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FPL Etch Solution Analysis



I work for a company that makes composite aircraft propellers. We have decided to use the FPL etch process to prep our components prior to being bonded with composite resins. We have tested the FPL etch and would like to use it in production. However, before we can do that we need to develop a procedure to ensure that the bath is always in good working order. That being said, here are my questions.

We are starting to use the FPL etch process and I was wondering how you should check the solution. Is a titration process the best process? If so, what would be the procedure to check chromic acid, sulfuric acid, and aluminum? And, how do you convert the titrated result into a solution concentration?


Pat Stumpff
- Piqua, Ohio, US


The recognised chemical control methods for the chrome sulphuric etch (FPL) is the relative density and free chrome content. The density can be monitored by a hydrometer and is given an upper limit of 1.45-1.5 before the solution is considered to be exhausted.

The free chrome content is continuously maintained between 40-50g/l by the regular addition of chromic acid. This is monitored by titration as follows.

Take 1millilitre of etch and add 8-10 millilitres of specker acid. Add a few drops of diphenylaminesulphonate indicator and titrate against 0.1N ferrous ammonium sulphate (FAS)

For 1 millilitre of etch the calculation is. Titre x N(FAS)x33.3=g/l CrO3.

For stable process control the use of continual anodic oxidation should be considered.

Hamish McWillie
- Northamptionshire U.K

August 26, 2010

What is the make up of specker acid. FPL Etch is becoming rare these days and I am beginning to deal with this process after twenty five years. Thanks

Dana Cote
- Andover, Massachusetts, USA

November 18, 2011

We are just starting to use an FPL Etch here and this is the procedure that I found for controlling the solution

For Sodium Dichromate
1. Pipette 2 ml of acid etch to be tested into a 250 ml
Ehrlenmeyer flask. Drop a magnetic stir bar into the
bottom and place on a stirrer.
2. Add 50 ml of deionized water by transfer pipette.
3. Add 12.5 ml of 10 percent H2SO4 (by pipette or buret).
4. Add 3 ml of potassium iodide [20 percent solution] (by
pipette or buret).
5. Titrate with 0.1N sodium thiosulfate until a straw color
appears -- do this very slowly so as not to overshoot
the end-point (from buret).
6. Add 2 ml starch indicator by pipette.
7. While stirring rapidly, add 0.1N sodium thiosulfate
dropwise very slowly until the color changes to a light blue.
8. Record the number of mL of Sodium Thiosulfate used.

Sodium Dichromate(g/L)= [(mL of Sodium Thiosulfate x 4.967)/2]

For Sulfuric Acid
1.Place a stir bar in a 250 mL Beaker.
2.Transfer 0.5 mL of Etching solution by pipette or buret
3.Add 100 mL of deionized water.
4.Rinse pH electrode with deionized water and blot dry.
5. Standardize the pH meter with ph 4 and 7 buffers.
6. Rinse electrodes and wipe dry.
7. Immerse electrodes in the acid solution which you have
placed on a stirrer.
8. Add NaOH (0.1N) dropwise until pH 3.5 is reached.
9. Record the number of ml of NaOH (0.1N) used.

Sulfuric Acid(g/L)=[(mL of 0.1N NaOH used x 49.04)/5]

Hopefully, this helps you a little bit.

Scott Merritt
- Eastman, Georgia, USA
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