I've an old school (but fairly new and lightly used, maybe 50 hours) 2 bar electric fire. One of the bars failed to come on a few months back, and recently started playing up, with half a bar lit, and then switching off.
Anyway, the problem appears to be a loose connection - both bars' securing bolt had come loose at the same end of the element. The one with the intermittent fault unwound a good few turns. I'd assume that's a case of cleaning up the connecting pins and retightening the nuts?
I also noticed, at the other end of the elements, both holders have corroded nuts/bolts. It seems a bit odd to me that it's only at one end, but is this cause for further concern?
Hot metals are bound to oxidise. Just clean up any electrical contacts. A basic safety check is always advisable on bar fires, many are hopelessly unsafe.
Once surfaces are degraded, "clean-up" does not last long. The two metallic items intended for low-resistance connection must be replaced. That stops the degradation from picking up where it left off.
I had a piece of lab equipment which cost around 5000-6000 or so, and the power supply in it had a burned connector. I cleaned it up the first time, and it lasted about 3 months before the same thing happened again. After a couple more "fixes", I made a permanent connection between the items, giving the shitty surfaces no chance to make more trouble.
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In modern toaster ovens, they changed from socket and rod heating, to "spot-welded connections everywhere". This stops the kind of problem in your picture, but at the price that typical DIY cannot repair common problems any more. You'd need a spot-welder that can re-make connections to new elements, for example. And it's a cold spot weld, rather than welding two frame components on your car. That allows working on thinnish materials without burning them.
Some kind of "dissimilar metals" or electrochemical cell seems to have formed on your item. It looks like a good effort was put into making contacts in this case, but something from underneath that area seems to have chemically attacked.
But all I can tell you, from experience, is using sandpaper on the contact areas, will not help. When metal connectors "cook", the composition of the metal changes, which is part of the reason it remains ohmic, runs hotter than normal, and accelerates any degradation process.
And for obvious reasons, you cannot use solder on heating elements. The insurance company hates paying out on fire claims, and finding somebody has tried to use solder (which melts and flows away, causing potential havoc and tripped protection devices and so on).
In the modern toaster ovens, all the electrical runs are stiff wires, they're spot welded to one another, and guides hold them in place. Because the metal to metal connections are permanent, there are no longer connectors to go ohmic. And to the manufacturer, that is the perceived advantage. The item is "disposable", but up to the point you're forced to throw it away, it is "safe".
Well, give an example. What have you buffed up with sandpaper, that lasted forever, carried a couple of kilowatts and likely idled at 100C or over ?
There are no guarantees when you do stuff like that.
There is a big difference between "there was a little oxide on the outside and no thermal damage" and "that baby was burned to a crisp, but I took a file and filed it down until I found metal". The items I've worked on, it's apparent when you file or sand them, the metallic composition is no longer the same. They're just not good conductors any longer. The damage is thru and thru.
You can touch up a flame sensor, and those seem to work OK afterwards. But they're also designed to sit in a low temp flame forever, so the materials are no stranger to the conditions.
Would you happen to know why each end experienced a different issue? One end of each element the nuts had loosened and caused the poor contact/tarnishing. The other the nuts tight/clean, but the mounting screws for the ceramic element holder severely corroded. Just curious.
I've not got any experience of this situation - so I think best to bin it. It was one of these, 4 years old:
Clean it up and away you go for years more use. Not recommended for an electric fire element but a little smear of grease also helps stop further problems, especially on something used outside.
If something is burnt then possibly replacement is a better option unless you can fully cut out carbonised insulating material.
Well silicone grease on the terminals would be good. I suspect that somehow its been stored near something that put corrosive fumes onto one end of it.
What bad contacts have I fixed? It's a very long list, and includes the occasional electric fire fwiw. Sandpaper is the last resort, better to leave a smooth finish. IME a small nut & bolt worked well on electric heaters. Whatever you do, a hot connection needs to be and remain gas tight. If it's not, it will fail soon.
not much in life is guaranteed. That is not a reason to do nothing.
Electric fires are simple devices. It's hard to imagine how you'd be unable to repair one due to a burnt out bit. Even the resistance wire bars can be repaired with nut & bolt and last decades.
Gold needs a bit more help than just plain oxygen to form an oxide.
I would suggest that "perchloric acid and an alkali metal perchlorate in a sealed quartz tube at a temperature of around 250 °C and a pressure of around 30 MPa" is beyond a natural process.
This is about some fool saying regarding oxidisation, "Stainless steel and nichrome doesn't".
Oh for the halcyon days when replacement spiral elements were sold at the electrical counter in Woolworths. (Every light bulb sold used to be tested before handing over to the purchaser)
Quite, and I said they didn't corrode any further.
Only an idiot would read that from my post. I said quite the opposite that a literate schoolboy chemist would have known. That stainless steel and nichrome oxidise very quickly in air to provide an insulating layer of oxide that prevents any further corrosion.
Only an illiterate would read from my post that you have to apply heat for a metal to oxidise.
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