Coating Mg with dicromate

[HoS]

I really have no clue on how to do this and can not find out how anywhere. Google has failed me

 

Thank you guys tons.

[rocket]

For stared what do you want to coat the Mg with?

There is linseed oil, paraffin and potassium

dichromate, there the only ones I know of but there may be more.

[HoS]

I was thinking KCrO7, has said in my title but I'll take any info I can get.

[Pretty green flame]

IIRC, for 500g of Mg you need 25g of K2Cr2O7. You mix the dichromate with hot water (i don't have any information on how much but was told it doesn't matter really, just keep it within normal limits). Slowly add the Mg (slowly as in not all at once). The magnesium is coated when all the foaming stops, after the foaming stopped keep stirring for a minute or two to ensure the Mg is fully coated. Strain the Mg out and let dry. The coated Mg should now be treated as it were potassium dichromate (toxic carcinogen)

 

Be safe.

 

Cheers

[HoS]

Hummm that easy huh, Thanks for the ratios.

[rocket]

Woops must have mist that, what PGF has post it pretty much covers it. With the amount of water, its 300ml per Kg of Mg. oh and other thing heat the Mg in an oven at 100c for 1 hour or so and add it to the K2Cr2O7 solution, that’s all I can add.

[HoS]

I can think of a few reasons chemicaly to heat it but none make sence for this, care to elaborate?

[rocket]

I would if I knew myself, I read about it on passfire.com.

The only thing I can think of is that in a way it burns the K2Cr2O7 onto the Mg.

[HoS]

Well thank you all when I finally get my chems in this will be very hand, and I'm off to bed, good night.

[TheSidewinder]

Also, please remember that the left-over Dichromate solution is HIGHLY TOXIC!

 

Even after coating the Mg, theres enough left in soultion to cause all kinds of problems.

 

Wear heavy rubber gloves (not cheap latex or vinyl "exam gloves") when mixing and coating, use a splash guard for your entire face, and a respirator AT LEAST during the mixing phase when the Dichromate is in powder form, and can become airborne. Its effects are cumulative.

Also, the gas given off during coating is inflammable. Obviously, keep away from a flame source.

 

Once done, store the remaining solution in an airtight (preferably Poly Ethylene) container. Assuming you're in the US, you can dispose of it through most local hazardous waste facilities as long as it's labelled as a Chrome Salt.

[HoS]

The gas is H2 right? Also what grade of respirator is needed, and should I wear throw away clothing?

[Mumbles]

Yes, it's hydrogen gas.

 

To me, the heating thing is useless and possibly harmful to the coating process. I know I have a big post about the chemistry behind the coating process. Probably in the general chem section. Basically what is happening is the solution is physically corroding the magnesium. Chromium oxides of various oxidation states is integrated into the oxide layer. Usually the magnesium oxide layer is pretty flaky and easily penetratable. The chromium integrated makes the oxide layer bind tighter and become much less impermeable to the various attacking agents.

 

To me heating would just add an unneccesary oxide layer to the magnesium that could compromise the binding of the protecting oxide layer. Perhaps it speeds up the reaction, hard to tell without some tests.

[Pretty green flame]

http://www.apcforum.net/forums/index.php?showtopic=106

 

Here's the post mumbles was talking about. Actually the first post in the pyro section

[Mumbles]

Didn't realise I made that post that long ago. I see I also never followed through on my tutorial promise or other experimentation. I actually haven't even coated the majority of Mg that I have. Interesting to read that again and get some ideas.

[oriansbelt]

I might have an answer to why they say to cook it. Chromium oxide is a type of ceramic so it would seem sencible to cook it to harden it on but I don't think would actually help. It was just a thought.

[Mumbles]

Chromium (III) Oxide isn't a ceramic material really. It is used as a colorant in glazes, not the actual ceramic material. The cooking is done before the coating process, so it doesn't cook it on either.

[FrankRizzo]

I believe heating the magnesium causes a uniform oxide layer on the suface of each particle that is then augmented via the chemical reaction with the dichromate solution. Or maybe the reason was to ensure that the magnesium *under* the oxide coating reacted...I honestly can't remember, but it *is* an important part of the process.

[cplmac]

Passfire says 1oz of water per 100g of mag. And like everyone else said, 5% Dich.

[Mumbles]

The coating isn't effective unless the corrosion happens in contact with the dichromate. You can't simply augment the oxide layer with chromium. The oxidation of the magnesium is possibly what causes the reduction from Cr (VI) to Cr(III).

 

My only guess is that it weakens the existing outside layer. This would allow better contact for the corrosion. Another possibility is that the heat serves as corroding agent, whereas you would need a chloride or sulfate salt for optimum protection. Read the thread that PGF pasted if you don't know what I'm talking about with the chloride or sulfate.

[ltf]

http://www.jackerypyro.com/dichromate.html

enjoy