Several different types of deicing chemicals exist. Those covered this section include chloride-based deicers, acetate-based deicers, and carbohydrates. A list of the chemicals approved for use by the MNDOT can be found here. This article presents information on non-environmental impacts from use of deicers. A discussion of environmental impacts can be found here.
Chloride based deicers are corrosive (Levelton Consultants Ltd, 2008; Shi et al., 2009a) and can impact vehicles, bridges, and roadways, (National Research Council, 1991). Magnesium chloride and calcium chloride may be more corrosive than sodium chloride due to a longer time of wetness, however the corrosivity of all three is still high (Nixon and Xiong, 2009; Shi et al., 2009b). Often times corrosion inhibitors are added to lessen the deicers corrosive effects. Every corrosion inhibitor has limitations as to which metals it can protect (Levelton Consultants, Ltd, 2008).
There are several ways in which chloride based deicers can negatively impact concrete, roads and bridges (Shi et al., 2009a; Levelton Consultants Ltd. 2008; Public Sector Consultants, 1993.):
Scaling of concrete is caused by the breakdown of the top surface of the road, which results in exposure of the aggregate below (Public Sector Consultants, 1993). While scaling can be the effect of many things, there are two main causes of scaling. The first is related to the temperature difference between the road surface and underlying portion of the road. The deicing agent reduces the freezing point at the road’s surface, but the layers below can still freeze. The resulting temperature differential can cause stress on the road and cause the top layer to chip away. Another cause of scaling could be related to the cracks. Crystals could begin to form in the cracks, causing the road surface to flake off (Public Sector Consultants, 1993). The occurrence of scaling is less dependent on the type of deicing agent, but rather on the quality of the concrete and the prevailing weather conditions (Levelton Consultants Ltd., 2008).
Chloride based deicers can affect the cement paste in both a physical and chemical manner. The physical effects were discussed above in the scaling section. The chemical effects are the result of chemical reactions between the deicers and cement paste, and through aggravation of the expansive aggregate-cement chemical reactions (Sumsion and Guthrie, 2013).
Of the various chloride-based deicers, magnesium chloride causes the greatest degradation of concrete (Cody et al., 1994; Lee et al., 2000; Sutter et al., 2008; Shi et al., 2009a). This is due to the way in which the magnesium chloride reacts with calcium-silicate-hydrate (C-S-H) and calcium hydroxide [Ca(OH)2], the two components of cement paste. The magnesium chloride will replace the calcium in the C-S-H compound with a non-cementitious compound called magnesium-silicate-hydrate (M-S-H) and calcium chloride (Lee et al., 2000). The M-S-H compound is not as strong as the C-S-H compound, and as such, the cement pasted is weakened and the concrete loses some of its strength. In addition, the magnesium will form magnesium hydroxide and calcium chloride when it reacts with the calcium hydroxide (Shi et al., 2009a). The magnesium chloride will cause expansive forces which can further accelerate the deterioration of the concrete (Levelton Consultants Ltd., 2008). Calcium chloride causes a similar degradation pattern in concrete but it occurs at a slower pace and to a lesser degree (Cody et al., 1994; Shi et al., 2009a).
Sodium chloride was found to have the least impact on cement paste (Cody et al., 1994; Sutter et al., 2008). Long-term use of this deicer does slowly accelerate the alkali-silica reaction but has not been shown to result in any significant loss in concrete strength (Shi et al., 2009a). It is important to note, while sodium chloride does not appear to have a significant effect on the cement paste, it does significantly corrode the reinforcing steel. Sutter et al. (2008) found sodium chloride had the highest rate of ingress into hardened concrete.
The reinforcing steel (rebar) embedded within concrete structures is usually protected from corrosion due to the high alkalinity of the surrounding concrete. This highly alkaline environment creates a chemical coating, often called a “passive” layer on the steel surface. This reduces the rate of corrosion to an almost negligible level (Levelton Consultants Ltd., 2008). The ingress of chloride into the concrete is one of the main driving forces behind the deterioration of reinforced concrete structures. Localized corrosion of rebar occurs when oxygen and water are able to access the rebar surface. This could be brought about by a decrease in the pH of a concrete pour solution, or the presence of enough water-soluble chloride ions (Shi et al., 2009b).
