Calcium chloride (\(\text{CaCl}_2\)) is widely used as an industrial desiccant and, most commonly, as a highly effective de-icing agent on roads and sidewalks. The compound is corrosive, and its use is a significant concern for the long-term integrity of infrastructure, vehicles, and metal components. Its corrosivity stems from its chemical composition and its ability to interact aggressively with metallic and cementitious materials.
The Mechanism of Chloride Corrosion
Corrosion is an electrochemical process where refined metals revert to their stable oxide forms. When calcium chloride dissolves in water, it dissociates into calcium ions (\(\text{Ca}^{2+}\)) and chloride ions (\(\text{Cl}^-\)), forming a strong, highly conductive electrolyte solution. This solution significantly accelerates the rate at which iron and steel oxidize, known as rusting.
The chloride ions are aggressive because they penetrate and disrupt the thin, protective oxide layer that naturally forms on metal surfaces. Once this passive layer is compromised, the metal dissolves much more quickly, leading to accelerated degradation. Calcium chloride is also highly hygroscopic, meaning it readily absorbs moisture from the air. This characteristic ensures the corrosive salt solution remains wet and active on surfaces for extended periods, lengthening the exposure time for metals.
Material Damage in De-Icing Applications
The chloride ions initiate localized corrosion, specifically a destructive form called pitting corrosion. This process creates small, concentrated holes in the metal surface, quickly compromising the structural integrity of thin materials. Automotive bodies, brake lines, and undercarriage components exposed to road brine often exhibit accelerated deterioration and rust-through damage.
Steel infrastructure, such as bridge components and guardrails, is heavily affected by this exposure. In reinforced concrete, chloride ions penetrate the porous structure to reach the internal steel reinforcement bars (rebar). The chlorides destroy the high-alkaline environment that normally protects the rebar, leading to rust formation.
As the rebar rusts, the resulting iron oxide occupies a much larger volume than the original steel. This creates immense internal pressure that causes the concrete to crack, flake, and break away—a process known as spalling.
Calcium chloride also causes chemical damage to the concrete matrix itself. It reacts with the calcium hydroxide present in the concrete to form calcium oxychloride. This reaction is expansive and induces internal stresses, leading to cracking and deterioration of the concrete. Calcium chloride reduces the alkalinity of the pore water more significantly than sodium chloride, increasing the risk of internal rebar corrosion.
Strategies for Preventing Corrosion Damage
The primary strategy for mitigating corrosive effects is the use of corrosion inhibitors, specialized chemical additives mixed directly into the de-icing solution. These inhibitors, often based on phosphates or organic acids, work by forming a protective film on the metal surface to block the electrochemical reaction. Inhibited liquid calcium chloride products have been shown to reduce corrosion rates on steel by 80 to 90 percent compared to uninhibited sodium chloride solutions.
For new construction, selecting corrosion-resistant materials offers a long-term solution. This includes using specialized coatings on infrastructure steel, such as galvanized steel or anti-corrosion paints. In concrete, epoxy-coated rebar provides a physical barrier that prevents chloride ions from contacting the steel surface and initiating rust.
Vehicle owners can reduce damage through diligent maintenance focused on removing the corrosive residue. Frequent undercarriage washing is the most effective measure, as it physically removes the salt brine before it causes extensive damage. Using high-pressure water sprays to rinse the wheel wells and chassis is recommended after periods of heavy de-icer application.
Biological Safety and Handling
Calcium chloride’s corrosive properties extend to living tissue, primarily due to its strong hygroscopic nature. When the compound contacts moist skin, eyes, or mucous membranes, it rapidly draws water out of the tissue. This desiccation process can cause severe irritation, inflammation, and chemical burns, especially if the exposure is prolonged.
Proper handling requires appropriate personal protective equipment, including safety goggles and impervious gloves, to prevent direct contact. In the event of skin contact, the affected area must be immediately flushed with large amounts of water for at least 15 minutes to remove the chemical and rehydrate the tissue. For eye contact, flushing with water for a minimum of 15 minutes is necessary, and medical attention should be sought immediately.