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Calculating sulfuric and phosphoric acid concentrations in a electropolishing bath and the criticalness of their ratios [Michigan] 

February 15, 2007

We currently have a electropolishing bath that is about 5000 gallons in volumn. How do we determind the phosphoric and sulfuric acid concentrations and therefore adjust and how critical is the ratio between the two acids relative to productivity?

TY VAN FOSSEN
metal finishing - Lake Orion, Michigan

The ratio of sulphuric acid to phosphoric acid is not particularly critical, but many electropolishing solutions have other additives in them. I therefore suggest you check the details in the process sheet supplied by your solution's supplier. This will tell you all the analyses you will need to do and what the acceptable ranges will be. Successful electropolishing is dependent on how much metal is dissolved in the solution, its viscosity and operating temperature - detila of these parameters will also be in the process sheet.

Trevor Crichton R&D practical scientist - UK

An analytical procedure is in the Metal Finishing Guidebook and Directory; hasn't changed in at least 15 years. Volumetric Analysis of Metal Finishing Solutions by Andrew McFadyen has a very similar procedure under 'Aluminum Electropolishing' (p. 210). Both use colored pH indicators; below is a procedure utilizing a pH meter:
Cool a sample of the well-stirred solution to room temperature.
1. Measure the specific gravity using a narrow-range hydrometer.*
2. Pipette** 2 ml of the solution into a 250 ml beaker and dilute to 100 ml using DI water.
3. Titrate with 1 N NaOH to pH = 4.0 and record as 'A' mls.
4. Continue titrating to pH = 8.5 and record the additional mls as 'B.'

H2SO4, wt. % = (A - B) x 2.452/SG
H3PO4, wt. % = B x 4.900/SG

FYI: 'A' is the titration for sulfuric + 1/3 of the phosphoric, 'B' titrates an additional 1/3 of the phosphoric.

*A 1.600-1.800 hydrometer and 300 ml hydrometer (non-graduated) cylinder are usually suitable for measuring SG.

**Use a TC 'to contain' pipet; it permits flushing out the sticky sample aliquot using DI water.

Making acid additions requires algebra, as the concentrated acids include water and the total solution volume increases. Agitate or stir very, very well before re-sampling. Excess water can be removed by evaporation (heating).

Productivity of an unspecified EP composition at unspecified temperature for electropolishing parts of unspecified geometry and metal using unspecified racking???

Some general comments (opinions):
High phosphoric baths cost more and may have ~infinite life but require closer anode-cathode spacing.
High sulfuric baths cost less and have better throwing power but are perhaps prone to etching at low current densities.
The acid-to-water ratio varies with phosphoric-sulfuric ratio and is perhaps more important. Some of Charles Faust's original phosphoric-sulfuric recipes for electropolishing stainless steels are given in US Patent 2,334,698 (1938).

Search this site for more electropolishing info. Also, several vendors offer electropolishing seminars.
Hope this helps.

Ken Vlach - Goleta, California