The chloride-induced corrosion causes a pitting corrosion on the rebar. The figure to the right provides an illustration of this process. Corrosion of the reinforcing steel leads to delamination and spalling, which ultimately decreases the strength of the structure.
The impact of road salt on motor vehicles can be broken into three categories: functional, structural, and cosmetic. Functional and structural damage result in a loss in operating performance while cosmetic damage affects only the appearance of the vehicle.
The cost of using road salt does not only include the price of materials and application. The cost associated with the damage to infrastructure and automobiles must also be considered. One study, Vitaliano (1992), estimated road salt adds an additional $332 per ton of salt per season for associated bridge maintenance. There is also an additional $1,460 per ton of damage to bridges in terms of corrosion. Corrosion to vehicles results in about a $133 per ton of salt. These associated costs could total toan extra $1,925 per ton of applied salt (Vitaliano, 1992). Actual costs will vary by season and location but generally the use of deicers will require additional infrastructure maintenance costs..
In 2014, Fortin Consulting compiled the results of seven studies conducted between 1976 and 2010 that looked at the damages associated with road salt, which is summarized below.
|Low Estimate||High Estimate|
|Extra Road Maintenance||$600||$615|
Studies have found that CMA is generally less corrosive to steel than chloride-based deicers, and is sometimes even used as a corrosion inhibitor (McCrum, 1988; NRC, 1991; Shi et al., 2009). It should be noted that many of the studies on the corrosiveness of CMA in regards to reinforcing steel were done on bars of coupons that were directly exposed to the CMA solutions, rather than embedded in the concrete (Levelton Consultants Ltd., 2008). It is thought that the steel embedded within the concrete could react in a different manner. Locke et al. (1988) found that the corrosion rate for steel in contact with CMA solutions was about 2 to 4 times less than that of NaCl solutions. There are, however, possible changes in the pore solution chemistry of the cement paste that could lead to macrocell corrosion due to the difference between different layers of steel.
Acetate based deicers can significantly damage asphalt pavement due to a combination of chemical reactions, emulsifications and distillations, as well as additional stress inside the asphalt (Shi et al., 2009). An investigation by Shi et al. (2009) found that, for deicers diluted at 3% by weight or volume, acetates were less corrosive to mild steel than chloride-based deicers but was similar to chloride-based deicers when looking at galvanized steel. In an experiment that looked at the effect of acetate deicers, specifically CMA and KAc, it was found that the lowest corrosion rate was for structural steel. They were equally or slightly less corrosive than chloride based deicers for magnesium and equally or slightly less corrosive than chlorides to magnesium. KAc’s had corrosion rates equal to chlorides for aluminum alloys while CMA’s had a much lower corrosion rate (Levelton Consultants Ltd., 2008).
Studies on the effect that acetate based deicers have on concrete structures show mixed results. Santagata and Collepardi (2000) reported on the effects of exposing good quality concrete to CMA continuously for eight months and found that there was a significant decrease in load capacity, mass loss, and some visual degradation of the concrete. There was a loss of up to 50-percent in compressive strength. However, when a slag-blended cement was used, the loss of compressive strength was not as significant. Lee et al. (2000) found that CMA solutions were the most damaging of all the solutions that were tested, which included sodium chloride, calcium chloride, and magnesium chloride. A study of the chemical interactions revealed reactions that occurred were similar to those that occurred with magnesium chloride. When the amount of magnesium was decreased, the effects were lessened. On the other hand, Slick (1987) found only a minimal effect to the concrete, though scaling did occur. Peterson (1991) found that CMA dissolved the cement paste at 68°F (20° C) but proceeded at a much slower pace at 41°F (5°C).
