Visco Posted October 16, 2016 Posted October 16, 2016 Can everyone help me on understanding why metals are coated? I understand steel for reasons of rusting, I guess my biggest questions are when do we have to coat magnesium? Does the same apply for magnaluim? Linseed oil or now I see possibly Parlon for water based comps and dichromate for AP. Does all magnesium and magnaluim need to be coated?
OldMarine Posted October 16, 2016 Posted October 16, 2016 Magnesium is a very reactive metal that doesn't readily form a protective oxide coating like aluminum does so it reacts quickly when wetted in the presence of oxidizers and very high and very low ph chems. Mgal is slightly less reactive due to the oxide coating provided by the aluminum. I've started using a dichromate solution for wetting both mg and mgal bearing comps just for safety's sake.I'm sure others can give a better explanation but that's what I've learned so far.
MadMat Posted October 16, 2016 Posted October 16, 2016 (edited) What OM said is pretty much spot on. As far as the coating process goes, dichromate reacts with the metal and creates magnesium chromate on the surface. This is a fairly inert material, in fact, it is used in plating processes. Ever see zinc plated screws, nuts or bolts that look yellowish? that is a chromate conversion coating added to the zinc plating to increase corrosion resistance. As far as coating your Mg with linseed oil, thats pretty much obvious. Basically, you are varnishing the surface of the metal. I prefer the chromate coating because the linseed oil, in addition to being messy, also adds a small but discernible amount of fuel to your reaction. I believe this can "dirty up" the color in your stars. To get brighter, more pure colors, I have gone so far as to replace magnalium with straight magnesium (you have to do the stoichiometry math as there is a slight difference to make sure your formula is balanced). Edited October 16, 2016 by MadMat
Visco Posted October 17, 2016 Author Posted October 17, 2016 Does magnesium need to be coated in comps that are bound by pvc or Parlon?
Mumbles Posted October 17, 2016 Posted October 17, 2016 Parlon has been shown to be protective to Mg. I would imagine PVC would be pretty similar if used in a true solvent. The biggest thing with doing this is making sure the acetone or solvent is sufficiently dry. I've heard accounts of Mg reacting with various things when wet acetone was utilized. I think this was specific to pourable go-getter slurries when those were popular if it rings a bell for anyone. That said, it never hurts to have a double ended attack against any reactions. I'd really only think about this for like Ammonium Perchlorate/Mg formulas. AP is very corrosive to Mg.
OldMarine Posted October 17, 2016 Posted October 17, 2016 I don't use much magnesium but I treat all of it with dichromate as soon as I get it. It only takes a few minutes to apply and then you just let it dry. Quick easy peace of mind.
Visco Posted October 17, 2016 Author Posted October 17, 2016 If I have 200 mesh magnesium and coat it what mesh do I use to re screen with? Does the dichromate layer protect against water? Or should I use Parlon or linseed oil with acetone.
OldMarine Posted October 17, 2016 Posted October 17, 2016 Using dichromate, you shouldn't need to screen it. Just keep out stirred up while it's drying to break up any clumps. It doesn't seem to cause adhesion between the grains of metal and mine flows freely after drying.
MadMat Posted October 17, 2016 Posted October 17, 2016 (edited) Dichromate treatment doesn't add any size to the particles of magnesium. The magnesium chromate coating goes into the metal rather than building up. So, as OM said, just stir it up as it dries to break up the clumps. And yes, magnesium chromate does not react with water. Keep in mind though, This coating is very thin and will break down if constantly subjected to harsh treatment. Edited October 17, 2016 by MadMat
Mumbles Posted October 18, 2016 Posted October 18, 2016 What OM said is pretty much spot on. As far as the coating process goes, dichromate reacts with the metal and creates magnesium chromate on the surface. This is a fairly inert material, in fact, it is used in plating processes. Ever see zinc plated screws, nuts or bolts that look yellowish? that is a chromate conversion coating added to the zinc plating to increase corrosion resistance. As far as coating your Mg with linseed oil, thats pretty much obvious. Basically, you are varnishing the surface of the metal. I prefer the chromate coating because the linseed oil, in addition to being messy, also adds a small but discernible amount of fuel to your reaction. I believe this can "dirty up" the color in your stars. To get brighter, more pure colors, I have gone so far as to replace magnalium with straight magnesium (you have to do the stoichiometry math as there is a slight difference to make sure your formula is balanced). Do you have any literature or more in depth sources on the treatment process or what actually happens on the surface? I've always been under the impression that it was more complicated than that. Just curious to dive a little deeper.
