Rust converters have many proponents, and some detractors too. Talk to a purveyor of this product, and he will extol the scientific basis of this "cure" for one of the most destructive foes a car owner may face. But scientists and those who have invested much time and effort into the fight against rust will sing a different tune. Rust inhibitors follow the same script.
How Rust Converters Work in Theory
Rust converters are specially formulated compounds designed to be applied to a rusty metal surface in order to render existing rust harmless, and to stop its advance. The ingredient in rust converters that does the job is usually a tannin, for which manufacturers often use tannic acid. According to an article by Susan L. Malthy that appeared on Britannica.com (and was republished at Alan.net), the tannin in a rust converter reacts with iron oxide, the main component of rust, to convert it to iron tannate, a stable compound with a bluish black colour.
Most manufacturers of rust converters include an organic polymer for a layer that serves as a protective primer, as well as an acid (usually oxalic acid or phosphoric acid) that speeds the action of the tannin. Rust converters are meant to be applied directly to the rusty surface without sanding. Preparation of the surface consists of removing any dust with a brush and vacuum, rinsing any salt deposits and degreasing, according to Malthy. You can then brush or spray on the rust converter and leave it to cure for 24 hours.
How Rust Converters Work in Practice
There is an abundance of claims made regarding the efficacy of rust converters, but most of these opinions come from a questionable source: the manufacturers or sellers. A scientific paper published in the December 1992 issue of the journal "Corrosion" (which is published by the National Association of Corrosion Engineers) found that "the slight inhibition [of rust] detected does not seem to be significant." Yet the same paper gives some credence to the claims of rust converter advocates by saying that the study does indicate that "the application of tannic acid...to rusted steel surfaces promotes the formation of ferric tannate complexes at the outermost rust surface." Compare those statements to some manufacturers' claims that rust converters will convert all the rust that's present.
Another neutral opinion comes from Standards Australia, a non-governmental agency, which says that in certain instances, using a rust converter can detract from the performance of some paints that are subsequently applied.
Rust inhibitors tend to be less controversial than rust converters. Yet the same pattern emerges when you look at the literature surrounding this product--manufacturers and sellers are full of glowing praise, and criticism comes only from parties that have nothing to gain from persuading people to buy their product.
For example, The Corrosion Doctors, a non-profit group whose website is dedicated to educating the public on rust-related issues, says that the gadgets being sold as rust inhibitors, irrespective of their successful use on other things such as bridges and ships, simply do not work on a car. The group likens those who sell such devices to "snake oil merchants."