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Deicing is the process of removing
snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughou ...
, ice or
frost Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above-freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, and resulting in a phase change from water vapor (a g ...
from a surface. Anti-icing is the application of chemicals that not only deice but also remain on a surface and continue to delay the reformation of ice for a certain period of time, or prevent adhesion of ice to make mechanical removal easier. Deicing can be accomplished by mechanical methods (scraping, pushing); through the application of
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
; by use of dry or liquid chemicals designed to lower the
freezing point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depend ...
of water (various
salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
s or
brine Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for ...
s,
alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...
s,
glycol A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is e ...
s); or by a combination of these different techniques.


Application areas


Roadways

In 2013, an estimated 14 million tons of salt were used for deicing roads in North America. Deicing of roads has traditionally been done with salt, spread by
snowplow A snowplow (also snow plow, snowplough or snow plough) is a device intended for mounting on a vehicle, used for removing snow and ice from outdoor surfaces, typically those serving transportation purposes. Although this term is often used to r ...
s or
dump truck A dump truck, known also as a dumping truck, dump trailer, dumper trailer, dump lorry or dumper lorry or a dumper for short, is used for transporting materials (such as dirt, gravel, or demolition waste) for construction as well as coal. A t ...
s designed to spread it, often mixed with
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class ...
and
gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...
, on slick roads.
Sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...
(rock salt) is normally used, as it is inexpensive and readily available in large quantities. However, since
salt water Saline water (more commonly known as salt water) is water that contains a high concentration of dissolved salts (mainly sodium chloride). On the United States Geological Survey (USGS) salinity scale, saline water is saltier than brackish w ...
still freezes at , it is of no help when the temperature falls below this point. It also has a tendency to cause
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
,
rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...
ing the
steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistan ...
used in most vehicles and the
rebar Rebar (short for reinforcing bar), known when massed as reinforcing steel or reinforcement steel, is a steel bar used as a tension device in reinforced concrete and reinforced masonry structures to strengthen and aid the concrete under tension. ...
in concrete bridges. Depending on the concentration, it can be toxic to some plants and animals, and some urban areas have moved away from it as a result. More recent snowmelters use other salts, such as
calcium chloride Calcium chloride is an inorganic compound, a salt with the chemical formula . It is a white crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with calcium hydroxide. Ca ...
and magnesium chloride, which not only depress the freezing point of water to a much lower temperature, but also produce an
exothermic reaction In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change Δ''H''⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC defines ...
. They are somewhat safer for
sidewalk A sidewalk (North American English), pavement (British English), footpath in Australia, India, New Zealand and Ireland, or footway, is a path along the side of a street, highway, terminals. Usually constructed of concrete, pavers, brick, stone ...
s, but excess should still be removed. More recently, organic compounds have been developed that reduce the environmental issues connected with salts and have longer residual effects when spread on roadways, usually in conjunction with salt brines or solids. These compounds are often generated as byproducts of agricultural operations such as
sugar beet A sugar beet is a plant whose root contains a high concentration of sucrose and which is grown commercially for sugar production. In plant breeding, it is known as the Altissima cultivar group of the common beet ('' Beta vulgaris''). Together ...
refining or the
distillation Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the he ...
process that produces
ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...
. Other organic compounds are
wood ash Wood ash is the powdery residue remaining after the combustion of wood, such as burning wood in a fireplace, bonfire, or an industrial power plant. It is largely composed of calcium compounds along with other non-combustible trace elements presen ...
and a deicing salt called calcium magnesium acetate made from roadside grass or even kitchen waste. Additionally, mixing common rock salt with some of the organic compounds and magnesium chloride results in spreadable materials that are both effective to much colder temperatures () as well as at lower overall rates of spreading per unit area. Solar road systems have been used to maintain the surface of roads above the freezing point of water. An array of pipes embedded in the road surface is used to collect solar energy in summer, transfer the heat to thermal banks and return the heat to the road in winter to maintain the surface above . This automated form of renewable energy collection, storage and delivery avoids the environmental issues of using chemical contaminants. It was suggested in 2012 that superhydrophobic surfaces capable of repelling water can also be used to prevent ice accumulation leading to icephobicity. However, not every superhydrophobic surface is icephobic and the method is still under development.


