Ultrasonic Acid Strip

[aspearin1]

I have NiCoCrAlY coatings that require concentrated HCl acid strip from workpiece tooling. Current process is to immerse, then pressure wash with a gun to remove smut and rinse. Operator yesterday requested ultrasonics in the tank so smut is removed in situ, claiming ergonomic reasons. Either way, there are many benefits to removing smut in situ. My concern is, though, is whether or not I can get compatible immersion transducers within the acid. Most available are Stainless, and that's not an option. One other option is to mount on the outside of the tank, but the tank is 1/2" HDPE. I am concerned that the tank may not withstand the stresses of the ultrasonics, if mounted on the outside. I have seen no OTC ultrasonic cleaning systems made in anything other than stainless components, tanks included. Does anyone have any experience with outer wall mounted transducers on a polymer tank? What about acid resistant immersion transducers?

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee

 

[kenvlach]

HDPE with HCl inside and external ultrasonic transducers will eventually get brittle and fail, but I doubt anyone can predict when.

I recall PVDF (Kynar)-coated aluminum used as alternative to SS tank, & PVDF has good acid resistance. Probably can get tranducers PVDF coated. Ti transducers may work, especially if you add a little nitric acid to maintain the Ti's passive film.

Here's a list with links for US ultrasonics mfrs:

http://www.ultrasonic-cleaning-equipment.com/

 

[aspearin1]

Bare Ti won't due, but I believe a TiN coating may. I have found an SS immersion type that can come TiN coated. Though, chemical compatability for TiN is tough to come by. Thank you for the info on HDPE.

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee

 

[kenvlach]

HASTELLOY C is recommended for HCl service; see if tranducers are available.

Bare Ti won't do in HCl unless an oxidizer such as nitric acid is added to protect the passive titanium oxide film. Nitric acid would protect the Cr of the NiCoCrAlY coating, but also definitely attack the Ni & Co.

TiN coatings are probably too thin for chemical resistance in such extreme condition. Cannot say re HCl, but TiN can be readily removed by hydrogen peroxide or nitric acid.

Do you use a surfactant (wetting agent or detergent) in the HCl? Surfactant + stress embrittle HDPE.

 

[unclesyd]

Do you have access to laboratory with an acid hood where you could try some different acid compositions?

 

[aspearin1]

RETRACTION: Tanks are Polypropylene, not HDPE. Chemistry is fixed at about 25% HCl, by weight. No other chemistries are permitted. I'm being pressureda little to "just give it a try" because "others are already doing it." So far it's hearsay, and I'm still concerned with long term effects to the integrity of my tanks. I'm a little more comfortable now that I know the material is PP. Less likely to suffer from ultrasonic deterioration, I presume. But I'm still apprehensive. Thanks for the input thusfar.

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee