[How do I?] Mill Scale Removal

mrjbinok

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Not sure where else I might post this question so I will start with this.

I am working with several small pieces 3" disks that are 5/16" thick. They come from a shop that will laser cut to size from plate but the outside edge as well as both flat surfaces have mill scale as well as spots that are moderately rusted (not really pitted, but rusted just the same). Up to this point, I have put on the lathe and used #36 sanding to knock off the glaze left from the laser cut and then use the lathe to finish off the edge.

I have read several articles about pickling and electrolysis and I think that pickling is the route I will go. The question I have is whether or not I would be able to use a pickling solution like the toilet bowl cleaners or Limeaway in an ultrasonic bath with a stainless tank. I use a citric based cleaner for other projects and this will work on the rust spots on these disks, but would take forever to clean up the mill scale.

Obviously I don't want to damage the stainless tank, but I would really like to speed up the de-scaling process by using the ultrasonic action.

Any suggestions?
 
Do you use the muriatic acid full strength? (as you get from the pool supply or box store?) I wonder how the acid would react in the ultrasonic...... Suppose I could suspend a plastic container of acid in the ultrasonic with water and get the agitation effect of the ultrasonic. I have like 300 pieces to do and I'm wanting to speed the process as much as I can.
 
I bought a gallon jug of it from the local hardware store, and yes I have been using it full-strength. Diluting it should work fine too though, just take a bit longer. I'm afraid I don't have any experience using ultrasonic cleaners so really can't provide much help on that front. Maybe it would make up for the loss in time to dilution?

-frank
 
Muriatic acid, aka hydrochloric acid, will attack stainless steel. Toilet bowl cleaners and Limeaway contain phosphoric acid which as far as I know, do not attack stainless steel.

Both acids will produce hydrogen gas when in contact with steel.

Muriatic acid is more aggressive towards scale than phosphoric acid because the chloride ion in the acid forms a stable complex with the iron in the mill scale (which is ferrous oxide) as opposed to forming ferrous phosphate.

I generally use full strength muriatic acid for derusting/descaling because it works faster but if I do, I derust/descale outside because of the hydrogen gas carrying acid droplets into the air. Weaker solutions can be used indoors but are far slower.
 
I have a jug of the muriatic acid that I used some time back on some masonry work, so I know about using with VERY good ventilation. Also don't want the fumes in the shop to destroy other tool surfaces etc. and after thinking about using it in the stainless tank of the ultrasonic..... came to the conclusion that it probably wouldn't be a good idea. I've got some glass beakers that I might try using the acid in those while putting that into distilled water so the ultrasonic can work to loosen the sludge a little quicker. I've read that using acid that to get the pieces really clean that a plastic or nylon brush serves that purpose.

I don't think I want to handle the pieces in acid solution so hopefully I can run the cleaner for a short time and transfer directly to my rinse tank. I suppose the same process would work with the toilet bowl cleaners without being quite so caustic.

I would like more input from others that might have had luck using an ultrasonic for this purpose.... to get some ideas on what I might try. If I had something that would work and not damage the tank, I could effectively clean 10 or so pieces at the same time.
 
If you have overnight/24 hours you can dump your stuff in a tub of vinegar and it will do the same thing as muriatic acid except without all the safety issues and worries. Pull it out the next day with a rinse and very light scrub (kitchen scrub pad) and its all gone.

If your really worried about it, put it in some water with baking soda to neutralize any remaining acid. However not really that critical since vinegar is so weak anyway.

I use this all the time whenever i'm welding.
If you have a really long piece you want to soak, a PVC pipe with caps makes a great soaking tub :)
 
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Would vinegar have the same problem as the acid if used indoors with regards to damaging fumes for other metals in the shop?
 
Not really. One of the uses of vinegar is actually to remove rust. If the fumes from it rusted anything it would take years and you would ahve to leave it right next to vinegar with the cap off and constantly refill the vinegar from the evaporation :)

Vinegar really does work great, but its slow, so if you plan ahead its easy to use. Bonus its STUPID cheap.
 
A leaking bottle of acid will rust everything in the shop, in short order. I have seen it multiple times. The latest was a high end stainless tool box set a friend has, with little bits of rust all over it, from a leak across the shop. Keep acids out of the workshop!
 
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