Yep, I was thinking of Tannic acid for rust conversion.

https://www.theruststore.com/Rust-Converter-FAQs-W48.aspx

Rust Converter FAQ's

What is Rust Converter? Rust Converter, a water-based primer, contains two active ingredients: Tannic acid and an organic polymer. The first ingredient, tannic acid, reacts with iron oxide (rust) and chemically converts it to iron tannate, a dark-colored stable material. Tannins are a group of water- and alcohol-soluble natural products extracted from fruits, trees and grasses. The second active ingredient, 2-Butoxyethanol, is an organic polymer that provides a protective primer layer. The overall chemical reaction converts rust into a stable, black protective polymeric coating that serves as an excellent primer for both oil and epoxy based paints.