iron oxide

Rust is a type of corrosion (the breakdown of materials due to reactions with their surroundings). Normally, when a material corrodes it becomes weaker, however some forms of high temperature corrosion can lead to the formation of protective compacted oxide layer glazes. Iron corrosion is called rusting. 

When exposed to air or water for a long time, iron slowly decomposes into other chemicals, because of a reaction with oxygen molecules (air and water contain oxygen). Many other metals undergo equivalent corrosion, but the resulting oxides are not commonly called rust.

Almost all metals rust, but they can be protected with paint. Alloys such as stainless steel, rust much slower than simple metals like pure iron. When a piece of metal rusts, it becomes a different color (for example, iron becomes red or brown), and the metal eventually decays (iron cannot be used or reused once it rusts). Some metals, such as aluminium, titanium, and stainless steel form a very thin coating of corrosion on the metal, which isolates the rest of the metal from environmental oxygen. This is why aluminium keeps its shine. It also makes aluminium seem very unreactive, even though it can react with water.

Rust is composed of iron oxides (chemical compounds of iron and oxygen, often used as a red pigment). Other forms of rust exist, like the result of reactions between iron and chloride in an environment deprived of oxygen – rebar used in underwater concrete pillars is an example – which generates green rust. Given sufficient time, oxygen, and water, any iron mass will eventually convert entirely to rust and disintegrate. Surface rust is flaky and friable, and provides no protection to the underlying iron, unlike the formation of patina (a coating of oxides, carbonates, and other compounds) on copper surfaces.

When impure (cast) iron is in contact with water, oxygen, or other strong oxidants, or acids, it rusts. If salt is present, for example in seawater or salt spray, the iron tends to rust more quickly, as a result of electrochemical reactions. Iron metal is relatively unaffected by pure water or by dry oxygen. As with other metals, like aluminium, a tightly adhering oxide coating, called a passivation layer, protects the bulk iron from further oxidation. The conversion of the passivating ferrous oxide layer to rust results from the combined action of two agents, usually oxygen and water.

However, other degrading solutions include sulfur dioxide in water and carbon dioxide in water. Under these corrosive conditions, iron hydroxide species are formed. Unlike ferrous oxides, the hydroxides do not adhere to the bulk metal. As they form and flake off from the surface, fresh iron is exposed, and the corrosion process continues until either all of the iron is consumed or all of the oxygen, water, carbon dioxide, or sulfur dioxide in the system are removed or consumed.

Galvanization refers to the coating of a metal with a more reactive metal to stop corrosion (normally zinc is used to coat iron). In more corrosive environments (such as salt water), cadmium plating is preferred. Galvanization often fails at seams, holes, and joints where there are gaps in the coating. In these cases, the coating still provides some partial electrochemical protection to iron, by acting as a galvanic anode and corroding itself instead of the underlying protected metal. The protective zinc layer is consumed by this action, and thus galvanization provides protection only for a limited period of time. More modern coatings add aluminium to the coating as zinc-alume; aluminium will migrate to cover scratches and thus provide protection for a longer period. In some cases, such as very aggressive environments or long design life, both zinc and a coating are applied to provide enhanced corrosion protection.

Cathodic protection is a technique used to inhibit corrosion on buried or immersed structures by supplying an electrical charge that suppresses the electro-chemical reaction. If correctly applied, corrosion can be stopped completely. In its simplest form, it is achieved by attaching a sacrificial anode (entry electrode), thereby making the iron or steel the cathode (exit electrode) in the cell formed. The sacrificial anode must be made from something with a more negative electrode potential than the iron or steel, commonly zinc, aluminium, or magnesium. The sacrificial anode will eventually corrode away, ceasing its protective action unless it is replaced in a timely manner. Cathodic protection can also be provided by using a special-purpose electrical device to appropriately induce an electric charge

Rust formation can be controlled with coatings, such as paint, lacquer, or varnish that 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 usually also contain rust inhibitors. Covering steel with concrete can provide some protection to steel because of the alkaline pH environment at the steel-concrete interface. However rusting of steel in concrete can still be a problem, as expanding rust can fracture or slowly ‘explode’ concrete from within. Iron bars were used to reinforce stonework of the Parthenon in Greece, but caused extensive damage by rusting, swelling, and shattering the marble components of the building.

When only temporary protection is needed for storage or transport, a thin layer of oil, grease, or a special mixture such as Cosmoline can be applied to an iron surface. Such treatments are extensively used when ‘mothballing’ a steel ship, automobile, or other equipment for long-term storage. Special anti-seize lubricant mixtures are available, and are applied to metallic threads and other precision machined surfaces to protect them from rust. These compounds usually contain grease mixed with copper, zinc, or aluminum powder, and other proprietary ingredients. Rust can be avoided by controlling the moisture in the atmosphere. An example of this is the use of silica gel packets to control humidity in equipment shipped by sea.

As rust has a much higher volume than the originating mass of iron, its build-up can also cause failure by forcing apart adjacent parts — a phenomenon sometimes known as ‘rust smacking.’ It was the cause of the collapse of the Mianus river bridge in 1983, when the bearings rusted internally and pushed one corner of the road slab off its support. Rust was also an important factor in the Silver Bridge disaster of 1967 in West Virginia, when a steel suspension bridge collapsed in less than a minute, killing 46 drivers and passengers on the bridge at the time. The Kinzua Bridge in Pennsylvania was blown down by a tornado in 2003, largely because the central base bolts holding the structure to the ground had rusted away, leaving the bridge anchored by gravity alone.

Rust is a commonly used metaphor for slow decay, since it gradually converts robust iron and steel metal into a soft crumbling powder. A wide section of the industrialized American Midwest and American Northeast, once dominated by steel foundries, the automotive industry, and other manufacturers, has experienced harsh economic cutbacks that have caused the region to be dubbed the ‘Rust Belt.’

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