Rusting is the common term for corrosion of elemental iron and its alloys such as steel. Many other metals undergo similar corrosion, but the resulting oxides are not commonly called rust “.
Other forms of rust include the result of reactions between iron and chloride in an environment deprived of oxygen. Rebar used in underwater concrete pillars, which generates green rust, is an example.
Rapid oxidation occurs when heated steel is exposed to air Rust is a general name for a complex of oxides and hydroxides of iron, which occur when iron or some alloys that contain iron are exposed to oxygen and moisture for a long period of time. Over time, the oxygen combines with the metal forming new compounds collectively called rust.
Although rust may generally be termed as “oxidation”, that term is much more general and describes a vast number of processes involving the loss of electrons or increased oxidation state, as part of a reaction. Many other oxidation reactions exist which do not involve iron or produce rust.
Iron or steel structures might appear to be solid, but water molecules can penetrate the microscopic pits and cracks in any exposed metal. The hydrogen atoms present in water molecules can combine with other elements to form acids, which will eventually cause more metal to be exposed.
If chloride ions are present, as is the case with saltwater, the corrosion is likely to occur more quickly. As the atoms combine, they weaken the metal, making the structure brittle and crumbly.
Iron metal is relatively unaffected by pure water or by dry oxygen. The conversion of the passivating ferrous oxide layer to rust results from the combined action of two agents, usually oxygen and 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 the iron is consumed or all of the oxygen, water, carbon dioxide, or sulfur dioxide in the system are removed or consumed. O 2 + 4 e + 2 H2O 4 OH Because it forms hydroxide ions, this process is strongly affected by the presence of acid.
With limited dissolved oxygen, iron(II)-containing materials are favored, including Few and black lodestone or magnetite (Fe 3 O 4). High oxygen concentrations favor ferric materials with the nominal formulae Fe(OH) 3 x O x 2.
The nature of rust changes with time, reflecting the slow rates of the reactions of solids. Furthermore, these complex processes are affected by the presence of other ions, such as Ca 2+, which serve as electrolytes which accelerate rust formation, or combine with the hydroxides and oxides of iron to precipitate a variety of Ca, Fe, O, OH species.
The onset of rusting can also be detected in the laboratory with the use of ferry indicator solution. Cor-Ten is a special iron alloy that rusts, but still retains its structural integrityBecause of the widespread use and importance of iron and steel products, the prevention or slowing of rust is the basis of major economic activities in a number of specialized technologies.
A brief overview of methods is presented here; for detailed coverage, see the cross-referenced articles. Special weathering steel alloys such as Cor-Ten rust at a much slower rate than normal, because the rust adheres to the surface of the metal in a protective layer.
Interior rusts in old galvanized iron water pipes can result in brown and black waterGalvanization consists of an application on the object to be protected of a layer of metallic zinc by either hot-dip galvanizing or electroplating. Zinc is traditionally used because it is cheap, adheres well to steel, and provides cathodic protection to the steel surface in case of damage to the zinc layer.
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 some cases, such as very aggressive environments or long design life, both zinc and a coating are applied to provide enhanced corrosion protection. Typical galvanization of steel products which are to be subjected to normal day-to-day weathering in an outside environment consists of a hot-dipped 85 µm zinc coating.
Cathodic protection is a technique used to inhibit corrosion on buried or immersed structures by supplying an electrical charge that suppresses the electrochemical reaction. The sacrificial anode must be made from something with a more negative electrode potential than the iron or steel, commonly zinc, aluminum, or magnesium.
The sacrificial anode will eventually corrode away, ceasing its protective action unless it is replaced in a timely manner. Flaking paint, exposing a patch of surface rust on sheet metal Rust formation can be controlled with coatings, such as paint, lacquer, varnish, or wax tapes that isolate the iron from the environment.
As a closely related example, iron bars were used to reinforce stonework of the Parthenon in Athens, 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 Coastline 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 antiseize 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. They are not effective when air circulation disperses them, and brings in fresh oxygen and moisture.
An example of this is the use of silica gel packets to control humidity in equipment shipped by sea. Rust removal from small iron or steel objects by electrolysis can be done in a home workshop using simple materials such as a plastic bucket filled with an electrolyte consisting of washing soda dissolved in tap water, a length of rebar suspended vertically in the solution to act as an anode, another laid across the top of the bucket to act as a support for suspending the object, baling wire to suspend the object in the solution from the horizontal rebar, and a battery charger as a power source in which the positive terminal is clamped to the anode and the negative terminal is clamped to the object to be treated which becomes the cathode.
The Kinda 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. It is one of the most common failure modes of reinforced concrete bridges and buildings.
^ Kermit, Bart; Griesser-Stermscheg, Martina; Sewn, Indie; Sutherland, Susanne. “ Rust Never Sleeps: Recognizing Metals and Their Corrosion Products” (PDF).
