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Stainless Solutions

IMOA’s »Stainless Solutions« e-newsletter covers a different stainless steel issue each month, with tips on design and specification, and links to technical resources.

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Frequently asked questions

General questions

What is the difference between Type 304 and Type 316 stainless steel?

The chemistries of these stainless steels are similar but Type 316 contains a molybdenum addition. The molybdenum gives Type 316 resistance to chlorides and improved resistance to urban and industrial pollution. If an application is within five to ten miles of a large salt-water body or exposed to deicing salts, Type 316 is often the most cost effective choice if corrosion-free performance is desired. Type 316 should also be considered if the location has low to moderate industrial pollution exposure or has high levels of urban pollution. Additional information about stainless steel selection can be found in the Nickel Institute brochure 'Stainless Steels in Architecture, Building, and Construction: Guidelines for Corrosion Prevention' and in the IMOA brochure »Which Stainless Steel Should Be Specified for Exterior Applications«:

Which Stainless Steel Should Be Specified for Exterior Applications?
released in 2005

What is the difference between 304 and 304L (or 316 and 316L)?

The “L” indicates low carbon.The chemistry requirements for Types 304 and 304L are identical except that Type 304L must have lower carbon levels.The low carbon version of the stainless steel should be specified if the design requires welding sections heavier than about 0.25 inches (6 mm) and the weld area will be exposed to a corrosive environment.Low carbon levels reduce the risk of sensitization and improve weld corrosion resistance.

Avoiding corrosion

What is the difference between Type 304 and Type 316 stainless steel?

The chemistries of these stainless steels are similar but Type 316 contains a molybdenum addition. The molybdenum gives Type 316 resistance to chlorides and improved resistance to urban and industrial pollution. If an application is within five to ten miles of a large salt-water body or exposed to deicing salts, Type 316 is often the most cost effective choice if corrosion-free performance is desired. Type 316 should also be considered if the location has low to moderate industrial pollution exposure or has high levels of urban pollution. Additional information about stainless steel selection can be found in the IMOA brochure MORELINK Which Stainless Steel Should Be Specified for Exterior Applications and the Nickel Institute brochure 'Stainless Steels in Architecture, Building, and Construction: Guidelines for Corrosion Prevention'

Are sheltered locations more or less likely to have corrosion problems?

If sheltered areas, such as building eaves, are not cleaned regularly, dust accumulates. Atmospheric dust is a concern if it contains corrosive marine and/or deicing salt and industrial pollution and particulate. Building eaves are microclimates and tend to have higher temperatures and more moderate humidity levels than exposed locations. Moderate humidity levels (above 50%) and high temperatures produce higher corrosive rates. Therefore, sheltered building locations, have a higher probability of corrosion than exposed locations on the same building. If sheltered locations are not cleaned on a regular basis and salt (chlorides) are present a more corrosion resistant stainless steel may be necessary.

What finishes provide the best corrosion performance in exterior applications?

Obtaining good corrosion performance is dependant on a variety of variables. Selecting the right finish can be as important as selecting the right stainless steel. Conversely, finish selection can make it necessary to use a more or less corrosion resistant stainless steel if corrosion free performance is desired. When a stainless steel has a pickled, electro-polished, or a mirror-like polished and buffed finish, it will generally provide the best corrosion resistance.

Light gauge sheet and strip with 2D, 2B, or BA (2R) mill finishes also provide very good corrosion performance. These finishes can be used as-is or patterns may be pressed into them. These patterns can include common coined and embossed finishes that look like fabric (cambric, linen), diamonds, and many other shapes. They can also simulate polishing and abrasive blasting. The performance of the patterned finishes varies with the finish pattern and its orientation. Some patterns are more easily washed clean by rain or manual cleaning and these will provide better performance than those which retain surface deposits. Vertical rather than horizontal finish orientation also encourages rain cleaning. These finishes vary in appearance from dull to mirror-like.

Is a rougher polished surface finish more likely to corrode?

Research has shown a direct correlation between the roughness of mechanically abraded or polished surface finishes and the likelihood of corrosion. Smoother surface finishes typically retain less dirt and debris, and provide better corrosion performance than rougher finishes. For that reason, European Standard EN 10088 recommends a surface roughness of Ra 0.5 microns (20 micro-inches) or less for polished finishes used in environments with high levels of particulate, corrosive pollution, and/or salt exposure and in applications where regular maintenance is unlikely. Similar guidelines should be followed for finishes produced by means other than polishing.

