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"Mill Scale Removal"

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October 9, 2021

When hydrochloric acid pickling mild steel what is the loss of weight owing to removal of mill scale and rust.

vikash goel
- kolkata, west bengal, India

October 2021

A. Hi Vikash.
I would not know how much mill scale and rust was on your steel, of course, but yes, the weight loss from pickling is equal to the amount of mill scale and rust that was on the steel assuming 100% of the mill scale and rust gets removed and assuming that the inhibitors employed are sufficient to insure 0% attack on the steel.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

October 22, 2021

A. Hi Vikash,
You could quantify the loses to the base metal by conducting an etch rate test.
Using a rust free, degreased, cleaned steel test panel of the alloy you process:-
While wearing gloves to prevent contamination of the panel, Measure its dimensions, calculate the surface area in centimeters.
Weigh the panel to at least 3 decimal places of a gram.
Record that weight.
Place the panel on a wire, then in the descale solution for the same length of time that the parts are in the descale solution.
Remove, rinse and dry the panel.
Weigh the panel again as before.
Subtract the second from the first weight.
Then calculate the weight lose per standard area, such as sq.ft (929.03 sq.cm), sq.decimeter (100 sq.cm), sq.metre (10,000 sq.cm), depending on your or any specification requirements.
This will tell you the etch rate of the base metal, and wether you need to add any inhibitor to the descaling solution.
Best of luck

Mark Lees
- A foresaken rock in the irish Sea

Closely related Q&A's, oldest first:


Q. Hi everyone.

We are trying to remove mill scale from steel prior to a powdercoat. What acid is the best to use for the removal of mill scale and rust? I seem to recall that Sulfuric acid is a good one for mill scale, but doesn't do the job on plasma edges. Just looking for some confirmation on my memory.


James Hanley
- Seattle, Washington


A. Hi,

The "most pleasant" acid to use would be an inhibited phosphoric acid based pickle. It can be used in stainless steel eqpt and is compatible with the following phosphate process. It is also less likely to rust stain compared to the other acids. If by plasma scale you are referring to laser cutting, then there is no problem.

Roger Bridger
Walterisation UK Ltd - Croydon, UK


A. Hi, James,

Yes, you are right. Sulfuric acid is one of the common acids used to remove mill scale and the another is hydrochloric acid. The former is cheaper and efficient in the most case, but highly dense scale, such as plasma edges. The critical difference among both acids is working mechanism. Sulfuric acid penetrate per crack in scale layer and works underneath scale rather than dissolving scale and rust, but, on contrary, hydrochloric acid does.

Jun Zhang
- Cincinnati, Ohio, USA

September 14, 2011

Q. I have a customer looking for a easy, quick test to determine if all the mill scale has been removed.

Joel Bialek
Sell Inspection Equipment - Lapeer, Michigan

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