^ Ramsay, Hosahalli S.; Marlette, Michele; Pastry, Sudhir; Abderrahim, Khalid (2014-02-14). CS1 main: archived copy as title (link) ^ Gupta, Lorraine Mira, Krishnakali.
This oxide layer resists corrosion, while at the same time prevents oxygen from reaching the underlying steel. When the alloy is exposed to water, a film of aluminum oxide forms quickly on the surface.
Copper oxidizes over time to form a green patina, which actually protects the metal from further corrosion. This type is carbon steel that has been galvanized, or coated, with a thin layer of zinc.
The zinc acts as a barrier preventing oxygen and water from reaching the steel, so that it is corrosion protected. Like aluminum, zinc is highly reactive to oxygen in the presence of moisture, and the coating prevents the iron in the steel from further oxidation.
Group of steel alloys designed to form a rust -like finish when exposed to weather In 1933 the United States Steel Corporation developed and patented a steel with exceptional mechanical resistance, primarily for use in railroad hopper cars, for the handling of heavy bulk loads including coal, metal ores, other mineral products and grain.
The controlled corrosion for which this material is now best known was a welcome benefit discovered soon after, prompting USS to apply the trademarked name Cor-Ten. Because of its inherent toughness, this steel is still used extensively for bulk transport and storage containers.
This is because the steel forms a protective layer on its surface under the influence of the weather. The corrosion-retarding effect of the protective layer is produced by the particular distribution and concentration of alloying elements in it.
The layer protecting the surface develops and regenerates continuously when subjected to the influence of the weather. In other words, the steel is allowed to rust in order to form the protective coating.
Weathering steel grade chemical composition (excl. Iron) by weight (%) Grade CSiMnPSCrCuVNi ATM A2420.120.25–0.750.20–0.500.01–0.200.0300.50–1.250.25–0.550.65 ATM A5880.160.30–0.500.80–1.250.0300.0300.40–0.650.25–0.400.02–0.100.40 The mechanical properties of weathering steels depend on which alloy and how thick the material is.
The original A242 alloy has a yield strength of 50 kilo pounds per square inch (340 MPA) and ultimate tensile strength of 70 ski (480 MPA) for light-medium rolled shapes and plates up to 0.75 inches (19 mm) thick. It has yield strength of 46 ski (320 MPA) and ultimate strength of 67 ski (460 MPA) for medium weight rolled shapes and plates from 0.75–1 inch (19–25 mm) thick.
The Type 2 steel, which is also called Cortex B, is used primarily in urban furnishing, passenger ships or cranes. A588 has a yield strength of at least 50 ski (340 MPA), and ultimate tensile strength of 70 ski (480 MPA) for all rolled shapes and plate thicknesses up to 4 in (100 mm) thick.
It is very widely used in marine transportation, in the construction of intermodal containers as well as visible sheet piling along recently widened sections of London's M25 freeway. A subsequent order in 1979 was built to similar specs, including weathering steel bodies, by Bombardier.
The cars were painted, a standard practice for weathering steel railcars. The durability of weathering steel did not live up to expectations, with rust holes appearing in the railcars.
If water is allowed to accumulate in pockets, those areas will experience higher corrosion rates, so provision for drainage must be made. Weathering steel is sensitive to humid subtropical climates, and in such environments, it is possible that the protective patina may not stabilize but instead continue to corrode.
For example, the former OFNI Coliseum, built in 1972 in Atlanta, never stopped rusting, and eventually large holes appeared in the structure. This was a major factor in the decision to demolish it just 25 years after construction.
The same thing can happen in environments laden with sea salt. The rate at which some weathering steels form the desired patina varies strongly with the presence of atmospheric pollutants which catalyze corrosion.
While the process is generally successful in large urban centers, the weathering rate is much slower in more rural environments. Uris Hall, a social science building on Cornell University's main campus in Ithaca, a small town in Upstate New York, did not achieve the predicted surface finish on its Bethlehem Steel Malaria weathering steel framing within the predicted time.
Rainwater runoff from the slowly rusting steel stained the numerous large windows and increased maintenance costs. Corrosion without the formation of a protective layer apparently led to the need for emergency structural reinforcement and galvanizing in 1974, less than two years after opening.
The initial weathering of the material resulted in a discoloration of the surrounding city sidewalks which is known as “bleeding” or “runoff”, as well as other nearby buildings. A cleanup effort was orchestrated by the corporation once weathering was complete to clean the markings.
A few of the nearby sidewalks were left uncleaned, and remain a rust color. This problem has been reduced in newer formulations of weathering steel.
^ Manual of Steel Construction, 8th Edition Second Revised Printing. “Fallen Cl egg Bradley's Leeds complex completed”.
^ “Photos of the South Shore Line in the Dunes' region of Northern Indiana”. “Donor Wrote Carson to Demand 'Old Rusty' Repairs Be Secret”.