Sciencemadness Discussion Board

Rusting on hotplate

JibbyDee - 16-12-2011 at 10:47

I (stupidly) left my hotplate (which cost me €300) out in my lab (which is not sufficiently waterproofed) for a few months without using it and today when I checked on my lab for the first time in months, I realized that my retort stands have started rusting heavily and the retort stand rod that I had screwed into the hotplate has rusted so badly, I can't screw it out. Thats not what I'm worried about though, I noticed that some visible parts inside the hotplate have rusted heavily. I brought the hotplate inside (which is where I'll keep it from now on) and I'm going to leave it dry properly before I plug it in to see if it still works properly. This hotplate is by far my most valued and expensive piece of lab equipment and I can't lose it. Since hotplates are used in environments where acid spills are not uncommon, are they designed so that the interior electrons are protected from any chemicals that may seep in and corrode them? I'm going to take it apart and evaluate the damage but I'm wondering if I should use phosphoric acid to remove the rust I can see inside (under the actual metal plate) before attempting to turn it on again.

Also, any tips on how to unscrew the metal rod which is stuck due to rust?

Arthur Dent - 16-12-2011 at 11:03

I bought a used Fisher hotplate many moons ago, and indeed, I made damn sure there was no trace of oxidation anywhere before I attempted to power it on.

I would suggest to disassemble it the best you can, or as far as you feel comfortable to (make sketches to help you reassemble it) and every single piece that's rusted should go in a bath of phosphoric acid, followed by a thorough rince in hot water and dunked in WD40. That should stabilize he degradation of the components. Then check out all the contacts and connectors, and don't hesitate to rince them in isopropanol 99% and use dielectric compouns if you really want to make it impervious to chemical degradation.

As for the rod... try to put some of the phosphoric acid (several times) directly on the edge of the thread in the hopes it will seep in and eventually loosen. Don't force it too much, because if the screw breaks in the hole, you're FUBAR.

Robert

mr.crow - 16-12-2011 at 11:28

You kept your HCl in plastic bottles, right?

MagicJigPipe - 16-12-2011 at 11:36

I've seen hotplates that were so badly rusted and corroded that even the electronic components were covered in oxidation. Yet, they still worked. I am often surprised by the amount of damage a hotplate can take. Just last night I dropped one onto a hard floor from a height of about 3 feet and it still works just fine (except a knob broke off).

Yes, reasonable measures should be taken to prevent rust but I think what Arthur Dent is suggesting is a bit excessive. Just try to keep it away from constant acid fumes and if you spill something on it and it gets inside just take it apart and wipe it up (or neutralize it). If a component goes out, just replace it. A lot of the older analog hotplates are so simple that, as long as the stirrer motor doesn't go out, you can keep them running forever for almost no cost.

They are easy to fix and not very complicated. So just take reasonable measures and simply repair something when it does break.

One more thing, as far as I can see these hotplates consist of the same components and circuit boards as all other electronic devices. They only design feature I have ever seen that is for protection from chemicals is a channel or guard that prevents spilled liquids from leaking into the hotplate.

Also, once I saw an analytical balance that had some sort of clear lacquer coating covering the components/boards inside. I've only seen that once and I'm not even sure it was done for chemical resistance.

peach - 18-12-2011 at 09:43

Quote:
Since hotplates are used in environments where acid spills are not uncommon, are they designed so that the interior electrons are protected from any chemicals that may seep in and corrode them?


They are usually designed such that any spills run off the top and sides rather than there being open holes or seams for it to run in through, but that's about it.
Quote:
Thats not what I'm worried about though, I noticed that some visible parts inside the hotplate have rusted heavily.


Sounds like Mr Crow is possibly right about acid gases, but only you'd know if that's ever happened.

Spills run off them, but having decent amounts of reactive gas floating around them, the gas will end up inside the unit and forming the corresponding acid or base solution with any moisture it meets in there.
Quote:
Also, any tips on how to unscrew the metal rod which is stuck due to rust?


Get a pair of these, line the jaws with some soft foam, rubber or tissue, grip the rod as tightly as possible, try unscrewing it.

If it won't budge, give it a few knocks in the tightening direction, then unscrew.