MadMat Posted October 18, 2016 Posted October 18, 2016 (edited) Sorry Mumbles, I'm just going on past experience in the metal finishing industry (a long time ago). But as I stated, it is a conversion coating, which implies that it is a conversion of the surface of the parent material. You may have some luck searching metal finishing/plating data. There was a "bible" of metal finishing, which gave out formulas for just about every type of plating/metal finishing processes available, but I can't remember, for the life of me, what the name of it was. The biggest problem you will encounter is that many of the formulas (brighteners, leveling agents ect.) are proprietary information of various chemical companies and they guard their secrets. Back when I worked in the metal finishing lab, three of the big guys in metal finishing chemicals were; Enthone, MacDermid and M&T chemicals. I don't know if these companies are even still in business (It was the early eighties), but just maybe they could help with some information. Edited October 18, 2016 by MadMat
OldMarine Posted October 18, 2016 Posted October 18, 2016 Google: action of dichromate on magnesium. I would have linked a couple but my phone goes straight to the pdf files.
MadMat Posted October 18, 2016 Posted October 18, 2016 As I remember it, in most of those chromate conversion coatings, the solution was made acidic (about 2% nitric acid). One of those companies I listed had a process called "Iridite" which was used on Al/Mg alloys and maybe even die cast. the biggest thing that stands out in my mind, was when the tech. rep.s would stop by the lab occasionally. There would invariably be an expense account lunch involved. Some of those reps were a lot of fun!
OldMarine Posted October 19, 2016 Posted October 19, 2016 Here's an interesting excerpt from an article I'll link below: "Dichromate treatment The dichromate treatment differs from chrome-pickle treatments in the following aspects: 1. Dichromate treatment itself removes no significant amount of metal and can be applied to finish-machined parts; 2. Coating formed by standard treatment is substantially thicker than that created by chrome pickles. This results in better stand-alone protection in mild atmospheres, allowing its use as a final coating on machined surfaces of sensitive components in computer disk drives; 3. The greater thickness and protective value of dichromate coatings is accompanied by higher electrical resistivity, and except for very brief treatments, the coating is unacceptable for grounding contacts in electrical applications; 4. The extended treatment times, high bath temperature, and the preparatory steps required for surface activation results in increased costs. While providing an excellent paint base, the dichromate treatment, is not used routinely for this purpose where lower-cost treatments are adequate. The dichromate treatment is applicable to all magnesium alloys and forms, except for LA141A, M1A, and alloys containing rare earths or thorium" Link: http://www.magnesium.com/w3/data-bank/index.php?mgw=182 And Mumbles must have Old-Timers disease since he seems to have forgotten this excellent post back from the dawn of time: http://www.amateurpyro.com/forums/topic/106-dichromate-corrosion-resistance-of-mg/
Mumbles Posted October 19, 2016 Posted October 19, 2016 I remember the details I made in the post, but forgot about that post. I was mainly looking for more documentation. I REALLY appreciate you bringing that post back up though. I've been looking for the file I linked to in that thread for several years, and finally found it tonight.
OldMarine Posted October 19, 2016 Posted October 19, 2016 I had found a patent for dichromate treating Mg while searching earlier today but the search is not showing up now that I'm on my laptop. Found it:http://www.freepatentsonline.com/2134830.pdf
Visco Posted October 19, 2016 Author Posted October 19, 2016 I have a 3M mask what kind of filters would be good for using dichromate?
MadMat Posted October 19, 2016 Posted October 19, 2016 Any mask rated for dusts should be fine. I don't particularly care for the paper disposable masks as they never seem to seal properly on your face. I bought a twin cartridge respirator type mask. They are available at Harbor Freight stores and are relatively inexpensive. Mine has cartridges for organic vapor (good when using solvents like acetone or MEK) and also has dust filters over the cartridges.
SharkWhisperer Posted January 19, 2020 Posted January 19, 2020 (edited) Apologies in advance for inadvertently rekindling a thread that went to sleep way back in 2016. I accidentally posted to this thread instead of a more current query posted today on the topic of coating Mg powder... Nonetheless, here's a decent link with a detailed protocol for protecting Mg with dichromate... http://www.pyrobin.com/files/Coating Magnesium.pdf Edited January 19, 2020 by SharkWhisperer
pyrodivision Posted April 7, 2020 Posted April 7, 2020 What is the ratio precentage? to treat magnesium with linseed oil? And can i dissolved linseed oil with aceton to facilitate the coating process
Recommended Posts