Protecting Al and MgAl - Glitters, Nitrates

[LiamPyro]

 

I have few questions regarding the use of aluminum and magnalium in glitters and other compositions containing nitrates:

1. In glitters containing sodium oxalate and aluminum, is adding boric acid sufficient to prevent degradation of the aluminum? I made some D1 glitter recently, and given the presence of sodium bicarbonate I omitted the boric acid, as the addition of acid to a basic mixture seemed pointless. The stars turned out... weird. I pumped them with a minimum of water and dried them slowly with plenty of airflow and minimal to no heat, initially. See the attached video of the 3" ball shell I made with them - the break was screwed up as I failed to prime the stars adequately. The tail on the stars was very lackluster (but the rocket was nice). There's also a video of a comet I made with the composition, which looked a bit better. Not sure if this was due to a nitrate/Al reaction or something else I screwed up on. I'm interested in using a sodium oxalate and aluminum glitter gerb composition and like to wet/rice/dry my compositions before pressing for ease of handling, but want to prevent any issues, which is why I asked about boric acid - I'll need to wet the composition significantly to rice it. Is pure alcohol a better choice instead of water?

2. I've heard that boric acid is not helpful in preventing degradation of MgAl, and is actually bad. I'm interested in making some MgAl glitters, as well as Sr(NO3)2 and Ba(NO3)2 compositions with MgAl. Are there any precautions that must be taken when wetting these compositions with water? Should I avoid water altogether in the color comps and simply bind with parlon/acteone for stars or rice with alcohol for pressing? Is there anything that can be added to prevent any attack of the MgAl by the nitrate, if this is even a concern? I'd like to rule out degradation of the metals as a factor before exploring these formulas further...

 

 

 

 

[Zumber]

If you are planning for a gerb then you are not going to use binder as it is necessary in case of stars. Boric acid solution in water instead of dry boric acid powder would be helpful to counter aluminium nitrate reaction also it has sodium oxalate which is basic in nature which may promote reaction.

Just be sure don't damp larger batches at a time. Try smaller batches and work in ventilated area and try to use minimum water as possible.

I often made red /green with magnelium as a fuel, sometimes my green may heat up sometimes it may not.

We have access to coated Magnelium and uncoated magnelium and uncoated magnelium often caused my batch to get warmer specially green. I often don't damp larger batches at a time, I work on small batches finish it and then proceed for next one.

[Mumbles]

The nitrate/metal reaction is base driven. If you want the full chemical equation, just let me know, but in the interest of clarity I'm leaving it out for now. Essentially what happens is that metal and nitrate in the presence of water converts to ammonia, hydroxide ions, metal ions, and heat released. Aluminum is actually a fairly reactive metal, but forms a thin protective oxide coating on the surface preventing further reaction or oxidation. It's why we can have so many things around the world made of aluminum that are fairly stable. The hydroxide ions and ammonia are capable of stripping off the protective layer naturally present on aluminum. This in turn reacts more, and it eventually can become self-sustaining or start to run away. This last part is what we observe and detect as a bad reaction.

It seems sort of counterintuitive, but boric acid and sodium bicarbonate don't actually react. It's easy enough to mix a little and find out if you want to prove it to yourself. It's hard to describe without getting into nitty gritty chemistry details, but boric acid is more of an acid in the sense that it absorbs hydroxide ions than in the sense that it pumps out H+ ions. It does tend to react with magnesium and to a lesser extent magnalium. I tend not to use boric acid in MgAl compositions. I just choose compositions that are known not to have issues.

High quality strontium and barium nitrates don't really react with metals. That base driven thing I mentioned earlier doesn't really happen with barium and strontium. The hydroxides of both are pretty insoluble, so it tends to really slow things down and not really let the reaction take off.  

[LiamPyro]

Thanks to both of you for the responses. Mumbles, I actually have a background in chemistry so I can’t believe I didn’t think this one through more throughly. Looked up the structure of boric acid and see what you mean, it should act preferentially as a Lewis acid in the presence of hydroxide ions. I’ll go ahead and use a boric acid solution to wet my next batches of aluminum glitters regardless of their composition. 

[kingkama]

I solved the problem of the interaction between nitrates and MgAl/Al by eliminating the dextrin and using pvb and ethyl alcohol

[DavidF]

There's no problem with MgAl and nitrate-based red and green stars with high parlon content when using acetone and making cut stars using the bag method. If the small amount of fallout from the high parlon is an issue, a small amount of charcoal or coarse metal can be added to hide it in the tail

[LiamPyro]

Yeah, I figured non-aqueous solvent/binder systems were safe. I asked about water specifically because I want to screen granulate colored gerb fuels before pressing them and therefore don’t want the mix to be strongly bound (imagine trying to press rubbery parlon bound granules into a solid fuel grain…)