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Removal of Rust from Iron

 

When iron begins to rust, it will puff up and expose clean metal to the open air, allowing rust to continue to the depths of the metal. The following methods can be applied to remove rust:

Sand Blasting or Bead Blasting

This is the most crude method of removing rust from metals and it is a standard practice in auto body repair-shops. Sand blast will also remove some good metal and will work – harden the metal surface.

Hence, glass beads blasting is used for more delicate parts. Immediately after any blasting, the metal surface is clean and exposed; so it is essential to rust proof it. In auto bodywork, the metal is often treated with acid-metal wash (a solution of phosphoric acid and alcohol) to remove moisture, followed by self etching primer.

Sandpaper and steel wool will also remove rusting, but they don’t get into tiny crevices. Rubber abrasive sanding blocks are good at removing a thin coat of rust and can also remove rust from minute pores in the metal.

Etching with Phosphoric Acid

Phosphoric acid has a unique property of dissolving hydrated iron oxide (i.e. rust) quickly while etching iron very slowly (by removing molecules of water from it). This means that metal can be left in the acid for much longer time than necessary with very little damage.

However, the acid will attack bare metal slowly and will start the process of hydrogen embrittlement, hence the minimum etch time that removes all rust should be used. Another advantage of phosphoric acid is that it leaves a fine coating of iron phosphate behind. (iron phosphate prevents rust) and leaves a hard, bright metal finish – this is because it will etch the surface slightly, exposing new, bare metal.

Common products which contains phosphoric acid are cola drinks, such as Coca cola. Cola drinks also contain carbonic acid, which etch rust as well. This is why when you leave a coin in some quantity of cola drinks, the coin comes up with shiny surfaces.

Other Acids (e.g. H2SO4, HCl and oxalic acid) will etch rust, but not as selectively. These acids will etch rust quickly, but if metal is left in them a bit too long, they will attack significant amount of metal. Also, the acids do not leave a protective film behind.

Oxalic acid operates to remove rust by forming a water-soluble complex ion. (called a Chelate) round each iron ion.

Note: All acids contribute some hydrogen to the metal structure by a process called hydrogen embrittlement – this weakens iron or steel.

Electrolytic Rust Removal

Rust can be electrically etched off iron or steel in a bath of mild alkali, such as sodium carbonate. The rusty part is connected to the –ve terminal of a 12V battery charger, and a scrap piece of steel or iron is connected to the positive terminal. A table-spoon of sodium carbonate per gallon of water is used.

Advantages of this method include:

(1). The alkaline solution is much safer than some acids mentioned earlier.

(2). It has no effect on good metal – the metal can be left in the alkaline for a long time and not damaged.

(3). There is no risk of hydrogen embrittlement.

Electrolytic rust removal will leave a black oxide surface, which is the result of a process that doesn’t remove any good metal but loose rust and embedded oxygen.   

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