Electrolytic Mill Scale Removal

[RJSakowski]

I use brute force. I just use hydrochloric (muriatic) acid as it works the quickest. FECl3 is one of the most soluble iron salts. I dilute the full strength (31%) HCl to 50/50 with water, mainly to avoid the acid vapors from the full strength acid. When finished, I rinse the parts thoroughly with water. If I am concerned about any residual acid on the part, I either heat the part or wash with a baking soda solution, followed by a thorough water rinse. The raw metal is extremely vulnerable to flash rusting when wet so I dry as thoroughly as possible. This is where the heating helps.

A second choice for removing scale is phosphoric acid, available in a gel formulation. It can be liberally painted on a large area or applied in spots. It isn't as fast as HCl but it does work.

Any acid wash will also attack bare metal so if dimensions are important, a process like electrolysis is a better option. If using NaCl for the electrolyte, be aware the chlorine gas is one of the products of the process so it is best done outdoors. Washing soda (Na2CO3) is a safer choice. I have tried the EvapoRust but I am unimpressed with the results. I haven't ever had enough molasses around to give it a go. Maybe a trip to the local feed mill?

I will often use mechanical means to hasten the rust and scale removal process. Used 40 grit or 60 grit sanding belts work well to scarify the scale. They are waterproof so they can be used wet. The final cleanup can then be the acid bath. If I am just trying to remove scale to prevent wear on my cutting tools, just breaking the scale with mechanical means is often enough as the scale is brittle and will flake off when the cutting tool hits it.

Not mentioned is removing anodizing from aluminum prior to machining. Anodizing, particularly hard anodizing, is extremely hard on cutting tools so I like to remove it. I use a dilute sodium hydroxide (NaOH), aka lye, solution to dissolve the anodizing. The process generates hydrogen gas which carries droplets of lye along with it so it is best done outdoors and wearing PPE. It vigorously attacks bare aluminum so the process needs to be monitored carefully. If the anodizing is colored, the part can be removed as soon as the color fades. Wash with a baking soda solution, followed by a thorough water rinse.

 

[rwm]

I have some Phosphoric acid. I will run a test with that. I hate storing Muriatic acid as I have no good place to do so.
I have had great results removing anodizing with lye. I'll see if I can find those pics.

Unfortunately I don't have a before pic. This shock tube was anodized gold originally


After


Here you can see the gold at the bottom which was not removed. This is covered up by the forks anyway.

 

[GeneT45]

I've had some bad staining after using household lye to remove anodizing. Caswell sells a "de-smut" solution that works better, but I favor mechanical removal if you plan on re-anodizing.

GsT

 

[Shotgun]

So far, I've just used mechanical means. Cheap chinese carbide bits. The scale (and interrupted cuts) chews them up, but when you can get them online for around $1/per, pfft!

BTW: A chipped insert will usually still cut, albeit poorly, but if you're just going through scale anyway. . .

 

[RJSakowski]

The staining is caused by oxidized alloying metals in the aluminum. It doesn't bond tightly to the aluminum and can be removed by scrubbing with a cloth or Scotch Brite pad before drying.

 

[jbaccell]

I don't really have a recipe.

Start with maybe 1/4 to 1/2 cup of citric acid to one gallon of hot tap water. If you aren't seeing mild activity (slow bubbling, yellowing then darkening of water) in 10 min, add another 1/4 cup.
(I have a short length of 4" pvc capped and turned up on end. I balance it in an old truck disc brake. Depending on metal load, maybe a gallon will fit. If I was cleaning hrs today, I'd probably start with "two scoops." guessing that is about 1/4cup. It's cold here, so I'd place the whole stack in a bin as a spill basin and do this indoors.)

In hot weather and relatively smooth and clean structural tube, you might be done in a few hrs. In cold weather or crusty and pitted hot rolled you might be at it overnight or for days. Wipe down with scotchbrite or wire brush periodically to gauge progress. I expect scrubbing/scratching helps undermine the oxide.

Thanks, I appreciate your help.

Joe

 

[pontiac428]

This all sounds like metal pickling processes to me. We have lots of resources that cover that. I've had it done professionally, it's a low-cost method. Don't reinvent the wheel, rediscover it.

 

[rwm]

Yes, just like acid pickling. However the original question was electrolytic. No harsh chemicals there. That was my motivation.

 

[rwm]

I have to report: concentrated Phosphoric acid was a fail. After 24 hours it definitely took off some of the scale but not to a satisfactory degree. I would try something stronger like Muriatic acid next time. The test piece was 5/8 round bar with typical hot roll scale. The acid was 35% ( I think?)

 

[SLK001]

I use about 1 cup HCl to 3 gallons of water. Your scale should be off in 30 minutes. Do it OUTSIDE, as the fumes will fall on metal items and rust them in no time.