How To Stop Corrosion

Knowing how to stop corrosion is exceedingly important as it can lead to disastrous failures. There are many ways to stop corrosion, here we will cover the 3 main methods.

3 Ways To Stop Corrosion

Intrinsic chemistry

Metals that are more corrosion resistant are those where corrosion is thermodynamically unfavourable. For example, metals like gold tend to break down into pure metal which is why we see them in the earth. However,  not many metals act like this.

Although corrosion is more thermodynamically favourable, some metals have slow reaction kinetics resulting in a tolerable slow rate e.g. zinc, cadmium and magnesium. An extreme sight of this is graphite, as it has extremely slow kinetics, this protects it against corrosion in normal conditions.

Passivation

Passivation stems from the material becoming more passive after being treated. The process involves creating a protective outer layer, applied as a micro coating. The protective layer is commonly created by either a chemical reaction with the base metal or allowed to build from spontaneous oxidation in air.

A good example of this is stainless steel. Untreated steel is reactive and tends to corrode easily and utilises a protective coat to minimise corrosion. The protective coat has chromium which is critical to the corrosion resistance. In this particular case, it can self-heal’. When disrupted, the chromium in the protective layers reacts more readily with the oxygen, producing chromium oxide and replenishing the lost material in the protective layer.

Another application of passivation is aluminium. Aluminium naturally creates a thin layer, upon contact with oxygen, of aluminium oxide (Al2O3) through the oxidation process giving the metal corrosion resistance.                                                    

Sacrificial Coatings

Sacrificial coating is a corrosion control method that applies a thin layer of a more reactive metal to a metal being protected. It is known as a sacrificial coating due to the protective layer attracting the corrosion towards itself, protecting the base metal.  It is like passivation in terms of a protective coating presence. The formation of the coating is from the use of a metal that is more reactive than the metal it is shielding.

This results in the protective layer becoming anodic and therefore corrodes as the base metal is cathodic, hence maintained. Galvanised steel is a great example. This is steel with a zinc-based coating. Disruption to the protective layer results in zinc readily donates its electrons to the iron in the steel therefore oxidising and making the iron cathodic. 

The most common way to apply a sacrificial coating is through the method hot-dip galvanisation. The metal, in this case, steel, is submerged in a bath of molten zinc at approximately 450 °C. When removed, the molten zinc reacts with the oxygen in the atmosphere to form zinc oxide (ZnO) which additionally reacts with carbon dioxide (CO2) to produce the final coating zinc carbonate (ZnCO3).

Galvanised steel is a commonly utilised metal when corrosion resistance is necessary. The metal is considered more superior in terms of life cycle and cost compared to stainless steel.

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