What am I doing wrong?

[Maadman]

I use the muriatic acid and then a baking soda rinse. Lately when I pull the metal out of the acid, it has a black film on it. It does not come off easily. You have to scrub to remove it.
This is newer muriatic acid, just changed it a couple weeks ago.
Can anyone tell me what I've done to make it do this! It's never done this before!

[WyoGreen]

Have you put any metal other than plain steel in it? If I put anything with copper on it in my bath, then later when I put steel in, my metal comes out copper plated. My thought is that the black is some other kind of metal that is plating your steel.

Steve

[Maadman]

Good thought, but not in this case. Nothing goes in there but hot rolled P&E, but thanks for the info. Didn't know that! Thank you for your reply.

[Gamelord]

Is your tank covered? If not your acid will evaporate. As it evaporates it gets weaker and weaker. Also, make sure that NO baking soda ever gets back into the acid tank. No double dipping without rinsing off extremely well or the soda will neutralize your acid tank. Usually if my acid stops working as good I just add a little more in to refresh it. Also, make sure you are using the same acid each time. Some brands are stronger than others. I usually mix my acid 10 to 1...but I purchased some from Home Depot and after mixing it was very weak and didn't work very well. The HD stuff I just used straight as it is already mixed like 7 or 8 to 1 compared to the commercial stuff I get from the pool supply people.

[Capstone]

Good thought, but not in this case. Nothing goes in there but hot rolled P&E, but thanks for the info. Didn't know that! Thank you for your reply.

Found this... "HRPO steel is still considered hot rolled steel even though it does not have any scale."

So you're dipping steel in muriatic acid that according to this really isn't necessary?

Also, baking soda is never needed. Simply rinsing the metal right as it comes out of the bath will neutralize the acid completely. It won't stop flash rust either, so just dry the piece with compressed air asap after rinsing in water.

[Maadman]

Thank you all for your thoughts. Gamelord, I did use muriatic acid from two different brands. Didn't think it would make a difference. Will buy my next ones from a pool supply, sounds like it is a lot more efficient. My tank does have a good lid, one I could lock if I found necessary. I know I haven't double dipped, so that wasn't it, but very good to know.
Capstone, I don't soak in the acid for scale removal. I soak because of the dross. When I cut product that has long flowing lines I have very little dross. But that is not what I cut a lot of. Most of my product is very detailed and often has a lot of text. That makes for a lot of dross, which the acid takes care of for me without having to spend a lot of time with the grinder.
Again, thank you everyone for the comments.