Skip to content
Home arrow Knowledgebase arrow Prevention of Rust
Prevention of Rust Print E-mail


Rust is the common term for corroded steel or iron. It is very common compound, iron oxide. Iron oxide, the chemical Fe2O3, is common because iron combines very readily with oxygen. It is so readily that pure iron is only rarely found in nature. The reddish oxide produced is caused by oxygen and moisture.

Iron (or steel) rusting is an example of corrosion, an electrochemical process involving an anode (a piece of metal that readily gives up electrons), an electrolyte (a liquid that helps electrons move) and a cathode (a piece of metal that readily accepts electrons). When a piece of metal corrodes, the electrolyte helps provide oxygen to the anode. As oxygen combines with the metal, electrons are liberated. When they flow through the electrolyte to the cathode, the metal of the anode disappears, swept away by the electrical flow or converted into metal cations in a form such as rust.

Image of Prevention of Rust


Rust consists of hydrated iron(III) oxides Fe2O3·nH2O, iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3.Rusting is the common term for corrosion of iron and its alloys, such as steel. Other metals undergo equivalent corrosion, but the resulting oxides are not commonly called rust. Given sufficient time, oxygen, and water, any iron mass eventually converts entirely to rust and disintegrates. The corrosion of aluminium is extremely slow because the resulting aluminium oxide forms a conformal coating, which protects the remaining aluminium. This process is known as passivation.

For iron to become iron oxide, three things are required: iron, water and oxygen. When a drop of water hits on iron,immediately two things begin to happen. First, the water, a good electrolyte, combines with carbon dioxide in the air to form a weak carbonic acid, an even better electrolyte. As the acid is formed and the iron dissolved, some of the water will begin to break down into its component pieces, hydrogen and oxygen. The free oxygen and dissolved iron bond into iron oxide, in the process freeing electrons. The electrons liberated from the anode portion of the iron flow to the cathode, which may be a piece of a metal less electrically reactive than iron, or another point on the piece of iron itself.

The chemical compounds found in liquids like acid rain, seawater and the salt-loaded spray from snow-belt roads make them better electrolytes than pure water, allowing their presence to speed the process of rusting on iron and other forms of corrosion on other metals.

Prevention

Rust is the cause of permeable to air and water, therefore the interior iron continues to corrode. Rust prevention thus requires coatings that preclude rust formation. Stainless steel forms a passivation layer of chromium(III) oxide. Similar passivation behavior occurs with magnesium, copper, titanium, and zinc.

An important rust prevention is galvanization, which typically consists of coating zinc by either hot-dip galvanizing or electroplating. Zinc is traditionally used because it is cheap and adheres well to steel (or iron). In more corrosive environments, such as salt water, cadmium is preferred. Galvanization often fails at seams, holes, and joints, where the coating is pierced. In these cases the coating provides cathodic protection to metal, where it acts as a galvanic anode rusting in preference. More modern coatings add aluminium to the coating as zinc-alum, aluminium will migrate to cover scratches and thus provide protection for longer. These approaches rely on the aluminium and zinc oxides protecting the once-scratched surface rather than oxidizing as a sacrificial anode. (Electroplating is the process of using electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material such as a metal)

Other methods are available to control corrosion and prevent the formation of rust, colloquially termed rustproofing:
(Rust-proofing is the process whereby the rate at which objects made of iron and/or steel begin to rust is reduced, so that the places in which they are rusting can be spotted in time and repaired. The term is particularly used for the automobile industry)
  • In a Cathodic protection, the iron is a cathode in a battery formed whenever water contacts the iron and also a sacrificial anode made from something with a more negative electrode potential, commonly zinc or magnesium. The electrode alone does not react in water but only provides electrons that are otherwise provided by the iron.
  • Bluing is a technique that can provide limited resistance to rusting for small iron /steel items, such as firearms; for it to be successful, water-displacing oil is rubbed onto the blued steel.
  • Rust formation can be controlled with coatings, such as paint. It isolate the iron from the environment. Large structures with enclosed box sections, such as ships and modern automobiles, often have a wax-based product (technically a "slushing oil") injected into these sections. Such treatments also contain rust inhibitors. Covering steel with concrete provides protection to steel by the high pH environment at the steel-concrete interface.
 
< Prev   Next >