The use of deicer is a widespread practice in winter maintenance, involving any substance applied to a surface to melt existing ice or prevent the formation of an ice-to-pavement bond. This intervention maintains safe conditions on roadways, sidewalks, and airport runways during freezing temperatures. Deicing agents work by chemically altering the properties of water, making it more difficult for ice crystals to form or remain stable. The materials used vary significantly in their chemical composition, performance characteristics, and environmental impacts.
The Common Salt-Based Deicing Agents
The most common deicing agent globally is sodium chloride, widely known as rock salt due to its low cost and availability. This compound is typically effective down to a pavement temperature of about 15°F (-9°C), though its melting performance slows substantially below 25°F. Sodium chloride is classified as an endothermic substance, meaning it absorbs heat from its surroundings to dissolve, which slows its action in severely cold weather.
For colder climates, calcium chloride offers superior performance, remaining effective at temperatures as low as -20°F to -25°F (-29°C to -32°C). Unlike rock salt, calcium chloride is exothermic, releasing heat upon contact with moisture. This accelerates the melting process and allows it to penetrate ice more quickly, making it a powerful, fast-acting solution for extreme cold.
Magnesium chloride is another chloride-based deicer, generally performing better than rock salt but less effectively than calcium chloride, with a practical melting temperature around -10°F (-23°C). It is often marketed as a friendlier alternative to sodium chloride, though it is still corrosive to metals and can harm vegetation. Potassium chloride, a less common option, is usually incorporated into blended products and has limited practical effectiveness, ceasing to work efficiently below 25°F (-4°C).
Specialized and Environmentally Preferred Compounds
Specialized deicing agents are used where corrosion or specific environmental factors are primary concerns. Acetate-based deicers, such as potassium acetate (KAc) and calcium magnesium acetate (CMA), are often deployed at airports and on sensitive bridge infrastructure. Potassium acetate is valued for its low corrosivity to steel and its effectiveness at low temperatures, with a practical range extending down to -15°F (-26°C).
Calcium magnesium acetate (CMA) is often used as an alternative to salt, with a practical melting point of about 20°F (-7°C). While acetates are biodegradable, their decomposition creates a high biochemical oxygen demand (BOD) in receiving water bodies. This elevated demand can severely deplete dissolved oxygen, potentially suffocating aquatic life.
Glycols, including ethylene and propylene glycol, are widely known for their use in aviation to remove ice from aircraft surfaces and prevent reformation. These compounds are highly effective anti-icers, but their application is limited to specialized settings due to cost and volume requirements. Urea, a nitrogen-based fertilizer, was historically used as a deicer, but its use has been phased out because the nitrogen runoff contributes significantly to water pollution and algae blooms.
The Science of Freezing Point Depression
All chemical deicers operate based on the principle of freezing point depression, a colligative property of solutions. When a deicing agent dissolves into the water layer on the ice, it separates into positively and negatively charged ions. These dissolved ions interfere with the natural tendency of water molecules to align into the crystalline lattice structure of ice.
The presence of these foreign particles disrupts the formation of the solid phase, requiring a lower temperature for the water to freeze. This lowering of the freezing point is directly related to the concentration of the dissolved substance. The melting action is sustained as long as the deicer can dissolve and form a brine solution.
For every deicer-water mixture, a specific concentration yields the lowest possible freezing point, known as the eutectic temperature. Below this temperature, the solution becomes saturated, and the chemical solute begins to precipitate out. Once this occurs, the deicer can no longer effectively melt ice, and the mixture may rapidly refreeze. Sodium chloride’s eutectic temperature is approximately -6°F (-21°C), although its practical utility ends much higher.
Practical Effects and Safety Concerns
The chemical action of deicers, while effective for safety, introduces significant practical risks to infrastructure and the environment.
Chloride salts accelerate the corrosion of metals by acting as electrolytes, speeding up the oxidation process in vehicles, bridges, and concrete reinforcements. This leads to the premature degradation of property.
Deicers also contribute to the deterioration of concrete surfaces through both physical and chemical mechanisms. The salts intensify the natural freeze-thaw cycle, allowing water to penetrate the concrete and create greater internal pressure upon refreezing, leading to scaling. Furthermore, certain chlorides, particularly calcium and magnesium chloride, can chemically react with the calcium hydroxide in concrete to form expansive compounds, such as calcium oxychloride, which causes the concrete to disintegrate from within.
Environmental runoff is a major consequence, as chloride ions are non-biodegradable and accumulate in soil and water bodies, leading to salinization. This process can be toxic to roadside vegetation and aquatic organisms, disrupting local ecosystems. High chloride concentrations can exceed regulatory limits in freshwater, impairing water quality.
The safety of household pets is also a concern, as deicing salts can irritate or burn their paws upon contact. Ingestion of these salts, particularly calcium chloride, can lead to gastrointestinal distress or more serious health issues. Property owners are advised to use deicers sparingly and consider pet-friendly alternatives or physical abrasives like sand for traction.