How to remove corrosion at home
Metallic, non-metallic coatings and other methods of protection
Corrosion is the spontaneous destruction of metal under the impact of the environment. Any factors can have a negative impact on metal – chemical, physico-chemical, electrochemical. Not only pure metals are subject to corrosion – many alloys may also rust.
How the rusting process takes place
Metal corrosion can be observed quite frequently in open air in a damp environment – insoluble compounds form on the surface, oxides (as a result of contact with oxygen), carbonates (from reaction with carbon dioxide CO₂ in the air), and sulfides of the corresponding metal. A typical example of rusting is the appearance of reddish-brown coatings on steel objects (the oxide and hydroxide of the trivalent iron forms). The equation of the process is the following:
4Fe + 6H₂O + 3O₂ = 4Fe(OH)₃ (iron(III) hydroxide – an insoluble base of a reddish-brown color)
When this reaction takes place, access of air and moisture making contact with the metal surface are sufficient. As Fe(OH)₃ in this form is quite an unstable compound, it swiftly loses water, forming the oxide Fe₂O₃:
2Fe(OH)₃ = Fe₂O₃ + 3H₂O
Iron(III) oxide is not an oxide film which can protect the metal from further destruction. The formation does not stop subsequent oxidation of the metal on the surface of Fe₂O₃, so if the rust is not removed in time, the metal may be destroyed completely.
This does not happen with all metals – in the corrosion process, some (for example aluminum) are covered with a thick oxide film, which protects the metal surface from further destruction. Only after its removal, the metal begins to react with the environment again
4Al + 2H₂O + 3O₂ = 4AlO(OH) (the white flaky substance aluminum metahydroxide forms)
Low reactive metals (for example, copper, which is covered with a patina, a greenish coating) corrode much more slowly and weakly than active ones. Noble metals are barely damaged at all under the impact of the environment – gold Au, silver Ag, platinum Pt etc.
How to remove rust at home
Traces of rust often appear on the surface of iron objects. If destruction has begun recently, and the surface of the metal is not damaged too greatly, then rust can be removed at home.
Often the simplest method is used to do this – the mechanical removal of rust by a rough metal brush. To achieve the effect more quickly, you can prepare a paste of baking soda or hydrogen peroxide (or water) with cream of tartar, rub the surface of the metal with the paste, and then rub it off. For large objects, rough sanding equipment and machines are used. They work according to the same principle as rough brushes – when the disk rotates, the rust is removed. It is important to remove the corrosion carefully, as it is possible that the machine may damage the clean surface of the metal.
Any acids are especially good for removing rust – by reacting with iron oxides and hydroxides, they dissolve unwanted formations.
Generally, there are a few simple methods for removing rust with acids:
- soaking in apple vinegar (you simply have to immerse the object in the vinegar solution overnight, and then remove the rust with a piece of crumpled foil; it can also be rinsed with vinegar);
- soaking in lemon or lime juice (to remove traces of corrosion more effectively, you can sprinkle the metal surface with table salt, rinse with lemon or lime juice, and leave for a long time; afterwards, as is the case with vinegar, rub the rust off with crumpled foil);
- soaking in hydrochloric or phosphoric acid (it is often encountered in everyday life, and works against corrosion quite effectively). The orthophosphoric acid contained in Coca Cola means it can also be used to remove rust from metal.
Rust can be removed from small objects by placing a potato on a metallic surface or sticking an object into a potato – the oxalic acid contained in potatoes also provides effective protection against corrosion.
Special liquids are useful for removing rust from metal – you can simply apply them to the rusty surface, and then remove the rust after a short time. They usually have quite a toxic composition, so follow instructions and observe safety rules.
Metal can also be cleaned using electrolysis – prepare a solution of baking soda (1 tablespoon per 4 liters of water). As an anode, you can use a piece of steel (it should be attracted by a magnet; aluminum or stainless steel won’t be suitable). To the part of the anode which is above the solution, attach the positive clamp of the cable, and to the non-rusty part of the cleaned object, the negative clamp (it is important that the clamp and cable are not in the solution). After connecting the cable to the battery, you can turn on the system. After 8–12 hours the reaction will take place, and the object can be removed, turning off the power beforehand – all the rust will come away from the metal and gather on the steel. But the metal will probably have to be cleaned with a rough brush from the remains of the oxides.
How to protect metals from corrosion
There are several methods for preventing the destruction of metals under the impact of the environment. They do not give a 100% result – corrosion processes can only be stopped completely in an atmosphere of inert gases (for example argon). In most cases this is impossible.
The main method of combatting corrosion is placing various coatings on the surface of metal (metallic, non-metallic, chemical). Metallic coatings come in two types:
- anode (covering a less active metal with a more active one – for example, covering iron with zinc);
- cathode (covering a more active metal with a less active one – such as covering iron with copper).
An important difference in these types of coatings is that when an anode coating is used, the protected metal is entirely preserved, until the coating is not completely worn away. With the cathode method, a single instance of damage to the film is sufficient for the protected metal to start to disintegrate.
Non-metallic coatings usually include polymer masses, non-organic and organic compounds – for example, varnishes, paints, cement and glass-like masses. They have a much lower reactive ability, and so they keep the metal intact.
Chemical protection from corrosion means creating a durable film on the surface of metal, which does not allow the metal to disintegrate. These films can be obtained by the following methods:
- nitrogenation – saturating the metallic surface with nitrogen;
- oxidation – creating durable oxide films (for example, Al2O₃, BeO);
- phosphatizing – creating a phosphate protective film (such as Fe₃(PO₄)₂).
Such methods of chemical protection from corrosion as burnishing and cementation are also widespread. In the first case, there is a reaction of the surface of the metal with organic substances, and in the second the formation of carbon compounds on the surface
Metal can be protected electrochemically. Sometimes a protective kind is mind – a plate of a more active metal is attached to the metal object (in this case this metal is the protector). The more active metal has a lower potential, and so is an anode. When a current is run it starts to disintegrate, while the protected metal (in this case the cathode) is preserved undamaged.
If you attach the protected metal to the cathode (negatively charged electron) of the source of the direct current, the protected metal itself becomes a cathode. In this case it is not the cathode that will disintegrate, but the anode (scrap metal is used as the anode – as it disintegrates, it preserves the cathode from destruction).
Other methods of protection
In some cases, it helps to add compounds to the metal which increase its durability to corrosion (the technical composition of the material changes, and the preserved metal does not corrode as strongly as it would in pure form). Other metals are also used as additives which are less prone to rusting, or non-organic or organic compounds.
Inhibitors of corrosion are substances that prevent and stop metal damage. Usually, adding them changes the properties of the environment the metal is in, making it favorable for keeping the surface intact. Various substances may be used as inhibitors – depending on the type of metal and cause of corrosion. For example, to prevent iron rusting, potassium bichromate or amines may be introduced into the environment, if it is damp. Click here for amazing experiments with iron.
In some cases, inhibitors are not introduced, but more aggressive elements that cause corrosion are removed from the environment (for example excess moisture). When using home methods of rust removal, it is important to observe caution, as this often involves working with caustic substances. All mechanical abrasives are quite rough, so one should work in protective gloves to avoid scratches.