Winter Maintenance

Jan. 4, 2002

For the melting of snow and ice from roadways, drives, walks and entrances to private and public buildings the use of chemicals has become a common practice. The basic objective is to achieve safe surfaces in the shortest time with the minimum cost.

For the melting of snow and ice from roadways, drives, walks and entrances to private and public buildings the use of chemicals has become a common practice. The basic objective is to achieve safe surfaces in the shortest time with the minimum cost.

There are several chemicals available on the market. While many of them are proprietary materials, sodium chloride, potassium chloride, calcium chloride, magnesium chloride, ammonium nitrate, ammonium sulfate and urea are well-defined chemical compounds that could be used as a deicer. Ammonium nitrate, ammonium sulfate and urea are manufactured chemicals. On the other hand, sodium, potassium, calcium and magnesium chlorides also are available naturally.

There are two main objectives of using these chemicals:

             To melt the fallen snow and the related ice; and

             To prevent the formation of ice at the interface between snow and pavement, so that the snow and ice can be removed efficiently.

In either case, the function of using these chemicals is to depress the freezing point of ice or snow and convert the mixture into a strong brine solution. The purpose of this article is to clarify some of the common misconceptions about deicers and anti-icers.

Relative performance

Apart from the freeze-point depression, the relative efficiency of deicers is determined by their ability to generate heat (thermodynamic properties). The reaction involved in the conversion of calcium chloride and magnesium chloride with water or ice into liquid is an exothermic reaction (liberation of heat). On the other hand, the reaction process in the conversion of sodium chloride, potassium chloride and urea into liquid form is endothermic (heat absorber). This means heat generated in the conversion of calcium chloride or magnesium chloride into aqueous solution aids the melting of snow and ice. Conversely, the endothermic condition of sodium chloride, potassium chloride and urea, which require heat, slows down the snow and ice melting properties.

At the laboratory scale, it is difficult to simulate the field conditions of the deicing process. However, the procedures developed by A. Dean McElroy have become standard testing methods for the evaluation of deicers. Using these methods, Henry Kirchner has compared the deicing performance of calcium chloride, sodium chloride, urea and calcium magnesium acetate (CMA).

Volume of Ice Melted and Degree of Ice Penetration: Test data on calcium chloride, sodium chloride and CMA reported by Kirchner are presented in Tables 1 and 2. The calcium chloride pellets used in the test contained about 3.8% water. This water will add to the measured volume of water from ice melting. In this analysis, the volumes of ice melted by calcium chloride in Table 1 are the “adjusted” volumes that were originally reported by Kirchner plus the volume of water already contained in the deicing chemicals. Under normal circumstances, the deicers fully convert into liquid in far less than 10 minutes. This is well within the initial test point of 15 minutes of contact period between the deicer and the ice surface given by Kirchner.

Data in Tables 1 and 2 show that at all temperatures calcium chloride melts more ice and is able to achieve a greater degree of ice penetration than sodium chloride and CMA. The difference between calcium chloride and other deicers is bigger at lower temperatures. At low temperatures, lower than 20°F, the effectiveness of CMA and sodium chloride has diminished drastically. In fact, at 5°F, CMA has failed in ice melting and in the depth of penetration tests.

The Midwest Research Institute used the same test method of Kirchner to evaluate the comparative deicing performance of calcium chloride and magnesium chloride. In this program, commercially available pellets of anhydrous (TETRA 94) and dihydrate (TETRA 80) calcium chloride and a hexahydrate magnesium chloride were used. The data showed that for the volume of ice melted at 15°F and 0°F the effectiveness of the three deicers evaluated is in the following order: TETRA 94 > TETRA 80 > magnesium chloride pellets. The ice penetration data for these deicers show a similar order of the degree of effectiveness. An anhydrous calcium chloride (TETRA 94) is noted to be better than a dihydrate calcium chloride (TETRA 80), and the two better than magnesium chloride hexahydrate pellets.

