Re: How do 'rust converter' chemicals work?

I am not familiar with this particular product. But it sounds as though it is very similar to the rust converter products I do know.

The reason why rusting is such a problem with iron and steel is that the oxide layer that forms does not adhere to the underlying metal, but powders or flakes off, leaving fresh metal surfaces exposed. Aluminium, which is a much more reactive metal than iron, does not corrode because the oxide layer that forms rapidly on an aluminium surface adheres closely to the underlying metal, and prevents further access of oxygen or other environmental oxidants.

Rust converter products are usually based on phosphoric acid; there are other additives as well to aid with flow and adhesion. The black coating that forms is hard, insoluble, ferric phosphate:

Fe2O3 + 2 H3PO4 ----> 2 FePO4 + 3 H2O

Unlike ferric oxide, ferric phosphate does adhere closely to the underlying metal, and protect against further access of oxygen or oxidants. If some excess phosphoric acid remains adsorbed or otherwise trapped in the phosphate layer, it will give some further protection against rusting at the site of small chips or scratches that damage the initial phosphate layer.

In using these products, it is usually best to clean the rust layer back to a fine powdery covering over the surface. The phosphate layer that forms in the presence of bare metal, or of sizeable rust flakes, is not usually quite so well-formed and effective.

I do not know the fine detail of how the additives work, nor what they are, nor even if the particular product you are using is phosphate-based. But this answer should at least give you a start on the right track.

John.