Why Is Rusting Common In Coastal Areas?

Everyone’s seen them – the old cars with unsightly, corroded bodies standing in coastal parking lots, the metal windowpanes flaking in seaside cottages. Iron and steel seem to rust more rapidly in coastal areas than inland, and people living there are engaged in a constant but unremarked battle to prevent it.

Rusting is common in coastal areas because they’re rich in ingredients that cause and accelerate rust, namely oxygen, water, and salt. There isn’t as much water and oxygen in the air at higher altitudes, and there’s hardly any salt. Many coastal areas are also polluted, further accelerating rust.

To appreciate why rusting is more common in coastal areas, you need to know something about rust and how it forms. The air in coastal regions generally has different environmental conditions than the atmosphere inland. This article explores the reasons why rusting is so common in coastal areas.

What Causes Rust In Coastal Areas?

Science is aware of some sixteen iron oxides, the best known being iron (III) oxide or rust. Rust is a flaky mixture of iron oxides on the surface of metal objects. As their name suggests, these iron oxides are a blend of iron and oxygen at the molecular level and form when iron atoms lose their electrons as they interact with oxygen.

Rust eventually converts iron and iron alloys into powder. Since these materials are prized for their rigidity and weight-bearing capacity, rust is highly undesirable. It also makes things look ugly. Water is an essential element for rust to occur, and it is present in much higher quantities at the coast.

Rust occurs when oxygen and water interact chemically with iron and indicates that it is degrading. It is a serious structural problem in buildings and other objects because it weakens the iron, increases its permeability to water, and eventually destroys its structural integrity.

Factors that speed up the rusting process include –

  • High humidity levels in the air – coastal areas are usually much more humid than deserts and other inland areas because water evaporates from the sea and is carried inland by air currents.
  • The presence of salt in sea air and water – salt ions increase the rate and frequency of electrochemical reactions in iron
  • The purity of the metal – iron alloys consist of a mixture of metals, including iron, that causes the process to speed up. If an object is made of pure iron, it usually rusts more slowly. 
  • Environmental acidity – low pH levels accelerate the rusting process. Seawater can be more acidic if too much carbon dioxide gas from the atmosphere dissolves into it. Therefore increased pollution by industrial plants in coastal areas could lower the pH of the water in the sea and air.

According to NOAA,  coastal acidification occurs due to changes in the chemistry of freshwater rivers running into the ocean that contain excess organic carbon and nitrogen from fertilizers and other industrial chemicals. These cause algal blooms in the sea that consume oxygen and release carbon dioxide when they die.

Coastal areas are the most highly populated regions on Earth, meaning there is inevitably increased pollution from human activity. The average population density of coastal areas is around eighty people per square kilometer, which is twice the population density of the entire world. Pollution is a common problem in coastal areas and speeds up the corrosion of iron and iron alloys like steel. 

There Is More Oxygen In Sea Air

The air is denser in coastal areas than inland and contains more oxygen due to its proximity to the ocean. Altitude sickness occurs in mountain climbers due to rapid decreases in air pressure and oxygen levels at high elevations. The air becomes ‘thinner” the higher you go above sea level.

Coastal areas are at or just marginally above sea level, which means they have the highest concentrations of molecules that make up the air. Oxygen is one of these molecules and is a necessary part of the chemical process that causes rust. Because the air is oxygen-rich at the coast, there is an increased chance of the formation of iron oxides.

Over Half The World’s Oxygen Comes From The Ocean

According to scientists, the ocean produces between fifty and eighty percent of the Earth’s oxygen. This is mainly due to mostly microscopic creatures called plankton that drift in the seas and are carried by tides and currents. Some plankton, such as certain crustaceans and jellyfish, can be seen with the naked eye.

Phytoplankton consists of tiny plants that, like all plants everywhere, absorb carbon dioxide and light and produce oxygen in the process called photosynthesis. It stands to reason that the oxygen they produce is more concentrated in regions closer to the ocean as the winds blow it inland. Coastal zones also have higher barometric pressure than inland, compacting the air molecules and making the atmosphere denser.

Corrosion and Pollution Are Interactive Processes

Carbon steel and reinforced concrete are typically used to construct marine and coastal installations, but other metals such as copper, stainless steel, and aluminum are also employed. Many pollutants accelerate corrosion which produces rust. Many rivers meet the sea in coastal areas, often carrying agricultural, municipal, and industrial contaminants with them.

Coastal areas also often have ports and fossil fuel power stations which further contribute to pollution. The polluted effluents of human civilization along coastal rivers and at the seaside increase the extent of corrosion of reinforced concrete and steel.

There is enormous industrial and economic activity on or near coastlines because of the importance of shipping, railways, and the need to transport vast quantities of goods to inland cities.

Salinity Increases Corrosion

Corrosion is an electrochemical process facilitated by the presence of salt. It increases the electrical conductivity of water in the sea and the air. Rust can only take place in the presence of water and oxygen, but the salt content of the water also makes a difference.

Rust is all about the exchange of electrons between iron and oxygen atoms, and salt makes it easier because salt acts as a catalyst, accelerating the chemical process. 

Why Cars Rust More In Coastal Areas

Cars kept less than ten miles from the sea tend to rust more because sea air contains millions of tiny salt particles that accelerate rusting. Sea air is salty because the wind constantly blows spray from the sea inland, which comes into contact with cars. There are many roads following the shoreline at the coast, and residents and tourists love to go for scenic ocean drives.

When the water droplets evaporate, they leave behind minute salt deposits on surfaces that build up over time. The oxygen in the air and water droplets interacts chemically with the metal to produce rust. Cars are mostly made of steel, an iron alloy susceptible to rust.

Conclusion

Rusting is common in coastal areas because they are rich in the ingredients necessary for the corrosion of metals to take place. The air is more humid and more oxygen-rich than inland. Water and oxygen are needed to make rust, but salt acts as a catalyst to speed up the process, and there is plenty of it in the sea air and water. Seaspray is carried onto the coastline by the wind and deposits salt on everything in the form of minute water droplets.

References

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