I M O A® INTERNATIONAL MOLYBDENUM ASSOCIATION Which Stainless Steel Should Be Specified for Exterior Applications? Stainless steel is one of the most durable materials used in architecture. It is instructive to examine nearby stainless steel installations and However, stainless steel is not just one material; there are many different determine their maintenance history before using the Site And Design types with different properties and, most importantly, different levels of Evaluation System inside this brochure. It is prudent to confirm the pre- corrosion resistance. If an appropriate stainless steel, surface finish, and dictions made with this evaluation system by exposing samples at the design are selected and it is properly maintained, its appearance will site, because some variables can be difficult to predict. It is suggested remain virtually unchanged over the life of the building, even if that life is that the sample simulate the proposed design and be exposed for four to well over 100 years. six months. It may be possible to reduce sample exposure to six weeks If an inappropriate stainless steel and/or finish are used, corrosion can in severe marine or industrial locations. Case studies that illustrate the be a problem. Although it may not adversely affect structural integrity, application of this evaluation system are available from IMOA. corrosion can be aesthetically undesirable and increase maintenance requirements. This brochure provides guidelines for evaluating a project and for determining which stainless steel should be specified. Site and Design Evaluation System Introduction Evaluation System In architectural applications, aesthetic and performance Score Points Section 1: Environment (Select the highest applicable score) requirements must be balanced against budget considera- tions for cost effective material specification. The guide- Rural 0 Very low or no pollution lines in this brochure assume that corrosion staining of Urban Pollution (Light industry, automotive exhaust) stainless steel is aesthetically unacceptable even if there is 0 Low no struc tural deterioration. 2 Moderate Identifying a stainless steel that meets this aesthetic 3 High* standard and is cost effective requires evaluation of the Industrial Pollution (Aggressive gases, iron oxides, chemicals, etc.) site environment, weather, finish, design and probable 3 Low or moderate maintenance schedule. The Site And Design Evaluation 4 High * System provides an initial evaluation of a project’s suscepti - Section 2: Coastal or Deicing Salt (Chloride) Exposure (Select the Score bility to corrosion and makes designers aware of factors highest applicable score). If there is exposure to both coastal and deicing salt, obtain assistance from a stainless steel corrosion expert that influence stainless steel selection Coastal or Marine Salt Exposure 1 Low (>1.6 to 16 km (1 to 10 miles) from salt water) ** Instructions 3 Moderate (30 m to 1.6 km (100 ft to 1 mile) from salt water) The Site And Design Evaluation System was developed 4 High (<30 m (100 ft) from salt water) based on practical experience and atmospheric exposure 5 Marine (Some salt spray or occasional splashing) * studies. Evaluating a site and application can be complex. 8 Severe Marine (Continuous splashing) * Nearby sites can have different requirements due to loca l- 10 Severe Marine (Continuous immersion) * ized conditions or microclimates. The score should be Deicing Salt Exposure (Distance from road or ground) No salt was detected on a sample from the site and no change in exposure viewed as a guideline that generates an initial evaluation 0 conditions is expected of stainless steel requirements. It is not a precise scientific Traffic and wind levels on nearby roads are too low to carry chlorides to the site and no 0 deicing salt is used on sidewalks determination. 1 Very low salt exposure (≥ 10 m to 1 km (33 to 3,280 ft) or 3 to 60 floors) ** The Evaluation System has five sections. Read the back- 2 Low salt exposure (< 10 to 500 m (33 to 1600 ft) or 2 to 34 floors) ** ground information for each section and then determine 3 Moderate salt exposure (< 3 to 100 m (10 to 328 ft) or 1 to 22 floors) ** the section score using the adjoining point system. Sum 4 High salt exposure (≤ 2 to 50 m (6.5 to 164 ft) or 1 to 3 floors) * ** Score all of the sections to obtain a Total Score. Compare the Section 3: Local Weather Pattern (Select only one) Total Score with Stainless Steel Selection on page 4. -1 Temperate or cold climates, regular heavy rain If the design characteristics, maintenance schedules, or -1 Hot or cold climates with typical humidity below 50% exposures of different components vary significantly, a 0 Temperate or cold climate, occasional heavy rain score should be determined for each component. 