Does the surface finish grain orientation have any influence on corrosion performance?

If a finish has an obvious directional grain, design or profile and it is oriented horizontally, the surface will collect more dirt and require more frequent cleaning. This is primarily a concern in exterior applications or dirty interior environments. A vertical grain orientation is more easily washed by rain and collects less dirt.

A carbon steel brush or abrasive pad was used to clean the welds on a stainless steel railing. Within a few days, there was rust on the surface. Why did this happen?

Carbon steel particles from the brush or pad probably became embedded in the surface. These embedded particles will rust as quickly as bare carbon steel. The corrosion rate will vary with the environment and corrosion could appear in as little as a few days or as long as a few weeks after the “cleaning”. This contamination should be removed. A stainless steel passivation product that contains nitric, citric, oxalic, or another suitable acid may remove the carbon steel if it is not deeply embedded in the surface. If corrosion returns after cleaning, finish restoration will require grinding and/or pickling, typically with pickling paste.

The best means of restoring the corrosion resistance of a weld is to pickle and grind the weld area. Stainless steel abrasive pads and brushes may remove the heat tint, but they may not remove the chromium-depleted layer and this will make the welded joint more susceptible to corrosion. If an abrasive pad or brush is used to remove heat tint, it should always be stainless steel and it is important to make sure that that brush or pad is only used on stainless steel.

Cleaning stainless steel

How can light to moderate rust stains be removed?

Corrosion or rust staining can be caused by coastal or deicing salts, atmospheric contaminants, inappropriate cleaning products, and superficial carbon steel or iron contamination.The mild abrasive products that are used for removing adherent or hardened deposits will remove most light to moderate rust staining.

Light staining can often be removed by the standard cleaning products used to remove dust and fingerprints from surfaces. Try one of these first before using more aggressive cleaning products.

In some parts of the world, you can obtain mild abrasive (200 mesh or finer calcium carbonate) household cleaners that contain dilute acids that are very effective in dissolving rust stains. The acids that are often used for this purpose are citric acid, nitric acid, phosphoric acid, and oxalic acid. Do not use any product that contains hydrochloric acid for this purpose because it can cause surface corrosion. The potential environmental impact of using acid must be assessed before use. They are the products that are most typically used for restoring neglected stainless steel building exteriors and are usually sufficient.

There are also commercial rust removal products that are specifically designed for use on stainless steel. Do not use a general “rust removal” product without identifying its ingredients and determining if they are acceptable for use on stainless steel. Test any new product on a small stainless steel surface prior to use to make sure that it does not cause color change.Follow manufacturers instructions for application and surface rinsing.These products typically contain phosphoric, nitric, citric, or oxalic acid.Determine the potential environmental impact before use.For example, the concentrations of nitric acid that are used may over-fertilize and kill nearby plants. Spray gels that allow controlled application and longer dwell times should be used instead of liquids.Some manufacturers have neutralizers that can be spray on top of the gel while it is in place to reduce the environmental impact.

If corrosion staining is removed but returns rapidly (within a few days or weeks), it is likely that the surface has been contaminated with heavily embedded carbon steel or iron. These generally have to be removed by grinding and refinishing and/or by pickling.

If welds have not been properly cleaned and heat tint was left on the surface, the welds will need to be ground and/or pickled to restore corrosion resistance. Information on restoring the corrosion resistance of a weld can be obtained from welding product suppliers or on one of the following websites: www.euro-inox.org or www.stainlessarchitecture.org.

How can paint and marker pen stains be removed?

Use a proprietary alkaline or solvent paint stripper after first testing it on a stainless steel sample or a low visibility area to make sure that it does not cause surface discoloration. Use a soft, bristle brush that will not scratch the surface to loosen the paint or marker pen residue.

Permanent marker pen stains may require repeated applications of paint stripper. They can be very difficult to remove from rough surfaces. A 200 mesh or finer calcium carbonate cleaning product may help to remove the final remnants of the stain. (See the standard cleaning sections.) In the worst cases, light refinishing may be necessary.

