Rust is the result of the corrosion process of metals, iron and steel, and is composed by different iron oxides. The red product that we see on a rusted metal object is in fact composed by an external oxide called hematite, while the internal layer is made of an iron oxide called magnetite.
The damaging effect of rust on a metal object is due to the nature of the iron oxides that are soft, not adherent to the metal surfaces, and have a volume that is approx three times the volume of metal corroded.
When a thick layer of rust products develop on a metal surface it can easily fall of exposing the metal surface to the corrosive environment, while other metals, like copper, develop oxide layers that are strongly adherent to the surface, are not easily removed and that protect the metal beneath from further corrosion. You could think for example of the copper roof on old building.
A metal rust remover is basically every method that can be used to remove rust and restore the metal surface to its original aspect, provided that the object has not been severely damaged by the corrosion process.
There are basically two different approaches to remove rust from a metal surface and reduce the effects of corrosion; mechanically remove rust or chemically convert rust into a more adherent and resistant material.
The first method, mechanical approach, includes the use of abrasive materials, abrasive papers, or powered mechanical sanding, grinding to mechanically remove the oxides from the metal surface. Abrasive blasting, a process to remove rust by directing a stream of abrasive particles against the surface offers the best level of oxide removal and leaves the metals surface with a roughness suitable for painting.
However mechanical techniques cannot be always applied due to the dimensions, geometry of the item to be treated and also because special tools, equipments like blasting machine, grinding machines are necessary.
The second method, chemical approach, is mainly based on acid like Phosphoric acid and tannins that react with iron oxides and convert them into a black ferric phosphate and other more stable material.
There is also a third method, the electrolytic approach that is based on the use of an electric current to convert the oxide back to the original metal. The object to be restored is immersed in an electrolyte and connects to a battery or generator.
A current is passed though the object and into the electrolyte causing a reaction that convert the metal oxide. The main drawback of this method is that only small object can be treated, like coins.
This method is also used to clean the surface of silver, in this case the object to be cleaned is connect to a piece of metal that works like a galvanic anodes.
The selection of the most suitable method should be based on a careful evaluation of the object to be treated, the extent of the corrosion damage and on the availability of equipments, tools.
The first thing to do should be an evaluation of the value of the object you’re going to repair, if it’s a valuable object, old coins for example, the best thing to do is to consult with an expert and then decide what to do. Sometimes a rusted object is more valuable than a clean, immaculate one.
Secondly you should clearly define the type of metal you’re going to threat because most people think that it is possible to apply the same method, mechanical or chemical, on every rusted metals surface. Using the wrong method can irreparably damage the item.
Third evaluate the cost associated with every possible applicable method. Take a look around and see what type of tools you already have and that can be used, this can save you a lot of time and money.