The minimum 12-percent chromium contained in stainless steel forms an invisible, protective, corrosion-resistant passive film on the surface. The film repairs itself quickly if it is damaged or removed during fabrication or polishing. If an appropriate stainless steel is selected, it will retain its original appearance with no corrosion or discoloration of surrounding materials.
The types of stainless steel most commonly used in architecture are 430, 304 and 316. Type 430 and other 400-series stainless steels are magnetic. Types 304, 316 and other 300-series stainless steels are not magnetic and can be strengthened by cold work. In most architectural uses, the stainless-steel sheets used have been heated and cooled in a process called "annealing." This is also called "dead soft" condition. In structural or sheet applications subjected to higher loads or impact, "cold working" can be an economical means of reducing weight and increasing strength.
Type 430 is used in indoor, rural or urban locations with little potential for corrosion. Type 304 is used in indoor, rural or moderately polluted urban areas, and coastal areas with low humidity and temperatures. Type 316 is used for marine, coastal, or de-icing salt exposures, and in industrial and in some urban locations. At some sites, a more corrosion-resistant stainless steel may be need
No coating neededIf an appropriate stainless steel is used, there is no reason to apply a protective coating. A variety of mechanical and colored finishes are available. Stainless steel is available in sheet, strip, plate, bar, tube, pipe, castings and extrusions.
Predicting a metal's performance in exterior applications, soil or concrete requires knowledge of the factors influencing corrosion. Atmospheric pollutants, wind-borne marine salt, de-icing salt exposure, temperature, humidity and rainfall must be considered when specifying metals. Sites only a few kilometers apart can have different levels of corrosiveness due to local pollution and the direction of prevailing winds.
For corrosion to occur, moisture from rain, humidity, condensation or fog must be present. That's why dry climates tend to be less corrosive. Very wet climates can also be less corrosive if the design takes advantage of rain's natural cleaning capabilities. When small amounts of moisture from very light rain, high humidity or fog combine with corrosive surface deposits, they can create a highly corrosive, wet film on the surface. Higher temperatures usually accelerate corrosion.
Service environments are classified as rural, urban, industrial or marine. Within each category, there are high-, medium- and low-levels of corrosion risk, which are determined by rainfall, air temperature, pollution and other factors. Future regional development should be considered when evaluating any site. No two environments are exactly alik
Test resultsThe National Institute of Standards and Technology has tested unprotected stainless steels in a variety of soils. Types 304 and 316 are highly resistant to pitting and general corrosion in most soils. In soils with high chloride levels, Type 304 was susceptible to pitting corrosion, but Type 316 showed good resistance to corrosion in all test soils including tidal marsh and clay. Type 316 only experienced pitting in oceanfront sand flooded by seawater.
There is growing interest in extending the useful life of concrete structures. The now-defunct U.S. Office of Technology Assessment once called corrosion-related, steel-reinforced bridge deck deterioration a "serious national problem."
The Illinois-based engineering firm Wiss, Janney, Elstner Associates Inc. conducted a five-year study for the U.S. Federal Highway Administration to identify material for use in a reinforcement bar that would not require a corrosion-related repair for 75 to 100 years. They tested black bars, epoxy-coated bars, stainless steel, copper-clad steel, galvanized steel and spray-metallic-clad steel. The corrosion rate of Type 316 stainless steel was 800 times lower than that of black bar. They concluded Type 316 should be used for installations where corrosion-induced damage is difficult or costly to repair, such as parking garages, sea walls, piers, tunnels and bridges with high traffic volumes.
The results of other studies have been similar. Depending on bridge size and complexity, stainless-steel rebar increases the initial project cost between one percent and 20 percent. When repair and disruption costs are considered, stainless steel provides substantial life-cycle cost savings over a 100-year period in uses where chloride exposure is likely
Environmental impactThe environmental impact of construction materials is a growing concern. If an appropriate grade and finish are selected, there should be no need to replace stainless steel, even if the building is around for hundreds of years. Stainless-steel scrap has a high value, so it is not discarded. Stainless steel is 100-percent recyclable and there is no limit to how much recycled scrap can be used to produce new stainless steel.
Due to the corrosion-resistant nature of stainless steel, protective coatings are not needed, so potentially harmful gases produced by the coatings are not released into the atmosphere. In addition, no acids or harsh chemicals are needed to clean stainless steel. A simple solution of water, detergent and a soft cloth will do.
When specifying a metal, it is important to identify the specific grade or alloy and the appropriate standard for the product. Corrosion resistance can vary significantly within a metal family. Specifying just stainless steel, aluminum, or copper is not adequate. The wrong grade or alloy within the metal family could be supplied, and it may not meet application requirements.
Metal performance can vary significantly with the environment. Avoid corrosion that could be detrimental to aesthetic, ventilation or structural requirements. The steel manufacturer or an industry association should be consulted to determine if the metal, coating, thickness or weights are appropriate for the given use.
(Catherine Houska is a senior market development manager with Pittsburgh-based building consultant TMR Stainless. This article originally appeared in the February 2002 edition of Architectural Metal, a newsletter produced by the Sheet Metal and Air Conditioning Contractors' National Association.)