Mumbles Posted February 5, 2006 Share Posted February 5, 2006 Posted by: Mumbles Posted on: October 4th, 2005, 3:46pm Ok, I felt it neccesary to share this post I made over at UK Rocketry. It isn't fair to leave you out of my geniushood . There is no set reaction for the dichromate/Mg interaction. Get ready because the following explanation will be highly chemistry based. I can see already one big misconception. This is that a layer of MgO or Mg(OH)2 is formed and this protects from corrosion. This is not quite the case. This layer will be combined to be refered to as the oxidation layer because I don't want to have to keep saying the names over and over. Well the oxidiation layer is not the same as Aluminum's, in fact they are very different. Al forms a strong bonded oxidation later. It is passive and non-porus. Al can withstand attack from concentrated HNO3. It forms the passive film of oxide, and the nitric acid is no longer able to get through it. Magnesium on the other hand has a weak oxidation layer. It forms in multiple layers, and is not strongly attached to the metalic nucleus underneath. Because of this the oxidation layer is flaky, easily attacked, and does not provide proper oxidiation protection. If you put Mg in water, it would completely react. The addition of chloride(and probably other electrolyte) ions increases the attack speed signifigantly. The dichromate addition does provides protection from water reaction, and chloride reaction. Because of this fact, it is obvious it does not simply provide an oxide layer on the Mg. If you have ever passivated Mg, you will know that you don't just soak the Mg in dichromate solution, and all is fine. You actually coat the Mg with the solution, and let it dry on the metal. this signifies that the dichromate (or derivitives of it) assist in the protection. Otherwise, we would just soak the metal, and use the remaining Dichromate to coat the next batch. Long story short, the chromium ions integrate as part of the oxidation layer. Inside the oxidation layer, spectromicroscopy has found Cr(III) ion, these come from the dichromate ion (Cr(VI) oxidation). The integration of Cr(III) ions, and Cr(VI) ions in addition to the oxidation layer makes a harder, less permeable layer. The same phenomenom is seen in Al. If Al is oxidised with dichromate, an even harder oxide layer is observed. The most resistant oxidation layers are found to be formed from pure metallic particles, not partially oxidsed ones. To get proper integration of the chromium ions, it needs to be combined during the formation of the layer. As far as practical use of this. I would say as long as your Mg is still sparkly or shiny, it will be fine to coat. If you have old corroded Mg, perhaps washing with very dilute HCl, and water DIRECTLY before coating with the dichromate would be acceptable. I almost forgot. Here is a journal article on this. If you don't understand it, as I'm sure there will be enough, just read the conclusion, points 2 and 4. It doesn't cover everything, but it will tell you all you need to know in a much more condensed manner. http://www.mse.eng.ohio-state.edu/~frankel/fcc/frankel69.pdf ----------------------- Posted by: BigBang Posted on: October 4th, 2005, 5:20pm wow, I always thought that It just formed a layer of dry dichromate. I didn't know that it actually interacted with the oxide layer in that way. Posted by: Mumbles Posted on: October 4th, 2005, 5:55pm I had trouble deciding where to put it initially. I feel it is more of a pyrotechnic issue than a general chemistry issue. I will also be making a tutorial for coating Mg, and will probably post it here too. ----------------------- Posted by: rogue_chemist Posted on: October 4th, 2005, 6:01pm I had thought dichromate would just be reduced to Cr3+ and then further reduction to a Cr layer on the Mg. I wonder if the used dichromate solution contains Mg2+? If so Mg is being lost when you passivate your Mg. Interesting AARG....you moved it in the middle of me posting.... ----------------------- Posted by: Kilo_G Posted on: October 4th, 2005, 9:05pm Completely apart from chemistry, cause I know nothing about it: I'm ascared to use dichromate Yuckey stuff. How bout a way to coat magnesium not using dichromate (linseed oil)? Wanna show a tutorial of that? ----------------------- Posted by: Mumbles Posted on: October 4th, 2005, 11:17pm Yeah, I can do that too, but it will go in a different thread. Dichromate is not actually all that bad anyway. There are much worse things. It is primarily used in solution, so there is no threat of inhalation. There is also some stuff about Ammonium Molybdenate(a fertilizer), and something else. ----------------------- Posted by: rooster Posted on: October 5th, 2005, 2:27am Quote:If you put Mg in water, it would completely react. You being all that genious, I had to look for something, dude Magnesium will only react with steam, not with water or even hot water. If you are talking about the oxide layer, it will also not react, since magnesium hydroxide and magnesium oxide are unsoluble in water(practically). Other than that, the post is purely genious. ----------------------- Posted by: Crazy_Swede Posted on: October 5th, 2005, 4:13am I think I've posted this before. Hmmm... maybe it was on Rouge Science? Anyway, there is an interesting article in Pyrotechnica XVI called "Corrosion Protection Of Magnesium Without The Use Of Chromates". The article describes to