Calcium chloride is a superior deicing chemical to magnesium chloride. This can be attributed to the difference in their thermodynamic properties. In 1981, Richard Tenu and Jean-Jacques Counioux reported the heat of solution for different hydration states of calcium chloride and magnesium chloride. For dihydrate molecules of calcium chloride and magnesium chloride the heat of solutions are 5.81 kJ/mole (17.14 BTU/lb) and 4.43 kJ/mole (14.68 BTU/lb), respectively. For their hexahydrate molecules the respective heat of solutions are 7.12 kJ/mole (13.44 BTU/lb) and 3.08 kJ/mole (6.57 BTU/lb). When a dihydrate calcium chloride (TETRA 80) is compared against the hexahydrate magnesium chloride pellets that were tested their respective heats of solutions are 17.14 BTU/lb and 6.57 BTU/lb. As the commercially available magnesium chloride pellets are primarily hexahydrate, calcium chloride  that is commercially available in anhydrous or dihydrate form would have comparatively much superior ice melting capability.

Pellets vs. Flakes: Solid deicers are commercially available as pellets and flakes. There have been arguments in favor of using one or the other. The effectiveness of pellet and flake deicers for melting ice and for the degree of ice penetration was compared in the laboratory as dihydrates.

In general, calcium chloride pellets melt ice and penetrate more effectively than flakes. However, at 0°F and up to 15 minutes of contact between the deicer and the ice surface, flakes have performed better than the pellet calcium chloride in ice melting and penetration. This is attributed to the larger surface area of contact between the flakes and ice. Initially, when the deicer is in the solid state, due to greater surface area of contact, there is greater degree of interaction between the deicer flakes and the body of ice. Once the deicer particles start converting into liquid, the differential between the pellets and flakes narrows. At deicer and ice surface contact periods of 20 minutes and higher, when the deicers are completely converted into liquid, the pellets outperform the flakes in both the volume of ice melted and in the degree of ice penetration. It suggests that in the real world situation, pellets are better deicers than flakes.

Undercutting of Ice: Deicers also are evaluated for the ease of undercutting of the ice at the pavement surface. The ease of undercutting is influenced by a number of variables, such as the type of pavement surface, its porosity or irregularities, heat transfer rates, brine concentration, density gradients and the diffusion rates of the chemical species. Susan Trost, F.J. Heng and E.L. Cussler reported that for sodium chloride, calcium chloride and magnesium chloride at -10°C (14°F) the rate constants for breaking of bond between ice and the pavement surface are 0.10, 0.11 and 0.07, respectively. This suggests that the use of calcium chloride as a deicer yields the most rapid cutting of the ice. This could be because calcium chloride is the fastest penetrating deicer.

Kirchner also has reported the comparative performance of calcium chloride and sodium chloride for the undercutting of ice. Data comparing the ice undercutting performance of these deicers show that at 25°F the difference in the effectiveness of calcium chloride and sodium chloride is marginal. It requires a slightly smaller amount of sodium chloride than calcium chloride to achieve similar performance. At 20°F, the calcium chloride requirement is slightly lower than sodium chloride. At lower temperatures, 5°F to 15°F, the differential is much greater in favor of calcium chloride. At the lowest temperature studied (5°F), the calcium chloride requirement is almost 1/3 of the sodium chloride requirement.

Corrosion inhibition

As it is difficult to simulate the conditions under which potentially corroding objects are exposed to the chemicals used in the snow and ice treatment, there is no standard test method for the evaluation of the rate of corrosion under the snow and ice conditions. However, a modified NACE Standard TM-01-69, first introduced by the Departments of Transportation from the Pacific Northwest States (PNS), is a widely accepted method.

The specification of the PNS requires that any corrosion inhibited deicer product have rate of corrosion less than 70% of the rate for sodium chloride. The result at 3 wt% solution of the deicer is compared against the corrosion rate of the coupon in the presence of 3 wt% sodium chloride solution. 3 wt% sodium chloride is the standard, because at this concentration sodium chloride solution is most corrosive. This also simulates the salinity of seawater.

To address the issue of corrosion, one of the earliest deicers developed was CMA. Being a non-halide alkali earth salt, while it is almost non-corrosive, its deicing properties are inferior to calcium or magnesium chloride. With its high production cost, and the product being less effective as a deicer, the market for this deicer has remained extremely limited. As DOT specifications restrict the range of additive species that can be included in the product, there are a limited number of corrosion inhibited deicers available on the market.

