Mill Scale

[Alex2005]

Hello,

I have some HRS angle that I would like to make into a rail, but i'm having trouble getting the mill scale off.

I know I can use brute force to remove it, but have read that its not good for the steel, rather I should use a chemical method for removing the scale.

I have tried soaking in spirit vinegar (fully submerged 48 Hours) to remove it but all it does is remove surface rust and the scale remains.

Am I using the wrong chemicals or is there a better way to remove it.

[Goat]

What you are trying to do is called "Pickling", chemically removing mill scale. The most popular chemistries to do this are hydrochloric or sulfuric acid. Prepare a bath with 8-10% HCl and water and place your parts into it. At room temperature to remove all the scale will take about 5-10min. If you heat the bath the pickling speed increases. Industry runs in baths at 60-70C just to give you an idea.

After pickling you must rinse the parts throughly, otherwise the acid will continue to etch away. Following the rinse, coat the parts with a light oil as they will start to rust very quickly.

Be very careful around HCl it is quite corrosive. I suggest that you not do this indoors. The hydrogen chloride effluent will cause metals to corrode, and the by product of etching Fe is H2. Wear chemical gloves and an acid apron as well as a face shield and remember "AAA". Always Add Acid to water not the other way around!!!

[Alex2005]

Thanks for your reply, do you know where I can get sulfuric acid as i'm not sure its very available in the UK, unless you have a good reason to need it.

Maybe there are some PCB type chemicals that have the right acid in them?

[Goat]

Use HCl, it is easier to find. It is used in masonary work, to etch & clean concrete. In the US, they call it Muriatic Acid, I don't know if it is the same in the UK.

[Zumba]

You can find muriatic acid wherever swimming pool supplies are sold.

[Alex2005]

Thanks guys, I'll be buying some brick and mortar cleaner, short of that I'll try the swimming pool supply house.

I'll post some images if its successful.

[Alex2005]

How does the acid from this company look abd which type would be best suited


http://www.thepoolshed.co.uk/70/

[Goat]

Originally Posted by Alex2005 How does the acid from this company look abd which type would be best suited http://www.thepoolshed.co.uk/70/

32% is too strong to use so you would have to dilute it. Which will give more solution for your buck...errr...pound.

They do sell 8% already in a bottle so you wouldn't have to dilute it.

[Alex2005]

OK, so after convincing people that i'm not a lunatic I managed to buy the 32% strength HCl.

I diluted with 50% water so about 16% strength, this stuff stinks.

I laid the steel in the solution and it does bubble away so to speak, but it seems to take longer than 5-10 minutes.

The finish I get is like a gun metal grey colour, is it possible to get a shiny metallic finish?

[Geof]

Originally Posted by Alex2005 OK, so after convincing people that i'm not a lunatic I managed to buy the 32% strength HCl. I diluted with 50% water so about 16% strength, this stuff stinks. I laid the steel in the solution and it does bubble away so to speak, but it seems to take longer than 5-10 minutes. The finish I get is like a gun metal grey colour, is it possible to get a shiny metallic finish?

No. The acid is etching metal away so it is making tiny pits in the surface.

That stuff does stink; I hope you did this outside. Your lungs will not thank you if you inhale too much.

Also keep both the 32% and your diluted mix outside and away from any metal. The fumes will settle on any metal and start corroding it. Even with the cap screwed on the original container enough will escape that if you put it in a confined space with some metal you will likely see corrosion in a few days.

To reduce the continuing corrosion on your treated metal wash it in a baking soda solution to neutralize the residual acid.

[Goat]

Originally Posted by Alex2005 I laid the steel in the solution and it does bubble away so to speak, but it seems to take longer than 5-10 minutes.

It depends on a number of things, how thick the scale is. It's composition and defects. If it is too slow, you can heat the solution. Increasing the acid concentration won't speed it up much. Heat is appropriate catalyst.

Unfortunately, what you see is what you get. I don't know of any chemical solution that will improve surface finish.

[Alex2005]

Well I believe I have come to a conclusion on the rails, that is to get bright steel or cold finished as its supposed to be more accurate and have a smooth finish from stock, I'm glad I got the acid though because I certainly learnt something and still have almost a gallon left which may be useful for the future.

Thank you everybody for your help on this.