Trains and rail switches

Trains and rail switches in Arctic regions can have significant problems with snow and ice build up. They need a constant heat source on cold days to assure functionality. On trains it is primarily the
brakes A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction. Backgroun ...
, suspension and couplers that require heaters for deicing. On rails it is primarily the switches that are sensitive to ice. High-powered electrical heaters prevent ice formation and rapidly melt any ice that forms. The heaters are preferably made of PTC material, for example
PTC rubber PTC rubberUS patent 8,367,986 is a silicone rubber which conducts electricity with a resistivity that increases exponentially with increasing temperature for all temperatures up to a temperature where the resistivity grows to infinity. Above this t ...
, to avoid overheating and potentially destroying the heaters. These heaters are self-limiting and require no regulating electronics; they cannot overheat and require no overheat protection.


Aviation


Ground deicing of aircraft

On the ground, when there are freezing conditions and
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
, deicing an aircraft is commonly practiced. Frozen contaminants interfere with the aerodynamic properties of the vehicle. Furthermore, dislodged ice can damage the engines. Ground deicing methods include: * Spraying on various aircraft deicing fluids to melt ice and prevent reformation * Using unheated forced air to blow off loose snow and ice * Using infrared heating to melt snow, ice, and frost without using chemicals * Mechanical deicing using tools such as brooms, scrapers, and ropes * Placing an aircraft in a warm hangar


In-flight deicing

Ice can build up on aircraft in flight due to atmospheric conditions, causing potential degradation of flight performance. Large commercial aircraft almost always have in-flight ice protections systems to shed ice buildup and prevent reformation. Ice protection systems are becoming increasingly common in smaller general aviation aircraft as well. Ice protection systems typically use one or more of the following approaches: * pneumatic rubber "boots" on leading edges of wings and control surfaces, which expand to break off accumulated ice * electrically heated strips on critical surfaces to prevent ice formation and melt accumulated ice * bleed air systems which take heated air from the engines and duct them to locations where ice can accumulate * fluid systems which "weep" deicing fluid over wings and control surfaces via tiny holes


Airport pavement

Deicing operations for airport pavement (
runway According to the International Civil Aviation Organization (ICAO), a runway is a "defined rectangular area on a land aerodrome prepared for the landing and takeoff of aircraft". Runways may be a man-made surface (often asphalt, concrete ...
s, taxiways, aprons, taxiway bridges) may involve several types of liquid and solid chemical products, including
propylene glycol Propylene glycol ( IUPAC name: propane-1,2-diol) is a viscous, colorless liquid, which is nearly odorless but possesses a faintly sweet taste. Its chemical formula is CH3CH(OH)CH2OH. Containing two alcohol groups, it is classed as a diol. It ...
,
ethylene glycol Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an o ...
and other organic compounds. Chloride-based compounds (e.g.
salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
) are not used at airports, due to their corrosive effect on aircraft and other equipment.
Urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...
mixtures have also been used for pavement deicing, due to their low cost. However, urea is a significant pollutant in waterways and wildlife, as it degrades to
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
after application, and it has largely been phased out at U.S. airports. In 2012 the U.S. Environmental Protection Agency (EPA) prohibited use of urea-based deicers at most commercial airports.


