Corrosion

Since the early years of salt use for deicing roadways in the 1930s, the "holy grail" of winter maintenance, mobility and safety has been the never-ending quest of finding the ideal deicer. Along with cost, effectiveness, and availability, the two key criteria of any highway agency's review of deicing materials have been (since the 1960s) corrosivity and environmental effects. However, after all the testing, and all the adjustments of equipment for minimizing application rates, most agencies have only found success in making a good deicer even better. After exploring high cost alternative deicing agents, agencies have found that the old "tried and true" NaCl agent continues to meet the instant mobility demands of today's drivers. That is why salt remains, "the deicer of choice."

The two areas of corrosion which agencies have to be concerned with are vehicular and infrastructure.

Vehicular

The best source of authoritative information about vehicle corrosion is NACE International (formerly the National Association of Corrosion Engineers) located in Houston, Texas. Every few years NACE International conducts a study of corrosion on older vehicles. Starting in 1976, it looked at corrosion on 6 year-old vehicles and found 90 percent of them had rust perforation. Automobile manufacturers, realizing their customers would agree this was an area for improvement in building cars to be sold in the snowbelt and coastal areas, implemented design, structural and coating improvements. These improvements consisted of: galvanized steel panels and wheel houses; anodic electrodeposition primer; anti-chip lower body coatings; crystal-size phosphate to obtain improved corrosion resistance; weatherpac electrical connectors; stainless steel exhaust systems; two-sided pre-coated steel products (i.e. two-sided galvanized, zinc-iron, zinc-nickel or composite steels) on exterior body panels to resist exterior surface rust and corrosion; which lead to better warranties against corrosion. The average warranties now run 5-7 years, with some lasting as long as 10 years. These improvements reduced rust perforation from 90 percent in 1976 to just 20 percent in 1980. By 1990, the percentage of rust perforation dropped to only 6 percent. The most recent survey found an insignificant percentage (less than 1 percent) of 6 year-old vehicles had any type of rust perforation. While spending scarce highway maintenance dollars for alternative deicers to combat auto corrosion was once considered a bright idea by some; it is now becoming more and more an unnecessary expense.

Infrastructure

Concerning infrastructure, new corrosion prevention techniques are continuing to evolve. Highway agencies specify epoxy-coated rebar, cathodic protection, mixing concrete with an admixture to reduce permeability to water and chlorides, equipping bridges with chloride sensors; using improved sealers to prevent chloride ion penetration, and (soon) replacing steel with plastic composite rebars. All of these processes have been explored, some more than others, and all have added longevity to concrete structures. With all these new technologies, FHWA forecast some bridges and parking garages will last 75-100 years.

Admixtures for concrete have been used on New York Department of Transportation projects. Specifically, they were approved to use MetaHRM (high-reactivity metakaolin) from Engelhard Corp., Iselin, New Jersey. "MetaHRM reacts with lime that forms when cement is mixed with water. Lime, which makes up about 25 percent of concrete by weight, is a soluble material having little structural strength. HRM converts lime to a hard, insoluble material that fills microscopic voids in the concrete; reducing permeability; and slowing corrosion of reinforcing steel"

The costs of cathodic protection, which prevents reinforcing steel corrosion, and reverses chloride penetration, have fallen to $43/m3, dropping by half from $75-$86/m3. Because of these cost savings, Donald Jackson, project manager for concrete bridge repair technologies at the Federal Highway Administration, predicts expanded use of cathodic protection and rehabilitation systems.

A field study of portland cement concrete sealers used on traffic bearing surfaces conducted by the Indiana Department of Transportation has been published. The study found sealers effective even after 3 years of exposure, demonstrating strong resistance to chloride ion penetration in concrete. Tests showed Silane and Silox 1 are the most effective of the seven sealers tested, with some sealers providing protection to concrete surfaces for up to 5 years.

Dramatic changes in coatings during the last 4-5 years, and improvements in manufacturing methods and handling techniques have reduced the incidence of delamination in coated reinforcing steel. But, the "primary durability is in the concrete itself" Even under severe exposure conditions, high levels of concrete durability can be achieved by controlling water content, proper placing, consolidating, finishing and curing.

The most recent report on the effects of deicers on concrete durability can be found here (pdf 6.95 MB) .