Rust converters are solutions or primers designed to be applied directly to a rusty surface to convert residual rust on steel surfaces to harmless and adherent chemical compounds. Unlike the standard scrape, prime, and paint regime, the user does not have to bring the surface down to bare metal. These in turn are able to develop a protective film (usually phosphates) on the metal surface that protect against rust.
The tannin is the heart of a rust converter. It reacts with the iron oxide, converting it to iron tannate, a stable blue/black corrosion product. Tannins are a group of water- and alcohol-soluble natural products extracted from a variety of plants. Little is known about their true structure as they are complex and variable. Industrial research in the effectiveness of tannin solutions as rust primers began in the 1950s. Since then, tannic acid (a tannin) has become a standard conservation treatment for corroded iron artifacts found on archeological sites. (reference)
The chemical mode of action of rust converters is the conversion of porous and loose iron compounds to sparingly soluble adherent compounds. The most frequently used formulations are based on phosphoric acid which cause the rust to be converted to firmly adhering iron phosphate layers. In addition, formulations based on naturally occurring polyphenols (tannin) are also used; due to a reaction with the iron ions, these lead to virtually insoluble iron complex compounds. Rust converters based on phosphoric acid are composed: (reference)
Rust converters are used in conjunction with primers and surfacers as
so-called rust primers. Whilst useful in theory, the use of rust converters is
not easy in practice because it is not possible to measure them accurately
according to the thickness and composition of a rust film:
The use of rust converters is not, therefore, permitted in many areas of steel construction. They are useful as a chemical rust removal measure after which excess solution is rinsed off before a coating is applied.