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[How do I?] Mill Scale Removal

mrjbinok

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#1
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?
 

francist

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#2
Not sure about the chemicals you've mentioned, but I find muriatic (hydrochloric) acid works really well for removing mill scale. In the photo below, the two larger angles have been "de-scaled" whereas the smaller bracket has not. I usually just use a plastic tupperware container and let the parts sit submerged for a couple hours. Does the trick for me. I also do it outside -- partly to keep from breathing the fumes but mostly to keep the vapours away from tools and machines. It will rust things very aggressively!

-frank

 

mrjbinok

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#3
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.
 

francist

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#4
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
 

RJSakowski

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#5
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.
 

mrjbinok

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#6
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.
 

Jmanb13

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#7
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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mrjbinok

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#8
Would vinegar have the same problem as the acid if used indoors with regards to damaging fumes for other metals in the shop?
 

Jmanb13

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#9
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.
 

Bob Korves

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#10
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!
 

mrjbinok

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#12
Well I did an experiment yesterday evening. I had some paint mixing plastic cups..... I put some Limeaway in one cup and some muriatic acid in the other. I put one of the plates that I am wanting to work with in the limeaway and a piece of A36 roundbar in the acid. Small amounts of each. Since the mureatic acid requires more ventilation, it was placed outside where the temps were falling below zero for my test and the limeaway was inside the garage and covered.

I left both pieces to soak for 3 hours and then rinsed and scrubbed with a nylon/plastic parts brush. The Limeaway piece was completely stripped of the mill scale and the slag left over from the laser cut. The acid piece was spotty and will require more time or the use of an abrasive. I rinsed both pieces and sprayed them down with some Windex w/ ammonia, dried and coated with some of the WD-40 Dry Lube.

My conclusion is that the Limeaway does an excellent job when used at somewhat of a room temperature, and the acid is less effective when used outside in cold temperatures. I am assuming that warmer temps would probably help with the acid. Since I have quite a few pieces that I will be wanting to strip in the near future, neither of these processes will do what I need done. I wasn't able to try the vinegar because mamma didn't have enough in the kitchen for me to try.

So for occasional need for removing the scale, I recommend the Limeaway, based on the fact that it is easier/safer to work with and will remove any trace of mill scale and rust in just a short time..... depending on the size of work piece and is easily disposed of.

For my needs and the fact that I will likely be needing a repeat process on a bigger scale, I guess I am going to have to look more closely at the electrolysis method and spend some $$ to get set up.

Many thanks for the advice and information. You've been a big help.

Jim
 

hman

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#13
Thanks for reporting on your test of Limeaway.

Since time usually isn't a concern for me, it'll most likely come down to what's cheaper or cheapest.
 

mrjbinok

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#14
I soaked the piece in the Limeaway for like 3-4 hours and could see that it was still coated with what I thought was the original mill scale. Once I took the part out and dipped in my rinse bucket, the smut started rubbing off. The brush removed all traces of the dark coating. My point is that it may not be necessary to leave in the Limeaway for 3-4 hours. I plan to try this in a larger tupperware pan that I have and try different time periods to see if the process will work faster..... the larger pan will allow me to do several pieces at the same time. I am fairly new to metal working so everything I try has a learning curve.
 

Martin W

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#15
Not trying to be a jackazz. Your original post said you tried to sand the scale off in the lathe. Couldn't you just turn a couple thousands off and save all the chemicals? I might be missing something
Martin W
Sorry I didn't read it properly and didn't realize they were so thin
 

mrjbinok

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#17
Not trying to be a jackazz. Your original post said you tried to sand the scale off in the lathe. Couldn't you just turn a couple thousands off and save all the chemicals? I might be missing something
Martin W
Sorry I didn't read it properly and didn't realize they were so thin
no problem. As I said earlier I am relatively new to metal working and trying to figure out a process for making some specialty tools to sell. When I got the plates from the laser cutter there was a hard mill type crust around the outside edge of the plates. That is what I was using the sandpaper on.... then a few thousands off with the lathe to true it up.

The mill scale is tough on carbide cutters. The two faces of the plate had the mill scale and rust spots, but I had already drilled and pressed in four pins in each plate and welded the back side of the pins. Now I am faced with trying to clean up the mill scale as well as grinding down most of the weld beads so I can face off the back side with the lathe. I have made a whole lot of extra work for myself getting things figured out on how I wanted to make these tools. What I have discovered on some of the welds, is that I didn't get a good penetration on a few of the welds, because of the mill scale and now have to do a little touch up on those few spots. This has been a huge learning process, but now I know what NOT to do.

My next batch of tool plates will be a lot easier to do now since I have a better idea of what is needed and an order for doing different steps.
 

Martin W

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#18
I remember shot blasting cutting edges to remove mill scale for loader buckets to get better penetration on the welds when I worked at John Deere. This helped with tracking of the seam for the robot also.
Cheers
Martin
 

george wilson

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#19
I have also just used vinegar to remove scale. That stuff is very hard,and very difficult to remove by mechanical means,like sanding,etc..

HOWEVER I would really caution you about keeping ANY container of muriatic acid in your shop. HCL gets through plastic containers,and can even get past glass bottles with bakelite lids and a gasket inside.

I would strongly advise that no container of HCL be kept in a shop. I had an unopened container in the garage back in the 50's. The chrome plated wrenches kept starting to rust. Finally,I realized that the HCL was getting vapors through the UNOPENED(You'd have to SNIP off a corner to open the plastic container). I took the plastic container and emptied it away from the shop. The rusting ceased. Lesson learned!