Endimion17 - 18-12-2011 at 23:26

That's why every time I leave my lab for an extended amount of time, I put my hotplate in its box and wrap it in a plastic bag.
Every now and then, when I see there is some corrosion on the plate itself, I use a steel brush to scrape it off, and apply that silver paste for hotplates... I have no idea what is it correctly called (will check it out), but it's quite old, still works. Smells like organics. Could be a mixture of aluminium powder and varnish or something similar.
Never had any problems. The plate is still good as new and looks great, even after few years.

JibbyDee - 19-12-2011 at 16:33

Thanks a lot for the replies. I've been a bit worried about it for a few days now because the hotplate one of the few instruments I can't afford to replace. I'm going to look for phosphoric acid and sort out this rust.

mr.crow: I must admit I was careless about storing hydrohalic acids (namely hydrobromic acid which I didn't store properly) so that might be the culprit.

MagicJigPipe: Thanks, that takes a load off my mind. I like tinkering with electronics so I have no problem replacing components. Hotplates shouldn't contain very complicated circuitry so it shouldn't be that difficult to obtain replacement components for them. When I bought the hotplate, I decided that I would make it last forever because I can replace any damaged electronic components but I don't know anything about the insides of a hotplate so I was having some doubts. Thanks for the info.

Endimion17: I know what you mean, I used to treat my hotplate like a baby, when I left the lab, it would leave with me and I'd wrap it up and store it in a box to protect it from anything that might damage it. I even read it bedtime stories at night. I got lazy and left it in the lab at the worst possible time (before a long period in which I didn't do any home chemistry at all).

[Edited on 20-12-2011 by JibbyDee]

JibbyDee - 25-12-2011 at 12:43

I opened up the hotplate to see what the damage is. Theres not a whole lot in there:

that green circuit board on the right is whats behind the temperature and stirring dials. I can only see 1 thing that was rusted. That square metal block thats circled in red. The sides of it are completely covered in rust. I don't know what this thing is. Its right in the center so it has something to do with the magnetic stirrer and maybe the heating element too. While it is probably the most important component of the whole device, it is only the sides of it that are rusted so I'm not too worried about it. The rusting that I could see before opening up the hotplate is here:

it only appears to be those cylinders. Those cylinders look like bolts which extend from the metal plate to the insides of the device. As you can see, under the actual hotplate, there is another, very thin, metal plate and below that is the plastic casing that houses the electronics. I'm glad that there isn't really a lot of rustable metal in there but I have to get the rust off that metal block and to do that I'm going to have to take the whole thing apart because there are all sorts of cables and connections that I have to disconnect before I can take off the actual hotplate part of the device.

[Edited on 25-12-2011 by JibbyDee]

Arthur Dent - 26-12-2011 at 10:45

@JibbyDee :

The circled thing is a simple line power induction AC motor, it's not catastrophic that there's a bit of rust on there, the metal blades making up the core are soft iron so they are prone to some rusting. The motor is connected to a magnet that makes up the stirring mechanism.

The heating element is completely separate from this, and is the two tabs sticking out of the top plate connected to the blue and brown wires.

You could just wet a piece of cotton with WD40 and gently rub the sides and top to remove most of the surface rust. Don't go too far disassembling because you might not be able to put it back together. Just leave it as it is, make sure the wires are not dusty or tacky with acidic vapor deposits and you'll be fine.

As for the acids, stuff like HCl and HBr, even in the best lab grade bottles, will vent some corrosive gases, and should be stored away from anything metallic. My personal stash of HCl is stored in a pail on my balcony outdoors and is unaffected by the cold. I learned it the hard way when I stored some HCl in my shed and all my gardening tools became quite rusty.

Robert


[Edited on 26-12-2011 by Arthur Dent]

peach - 29-12-2011 at 18:20

Ooooooo Jibby, I'm afraid that rust means it's a gonner. Fret not! For I have an awesome deal I can offer you.



I'll swap yours for one of mine (slight cosmetic wear and tear), and there's only a $50 fee to cover administration costs.

