Base Formula
100ml Distilled Water
6g Naval Jelly (1tsp) (30% phosphoric acid)
4g tannic acid
Actual Chemical Formula
100ml Distilled Water
1.8-2g phosphoric acid
4g tannic acid
From the Canadian Conservation Institute
https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes/tannic-acid-rusted-iron-artifacts.html
100 g tannic acid
900 mL deionized or distilled water
50 mL ethanol
approximately 2 mL dilute phosphoric acid (H3PO4)
Explanation (this is from an AI summary)
Tannic acid and Naval Jelly (phosphoric acid) work together to chemically convert active iron oxides (rust) into a stable, water-insoluble, dark blue-black protective layer.
The Core Chemistry
Naval Jelly (Phosphoric Acid - H₃PO₄):
Action: Phosphoric acid reacts directly with red or orange iron oxides (such as Fe₂O₃ and Fe₃O₄).
Product: It converts iron oxides into ferric phosphate (FePO₄), a hard, grayish-white or dark crystalline layer that bonds to the underlying steel.
Role in the Mix: It lowers the local pH, etches the surface slightly, and makes free iron ions (Fe²⁺ and Fe³⁺) readily available to react with the tannin. [1, 2, 3]
Tannic Acid (Polyphenol / Tannins):
Action: Tannic acid contains multiple phenolic groups that chelate (bind) strongly with iron ions (Fe²⁺ and Fe³⁺) present in the rust layer.
Product: It forms an organo-metallic complex known as ferric tannate (iron(III) tannate).
Role in the Mix: Ferric tannate is a dense, water-insoluble blue-black compound that passivates the metal surface, stopping further oxidation and oxygen/moisture contact.
How the Reaction Looks and Works
Color Change: Within 10 to 15 minutes of application, the surface shifts from orange-brown rust to dark gray and deep blue-black as the ferric tannate and ferric phosphate matrix forms.
Bonding: Rather than stripping the metal down to bare steel (which requires abrasive grinding or sandblasting), the chemistry uses the existing rust as a direct reactant to build an armor-like conversion coating.