Today’s world is more connected than ever before. We are all global citizens that reap the benefits of this well-established trade infrastructure of the 21^st century. The computer I am typing on is made in China. The chair I am sitting on is made in Vietnam. My car is made in Germany and my TV is made in Japan. My clothes are made in India. The cheese I ate this morning was from Holland, and the lamb I just cooked was imported from Australia.

About 90% of this global trade volume occurs by sea since it is the most cost-efficient means of transport. The containers ships that carry these goods over the vast water bodies are subjected to some of the harshest and most dynamic conditions.

Consider a container ship traveling from Mumbai (India) to Toronto (Canada). It’s typical maritime route would be as follows:

1. Depart from Mumbai Port: The container ship would depart from Mumbai Port on the western coast of India. Mumbai Port is a major seaport and serves as a gateway for international trade in the region.
2. Arabian Sea: The ship would navigate through the Arabian Sea, heading northwest.
3. Red Sea: The vessel would pass through the Bab al-Mandeb Strait, which connects the Gulf of Aden to the Red Sea.
4. Suez Canal: The ship would transit through the Suez Canal, a man-made waterway that connects the Mediterranean Sea to the Red Sea. The Suez Canal is a vital trade route, allowing ships to avoid the longer journey around the southern tip of Africa.
5. Mediterranean Sea: After passing through the Suez Canal, the ship would sail across the Mediterranean Sea, heading north.
6. Atlantic Ocean: Upon reaching the Mediterranean, the ship would enter the Atlantic Ocean.
7. St. Lawrence River: The ship would enter the St. Lawrence River, a major waterway in eastern Canada.
8. Port of Toronto: Finally, the container ship would arrive at the Port of Toronto, located on Lake Ontario, where cargo would be unloaded for distribution in the Toronto area.

Considering the above path, the total route from Mumbai to Toronto could span around 12,500 to 15,000 nautical miles (23,150 to 27,780 kilometers) and would take approximately 30-40 days to complete. During this travel, the container ship would be subjected to various environmental conditions and water bodies with very distinct compositions and ecosystems.

From a coatings perspective, the container ship (the asset to protect) is predominantly made from steel plates that must be able to withstand these varying conditions. Some of the challenges presented by the marine environment are as follows:

1. Corrosion: Ships are in constant contact at length with one of the harshest environments in nature- sea water.

• High Salinity or concentration of salt in the water
  Amount and composition of dissolved salts in water bodies vary and these salts provide free ions which allow for a highly conductive environment in the immediate surroundings of the vessel making it very conducive for corrosion to occur.
• Microbially Influenced Corrosion (MIC)
  Bacteria adhere onto substrates by secreting a slimy substance. This biofilm enables more bacteria and fungi to colonize onto the substrate. Metabolic processes of these microorganisms create a highly corrosive environment that influence the cathodic and anodic reactions that occur during corrosion.
• pH of the water
  Dissolved in the waters, especially at the surface, are various gases such as oxygen, carbon dioxide, hydrogen sulphide and ammonia. These can create concentration cells that can further accelerate corrosion.

2. Biofouling: Open waters are highly biodiverse, even more so than the species we see on land. The chemical nature of these plants, animals and microorganisms in the marine environment also varies on a spectrum that ranges from hydrophobic to hydrophilic. Therefore, trying to inhibit one type of organism may help another to thrive. Some of these are mentioned below:

• Barnacles
  Barnacles are crustaceans that attach themselves to ship surfaces and form hard, shell-like structures. They can significantly increase drag and fuel consumption by creating roughness on the hull.
• Mussels
  Mussels are bivalve mollusks that adhere to ship surfaces using byssal threads. Their attachment can lead to surface roughness and increased frictional resistance.
• Seaweeds and algae
  Various species of seaweeds and algae can grow on ship surfaces, forming slimy layers and contributing to fouling. These organisms can reduce the efficiency of the ship’s propulsion system.
• Tube worms
  Tube worms create tube-like structures attached to ship surfaces. They can cause localized roughness and increase hydrodynamic drag.
• Bacteria and biofilms
  Various types of bacteria and microorganisms can colonize ship surfaces and form biofilms. These biofilms can provide a suitable substrate for other fouling organisms to attach and grow, exacerbating fouling effects.

3. Other Environmental Variations: We experience the effect of the earth’s rotation and revolution in the form of day/night and changes in seasons respectively. These effects are further enhanced for a vessel that is moving across the globe over various longitudes and latitudes. The coating on the ship must endure these environmental changes encountered during the voyage.

• UV exposure
  The sun’s rays hit the earth more directly (perpendicularly) at the equator compared to the poles where the rays hit at a slant. The rays near the equator thus have a shorter path to travel through earth’s atmosphere resulting in these areas experiencing more intense UV radiations. On the other hand, the rays at the pole travel a longer distance through the earth’s atmosphere that absorbs and scatters most of the UV radiations making them relatively less intense.
• Temperature variations
  Similar to the effect of UV exposure, the variation in the sun’s radiations results in temperature variations across latitudes. The temperatures at the poles on an average is -2°C while at the equator is 35° Further still, there are annual temperature variations of 10°C.
• Air and water currents
  The earth’s spin causes currents in the air and water. Consequently, these currents affect the velocity of sea water and consequently the drag experienced by the vessel resulting in physical challenges such as abrasion.

In conclusion, the marine environment is one of the challenging environments from a coatings perspective. A variety of factors must be accounted for when formulating marine coatings. The marine environment presents the challenges of corrosion, bio fouling and variations in the environment.

Further Readings:

• The Electrochemical Society for how corrosion works.
• Corrosionpedia for marine coatings.
• Coatings World for marine coatings market.
• OECD for trades by ship and ship-building.