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Addition of ammonium bifluoride to increase titanium etch rate

I am the lab technician at a finishing division and I am considering an ABF addition to my titanium etch tank to increase the etch rate. I must conform to BAC5753 and have not made an ABF addition to this tank since it was installed. I need to keep the etch rate within the shop range 0.002 - 0.004 ml/surf/hr. I would like to know if anyone has an established correlation between percent addition of ABF to titanium etch rate. I'm looking to increase my titanium etch rate, but do not want to overshoot. The rate is now 0.0021 ml/surf/hr and Ti concentration is about 0.7 oz/gal. I'd like to bring it up to 0.0030 or so. I don't have any recommended additions as I would with some of my suppliers like HST, since the tank is commercial HNO3 and ABF. Any guidance anyone can offer will be greatly appreciated as I am very hesitant to 'wing it' (which would be a 25# addition to my 300 gal tank, which I fear would be too much and then I'd have a removal issue to bring the etch rate back down ;<{. Anyway thanks for the great site, I wish I had more time to spend here.

Fred DeMuth
finishing lab tech - Bensalem, Pennsylvania, USA
February 3, 2010

Hi, Fred. Thanks for the kind words. Although I can't help you on the amount of ABF to add, I would suggest that you spend your limited available time on the site on the subject of Hull Cell testing :-)

The general principle is to do your best to try the additions on a lab size basis, and only then scale up to your production tank. Good luck. /P>

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 4, 2010

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First of two simultaneous responses -- February 5, 2010

I really do not think that BAC will allow you to make additions of ABF in the blind. Normal aerospace requirement is to analyze and make additions from there.
The etch rate will be dependent on the ABF concentration, the amount of dissolved Ti and the tank temperature. As the dissolved Ti goes up, the rate decreases even if there is only a modest lowering of the ABF concentration.

If you are going to wing it, I would use a 5 pound addition every time you get etch rate data back.

Teds suggestion of a Hull cell test, normally implies an application of electricity. A jar test with about 2 liters of solution and get an etch rate. Then add and well mix an estimate of a reasonable amount would be and do another etch coupon and see what rate difference there is. If you like the results, just do the calculations for a similar addition for the main tank. You are within spec now, so I would not get carried away. If your test did not give the desired results, get a new known amount sample and make an addition to the test jar of double the amount that you tried first.

James Watts
- Navarre, Florida

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Second of two simultaneous responses --

If your etch rate is correct, your addition will probably be about 15 pounds. As Ted says though, do it in the lab or sneak up on it by making a series of small additions. Before you do the add though, you should really do an etch uniformity survey of your tank. You do this by doing the same etch rate test, but with numerous panels all at different places within the tank solution. If you haven't already done this, you will likely find that the etch rate result varies significantly depending on where the panel was placed and what form of agitation you use. If you use a pump to agitate the bath, then the differences can be as much as a factor of 10. In the end, your etch rates will be the most uniform if you use moving rod agitation or no agitation during etching.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

February 5, 2010

Thanks for the advice guys. I've been checking etch rate with air agitation and want to get away from agitation completely to insure uniformity (and of course safety with the HF). It looks to be a factor of 4 times higher with agitation than without, and hence going without agitation will drop me out of spec. Etching without agitation lends itself to a lab batch and scale up, so I'll be doing that. I try to be "one and done" with additions here, so I'll let you know how it works out. Thanks again.

Fred DeMuth
- Bensalem, Pennsylvania, USA
February 13, 2010

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Hello everyone. Sorry I haven't followed up for a while. I made a series of lab batch additions of ABF to my process titanium etch solution. The result was an addition of 38 lbs. to the process tank which yielded an etch rate of 0.0030 in/surf/hr.
Thanks for your help to everyone who responded to my initial query.

Fred DeMuth
- Bensalem, Pennsylvania, USA
May 8, 2010