View Full Version : Hot roll and scale removal

Forrest Addy
09-27-2003, 12:43 AM
Hot roll and scale removal

Some time ago I posted some info on removing the hard black mill scale from hot roll steel. Here's a reprise of the weak pickle method for those who might have a need.

Anyone who's ever machined hot roll steel has had to deal with its hard adherant scale. The scale is a form of iron oxide (FeO) and is hard and strongly adherant to the base metal. Scale quickly blunts high speed steel and if the finish size is only a skim cut below the scale size holding can be a PITA.

Furthermore, some two part paint systems require a scale-free surface to bond properly to the base metal.

The simplest way to remove scale is with an acid pickle. Sand blasting is fine but it required a beefy compressor and a booth of some kind. Grinding is labor intensive. Wire brushing works hardly at all. With the pickle you drop the material in the solution and do something else for a day. The capital investment is a plastic bucket and a quart of muratic acid and an all plastic scrub brush.

Caution: Acid. Never cover and store acid or solutions in a machine shop. Use and store acid only out of doors. Acid fumes (even from dilute acid) drift a long ways and causes chronic rust problems possibly for years after the original chemical attack.

Mine is the weak pickle method. The acid is very dilute. I use a clean 5 gallon plastic pail about 3" short of full with cold tap water. Add 1 quart of muratic acid from the home center. Observe acid precautions on the container label.

The resultant pickle is abut 1 1/2% acid concentration. Vinegar is technically a stronger concentration. I work with the stuff bare handed (naturally oily skin).

Dilute muratic (hydrochloric) acid will attack iron oxide and the underlying steel barely at all. Be sure the scaled material is free from oil and grease before you drop it in the pickle.

It takes about 24 hours to de-scale a batch of scaly strucural steel or bar stock. If the material sticks above the pickle level you can end for end it and stick it back in. The end for end trick works for stuff up to 2 ft long in a standard 5 callon pail.

When the pickling is done remove the material and wash it off with the garden hose. Run-off will be very dilute acid and iron chloride, safe enough for the gutter and storm sewer. There will be a soft residue on the pickled material that scrubs off with a stiff brush leaving clean bare metal. Give it a final wash with any bathroom cleanser to neutralize the surface and let it dry. You have clean scale free stock ready for machining or painting.

This is not the movies where the heroine gets splashed with a chemical and dissolves in seconds before your eyes.

Momentary skin contact with the fresh weak pickle is of no particular safety concern. promptly wash the splash off with the garden hose you keep charged and convenient to the pickling operation. A splash in the eyes won't happen because you're wearing a face mask - aren't you? A neglected splash on the clothes may cause bleached spots and later the fabric (especially cotton) may fail and a hole result.

When the pickle peters out and takes several days to loosen the scale, neutralize it with a cup of powdered laundry detergent for disposal. Be sure to read the detergent lable to confirm it contains washing soda or sodium carbonate. When the stuff no longer reacts by foaming it's safe to dump down the john, followed by two flushes.

The disposal products are scale in the form of black grit, iron chloride from the acid reacting with the scale, and sodium chloride from the acid reacting with the soda in the powdered wlaundry detergent.

Chemically the neutralized pickle will be neutral to a bit alkaline. Because of the iron chloride its not suitable for human consumption. It's certainly safer than the stuff used for dying hair and for permenents, graywater containing bleach from the laundry, and water-based paint wash up water.

This is wet work and a bit messy; about like washing a car. Work safely

09-27-2003, 01:00 AM
Have you had any experience with Phosphoric acid in removing rust from steel?

09-27-2003, 01:40 AM
Thanks for reminding me of this. I am not concerned about my eyes or hands with that concentration of HCL. I've barfed much stronger.

Forrest Addy
09-27-2003, 03:22 AM
Cass, the reason I chose muratic acid is because it's far lower in cost measured in terms of the surface descaled.

09-27-2003, 03:30 AM
Muriatic acid is cheap like Borscht. Maybe Borscht would work.

09-27-2003, 03:33 AM

I use phosphoric acid on steel, but only for slight rusting or when I desire a "Phosphated Finish" - slightly more attractive than plain steel, not as nice as bluing or blackening.

This is also the phosphated base treatment of steels before priming and painting.

Bleach will decompose in sunlight into salt & water & Oxygen (providing it is just a Sodium Hypoclorate solution with no additives), however it usually does a fair amount of damage to organics (plants & animals) before that happens. You are right about being careful what you dump done the sewer - we all should be more aware.

If the residue is Ferric Cloride it needs to be disposed of carefully - this is the stuff that etches copper circuit boards - so I would not dump it down the sewer.

09-27-2003, 03:39 AM
Also serious exothermic reaction when mixing ferric chloride with water. Little bit at a time. Also stains worse than a bad reputation.

Forrest Addy
09-27-2003, 05:03 AM
Even, read the post. The ferric chloride is a pickling by-product. You don't add it from scratch.

Also the pickle starts out clear but when spent is very pale blue. Balance the reaction for 3/4 exhaustion and you'll find about 1/2% total ferric chloride. The stuff used for PC board etching is about 15% by weight and looks like strong coffee.

[This message has been edited by Forrest Addy (edited 09-27-2003).]

09-27-2003, 10:27 AM
On Ferric Chloride...........

To neutralize, dilute it and allow it to "work on" aluminum shavings or pieces until it is finished reacting.

The liquid remainder is safe, and the copper (if it was used on PC boards) is recovered in the form of a sludge at the bottom of the tank.

DILUTE first because the action on aluminum is very vigorous.

Also do this in a ventilated area.

Much cheaper to dispose of the safe liquid and the somewhat purified copper sludge.

09-27-2003, 10:36 AM
I saw a guy restoring an old Jaquar sedan in an Army hobby shop who was saturating cloth with phosphoric acid that he laid on the heavily rusted floor board of the car. I imagine he got the phosphoric acid from a paint store. It worked very well and left the surface "phosphated" according to the guy so he could paint it easily. Years later I bought a gallon of phosphoric acid rust remover from a paint store and it worked pretty well. I still have some of the stuff and use it now and then with rubber gloves. I think it is not very concentrated and not a real strong acid in any case but I am not a chemist and I haven't looked at the information on the plastic jug in a long time. I did notice that the paint store no longer has it for sale. I have a large weldment (10 feet) that I want to paint and I am thinking of using something like the phosphoric acid to get the light rust off the structural tubing and prepare it for painting.

09-27-2003, 02:04 PM

I understood that. I just threw that in for anyone thinking of making PCBs.

09-27-2003, 02:06 PM
Coca Cola contains phosphoric acid.

Al Messer
09-27-2003, 08:40 PM
I thought that the acid in Coca-Cola was Carbonic.

09-27-2003, 11:39 PM
Carbonic acid is just CO2 dissolved in water, and yes, Coca Cola does have it. Phosphoric acid is the fourth ingredient according to the can I am holding.