Summary for ferric nitrate bright pickle Discussion
ferric nitrate bright pickle of magnesium is the thread’s focus, but the only reply is cautious and largely discouraging. Casey Hill asks for help reproducing Dow’s Chemical No. 21 ferric nitrate bright pickle for magnesium so he can get a bright, stain-free surface before e-coat and match an aluminum anodize color finish. He reports trying nitric bright dip, hydrofluoric-sulfuric pickle, and a ferric nitrate/potassium fluoride/chromic acid mix per Dow 1952 instructions, yet AZ80A magnesium still turns dark brown rather than bright. The answerer says he has no direct process experience, but notes that magnesium has historically been hard to keep shiny; early cast automobile wheels were magnesium, and the industry moved to aluminum because magnesium was essentially impossible to keep bright for long, even after careful polishing. He suggests the issue may be inherent to the metal’s appearance stability and points readers to another thread for more background.
Q. I have been attempting to develop a pre-treatment process prior to e-coat over magnesium that can offer a bright surface allowing me to achieve an aluminum anodize color finish. I have tested with the nitric bright dip, hydrofluoric-sulfuric pickle, and the ferric nitrate bright pickle processes originally published by DOW. Variations to the nitric bright dip and hydrofluoric pickles ultimately leave a dark or pitted surface.
Most recently I attempted to replicate the ferric nitrate process using ferric nitrate (reagent-Sigma Aldrich), potassium fluoride (reagent-Sigma Aldrich) and a combination of sodium dichromate and sulfuric acid (separately mixed) to yield chromic acid. The concentrations of each follow the DOW 1952 work instructions for chemical no. 21 treatment. Under ambient conditions, for a range between 30 seconds and five minutes, I can only achieve a dark brown surface on AZ80A magnesium similar to the outcome of the Dow 9 galvanic anodize process.
Can someone comment as to any experience or optimized work instructions for the ferric nitrate bright pickle process by Dow (or another entity)? Any other recommendations to achieve a bright, stain free, surface on any commercial grade magnesium alloys?
Casey Hill
– Cleveland, Ohio
July 24, 2024
A. Hi Casey,
Hopefully another reader will relate a positive experience, but my only knowledge of this is that early cast automobile wheels were magnesium — which is why cast aluminum wheels are still called “mag wheels” by many — but production was switched to aluminum because it was essentially impossible to keep them shiny for long even after careful hand polishing. Please see thread 0003 for further info on this. You can still see real magnesium wheels offered for sale, but my limited understanding is that they continue to be popular only for actual racing because of this aesthetic issue.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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