0 Tropical or subtropical, wet, regular or seasonal very heavy rain If the presence of corrosive pollution or salt (chlorides) can- 1 Temperate climate, infrequent rain, humidity above 50% not be determined by site examination alone, a laboratory 1 Regular very light rain or frequent fog 2 Hot, humidity above 50%, very low or no rainfall *** could test the surface of an unwashed sample from the Score site. The sample must have had long-term exposure with- Section 4: Design Considerations (Select all that apply) out cleaning. Care must be taken during sample collection 0 Boldly exposed for easy rain cleaning and handling to prevent removal of surface deposits. 0 Vertical surfaces with a vertical or no finish grain -2 Surface finish is pickled, electropolished, or roughness ≤ Ra 0.3 µm (12 µin) -1 Surface finish roughness Ra 0.3 µm (12 µin) < X ≤ Ra 0.5 µm (20 µin) When do I need a stainless steel corrosion expert? 1 Surface finish roughness Ra 0.5 µm (20 µin) < X ≤ Ra 1 µm (40 µin) If there is uncertainty about the project evaluation or if the 2 Surface finish roughness > Ra 1 µm (40 µin) location appears to be particularly corrosive, a stainless steel 1 Sheltered location or unsealed crevices*** corrosion expert with architectural experience should evaluate 1 Horizontal surfaces the site and architect’s drawings to recommend an appropriate 1 Horizontal finish grain orientation Score stainless steel. The Nickel Institute or a stainless steel market Section 5: Maintenance Schedule (Select only one) development association can identify an expert (See page 6 for 0 Not washed contact information). Particularly corrosive conditions include: • Occasional or regular seawater or deicing salt splashing, -1 Washed at least annually immersion or salt spray -2 Washed four or more times per year • High industrial or urban pollution levels -3 Washed at least monthly Total Score • Exposure to both coastal and deicing salt Total Score • Hot, humid locations with salt or pollution exposure and very low or no rainfall * Potentially a highly corrosive location. Have a stainless steel corrosion expert evaluate the site. • Sites and designs producing a Total Score of 5 or higher ** The range shows how far this chloride concentration has been found from small rural and large high traffic roads. Test surface chloride concentrations. *** If there is also salt or pollution exposure, have a stainless steel corrosion expert evaluate the site. 2 Section 1: Environment If you are uncertain about the pollution levels for a region, data can usually City Suspended Particulate µg/m3 Sulfur Dioxide µg/m3 be obtained through the Internet or by telephone from local or national Beijing 377 90 Calcutta 375 49 governments. The data in the table was issued by the World Bank in 1998 Helsinki 40 4 and compares two pollution factors in a sampling of cities. If pollution levels Los Angeles 46 9 Moscow 100 109 are suspected to be high, a stainless steel corrosion expert with architecture New York 23 26 experience should be consulted. Paris 14 14 Rio de Janeiro 139 129 Sydney 54 28 Rural Tokyo 49 18 Toronto 36 17 Rural or suburban areas with low population densities and light, non- 1995 Urban Pollution Levels, World Health Organization polluting industry are included in this category. Migrant air pollution from industry upwind of the site could change this classification. Industrial sites Sulfur and nitrogen oxides from coal combustion, and gases released from Urban sites chemical and process industry plants are typical in industrial sites. Urban sites include residential, commercial, and light industrial locations Airborne particles, such as soot from incompletely burned fuel or iron with low to moderate pollution from vehicular traffic. High urban pollution oxides, increase the site’s corrosiveness. High industrial pollution levels levels are generally found in areas with little or no air pollution controls or are generally found in areas with little or no air pollution controls or are are caused by localized conditions that concentrate pollution. caused by localized conditions that concentrate pollution. Section 2: Coastal or Deicing Salt Exposure Salt can corrode architectural metals, including some stainless steels. If the Deicing salt exposure humidity and temperature are high enough or if there is regular light rain or Sodium chloride, calcium chloride and magnesium chloride are used for fog, salt deposits on the surface will absorb moisture and form a highly con- deicing. Deicing salt accumulations make the environment near roads and centrated and corrosive salt solution. 1 walkways corrosive. Vehicle and wind speeds and traffic levels are the A stainless steel corrosion expert should be consulted if a component is most important factors in determining how far deicing splashed, sprayed or immersed on an occasional or regular basis with sea- salt laden road mist will travel. Salt concentrations are water or a deicing salt. An expert should also be consulted if a location is ex- higher closer to where it is applied to roads. Unfortu- posed to both deicing and coastal salt, because the combined salt exposure nately, salt contamination is not always limited to sites can make the location more corrosive than exposure to only one salt source.