct. We did spend one overnight "boiling" off the H2O and the bright dip normalized. I also learned the method to titrate for g/l of dissolved aluminum. Result was midline amount of Al. And my typo was discovered...I did mean ferrous ammonium sulphate.

Thank you for the advice. Great to know there are experts out there willing to lend some help!

David Reece [returning]
- Portland, Oregon, USA
May 8, 2010

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