What Creates Rust?
Rust is an iron oxide, a product of the reaction between iron and oxygen in the presence of water or air moisture. The oxygen reacts with the iron to form iron oxide, which is also known as rust. The water helps the oxygen to react with the iron by providing a source of moisture.
Rust Formation Processes
Rust is iron oxide, which forms when iron or an alloy that contains iron is exposed to oxygen and moisture for a long time. The resulting reddish-brown appearance is due to hydrated iron(III) oxide formation.
Rusting is an electrochemical process that begins with transferring electrons from the metal to oxygen. This creates an electron deficiency in the metal, which must be filled by the flow of electrons from a more electronegative substance.
Three things must be present for rusting to occur: iron, oxygen, and water. Rust forms when these three elements combine and create an electrochemical reaction. iron + water + oxygen → hydrated iron(III) oxide. It is the hydrated part that makes rust brown instead of red.
Factors Influencing the Rate of Rusting
When it comes to rust, not all metals are created equal. The rate of rusting depends on the various factors that can be summarized as follows:
The Type of Metal
Some metals rust more easily than others. For example, iron and steel rust more readily than aluminum or stainless steel. The difference is due to the electrochemical potential of the metal. Pure metals are usually more reactive and therefore more prone to rusting.
Stainless steel alloys contain at least 10.5% chromium. The chromium forms a thin layer of oxide on the surface of the steel that protects it from further rusting. If your appliances are rusting, it may be due to the fact that they’re made with a less corrosion-resistant steel alloy.
The Presence of Moisture
Rust can’t form without water. The reason why iron pipes often rust from the inside is that water vapor condenses on the metal. The presence of moisture speeds up the rusting process because it provides the electrolyte needed for the reaction to occur.
Non-alloyed metals are particularly susceptible to rusting in humid environments. That’s why you’ll often see galvanized steel or wrought iron rusting faster than other types of metal.
The Presence of Oxygen
Rust can only form in the presence of oxygen. If you live in a dry climate, your metal surfaces may be less likely to rust. This is because oxygen is required for the reaction to occur, and it’s more readily available in the humid air.
What you need to remember is that not only oxygen is found in water or humid areas. Carbon dioxide in the air can also provide the oxygen necessary for rusting. In the presence of water, carbon dioxide reacts to form carbonic acid. This electrolyte can speed up the rusting process and cause corrosion.
Therefore, even if you don’t live in a humid climate, your metal surfaces may still rust if they’re exposed to carbon dioxide and water.
The Presence of Other Chemicals
Rust can also form when other chemicals are present. For example, salt accelerates the rusting process by corroding the metal. This is why you’ll often see iron railings and fences rust more quickly near the ocean.
Acids can also speed up the rusting process. This is because they break down the oxide layer that forms on the surface of the metal, exposing the metal to oxygen and water hence accelerating the rusting process.
Bases can also promote rusting. This is because they neutralize the acids that form on the surface of the metal, exposing it to oxygen and water. Therefore, if you live in an area with high pollution levels, your metal surfaces may be more likely to rust.
The Temperature
The
temperature can also influence the rate of rusting. In general, the higher the temperature, the faster the rusting process. This is because the molecules of water and oxygen are more active at higher temperatures, and they can more easily penetrate the metal surface and react with the iron.
Source: Ignite Industrial Technologies. This is a pretty understandable summary of why metal rusts. A lot more simplified than the way my professors explained it during my Metallurgy and Chemical Engineering Technology program in the 1990's.