moderate depth how chromate conversion coatings work and also suggests an alternative method based on ammonium metamolybdate. (I've tried this by the way, it's ok with Mg ribbon but doesn't work well with powder!) One aspect that people in the fireworks business seem to have missed, is that it is ineffective to treat magnesium with pure dichromate solutions. You also need an anion activator, for example acetate, formate, sulphate, chloride, fluoride, nitrate, phosphate or sulphamate. The activator corrodes the Mg metal and provides the hexavalent chromium with electrons to facilitate its partial reduction to trivalent chromium. The protective layer formed consists of an amorphous and complex mixture of hydrated basic chromium and magnesium oxides. The reasons why chromate coversion coatings give superior corrosion protection are: The film is less reactive or completely unreactive with the environment to which the magnesium metal is being exposed. It is also less porous and thicker compared to the untreated surfaceNot all chromium is reduced to trivalent forms, the film still contains some soluble chromate which can leak out when a scratched film is exposed to humididty or agressive chemicals, providing a self healing effect! To answer rouge_chemist's question: Yes, some Mg is lost in the process. One should also be aware of the fact that treated Mg will react more slowly in pyrotechnic compositions compared to the untreated product! Also, don't forget that the superficial layer of treated magnesium powder still contains some hexavalent chromium. Don't inhale any dust rasing from handling such powders and don't touch them with bare hands! ----------------------- Posted by: Mumbles Posted on: October 5th, 2005, 9:00am That is very interesting with the anion activator. The article I posted mentions the anions, specifically chloride, but from reading the article I thought that was just there as a corrosive agent. Something that normally corrodes Mg with great ease is stopped by the chromate activation. I never realised it actually played a roll on the coating process. Out of curiosity I was looking through some scientific journals for more information. Of course anything that would actually provide good information is in journals my university doesn't subscribe to. Most of the stuff I found was on laser passivation or coating the Mg with Ni. There were a few things that I could get that provided a bit of insight. There was one article on protection of Mg with sulfate. To my dissapointment, it was determined that it didn't work at all. There was one on how ballmilling Mg actually reduced it's tendancy to corrode. I found this very interesting and I will have to read it again. After reading through somethings, something just came to me. It just put a big smile on my face. I had actually discovered a legitimate use for Permanganate in pyrotechnics. Cr, Fe, Ni, and Mn all help in the corrosive resistance of Mg by integration into the oxidation layer. After some more searching, I found my idea wasn't so new or groundbreaking as there were already several articles on the subject. Perhaps I will do a comparison of all the different Mg corrosive resistance method(sans the laser and micro-arc passivations). I just don't really know of a way to test it. With Al I would use the nitrate/Al reaction and check for ammonia. With Mg though, I don't know, maybe dampening it and looking for hydrogen or heat production? I don't think reaction time with HCl or the like would be appropriate. While it may stand up to water and chloride, it probably won't stand up to acidic hydrogen. Eh, I have enough Mg that wasting 5g won't be a problem. ----------------------- Posted by: pyrohawk Posted on: October 5th, 2005, 2:10pm on October 5th, 2005, 2:27am, rooster wrote:You being all that genious, I had to look for something, dude Magnesium will only react with steam, not with water or even hot water. If you are talking about the oxide layer, it will also not react, since magnesium hydroxide and magnesium oxide are unsoluble in water(practically). Other than that, the post is purely genious. You sure about that? I thought that the oxide would just crumble off of Mg and the Mg continue the reaction unlike Al or whatever... Thought it had something to do with the speed of the reaction and how well the oxide layer "stuck".... So I guess I'm confused... ----------------------- Posted by: Crazy_Swede Posted on: October 6th, 2005, 1:25am Mumbles, the biggest problem in pyrotechnics is to protect magnesium from corrosion when in contact with ammonium perchlorate. Therefore, I suggest that you use a concentrated solution of ammonium perchlorate instead of any acid if you would like to do some research. The corrosive reaction causes heat, hydrogen gas and also raises the pH. Depending on what kind of equipment you have at hand, some of those could be monitored to compare degree of corrosion in a test series. Here are some more info from the article I mentioned: "...The mechanism of the film formation is not exactly determined, but a simplified reaction for the treatment of magnesium with a sulfate as the activator could be: 6Mg(s) + 6SO42-(aq)->6Mg2+(aq) + 6SO42-(aq) + 12e- 12e- + 2Cr2O72-(aq) + 2Mg2+(aq) + 12H+(aq) ->2MgCr2O4(s) + 6H2O(l) This also explains the subsequent rise in pH during the treatment..." Link to comment Share on other sites More sharing options...
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