Originally Posted By: Stallones
I use pure water, either rain water or deionized
In my experience, some steels don't want to change from brown rust to black rust (Fe2O3 to Fe3O4) when boiled in pure water. Apparently there isn't enough hydronium ions present in the bath. In such cases, I add a few (2 or 3) drops of acid, such as hydrochloric, sulfuric or nitric, to the bath. It works for me.