Removing Heavy Rust from Very Old Carbon Steel Carving Knife on the Cheap
First, read my answer here where I cite the electrochemistry background. The latter include source links to galvanic corrosion and, in particular, in the current instance, a classic case of pit corrosion.
Stop long-term soaking, as this in the presence of salts and an H+ together with dissolved oxygen will just further promote the slowly evolving electrochemistry.
I, personally, ignored stake-knives left in a sink (water and oxygen exposure) that were used to cut a mango. The sticky mango juice, rich in Ascorbic acid (Vitamin C) is apparently particularly notorious for promoting pit corrosion.
Never have completely removed the stains, so do not have high expectation here,