What is the best way to remove adhesives?

If possible, contact the adhesive manufacturer and obtain their advice. The cleaning products that are necessary to remove specific adhesives can vary considerably. Solvents are generally used in combination with a soft bristle plastic brush and a soft clean cloth for applying the solution. After using the solvent, it is usually best to wash the stainless steel with a mild detergent solution to remove any residual solvent (see the questions on standard cleaning).

  • Remove the tape, adherent stickers, strippable film and other deposits by hand.Avoid the use of abrasives or brushes that could scratch the surface.Plastic scrapers may be used to gently remove deposits without scratching the surface.
  • Initially try rubbing alcohol, a citric acid cleaner or a product that combines these ingredients. Other less hazardous solvents and adhesive removers may be tried.
  • Methyl ethyl ketone (MEK) or products containing this chemical are usually the most effective means of removing adherent adhesives, particularly those that have been on a surface for some time. They are hazardous and should be avoided if possible. Use the chemical sparingly with appropriate precautions as described in the manufacturer’s literature.

How should I remove adherent or hardened grime deposits (not containing adhesives)?

Adherent deposits can range from hydrocarbon or oil and dirt mixtures to bird droppings. Degreasers can effectively loosen deposits containing hardened hydrocarbons or oil. Mild abrasives can be useful for a range of deposits but they may not be suitable for colored or mirror-polished finishes. Test in a low visibility spot before use. Follow manufacturers application and rinsing instructions.

  • Use an acid free liquid degreaser that is designed for use on stainless steel.
  • Use a pH neutral, acid free cleaning product that contains a 200 mesh or finer calcium carbonate abrasive and detergent. There are various proprietary products sold around the world with this basic composition. These products are typically mixed with enough water to create a paste and then applied to the surface. Some of these products are more effective if they are allowed to sit on the surface for 10 -15 minutes before rubbing.Rub gently until deposit is removed.Rinse thoroughly to remove all of the cleaning product residue.
  • Some mild abrasive cleaners contain acid to help loosen deposits or remove corrosion staining. Others contain chlorides. Determine whether the acid is acceptable for use on stainless steel prior to use. If chlorides are present, thorough rinsing is even more important.

How can I remove heavy fingerprints, grease or oil?

Hydrocarbon solvents are necessary for complete removal of heavier grease and oil deposits. This may include alkaline formulations with surfactant additions. .It is always a good idea to test the cleaner on a sample to make sure they will not harm the surface. Follow the manufacturers instructions for application and surface rinsing.

  • Use an oil-free citric acid cleaner that is designed for grease removal.
  • Hot water power washing with a mild detergent, oil-free citric acid cleaner, or a degreasing solution designed for this purpose can cut through many grease and oil deposits.
  • Apply an acid free, liquid degreaser that is specifically designed for use on stainless steel.

How can I remove fingerprints from small areas?

Use the same products that are used for cleaning larger surfaces (see previous question). Dilute dishwashing detergent is typically a more effective degreaser than a window cleaner. Proprietary oil, hydrocarbon, and wax free stainless steel degreasers are effective and are generally less messy than a detergent and water solution. It is best to check the ingredients for anything that might be corrosive to stainless steel and to test any cleaner in a less visible spot before use.

How can light surface contaminants, such as dirt and light fingerprints, be removed from an exterior application? (no corrosion)

It is not uncommon to clean stainless steel when the windows are cleaned to keep it sparkling clean. Vinegar or ammonia-containing window cleaning products will remove light dirt and fingerprints from stainless steel

Liquid dishwashing detergent or automotive detergent can be used as long as the product does not leave a coating on the surface. Coatings can adversely affect appearance and corrosion performance over time. These products can remove heavier dirt deposits and fingerprinting than window cleaners. The detergent should contain a degreaser. Some liquid detergents contain chlorides. Use chloride-free, pH neutral products if available. If the product contains low levels of chlorine or chlorides, it is important to thoroughly rinse off any residual detergent or it may increase the probability of corrosion.

If water infiltration is a concern, handwash the surface. If water infiltration is not a concern, hot water power washing can be used. This is a common means of keeping railings, benches and similar exterior applications attractive while minimizing costs. The best results are obtaining by power washing with a cleaning product that contains a detergent and degreaser.

