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Stainless Steel Helps Prevent Deicing Salt Corrosion

By Catherine Houska, TMR Stainless (Consultants to IMOA)

Road deicing salt has been used since the 1960’s to prevent commerce disruption and accidents in cities with regular winter snow (e.g. Düsseldorf and Chicago) or freezing rain exposure (e.g. Beijing or Dallas). Total deicing salt tonnage has continued to increase worldwide and corrosion damage is not limited to highways and vehicles. Building material corrosion and premature failure is a significant and growing problem.

Road salt mists are formed as the tires of moving vehicles spray tiny particles of salt water or dry salt particles into the air. Hundreds of deicing salt studies have been done but most focused on areas close to roadways. New US research has documented seasonal deicing salt accumulations as much as 1.9 km (1.2 miles) downwind of a major highway outside of Chicago. The researchers collected data between 1997 and 2004 and produced a model (Figure 1) covering a 5 x 5-km (3.1 x 3.1-mi) area.1 It shows that sites within 100 m (328 ft) of busy roadways can be comparable to moderate to severe coastal areas.

Figure 1:
Model projection of annual deicing salt deposition around highways I-55 and I-355 and small surface roads with the spacing of some test sites shown. Illinois Department of Transportation (DOT) in collaboration with Argonne National Laboratory (ANL) and the U.S. National Atmospheric Deposition Program/National Trends Network (NADP/NTN)

The researchers theorized that urban areas with busy roadways might have regional deicing salt plumes or mists for several days after a snow event. This increases total salt accumulation on buildings. In Chicago, deicing salt-related metal corrosion has been found up to the 60th floor of one tall building.2 Exposure levels are likely to be similar in any major city with regular winter snow exposure.

Internationally, it is estimated that over 60 million metric tons (66 million tons) of salt are used for deicing.3 China, which previously used very little deicing salt, has had a considerable increase in usage and recently became the world’s largest producer.

In the US, about 70% of the roads and population areas receive at least 13 cm (5 inches) of snow annually and additional areas are affected by seasonal freezing rain. In recent years, the United States used 13.6 to 18 million metric tons (15 to 20 million tons) of deicing salt per year and Canada used another 3.6 to 4.5 million metric tons (4 to 5 million tons).4

Many Europeans incorrectly assume that their deicing salt use is quite minimal, but it is actually similar to North American use. Deicing salt tonnage is increasing in both regions. For example, the salt industry association, Verband der Kali- und Salzindustrie e.V., reported that over 3 million metric tons of deicing salt were used on German roads during both 2005 and 2006, which is a 50% increase since 2002.5

The most comprehensive European study was done in 2002 when twenty countries participated in a task group that studied deicing practices. Table 1 summarizes the deicing methods used in each country, and, where available, the annual tonnage. It is evident from this and other data sources that deicing is standard practice throughout Europe.

Deicing methods by country
CountryDeicing products usedTons (1,000)Deicing period
Austria NaCl, CaCl2 NA Nov. - March
Belgium NaCl, CaCl2 113 Oct. - April
Croatia NaCl NA NA
Czech Republic NaCl, CaCl2, NaCl, MgCl2 215 Nov. - April
Denmark NaCl 115 Oct. - April
Finland NaCl NA Oct. - April
France NaCl, CaCl2 400 - 1,400 Nov. - March
Germany NaCl, CaCl2, NaCl, MgCl2 2,000 Nov. - March
Great Britain NaCl, CaCl2 2,200  
Hungary NaCl, CaCl2 NA Nov. - March
Iceland NaCl, CaCl2 NA Oct. - April
Ireland NaCl 30 - 70 Nov. - April
Norway NaCl 83 Oct. - April
Poland No details NA NA
Romania NaCl 108 Nov. - March
Slovenia NaCl, CaCl2, NaCl, MgCl2 NA NA
Spain NaCl, CaCl2 80 Oct. - April
Sweden NaCl 300 Oct. - April
Switzerland NaCl, CaCl2 NA Oct. - April
The Netherlands NaCl, CaCl2 135 Oct. - April
NA - total tonnage data not available

Table 1:
Road and bridge snow and ice control procedures in countries that participated in a 2002 European study of Winter maintenance practices
3

Unfortunately, many designers are not aware that deicing salt is very corrosive to architectural materials, and inadequate knowledge can lead to inappropriate material selection and premature failure. Molybdenum containing stainless steels are often a cost-effective choice when long-term performance, aesthetics, security, and/or minimal maintenance are important because the molybdenum alloying addition improves resistance to chloride (salt) corrosion.

