coating magnesium

[lloyd]

In most cases, one need not even "pre-treat" Mg powder.

 

As a matter of course rather than exception, I've routinely used a saturated solution of potassium dichromate to wet water-bound powders containing magnesium. In almost every case, such 'kool aid' suppresses adverse reactions as well as does pre-treating the Mg powder.

 

LLoyd

[Mumbles]

Hi Crazy Swede, long time no see. Glad to see you still around. Do you happen to have any other information or other resources about the addition of an electrolyte or electron donor. I read about this long ago in some paper, but can no longer find it. I've been searching high and low, as it had additional information on the actual make up of the coating and protective layer.

[Crazy Swede]

Hi Mumbles🙂

Phosphates can also be used.

 

I can check the references I used in my article in Pyrotechnica to see if I find the source.

[Crazy Swede]

My original source regarding the activators was an article called "Chromate Conversion Coatings" published in Metal Finishing Handbook and Directory from 1977. I do not have that article scanned but I did a quick search on the Internet and there seems to be a wealth of information on this subject on the same level although it seems that actual solutions being in use are secret.

 

Here are som links to articles that look interesting but I have not read them myself:

 

https://materialsdata.nist.gov/dspace/xmlui/bitstream/handle/11115/198/Chromate%20Conversion%20Coatings.pdf?sequence=3

http://www.plateworld.com/editorial62.htm

https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxjb3Jyb3Npb25mcmJhdXRufGd4OjExZTAyMDRiMGIzNmVmZGU

http://cdn.intechopen.com/pdfs-wm/12726.pdf

http://benthamopen.com/contents/pdf/RPTCS/RPTCS-2-55.pdf

[Fulmen]

The only conversion coating I've done on magnesium was straight chromic acid (CrO3). Nasty stuff to handle in kilogram quantities.

[lloyd]

No hydrogen, no acid! CrO3 is a solid oxide (CrIII oxide)

 

H2CrO3 is chromous acid. Chromic is H2CrO4

 

Lloyd

[WSM]

I can vouch for the Mg not changing color with the dichromate treatment. Mixed up a measured aqueous solution of the dichromate, added a mix of milled 60-120 mesh MgAl + 325 mesh Mg 50:50 which bubbled profusely at first but then halted after about 10 minutes. Saw no color change so let it sit with occasional mixing for about 4 hours and still no color change. I guess the dichromate solution turned a little more yellow/green instead of orange...

Anyway, filtered the Mg out in some paper towel and dropped some in an AP solution with a little mixing and there was no reaction. To make doubly sure I added a pinch of untreated Mg powder to the AP solution and it began mildly fizzing and got cloudy. Strange since I have read the treated Mg should turn "brown" but eh... at least it worked.

 

It's my understanding that when magnesium is coated with dichromate, the dichromate disassociates and the results are magnesium chromates coating the metal and potassium chromate solution. There is a slight perceivable color change on dried, treated magnesium and the potassium chromate is bright yellow (which looks greenish when the magnesium particles are still there).

 

If you vacuum filter the mixture and separate the treated magnesium from the liquid, the filtrate will be bright yellow in color. To deal with the hexavalent chromium K₂CrO₄ solution, I've combined it with copper nitrate solution and dropped out a brown precipitate of copper chromate. When vacuum filtered and dried, it can be heated in a covered crucible and converted to the much-sought after catalyst, copper chromites. Also, by similar means (not the roasting in a crucible part), it can be converted to barium or lead chromate.

 

If you have no need for such things, there are chemical means to convert the potassium chromate residue to trivalent chromium compounds, which are much less toxic.

 

WSM

Edited August 24, 2016 by WSM

[WSM]

No hydrogen, no acid! CrO3 is a solid oxide (CrIII oxide)

H2CrO3 is chromous acid. Chromic is H2CrO4

Lloyd

 

I'm not sure about the solubilities, but isn't chromic oxide mixed with water chromic acid?

 

CrO₃ + H₂O --> H₂CrO₄

 

Whether or not the process requires special facilitation, I haven't researched, but it may be so. If I needed any such thing, I would have studied it out.

 

WSM

[rsambo]

The issue of coating, or not coating magnesium and similarly how one should set about formulated a "stable" composition has fascinated me for sometime. I'm working with magnesium, ammonium perchlorate, guanadine nitrate and bromates on a regular basis and I follow the principles laid out by Dr Takeo Shimizu in Proceedings of the 19th International Pyrotechnics Seminar, February, 1994. Stabilizing Firework Compositions – I. Minimum Solubility Law to Foresee the Degeneration, II. A New Chemical Method of Magnesium Coating.

 

That is to say, in his opening paragraph in that paper the good doctor states :

 

"...the mixture often causes chemical degeneration to remove into a more stable state which is opposite to the purpose. The direction of the change has been unknown without experiences. It has been a great difficulty on selecting materials. I have found a rule to foresee the direction: the component materials in a mixture gradually decompose with each other to create the most water insoluble material. This tendency should be called the “minimum solubility law”. A table was prepared to foresee the direction of the degeneration reactions arranging materials in the order of their solubilities. When a magnesium flake is soaked in a solution of dichromate and sulfate, the flake is gradually coated with a thin black film. It may be CrO2 and have a high corrosion resistance. The effect was tested with several dichromates and sulfates against mainly ammonium perchlorate using magnesium ribbon and powder. In addition an effect of guanidine nitrate on the coating was observed because it gave a good result of corrosion resistance when it was used as a blinker (strobes) in the past."