There are several corrosion inhibited liquid deicers on the market that contain biodegradable byproducts from the corn syrup manufacturing industry. Products containing these additives are difficult to handle in field operation. The corrosion inhibitor additive component of the composition contributes to sludge formation in the deicer. This sludge, along with the frequently encountered sedimentation of sulfates in commercially available magnesium chloride, presents a problem to an efficient sprinkling of the deicer through the spreader nozzles. Despite continued agitation in the deicer spreader tanks to reduce sedimentation, the blockage of the spreader nozzles still is a major complaint from the operators. These deicers have short shelf life and cannot be stored for a long period of time.

Environmental issues

There are several misconceptions about the environmental aspect of commercially available deicers. Major issues are related to their effects on the vegetation, wildlife and concrete pavements.

Effects on Concrete Structures: Points of view in the literature on the effects of sodium, calcium and magnesium chlorides on concrete pavements are contradicting. A recent study (Nadzehdin et al.) compared the spalling of concrete after its treatment with various deicers and concluded that sodium chloride imparted maximum loss in weight of the concrete due to spalling. The respective weight losses for different deicers indicate that the use of calcium chloride as a deicer is significantly less harmful to the concrete structure than sodium chloride.

Robert Cody, Anita Cody, Paul Spry and Guo-Liang Gan compared the effects of sodium chloride, calcium chloride and magnesium chloride on the deterioration of highway concrete cores being exposed to the wet/dry, freeze/thaw and continuous soak conditions. They concluded that magnesium chloride was the most destructive deicer with severe deterioration produced under almost all of the experimental conditions. Sodium chloride was reported to be least destructive, followed by calcium chloride and then magnesium chloride.

There is some agreement among researchers that the major damage to the pavements is due to frequent freeze/thaw cycles. Deicers with lower freeze points reduce the number and frequency of freeze/thaw cycles the pavement would go through during the winter months. Both calcium chloride and magnesium chloride have lower freeze points. They are less damaging to the pavements when compared with sodium chloride. Because calcium chloride has a lower freezing point than magnesium chloride, there will be fewer freeze/thaw cycles and less spalling.

Effects on Vegetation: Sodium chloride, calcium chloride and magnesium chloride are generally non-toxic to humans, wildlife and aquatic life. However, the effect of these deicers on the health of soil and vegetation is frequently misunderstood. Their effects need to be examined in terms of their cationic and anionic species. Sodium, calcium and magnesium chlorides, when they are in solution, have free chloride ions and their respective cations. When these deicers are applied to the pavement, chloride ions will stay in the liquid phase. The melted ice running into the roadside or streams will contain chloride ions which will contact the soil and vegetation. Because they are anionic, chloride ions do not adsorb at the soil surface due to the fact that the soil is negatively charged and will repel the like charged species of chloride ions. For cations of the deicers, such as sodium, calcium and magnesium, the negatively charged soil surfaces are suitable sites for their adsorption. Consequently, a significant portion of the cationic species of these deicers will adsorb in the soil matrix, reducing their concentration in the runoff.

There is a misconception that chloride ions are toxic to the vegetation. In fact, chloride fertilizers are used for controlling root and leaf diseases in crops, including wheat, rice, barley, potatoes and coconut palms. It is an essential micronutrient required in small quantities by plants. Like other elements, there are threshold limits for the concentration of chloride ions for different crops and vegetation.

Among the cations of these chloride deicers, both calcium and magnesium are essential nutrients for plants and vegetation. For the growth and sustaining of plants and vegetation calcium requirement is much higher than that of magnesium. Sodium is considered toxic. Sodium also disperses clay, which makes the soil structure impermeable for moisture and air, cutting them off from the root zone.

Calcium-containing chemicals, particularly calcium chloride, are widely used for the amendment of sodium chloride-affected soil. The calcium species of these chemicals preferentially adsorb in the clay structure, replacing the adsorbed sodium. This, in turn, displaces the toxic sodium species from the root zone of the plants and vegetation. The adsorbed calcium changes the otherwise dispersed and impermeable soil to a more permeable structure that allows the root zone to receive moisture and air. The calcium available in the soil structure also provides nutritional value to the plant for its growth. Consequently, the amount of calcium available from the deicers has a positive impact on the soil and vegetation of the surrounding areas. Conversely, with the continued use of sodium chloride as a deicer, vegetation will be destroyed.

About The Author: Mishra is manager, business development, with Tetra Technologies, Inc., The Woodlands, Texas.

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