I kid, I kid... Arthur is correct. Rust on core laminations is so common that it can already be present when a motor or transformer is bought new. You don't even need to clean it off, it's absolutely fine. Splashing around in there with water may make things worse. I don't use it for much but the plate in the photo still works, like a zombie rabbit with myxomatosis. Ahhhh, we spent some excellent time together that plate and I. I don't think I could bring myself to bin it even if it was dead.

[Edited on 30-12-2011 by peach]

Old post but a tip

zenosx - 5-11-2012 at 15:18

When I purchased my used PC-320D corning hotplate, the stir motor was almost seized up. It would work after the plate got a bit hot, but only on higher speeds. I too it apart, removed the stirring motor (like above circled in red), (the top was a large circular doughnut magnet almost like a speaker magnet.)

Anyways, I took apart the motor carefully (the two obvious black looking bolts/screws above in the read circled area), and popped the bearings out of their channels and repacked them with lithium grease with a bit of WD-40 thrown in.

To pack the bearings, put your grease/mix etc, into a plastic zip lock bag, then using your fingers and the corners of the bag, force the grease into the bearing if you can (some bearings are sealed up, but you can still usually pack them a little bit. Obviously you should put the bearings INTO the bag, I know of someone that somehow mixed that up...

They actually make bearing packers for hobbyist, but not worth purchasing one just for this kind of thing.

If all else fails, you can usually find replacement bearings at your local hardware store, just bring the old ones with you, and look through the drawers.
They are usually measured by O.D (of bearing), I.D (of center channel) and thickness.

My stirring system works great now. Still a bit slow on the lowest speed till it warms up, but that's better than Nothing at all moving :)

Dr.Bob - 5-11-2012 at 20:14

I have used stirplates that were very rusty many times. Most will keep working just fine. The biggest problem I ever have is the speed controller getting confused by dirt or rust and getting stuck on high speed, which is tough as it wants to make the stir bar go flying out of the flask. But I have never worried about a bit of rust, dirt or corrosion, I just wipe the top and front off with isoPrOH or EtOH. Of course they get used a lot, but acids do tend to visually mess then up a bit, especially HCl and TFA, but most of them are pretty sturdy and hold up well.

Endimion17 - 28-12-2012 at 11:12

As I've promised, here's the paste I was talking about.



It's Metalcrom's paste and I see the company's website is under construction. I have no idea whether this paste is even manufactured today. My tube is totally ancient and it's just fine. I've just given my hotplate a smooth shave using steel wool, applied this with a paper tissue and turned it slowly and gradually to high heat to evaporate the organic part. Now it's good as new. :)

[Edited on 28-12-2012 by Endimion17]

elementcollector1 - 28-12-2012 at 11:33

Quote: Originally posted by peach  
Ooooooo Jibby, I'm afraid that rust means it's a gonner. Fret not! For I have an awesome deal I can offer you.



I'll swap yours for one of mine (slight cosmetic wear and tear), and there's only a $50 fee to cover administration costs.

I kid, I kid... Arthur is correct. Rust on core laminations is so common that it can already be present when a motor or transformer is bought new. You don't even need to clean it off, it's absolutely fine. Splashing around in there with water may make things worse. I don't use it for much but the plate in the photo still works, like a zombie rabbit with myxomatosis. Ahhhh, we spent some excellent time together that plate and I. I don't think I could bring myself to bin it even if it was dead.

[Edited on 30-12-2011 by peach]


What is that recently solidified metal lump?

Lambda-Eyde - 28-12-2012 at 12:11

Quote: Originally posted by elementcollector1  

What is that recently solidified metal lump?

Seeing as Peach hasn't been logged in since october (I wonder what happened to him..? I miss him greatly :( ), I'll permit myself to tell you that it's in fact aluminium. IIRC he actually managed to melt a foil skirt with his hotplate, so watch those skirts!

elementcollector1 - 28-12-2012 at 12:17

Quote: Originally posted by Lambda-Eyde  
Quote: Originally posted by elementcollector1  

What is that recently solidified metal lump?

Seeing as Peach hasn't been logged in since october (I wonder what happened to him..? I miss him greatly :( ), I'll permit myself to tell you that it's in fact aluminium. IIRC he actually managed to melt a foil skirt with his hotplate, so watch those skirts!

Really? I've never managed to melt foil before, it always just blackens and oxidizes itself to death.