Protecting MgAl against corrosion by NH4Cl

[Voryon]

Hi All,

 

I´ve recently discovered NH4Cl as chlorine donor and trying to find its use in pyrotechnics. Up to now I did not find much information about NH4Cl and its compatibilities and especially incompatibilities in Pyrotechnics and was wondering why as it is OTC and very cheap with app. 66% of Cl. Upon heating it decomposes to NH3 and HCl which should make it a great source for forming the all desired chlorine species of Ba, Sr, Ca, Cu, etc... . The MSDS for NH4Cl states incompatibility with Al due to corrosion making it difficult to use in metallic fuelled compositions, I´d assume the same is true for Mg or the alloy of the two. Please be aware to not mix NH4Cl with NH4NO3 and Zinc or other metal powders to accidentially produce a "Negative X" like mixture, that catches fire upon contact with water (i.e. humidity).

 

In the formulary Database the following can be found:

"This chemical isn't very widely used anymore. Commonly seen in the "negative X" self ignition composition, and in some older smoke compositions. There are some rather outdated formulas where it is used as a chlorine donor."

 

Up to now I´ve experimented with small amounts of mixtures of Sr or Ba(NO3)2, Sulfur, MgAl, NH4Cl, K2Cr2O7 and using NC/Acetone (dry) to make coloured "stars" which worked nicely from colour perspective but during drying from acetone there was always a significant smell of ammonia until dry. Even at priming using dextrine based prime and wetting with saturated aqueous K2Cr2O7 solution the ammonia smell returned. No detectable heat was produced during wetting and after drying the ammonia smell disappeared, the compositions still "worked". The heating might not have been significant as the amount of composition I was working with was always very small and the temperature is low at this time of the year. As I am quite pleased with the results I´d be curious if someone has an idea on how to prevent the reaction between MgAl and NH4Cl that releases NH3? Potassium dichromate solution does not do the trick, I´d assume "coating" the MgAl with dichromate prior to adding it to NH4Cl won´t do so either.

 

Any input would be very appreciated!

 

Best regards,

 

V

 

[NeighborJ]

I can't speak on the chemistry involved here but coating the MgAl with NC might do the trick. My first thought was to coat it with a parlon solution but that may skew your experiments. It would seem to me that a inhibiting layer of any type of waterproof laquer should work. Dont put too much weight in my opinion because i have no idea of any possible incompatibilities.

[solkompleksowa]

Tried it once in a mix with bright aluminium - stearine coated (as I didn't have any PVC or parlon then), experienced problems with its hygroscopic nature. The stars were binded with NC laquer, but fallen apart during storage. As far as i can remember i still have some of this composition around in a jar, it's quite stable, might revisit it someday.

Comp:

Barium nitrate - 65%
Aluminium (bright) - 19
PVC - 8 - replaced with NH4Cl
Sulfur - 8
Dextrin- 3,5 - used NC instead

Note it's difficult to ignite

Edited November 16, 2017 by solkompleksowa

[Mumbles]

Barium and strontium nitrate are alleged to essentially negate the dichromate coating. Barium and strontium nitrate will react with soluble chromates and dichromates or leach it from the surface to form the very low solubility respective barium or strontium chromate.

[Voryon]

Thank you all for the discussion!

 

According to the information Mumbles gave there is no hope for dichromate when using Ba/Sr with NH4Cl. As I'd like to bind with acetone I'd need to find a coating that is not acetone soluable. Parlon & NC are therefore not ideal, what about Linseed oil? May it add protection and is it insoluable in acetone after "drying"?

[WSM]

Thank you all for the discussion!

According to the information Mumbles gave there is no hope for dichromate when using Ba/Sr with NH4Cl. As I'd like to bind with acetone I'd need to find a coating that is not acetone soluable. Parlon & NC are therefore not ideal, what about Linseed oil? May it add protection and is it insoluable in acetone after "drying"?

 

To avoid further moisture problems while using acetone (which is prone to absorbing moisture from the atmosphere, as is alcohol), treat the acetone with dry calcium chloride (Damp-Rid, here in the US) to remove moisture, before using it in your pyrotechnic mixes.

 

Linseed oil is a drying oil, which slowly polymerizes as it dries. It also heats up as it goes through the polymerization process. Use caution. I'm not certain, but even if the cured linseed oil isn't soluble in acetone, it may swell while exposed to it. More research is needed.

 

Good luck and have fun (but also be safe).

 

WSM

Edited November 18, 2017 by WSM

[Mumbles]

Your best bet is probably to ditch the acetone binding. Coating the MgAl with parlon or linseed oil, and then binding with water is perhaps your best bet. Parlon is said to protect Mg from HCl, so surely it'd do the same for ammonium chloride. When ammonium chloride was used, they primarily used organic stars so these sorts of issues never really came up.

[NeighborJ]

Just thinking outside the box here but could the ammonium chloride be coated to prevent it from reacting with stuff? Maybe linseed oil then granulating it as fine as possible.

[Voryon]

Is there a "best practice" so to say to coat powders with linseed oil? Is there a Special linseed oil needed or is any Food grade linseed oil suitable?

 

NeighborJ I like the idea of coating the NH4Cl instead of the metal, some references state hygroscopicity to be an issue, so this could also help to prevent that if it works!

[MrB]

Is there a "best practice" so to say to coat powders with linseed oil? Is there a Special linseed oil needed or is any Food grade linseed oil suitable?

 

"Food grade"? Anyway, paintshop stuff servers you well.

[WSM]

Typically, boiled linseed oil is the one to use. It dries/cures slowly though (it takes almost a month to "air polymerize").

 

If a few drops of Japan drier are added to the solvent used to thin the linseed oil, it accelerates the cure time to a few days.

 

Be aware that the linseed oil heats as it dries and could cause a runaway reaction if large quantities are treated at once.

 

WSM