48 replies to this topic

[digger]

Now then, I have made a whole bunch of ematches in the same manner that Paul does his in this thread Pauls ematches.

I then dipped them in a slurry of BP and NC lacquer, after drying they worked very well, however after leaving them for a few days I tested them again and nothing. I removed the BP from a couple of them to see what was going on and I found that the nichrome wire had corroded away.

Does anyone have an explanation for why this may have occurred and how to stop it happening again (do I need to use a different type of slurry?).

Any help would be greatly appreciated.

[T-sec]

Does anyone have an explanation for why this may have occurred and how to stop it happening again (do I need to use a different type of slurry?).

Any help would be greatly appreciated.

Home made bp normally contains a small amount of water, like 2-3%, maybe this caused your nichrome wire to corrode.
Next to this you made a bp/nc laquer based slurry, this will also contain 1 or 2% water, just enough to maybe get the corrosion on your nichrome wires. If the wires you are using are real nichrome wires...

An other cause can be the difference in metal used to make the ematch. When for example your ematch base is a small piece of PCB with double copper layer, and the wire nichrome, welded together with lead based soldering wire, this can cause one metal to corrode when attached to the other. You can try to use 4% silver based solder wire instead.

Giving the nichrome wires a pre-coating with a water resistant layer will probably not work, because the bp/nc laquer slurry will probably solve this layer.

[Arthur Brown]

Given the corrosion resistance of nichrome alloys I'd look at whether the wire you have is nichrome.

[digger]

Thanks for your answers chaps

Yep, both things I have thought about. Arthur I do have another source of nichrome wire so I will try that (however the wire that I used has the right resistance for nichrome, so I do not think it is constantan). As for galvanic corrosion as you mention T-Sec I assume this is possible, however the speed at which this occurred I think would rule this out. The only plausible thing I can think of at the moment is that it is oxidation due to water along with the action of the nitrate.

As you mention when using a nc slurry it would probably dissolve any coating I try to use, so I think I will try to coat the wires with a nc lacquer first then use a water based slurry to see if that helps (maybe even a white glue based slurry).

[Andrew]

Nichrome, will not corrode because of water. Would would not be able to weld nichrome wire to copper, not without laser equipment that would cost millions. You would "solder" it. Anyway...

The corrosion is definitely because of the oxidiser present in the composition, without a shadow of a doubt.

There is another possibility, if the wire was not soldered to the leads you have what is called dry contact. This is big problem because powder can get into the gaps and cause a dry joint (no conductivity).

Anyway, back to corrosion. I've seen my 216 stainless steel thrusters get corroded by HTP, it rusts! Nichrome is not that impervious to a powerful oxidiser, especially when there is whetted contact, like in your case.



There are some solutions.

Firstly the nice simple one, put a THIN coating of NC on the wire. This can then be inserted directly into your device once dry. A further improvement, would be to wait until it has dried and apply another coat of NC lacquer and then dip it in loose powder. You could always use a slurry in your second coat, but I find that loose powder dipped performs better, they are also smaller (but also more delicate).

The second way to reduce corrosion is to reduce the surface area exposed to the oxidiser. I've made e-matches using a very fine fraction of grain powder (80# to 120#). This is the really small stuff that you should really put back into the mill. But I'm all for reuse.

Most important of all, always solder your wire to the leads!

[digger]

Nope the wire was not soldered to the nichrome wire, however the method of construction puts a large area of the nichrome under pressure from the bell wire (0.6mm) to ensure a good contact. On disassembly of the ematches, the nichrome wire has completely dissolved (no trace left), so nitrate corrosion must be the one (maybe with a bit of help from acidity from the sulphur used in the bp).

I will try the double coating method to see how that turns out.

[Andrew]

(maybe with a bit of help from acidity from the sulphur used in the bp).

Doesn't help no, try the coating method with soldered wires, I know soldering is a pain but it really helps. As an extra you could use a less acidic source of Sulphur, i.e. not flowers!

[marble]

Dip the nichrome in some sort of lacquer or even wax

[digger]

Doesn't help no, try the coating method with soldered wires, I know soldering is a pain but it really helps. As an extra you could use a less acidic source of Sulphur, i.e. not flowers!

I use milled GD sulphur chips, however I do not know how these are made and hence may have an acidic content. I may try washing my sulphur in future by using a Buckner funnel (I have just managed to acquire an Edwards vacuum pump).

Edited by digger, 17 August 2007 - 03:57 PM.

[Andrew]

I use milled GD sulphur chips, however I do not know how these are made and hence may have an acidic content. I may try washing my sulphur in future by using a Buckner funnel (I have just managed to acquire an Edwards vacuum pump).

That's Rhombic Sulphur, it's made by freezing molten Sulphur. It's acidic content is minimal.

[T-sec]

Maybe a stupid question, of topic, but why don't you guys just buy ematches?
The Chinese versions almost costs nothing when you buy them per box...

[Andrew]

Where can you buy them from?

Also it's very satisfying when you made a huge bunch of your own and you have a store of them!!!

[Mumbles]

It might be different in the UK, but you need an ATF licence to purchase e-matches here in the US. I've always been a fan of bridgeless e-matches. No soldering, no corroding, less time. How I do it is a layer of graphite/NC slurry over the two wires which are approximatly 1mm apart. After that dries, I use another layer of WATER based blackpowder slurry. I had problems using a NC layer of BP. It took longer to ignite, and in some cases ceased to ignite at all. I think the 2nd NC layer dissolved the first, and some of the BP leached in. I usually do a final dip in NC just to keep everything sealed up. It doesn't take too long to make the matches.

[digger]

It might be different in the UK, but you need an ATF license to purchase e-matches here in the US. I've always been a fan of bridgeless e-matches. No soldering, no corroding, less time. How I do it is a layer of graphite/NC slurry over the two wires which are approximately 1mm apart. After that dries, I use another layer of WATER based black powder slurry. I had problems using a NC layer of BP. It took longer to ignite, and in some cases ceased to ignite at all. I think the 2nd NC layer dissolved the first, and some of the BP leached in. I usually do a final dip in NC just to keep everything sealed up. It doesn't take too long to make the matches.

I think you can buy them in the UK, however they cost around £1 each and I can roll up 200 in an evening. I like the sound of the bridgeless ematches. What voltage does it take to set yours off? Also what is the resistance of the finished item?.

[T-sec]

When you want to buy ematches, the general rule is: "The more you buy, the cheaper the price.

Ofcourse you can spend al you money on the expensive Bickford ones, but try the ones from Cosmos for example.

And for the legal part, over here in Europe ematches are classified under UN 1.4S. This is the lowest cathegory in pyrotechnic products, free in transport or stock, and not to hard to purchase. Never the less Europe has some countries which do not allow citizens to purchase them without the required certifications. 1.4S post oders or prarcels is a complete different story...