THE corrosion that we will analyze in this text refers to the deterioration process of metals through electrochemical processes through redox reactions. This type of corrosion is very common in our daily lives, such as the formation of rust, which is actually the iron oxide compound (Fe (OH)3), which is formed when iron (reducing agent) is oxidized by oxygen (oxidizing agent) present in air, while the latter is reduced.
If the air is moist, this reaction proceeds even faster.
The semi-reactions and the overall reaction involved in the formation of rust are:
Anode (iron oxidation): Faith(s) → Fe2+ + 2e-
Cathode (oxygen reduction): O2 + 2 H2O + 4 and- → 4 OH-
Global Reaction: 2 Fe(s) + O2 + 2 H2O → 4 Fe(OH)2 →Fe(OH)3 (rust)
In cities located in coastal or coastal regions, this process takes place much faster. This represents a huge economic, social and technological loss for the population and the government. Houses, cars, ships, metallic structures of buildings and bridges, statues, among others, suffer from corrosion.
Studies indicate that around 80 billion dollars a year is spent in the United States on damage caused by corrosion. Even one of New York's biggest sights, the Statue of Liberty, is at risk from corrosion. It is made of an iron frame on copper plates. Since it is in a marine environment, copper comes into contact with moist air, leading to the formation of hollywood (a toxic mixture of copper hydroxide I, copper hydroxide II, copper carbonate I and copper carbonate II), which over time coats the statue (green layer). However, iron is more reactive than copper, so it reacts with the mulberry formed, displacing the copper cations from the mulberry. This leads to corrosion and shakes the statue's structure.
In Brazil, the losses are no different. Cepel (Eletrobrás Research Center), in Fortaleza, indicated that the useful life of a pole is less than 5 years in coastal cities, whereas in other places it can reach 30 years. Not to mention the constant maintenance that must be done on transformers and other electrical equipment.
It is estimated that around 20% of the iron produced every year is used to replace corroded iron.
But why is the process of metal corrosion accelerated in coastal cities? Is it just because the air is wetter?
In fact it's not just that. In these places there is the phenomenon of salty air, which for the population is usually associated with a strong and characteristic smell that emanates from the sea at the ebb. However, chemically, it can be said that the salt spray would be the oxidizing action of sea water due to the substances dissolved in it.
Seawater is not pure and due to the action of wind and other environmental conditions, the seawater particles spread, coming into contact with metallic objects that corrode. This process is accelerated by presence of ions in seawater and evaporites, which are microparticles present in the air that contain salts. These ions form a salt bridge, which enables the phenomenon of redox between the oxygen in the air and the metals.
Some processes are used to prevent rust and reduce damage from corrosion. See some of them by reading the text “Iron Corrosion Protection” .
By Jennifer Fogaça
Graduated in Chemistry
Source: Brazil School - https://brasilescola.uol.com.br/quimica/maresia-corrosao-dos-metais.htm
