A slow cooker on high, with white vinegar usually works for me.

It works cold, but works faster hot.

For scale (and occasionally flux) removal I used vinegar in the past, moved to HCl for a while, and now am having good luck with Sparex #1 (although it seems to pit a bit if left in long enough) and Derustit B-P Metal Cleaner. all are extended soak times for me right now as I got the last two after it got cold here in central Oregon and they all live in a tack room in the barn where the average temp is below freezing. I expect they'll work much better come spring. An industrial tumbler and SS media are on my shopping list. Local jewelers and metalsmiths have been using ultrasonic cleaners with good success for cleaning scale and flux off of various metals, but effectiveness probably depends on the size of what you're cleaning...

I have really good luck with just straight vinegar and an over night soak. The next day, I wire brush the blade and most of the scale comes off except for the most stubborn spots. A flap disk easily cleans those stubborn scale spots off. Be sure to wear rubber gloves as the vinegar/iron mix will turn your fingers black.

Rusty in Ohio

HCl or muratic acid and a wire brush. 20-30 min. It is the cheapest imo. Be sure you wear good rubber gloves and eye protection.