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Metal spray stripping via hydrochloric acid
Q. Hello all,
I work in a aerospace overhaul and repair facility and the owner wants to get into stripping the metal spray from old parts so we can re-spray and finish them.
My question is what equipment would we need to do this and is a air scrubber needed to process the hydrogen gas given off? I know we will need a "clean" wash system to make sure no items that can cause a reaction can get into the stripping tank.
Does anyone out there do this and if so how difficult of a system is this to set up?
I do all the chemical testing for the plant and keep the tanks in good order but this is my first experience with HCl stripping process and want to make sure it's done RIGHT.
Thank you much
Overhaul and Repair Chemist and Resident Mad Scientist. - Phoenix, Arizona, USA
June 2, 2008
June 3, 2008
A. Question 1: Does the overhaul manual permit stripping?
Q2: Will HCl strip the metal spray, I.E.: what is the metal that you want to strip? Not all spray coatings can be stripped.
Q3: What is the base metal. Sometimes HCl will attack or pit the metal.
Comment -- metal spray is normally rather thick. This means long strip times and probably removal from the strip and blasting the surface to get the acid to strip well again.
The initial setup is a bit expensive for a small company because of air quality. Waste disposal is an ongoing expense that may not be cheap, depending on your area and rules.
Remember your chemistry: the more metal in solution, the slower the strip rate, until it actually stops.
Most companies do not have the expertise to do a true cost of operation and thus a cost benefit study.
- Navarre, Florida
June 4, 2008
Q. I apologize for mis-stating the question. the metal spray (Medco^Metco 601ns Al 12Si 40 Polyester) itself is removed via blasting. It is the bond coat (Medco^Metco 450ns Ni 5Al) Nickel aluminum mix) that must be removed via chemical stripping.
"Question 1: does the overhaul manual permit stripping?"
Yes it does, the process was done in house by one of our large customers and since they no longer do so they are having it included as part of the repair work order. It's not possible to machine the bond coat off so chemical stripping is our only option.
"Q2: Will HCl strip the metal spray, IE: what is the metal spray that you want to strip? Not all spray coatings can be stripped."
The actual substance being removed is a Medco^Metco 450ns bond coat that is comprised of nickel and aluminum (Ni 5Al)
The metal spray itself (Medco^Metco 601ns Al 12Si 40 Polyester) is blasted off with an abrasive alum oxide grit blast media.
"Q3: what is the base metal. Sometimes HCl will attack or pit the metal."
Inconel 718, Inconel 625 Magnesium, Stainless steel and some alum parts. The ones that are being stripped have been stripped before so as far as I know, no untested materials are being used.
I know there are ways to recover the spent HCl from the bath but there are so many options and I am not sure exactly which would be the best to use in our shop. Any input on that also is greatly appreciated as I am very environmentally conscious (and also responsible for the Hazmat related responsibilities, and I prefer to limit the amount of toxic anything I have to have disposed of.
I am working on a report with startup cost, operating cost and cost benefit report as best I can with the limited information they are giving me.
I appreciate the information very much.
Overhaul and Repair Chemist and Resident Mad Scientist. - Phoenix, Arizona, USA
June 6, 2008
A. Picky me, it is Metco, not Medco.
Yes it can be stripped, but I do not remember if we used HCl to strip it. The OEM repair manual will specify what you may use.
HCl or Nitric require a good exhaust system with considerable attention paid to the hood. You will need a fume scrubber and I would have a caustic addition controlled by a pH controller attachment. You do not need the amount of water flow that the hood Mfgr. will recommend. Half will work and you will have less waste water to treat.
After the parts have been stripped, do a very light blast to see if you got it all. It is a very good check.
I would use a caustic clean step before the acid. Reverse polarity (anodic) will help clear any unalloyed aluminum in the bond coat.
There is a good reason that your customer does not want to strip the parts. It costs a lot more than it appears to on the surface of the operation.
There are recycle methods for HCl, but it is an expensive initial setup. Neutralize and waste treat is a lot cheaper, unless you are using hundreds of gal per month. The by-stream still has to be treated and it generates a fair amount of it.
- Navarre, Florida
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Ed. note: Thanks for the great insights, James. Regarding the recycling of HCl, please see letter 26974, "Disposal/recycling of muriatic (hydrochloric/HCl) acid"
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