CMA is less corrosive to bridge materials than salt. In general, CMA is likely to be 1/2 to 1/5 as corrosive as salt to structural steels, 1/10 as corrosive to bridge aluminum, and significantly less damaging than salt to reinforced concrete. It is thought that CMA could accelerate the corrosion of reinforcing steel in concrete that has already been contaminated by salt (Peart and Jacoby, 1991) but there is insufficient evidence to determine if this is actually the case (National Research Council, 1991).
In regards to automobiles, CMA had less of an effect on the corrosion of brake line tubes and other automobile parts than salts did (Slick, 1987).
Like chloride based deicers, acetate based deicers also affect asphalt pavement. The damage comes about through the combination of chemical reactions, emulsifications, and distillations, as well as generation of additional stress inside the asphalt (Shi et al., 2009).
There are many chemicals associated with deicing that have both similar and unique properties and environmental effects. The following table summarizes the corrosion and environmental impacts of the deicing agents described in this article. Care should be taken when determining which chemicals are best for the intended application and for the environment surrounding the application area.
|Category||Type||Lowest Practical Melting Temperature 1,2||Potential for corrosion impairment3||Environmental Impact3|
|Atmospheric Corrosion to Metals||Concrete Matrix||Concrete Reinforcing||Water Quality/Aquatic Live||Air Quality||Soils||Vegetation|
|Chloride Based Deicers||Sodium Chloride||15°F||High; will initiate and accelerate corrosion||Low/moderate; Will exacerbate scaling; low risk of paste attack||High: Will initiate corrosion of rebar||Moderate: Excessive chloride loading/metals contaminants; ferrocyanide additives||Low: Leads to reduced abrasives use||Moderate/High: Sodium accumulation breaks down soil structure and decreases permeability and soil stability; potential for metals to mobilize||High: Spray causes foliage damage; osmotic stress harms roots, chloride toxicosis|
|Calcium Chloride||-20°F||High; Will initiate and accelerate corrosion; higher potential for corrosion related to hydroscopic properties||Low/moderate; Will exacerbate scaling; low risk of paste attack||High: Will initiate corrosion of rebar||Moderate: Excessive chloride loading; heavy metal contamination||Low: Leads to reduced abrasives use||Low/Moderate: Improves soil structure; increases permeability; potential for metals to mobilize||High: Spray causes foliage damage; osmotic stress harms roots, chloride toxicosis|
|Magnesium Chloride||-10°F||High; Will initiate and accelerate corrosion; higher potential for corrosion related to hydroscopic properties||Moderate/high: Will exacerbate scaling; risk of paste deterioration from magnesium||High: Will initiate corrosion of rebar, evidence suggest MgCl2 has the highest potential for corrosion of chloride produces||Moderate: Excessive chloride loading; heavy metal contamination||Low: Leads to reduced abrasives||Low/Moderate: Improves soil structure; increases permeability; potential for metals to mobilize||High: Spray causes foliage damage; osmotic stress harms roots, chloride toxicosis|
|Acetate Based Deicers||Calcium Magnesium Acetate||20°F||Low/moderate; Potential to initiate and accelerate corrosion due to elevated conductivity||Moderate/high: Will exacerbate scaling; risk of pate deterioration from magnesium reactions||Low; probably little or no effect||High: Organic content leading to oxygen demand||Low: Leads to reduced abrasives use||Low/Moderate: Improves soil structure; increases permeability; potential for metals to mobilize||Low: Little or no adverse effect; osmotic stress at high levels|
|Potassium Acetate||-15°F||Low/moderate; Potential to initiate and accelerate corrosion due to elevated conductivity||Low; probably little or no effect||High: Organic content leading to oxygen demand||Low: Leads to reduced abrasives use|
|Carbohydrates||Beet Juice||NA||Low; Potential to initiate and accelerate corrosion due to elevated conductivity clams of mitigation of corrosion require further evaluation||Low; Probably little or no effect||Low; Probably little or no effect; claims of mitigation of corrosion require further evaluation||High Organic matter leading to oxygen demand; nutrient enrichment by phosphorus and nitrogen; heavy metals||Low: Leads to reduced abrasive use||Low: Probably little or no effect; limited information available||Low: Probably little or no effect|