Hmmm, not so sure about the metal bucket.
Metal buckets introduce Zinc as a sacrificial metal; which would donate electrons to the iron to retard corrosion. I am just a silly engineer, so metallurgy was one of my college fetishes; stainless steel sinks have some trace amounts of Nickel and possibly Zinc (depending upon SS formulation).
My 12 hours of Chem don’t help me here.
He advised using an old galvanized bucket. My last treatment of a hard disk was a year ago May/June, and he still recommended it.