Frequently Asked Questions

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 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 R_a 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.