There are also proprietary spray-on industrial oil and wax free stainless steel fingerprint removers designed for use on stainless steel that do not leave a coating on the surface. These products will also remove light dirt deposits. Check the ingredients carefully and follow manufacturer.s instructions.

Are all "stainless steel cleaners" safe for stainless steel?

No, some products whose labels identify them as a "stainless steel cleaner" contain chlorides or acids that can cause stainless steel corrosion. It is especially important to avoid any cleaner that contains hydrochloric acid (also called Muriatic acid). Other “stainless steel cleaners” contain coarse abrasives that will scratch the finish.

Do not assume that a product is appropriate because of its "stainless steel cleaner" label. Check the ingredients and, if necessary, test the cleaner on a low visibility spot before use.

How can I remove resistant water spots?

Resistant water spots are usually caused by using rinse water with a high contained-solids content and allowing water to dry on the surface. The water leaves mineral deposits on the surface when it dries. There are proprietary cleaners designed specifically for removing these deposits from stainless steel.

How can I avoid water spots?

If rinse water is needed, use clean potable water, preferably with a low contained-solids content. Remove the water from the stainless steel surface with an air blower or a clean, soft plastic or rubber wiper that is designed for removing water from glass. If the rinse water will be allowed to dry on the surface, use de-ionized water.

A carbon steel brush or abrasive pad was used to clean the welds on a stainless steel railing. Within a few days, there was rust on the surface. Why did this happen?

Carbon steel particles from the brush or pad probably became embedded in the surface. These embedded particles will rust as quickly as bare carbon steel. The corrosion rate will vary with the environment and corrosion could appear in as little as a few days or as long as a few weeks after the “cleaning”. This contamination should be removed. A stainless steel passivation product that contains nitric, citric, oxalic, or another suitable acid may remove the carbon steel if it is not deeply embedded in the surface. If corrosion returns after cleaning, finish restoration will require grinding and/or pickling, typically with pickling paste.

The best means of restoring the corrosion resistance of a weld is to pickle and grind the weld area. Stainless steel abrasive pads and brushes may remove the heat tint, but they may not remove the chromium-depleted layer and this will make the welded joint more susceptible to corrosion. If an abrasive pad or brush is used to remove heat tint, it should always be stainless steel and it is important to make sure that that brush or pad is only used on stainless steel.

Surface finish

What finishes provide the best corrosion performance in exterior applications?

Obtaining good corrosion performance is dependant on a variety of variables. Selecting the right finish can be as important as selecting the right stainless steel. Conversely, finish selection can make it necessary to use a more or less corrosion resistant stainless steel if corrosion free performance is desired. When a stainless steel has a pickled, electro-polished, or a mirror-like polished and buffed finish, it will generally provide the best corrosion resistance.

Light gauge sheet and strip with 2D, 2B, or BA (2R) mill finishes also provide very good corrosion performance. These finishes can be used as-is or patterns may be pressed into them. These patterns can include common coined and embossed finishes that look like fabric (cambric, linen), diamonds, and many other shapes. They can also simulate polishing and abrasive blasting. The performance of the patterned finishes varies with the finish pattern and its orientation. Some patterns are more easily washed clean by rain or manual cleaning and these will provide better performance than those which retain surface deposits. Vertical rather than horizontal finish orientation also encourages rain cleaning. These finishes vary in appearance from dull to mirror-like.

Is a rougher polished surface finish more likely to corrode?

Research has shown a direct correlation between the roughness of mechanically abraded or polished surface finishes and the likelihood of corrosion. Smoother surface finishes typically retain less dirt and debris, and provide better corrosion performance than rougher finishes. For that reason, European Standard EN 10088 recommends a surface roughness of Ra 0.5 microns (20 micro-inches) or less for polished finishes used in environments with high levels of particulate, corrosive pollution, and/or salt exposure and in applications where regular maintenance is unlikely. Similar guidelines should be followed for finishes produced by means other than polishing.

Does the surface finish grain orientation have any influence on corrosion performance?

If a finish has an obvious directional grain, design or profile and it is oriented horizontally, the surface will collect more dirt and require more frequent cleaning. This is primarily a concern in exterior applications or dirty interior environments. A vertical grain orientation is more easily washed by rain and collects less dirt.