Hot Rolled Steel Coil Pickling Standards

You would have to do a titration of the tank and follow you chemical suppliers recommendations.

I wanted to mean replacement of HCl acid as its concentration get decreased gradually with each coil get pickled and thus oxide removal capacity of acid from hr coil surface also get decreased.

See #1

You might try a pH meter.

Do you not have a process engineer, or someone who knows something about quality control?

You have to titrate, to measure acid concentration, free acid, and iron % and PH. There is no rule of thumb. Your chemical supplier will have parameters. We pickle steel as part of pretreatment before coating in our plant, we change tank maybe once every year (to remove contaminates). We titrate twice daily and adjust concentration accordingly.

If you don't titrate and just want to dump tank every so often, so be it, it's going to get expensive.

<edit> A pickling tank is a mixture of HCL and water, you don't just dump it when concentrations get low, titrations will tell you when to add more HCL.

Contact your HCL acid supplier, they will have the info you need.

How about just changing it when it takes on an "off" odor?