We know that rust is that layer that forms on the surface of metals, with different characteristics depending on the metal on which it forms. But do we know what causes it and why? Do all metals rust? Is corrosion the same as oxidation?
There is some confusion when it comes to distinguishing between the two concepts, as well as not knowing whether all metals oxidise or not. In this blog post we explain in detail that all metals rust and why.
Difference between oxidation and corrosion
Oxidation occurs in metal when the surface of the metal comes into contact with oxygen, transforming it into complex oxides. This is a surface reaction that produces a layer of stains known as rust, which can have different characteristics depending on the metal on which it is formed. This reaction does not affect the metal to any great extent, and the metal material does not deteriorate. However, this oxide layer can also protect the surface of the metal, as in the case of the layer of cupric oxide and cupric carbonate that forms on copper, which means that the metal is not in direct contact with air or water.
This reaction occurs in any type of metal exposed to oxygen, which means that all metals oxidise, although depending on their characteristics and composition they take longer or shorter to do so. Aluminium and copper are the metals that oxidise first, and it is the oxides of these metals that protect them from corrosion.
Corrosion, on the other hand, is defined as ‘the chemical or electrochemical reaction between a material, usually a metal, and its environment, which results in deterioration or degradation of the material and its properties’. This degradation occurs because every metal shows an inherent tendency to react with the environment.
Therefore, oxidation is usually the process prior to corrosion, and if we allow oxidation to continue to act, it may be the case that the metal ends up deteriorated. Thus, we can summarise that corrosion involves some form of oxidation, while not all oxidation leads to corrosion. In fact, aluminium, copper, zinc and tin oxides when produced in contact with the environment protect the metal from corrosion.
In addition, we must consider that oxidation can occur in many materials, while corrosion is limited to metals and has a more direct impact at the structural level.
Types of metal oxidation
Taking into account that all metals oxidise, we can distinguish mainly two types of oxidation.
Slow oxidation
Slow oxidation is the oxidation caused by water or air, which means that metals lose their shine and suffer corrosion when they are exposed to the environment for too long.
Rapid oxidation
Rapid oxidation occurs in violent chemical reactions such as combustion, generally exothermic (releasing energy in the form of heat), and occurs mainly in organic elements (containing carbon and hydrogen).
Corrosion prevention
To prevent metals from corroding, they can undergo certain processes that coat and protect their surface, forming a barrier between the metal surface and the corrosive environment. These coatings may contain corrosion inhibitors, which are chemicals that reduce the rate at which the metal corrodes.
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