Type 316 stainless steel (with 2% molybdenum) with a smooth finish is preferred for many applications exposed to deicing salt. In applications with very high salt exposure levels (i.e. embedded sidewalk lighting and entrance gratings) and areas regularly exposed to salt water splashing, a more corrosion resistant stainless steel or regular cleaning may be necessary. Stainless steels with higher molybdenum contents, like Types 2205, 317LMN, and 904L, are generally sufficient for these seasonal high deicing salt exposure applications.

The Type 316 stainless steel exterior of Frank Gehry’s Weisman Art Museum (Figure 2) illustrates that success is readily achievable when an appropriate stainless steel is selected, while Figure 3 shows unacceptable staining on a Type 304 stainless steel panel with less significant deicing salt exposure.

Figure 2:
Photo: Catherine Houska, Nickel Institute
The Frederick R Weisman Art Museum has a smooth No. 4 finish on its Type 316 stainless steel exterior and is easily rain-washed. Despite five-year gaps between cleanings, no corrosion staining is evident but there is some dirt accumulation.

To avoid problems, specifiers must be made aware of the severity of the deicing salt problem, comparative metal corrosion rates, and common deicing myths. This makes it necessary to provide information on both stainless steel selection and the relative performance of competitive architectural metals. For example, there is a common misconception among architects that aluminum’s corrosion resistance is similar to that of stainless steel. This is incorrect. In locations that are exposed to chlorides (coastal or deicing salt), the corrosion rate of aluminum is typically 10 to 100 times that of stainless steel. The white to grayish white color of aluminum corrosion product may not bring attention to the problem (as the rusting red of steel does), until permanent aesthetic or structural damage has been done. This lack of knowledge makes aluminum a common replacement for stainless steel when construction costs must be reduced. Figures 4 and 5 show severe aluminum corrosion in a deicing salt laden environment and provide a significant contrast to Figure 2.

Figure 3:
Photo: Catherine Houska

This Type 304 panel is on the second floor of a building in downtown Pittsburgh. Although it is cleaned more frequently than the building in Figure 2, it is exhibiting significant deicing salt corrosion staining.

Figure 4:
Photo: Catherine Houska
Severe deicing salt corrosion has caused perforation of this aluminum door threshold.

Figure 5:
Photo: Catherine Houska

Deicing salt corrosion of charcoal grey anodized aluminum panels on the second floor of an office building lead to permanent surface damage.


International “sustainable” design trends have made the specification of long lasting materials even more important. Most of the world’s major population centers are exposed to coastal or deicing salt or both. IMOA has developed and posted a new web article on the international impact of deicing salt (see www.imoa.info). It contains the information shared in this article as well as a more in-depth discussion of the relative performance of competitive materials. IMOA is providing decision makers with the information necessary to understand the problem and justify the use of high performance Mo-containing stainless steels.

More information can be found in our publication:

Deicing Salt - Recognizing The Corrosion Threat

Notes

1. Allen L. Williams and Gary J. Stensland, “Atmospheric Dispersion Study of Deicing Salt Applied to Roads,” Part II Final Report For Period July 2002 to June 2004, in the January 2006 issue of Physical Research Report No. 140, Illinois Department of Transportation.
2. Houska, Catherine, “Which Stainless Steel Should Be Specified for Exterior Applications?”, International Molybdenum Association, architecture, building and construction series
3. European Task Force TG3, ‘Snow and Ice Control on European Roads and Bridges’, August 2002
4. Salt Institute, www.saltinstitute.org.
5. The European Salt Company, www.esco-salt.com

Additional Literature
The Swedish Corrosion Institute and Outokumpu conducted research to determine the performance of various stainless steels in locations that are exposed to deicing salt. The samples were placed in tunnels, on bridges, and alongside roadways.

Acom 2008 Ed:1 "The Suitability of Stainless Steels for Road Constructions"

Case studies

Case Study 1: Pittsburgh and Chicago Handrails and Street Furniture
Case Study 2: Minneapolis Exterior Wall Panels and Window Frames