 

I follow these principles and use his tables accordingly. He noted that when storing stars for long period of times ( I think 8 years ) those that contain a sulphate stored significantly better - in that paper he similarly states " Sulfates should be the most important material to avoid corrosion of magnesium" - he is referring to strobes and similarly the next most important ingredient is guanidine nitrate.

 

The sulfate is thought to react with the magnesium to generate hydrogen bubbles which then react with the dichromate to form CrO2 which forms the black film on the surface of the magnesium.This formation of black film only occurs with the sulfate.If you are coating only with dichromate but THEN adding to a strobe composition that contains a sulfate the black film will eventually form, albeit slowly, and afford greater corrosion resistance.

 

He goes on to make an entirely different statement altogether considering an alternative method :

 

220 g of a hydrate of magnesium sulfate, MgSO4•7H2O,
90 g of ammonium dichromate, (NH4)2Cr2O7,
and 20 g of guanidine nitrate, CN3H5•HNO3,

 

are dissolved in 500 cc of water. 1000 g of magnesium powder is placed in a fairly large aluminum bowl and the solution is added to it. It is mixed, stirring by hand with gloves, until the powder colors uniformly brown. Then the powder is spread on a sheet of Kraft paper and dried well in the sun. When dried, it is passed through a 30 mesh sieve. The dust must not be inhaled because ammonium dichromate is poisonous. With this method, the resistant black film is not formed in a short time even when the powder is soaked in a larger volume of the solution than the above. However, when the coating materials exist together with the magnesium powder, this method was far more effective at a corrosion test against ammonium perchlorate than
the mechanical coatings using paraffin, linseed oil or polyester, which gave only a short time resistance of a few seconds or minutes.

 

Sam

[Fulmen]

Technically CrO3 is the anhydride to chromic acid, I forgot to mention it was in aqueous solution. Warm chromic acid, yummy.

[lloyd]

Yeah... just foolin' with you. Somebody might have tried adding the anhydrous chromic oxide to magnesium... and been AMAZED! Grin!

 

Lloyd

[Fulmen]

Aw crap! That would be something to watch.

.

.

.

.

.

.

 

From a distance.

 

 

But you're right in ripping me, the devil is in the details. Especially when dealing with energetic materials.

[Crazy Swede]

...In addition an effect of guanidine nitrate on the coating was observed because it gave a good result of corrosion resistance when it was used as a blinker (strobes) in the past...

Well, I tried that when doing all those tests on the magnesium ribbons before writing the article in Pyrotechnica XVI but I could never prove that the guanidine nitrate really helped.

 

I do believe though that anything that can secure or seal the protective film is beneficial for minimizing the corrosion and best is of course an additive that is already there in the treating solution.

[MadMat]

Yeah chromium trioxide is some nasty stuff. Mixed in water at about 2 lb.s per gallon with sulfuric acid (adjusted to a ratio of 150-200:1 sulfate ion) and you have chrome plating solution.

[pyrojig]

I read something I believe was from Fireworkings site about coating Mag with parlon. I believe it was related to the rainbow stars and other magnal/ Mag stars.

In past I have had luck with ballmilling dichromate with the AP, then using that in the strobe or star comps/. didnt see any adverse reactions .

[lloyd]

I, also, use dichromate with AP, and like you, I add it during the milling cycle. It's always worked for me, too.

 

Lloyd

[Mumbles]

I read something I believe was from Fireworkings site about coating Mag with parlon. I believe it was related to the rainbow stars and other magnal/ Mag stars.

In past I have had luck with ballmilling dichromate with the AP, then using that in the strobe or star comps/. didnt see any adverse reactions .

 

 

It's been known since around the time of Troy Fish and go-getter that dissolved parlon provides some extra protection to metals, at least as long as the solvent is dry. A member here did some experimenting with it several years ago.

 

http://www.amateurpyro.com/forums/topic/3835-coating-mg-with-parlon/

[GermanPyro]

Hello together,

 

I have a question, in other treads I've read, magnesium can be coated with parlon. I hope, anyone of you can help me, I can't find the correct ratio of magnesium to solved parlon and the ratio of parlon and mek.

 

Thanks in advance & greetings from germany

stefan

[Mumbles]

The thread I linked to in the post just above yours is about as thorough of testing that I'm aware of on this forum.

[GermanPyro]

The thread I linked to in the post just above yours is about as thorough of testing that I'm aware of on this forum.

 

Yes, I've read this tread. But, a couple of ml --> how many ml are a couple?

[OldMarine]

The thread posted above states 10% parlon per kg of Mg dissolved in enough MEK added to just cover the metal in a shallow dish. You can pour off and save the remainder of the solution for reuse. I still wonder how even that small amount of Parlon will affect the comps.

[GermanPyro]

Thanks!

 

Greetz

Stefan

[schroedinger]

Since your in the EU MEK can be a problem to get, you can also use pure Acetone for this process. Dry it over pottasium carbonate or molecular sieves.

 

After coating it, take a test tube add some of the coated metall and check the coating with hydrochloric acid.