Mill Scale Removal

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,

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

A. Hi again. Okay, it sounds like you're not really talking about HCl or inhibitors, or pickling or etching then ... you're asking how much mill scale or yellow rust should be expected/anticipated/allowed on freshly rolled steel.
• https://weldingacademy.online/2019/10/01/the-mill-scale-on-steel-and-welding/ says the thickness and composition will vary based on rolling temperature.
• medium.com/@bestadwise/everything-about-mill-scale-9bf2a59fec85 and www.corrosionpedia.com/definition/780/mill-scale say it tends to be about 1 mm thick.
• millscale.org claim to be world experts on this and should be able to help you.

But please tell us your situation rather than keeping this so abstract and hypothetical :-)

Luck & Regards,

A. Hi Vikash. You're asking the same question about mill scale and rust a third time, although it has already been answered. And you already asked about the weight gain from galvanizing under topic 49676 and it was answered there. Maybe the problem is that you are not appreciating that metal finishing, including pickling of steel and galvanizing it) is "surface finishing": how much gets removed or added doesn't depend on the weight of the pieces you are processing, it depends on their surface area.

A mooring bollard might weigh a metric ton and have a surface area of a square meter. That same metric ton of steel if rolled into sheet may have a surface area 50X as great. Even after estimating the scale as being 1 mm thick, which it may or may not be depending on many factors, 1 mm of scale on the surface of the bollard surface would weigh under 1 kg and the mill scale on the sheet steel may weigh over 50 kg.

You ask about weight of galvanizing vs. various microns of coating. But this question usually applies only to coils which are galvanized in continuous coil-to-coil lines, complete with wipers of some sort, such that thickness is controllable to x grams per square meter. In the case of hot dip galvanizing of discrete components though, there is no such control; there is some dependence on silicon content and so on, but in general you get what you get, and Geoff Crowley estimated that in topic 49676, but you will get something like 50X as much weight gain on the sheet metal than the bollard because the surface area if 50X bigger.

Luck & Regards,

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.

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.