Deicing chemicals

All chemical deicers share a common working mechanism: they chemically prevent water molecules from binding above a certain temperature that depends on the concentration. This temperature is below 0 °C, the freezing point of pure water ( freezing point depression). Sometimes, there is an
exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity ...
dissolution reaction that allows for an even stronger melting power. The following lists contains the most-commonly used deicing chemicals and their typical
chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
. ;
Inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemist ...
salts In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...
*
Sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...
(NaCl or table salt; the most common deicing chemical) * Magnesium chloride (, often added to salt to lower its working temperature) *
Calcium chloride Calcium chloride is an inorganic compound, a salt with the chemical formula . It is a white crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with calcium hydroxide. Ca ...
(, often added to salt to lower its working temperature) *
Potassium chloride Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a salt ...
(KCl) ;
Organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s * Calcium magnesium acetate () * Potassium acetate () * Potassium formate () * Sodium formate (HCOONa) *
Calcium formate Calcium formate is the calcium salt of formic acid. It is also known as E238. Under this E number it is used as an animal feed preservative within EU, but not in foods intended for people. Calcium formate is stable at room temperature, is inflam ...
() *
Urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...
(), a common fertilizer * Agricultural By-Products (generally used as additives to
sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...
) ;
Alcohols In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a saturated carbon atom. The term ''alcohol'' originally referred to the primary alcohol ethanol (ethyl alcohol), which is ...
,
diols A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is e ...
and polyols (these are antifreeze agents and scarcely used on roads) *
Methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...
() *
Ethylene glycol Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an o ...
() *
Propylene glycol Propylene glycol ( IUPAC name: propane-1,2-diol) is a viscous, colorless liquid, which is nearly odorless but possesses a faintly sweet taste. Its chemical formula is CH3CH(OH)CH2OH. Containing two alcohol groups, it is classed as a diol. It ...
() *
Glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
()


Environmental impacts and mitigation

Deicing salts such as
sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...
or
calcium chloride Calcium chloride is an inorganic compound, a salt with the chemical formula . It is a white crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with calcium hydroxide. Ca ...
leach into natural waters, strongly affecting their salinity. Ethylene glycol and propylene glycol are known to exert high levels of
biochemical oxygen demand Biochemical oxygen demand (BOD) is the amount of dissolved oxygen (DO) needed (i.e. demanded) by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period. ...
(BOD) during degradation in surface waters. This process can adversely affect aquatic life by consuming oxygen needed by aquatic organisms for survival. Large quantities of
dissolved oxygen Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It ca ...
(DO) in the
water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
are consumed when microbial populations decompose propylene glycol. Sufficient dissolved oxygen levels in surface waters are critical for the survival of fish,
macroinvertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordat ...
s, and other aquatic organisms. If oxygen concentrations drop below a minimum level, organisms emigrate, if able and possible, to areas with higher oxygen levels or eventually die. This effect can drastically reduce the amount of usable aquatic habitat. Reductions in DO levels can reduce or eliminate bottom feeder populations, create conditions that favor a change in a community's species profile, or alter critical
food-web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is consumer-resource system. Ecologists can broadly lump all life forms into one o ...
interactions. In one case, a significant
snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughou ...
in
Atlanta Atlanta ( ) is the capital and most populous city of the U.S. state of Georgia. It is the seat of Fulton County, the most populous county in Georgia, but its territory falls in both Fulton and DeKalb counties. With a population of 498,7 ...
in early January 2002 caused an overflow of such a system, briefly contaminating the Flint River downstream of the
Atlanta airport Atlanta ( ) is the capital and most populous city of the U.S. state of Georgia. It is the seat of Fulton County, the most populous county in Georgia, but its territory falls in both Fulton and DeKalb counties. With a population of 498,715 ...
. Some airports recycle used deicing fluid, separating water and solid contaminants, enabling reuse of the fluid in other applications. Other airports have an on-site wastewater treatment facility, and/or send collected fluid to a municipal
sewage treatment Sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable for discharge to the surrounding en ...
plant or a commercial wastewater treatment facility. The toxicity of deicing fluids is another environmental concern, and research is underway to find less toxic (i.e. non-glycol-based) alternatives.SAE International (2011)
"Issues and Testing of Non-Glycol Aircraft Ground Deicing Fluids."


See also

*
Atmospheric icing Atmospheric icing occurs in the atmosphere when water droplets suspended in air freeze on objects they come in contact with. It is not the same as freezing rain, which is caused directly by precipitation. Icing conditions can be particularly dang ...
*
Winter service vehicle A winter service vehicle (WSV), or snow removal vehicle, is a vehicle specially designed or adapted to clear thoroughfares of ice and snow. Winter service vehicles are usually based on a dump truck chassis, with adaptations allowing them to carr ...


References


External links

* {{commons category-inline Aviation safety Aviation risks Transport safety Chemical processes Ice in transportation NASA spin-off technologies