The enormous damage caused by corrosion processes to materials, particularly steel, is well known. The causes of corrosion are manifold, but its consequences always lead to economic losses and, in the most serious cases, even damage to the safety of industrial processes, structures and buildings.
Exposure to water, high temperatures and aggressive substances can produce different types of corrosion. Any metallic element located outdoors is susceptible to corrosion. To prevent damage, it is recommended to use aluminium and stainless steel, to keep the surface as dry as possible, to use a grease coating or to apply an anti-corrosion paint. It is advisable to use one or more of the protection systems described below, or to use them in combination.
Protection by coating.
Something as simple as forming a protective film on the metal to insulate it from the environment can be very effective. These can be non-metallic coatings such as paints, varnishes, plastics, enamels and ceramics. There is also the option of a metallic coating, which can be applied in different ways.
By immersion: the metal to be protected is immersed in a bath of molten metal which, when cooled, forms a solid protective layer. Usually tin, zinc (galvanised), aluminium and lead are used.
By electrodeposition: Electricity is transferred between two different metals, immersed in a conductive liquid that acts as an electrolyte. The metal to be protected acts as the cathode and the other as the anode. The protective film is formed on the first metal. An example of this system is nickel plating.
By chemical coating: This is achieved by reacting the metal with a chemical agent. This produces a thin compound on the surface that forms a protective layer. This is the case with processes such as chromatisation (application of chromic acid) and phosphatisation (application of phosphoric acid and phosphates).
Corrosion protection methods
Corrosion inhibitors
These are chemical substances which, when applied to the surface of a metal, produce a protective layer that prevents corrosion. Inhibitors are widely used as a temporary coating for materials, for example during storage or transport. Although the most commonly used inhibitors were initially oil-based or solvent-based, the use of water-based inhibitors has increased in recent years. There are two types: absorbent inhibitors, which form a protective film, or scavengers, which remove oxygen.
Cathodic protection
With this technique, the metal to be protected behaves as a cathode by supplying electrons. To achieve this, another metal is used which is more negatively charged than the first, the so-called sacrificial anode, which oxidises by passing the electrons released by the process to the metal to be protected. This option is widely used in the shipbuilding industry. A variant of cathodic protection is potential difference protection, which increases the passage of electrons by connecting a voltage source that maintains the current between the two metals. It is used in metro applications.
The key to preventing corrosion from occurring
However, there is no doubt that the best option for preventing corrosion is to invest in a high-quality design and to select good materials according to the application. To do this, it is necessary to consider several factors, such as the corrosion penetration capacity of the metals used and an analysis of the mechanical stresses they will be subjected to. In addition, it is advisable to choose metals that are close to each other in the electronegativity table. If this is not possible, it is preferable to reduce electrical contact by interspersing non-metallic materials. Also, to avoid crevice corrosion, it is preferable to choose welding rather than riveting. It is also a good measure to avoid stress concentrations in areas prone to corrosion, paying particular attention to stainless steel, brass and materials with a tendency to corrode. Likewise, especially in tanks and pipelines, it is vital to prevent erosion corrosion and the build-up of corrosion-producing substances.
On the other hand, considering that environmental conditions are key to corrosion, influencing them will make it possible to control the process. It goes without saying that reducing the temperature, the speed of the corrosive solution, eliminating oxygen from aqueous solutions and avoiding stagnant solutions are some of the options for preventing corrosion of metallic materials.