Winter Maintenance

A mixture for snow and ice

Surendra K. Mishra / January 04, 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.

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: style='font-weight:normal'>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 style="mso-spacerun: yes">  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: style='font-weight:normal'> 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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