Reaction with Water

• Metallic or steel bodies rust when exposed to water, especially rainwater. When metal meets water, it reacts with water and oxygen in air, and forms an acid by combining dust and dirt in a mixture along with the iron oxides. When water reaches an iron body, the oxygen in water combines with iron atoms to form compound molecules known as iron oxides. The particles of iron oxide remain suspended in water until it evaporates. After evaporation, the particles still remain on the body, forming a red coating on the iron known as rust. Alternatively, patina, rust on copper or bronze, turns green in color instead of the red color of rusted iron.
  
  

Chemicals and Compounds in Water

• Rust also occurs when iron comes in contact with carbonic acid. Some compounds and chemicals found in water also cause rust to occur on iron and its alloys like steel. Common chemicals include dissolved solids, carbon dioxide and oxygen. Oxygen and hydrogen gas react with each other at cathode and speed up the process of rusting. This leads to water becoming corrosive, causing rust on the metal. Chlorine and nitrates are other oxidizing agents commonly found in water, which can accelerate rusting.
  
  

Carbonic Acid

• Carbon dioxide and water contact can form a carbonic acid that can also lead to rust on metal. This is because carbonic acid is responsible for lowering the pH level of water. Moreover, water with low pH level is acidic and becomes highly corrosive, resulting in rust.
  
  

Contact with Seawater

• Seawater has elements that conduct better electrons than other forms of water to form rust. This is why when a nail falls in seawater, it rusts more quickly than a nail in any other type of water.