One method used to prevent rust, is to insulate the metal from moisture and oxygen by covering it with protective chemical layers or paint. This method is effective as long as the protective surface is not damaged, for example, stone chips, scratches, abrasions in the joints. In real terms this is not possible and where damage occurs, no matter how small, rust will occur. It is also impossible to cover every area on the vehicle with this method and hard to reach places are left unprotected, often these are the areas that are most prone to corrosion.
Another method is Capacitive Coupling, used by some products, which relies on the paint as a dielectric or barrier to the free electrons, causing the metal to be at a lower potential difference, theoretically slowing rust formation. There are a number of major problems with Capacitive Coupling for vehicles. Firstly, if there are stone chips or cracks in the paint, the dielectric is broken and so the charge is lost. So you need to make sure that there is no bare metal, chips or cracks on the vehicle, which is impractical. Another problem is that it is almost impossible to hold a charge (potential difference) on a sharp thin edge of metal, and so this technology is not really suited for vehicles, which are made up of many irregular shapes. Because of this, it is also ineffective on existing rust and the areas surrounding it, as rust always causes sharp edges and damaged paintwork.
Electronic rust prevention is another method used to prevent rust using one of two technologies; either Impressed Current or Sacrificial Anode. Impressed Current units are extremely effective in dry conditions but ineffective in the wet. Sacrificial Anode units, like those used on ships, are very effective in wet conditions but cannot inhibit rust in dry conditions. Obviously vehicles experience varied conditions and so neither of these types of products are suited for vehicle rust prevention.

Under normal conditions the metal of a vehicle contains a mass of Iron Atoms, Impurities and Free Electrons. The impurities are slightly positively charged and therefore there is a potential difference (voltage) between the impurity atoms and the iron atoms. This means that the Iron Atoms become the Cathodes (-) and the Impurity Atoms the Anodes (+). The potential difference usually averages at about 1.1 Volts
Because the Impurity Atoms are at a higher potential (anodes) there is an attraction of the free electrons, which are negatively charged (positive attracts negative). As soon as there is moisture on the vehicle, the moisture becomes the electrolyte, which provides the path for the free electrons to travel from the iron atoms (-) and attack the positively charged impurities/anodes in the metal; this reaction is called oxidization (rust). Along coastal and surrounding areas there is a high salt content in the air due to the sea, which intensifies the rusting process as it makes the moisture even more conductive. The same situation occurs in areas where salt is used to melt icy roads.

So in order for corrosion to form, there needs to be water and free electrons present.
By eliminating any one of these agents rust will not be able to form.

The Proof -

\".....If a new car were to reach a certain stage of rustiness under severe conditions at 3 years of age, then with the (Rust Protection) device fitted the vehicle would take 12 years to experience the same degree of rust.\"

- The Motorist Magazine