Mill scale removal

[Essex_29]

I have no idea how well known this is, but I learned it pretty recently, how to easily remove mill scale from hot rolled sheet metal.
All you need is a (preferably plastic) container, big enough to hold the pieces of sheet metal you need to get cleaned, and enough Apple cider vinegar or acetic acid to cover the parts. I found 12% acetic acid for weed removal, and it works nicely and about the same as vinegar.
At about 30 degrees C (85 F) four hours is enough, but in cooler temperatures, say, like under 20 C (70 F) the process can take up to a couple days. If in a hurry on a cool day, you can put the container in a bigger container and fill that up with hot tap water.

For me, doing a lot of fabrication laterly, it's a real work saving way to get rid of mill scale. You just have to do it outsid, the smell is not all that pleasant.


After four hours, millscale has fallen to the bottom of the container

The metal looks bright and clean

After a rinse in water, and wash in a weak soda solution to neutralize the acidity the parts are sanded and look perfect, not much elbow grease needed.
Stronger acids would work faster, but using weak acids is safe and  easy.

[TFoch]

Great tip!  Welcome to the Rusty Bowtie Essex_29!

[EDNY]

I have been using apple cider vinegar in my ultrasonic cleaner and it works great.  A word of caution if you have for example "blued" gun parts....both the bluing and rust disappear

Gun bluing is actually a type of rust made with hot salt baths.

Welcome to the forum!

[Essex_29]

Thank you TFoch and EDNY, both!

I'm amazed at how easy the mill scale comes off with weak acids like vinegar or acetic acid, as it's so hard to grind.

[sixball]

I did a demo at the Inliners International convention using food grade citric acid & water to remove rust. It won't harm paint and some stickers. It is good to know what other acids work for us. Thanks!

[Essex_29]

Another few parts de-scaled. 12 hours in a cool average 65 degrees, and the mill scale just washes off. Engine and transmission mount parts here, just rinsed with water and hit with 80 grit on an orbital sander.
Okay, I'll leave this be now. It absolutely works, and is worth the while, especially with smaller parts and if you don't mind waiting a few hours or a day.

[EDNY]

Looks great...you are ready for some powder coating!

[sixball]

A great tip, Thanks. We all know the advantages of working with clean metal.