Stainless Steel Corrosion

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Stainless Steel Corrosion is a complex subject and an in-depth discussion is beyond the scope of this blog. The focus here is on several high-level considerations for exterior exposed stainless steel used for architectural purposes in the North American market.

An excellent resource for architects and specifiers is a 6-page brochure written by IMOA consultant Catherine Houska, TMR Consulting, and published by the International Molybdenum Association (IMOA). Titled “Which Stainless Steel Should be Specified for Exterior Applications?”, Ms. Houska introduces a Site And Design Evaluation System to help determine what type of stainless steel is best for the project. The System lists a series of influencing factors, assigning points to each, with the Total Score suggestive of the stainless steel Type recommended in the instance evaluated. She cautions that a corrosion expert should be engaged if there is uncertainty or the location appears to be particularly corrosive. Click on the following hot link to download a pdf copy of the article: IMOA Article.

In introducing the System, Ms. Houska adds the caveat: “It is instructive to examine nearby stainless steel installations and determine the maintenance history before using the Site And Design Evaluation System . . . ” presented. She also notes that samples taken at the intended location can be tested for chloride content, adding useful information for selection. Listed influencing factors include air pollution, distance from salt water (coastal or marine salt exposure), distance from roads or surfaces where deicing salts are regularly used, local weather patterns, design considerations (surface roughness, finish grain orientation – vertical orientation is better, extent of exposure, and existence of crevices where contaminants remain trapped), and maintenance schedule (“frequent cleaning by heavy rain or manual washing prevents corrosive deposit accumulation and corrosion”).

The most commonly specified stainless steel in North America (except for coastal regions) for exterior architectural purposes is SAE Type 304 (S30400). A Specialty Steel Industry of North America (SSINA) No. 4 finish is typical, though other finishes are also used. As is clear from the cited IMOA article, though, Type 304 might not be appropriate in a specific location, and SAE Type 316 (S31600) might be a better choice. If welding is involved, then consideration of the “L” series (lower carbon content with increased weldability  but somewhat reduced strength): SAE Type 304L (S30403) or SAE Type 316L (SS31603). The designations within the brackets are the unified numbering system (UNS) designations, which is a system jointly managed by ASTM International and SAE International, and widely accepted in North America. If a SSINA No. 4 finish is specified, then adding a requirement that the grain be vertical reduces corrosion risk to an extent.

As a result of higher nickel prices, there has been increased interest in low-nickel or no-nickel grades of stainless steel. One such family of stainless steels is the 200-series and use of these has increased. It can happen that a 200-series product, with its lower corrosion resistance, can turn up on your project. Site verification of stainless steel Type is difficult and requires expert testing agency involvement. A hand-held X-ray fluorescence machine can be used, for example, to help identify alloy. The trade contractor, or even their supplier, may not know that the product is a Type 200-series material; its use may have been undisclosed and due to an upstream source error or substitution.  If corrosion becomes apparent on a project, the first thing to due is verify the material.

Shop or yard contamination of stainless steel products, as well as during shipping and on-site storage and handling can contaminate stainless steel surfaces with carbon steel product or other surface contaminating particles. Contamination from carbon steel shows up as a red “dusting” on the surface that looks like the stainless steel is rusting, but is the byproduct of corroding surface contamination. If surface dusting is apparent, then the installer can be directed to clean and passivate the stainless steel and welds using industrial/commercial grade cleaners and passivating agents. One cleaning product that can be considered is CitriSurf® 2310, but there are others that may be more appropriate regionally.

Standards change and evolve over time, and old office standards for stainless steel may no longer be appropriate or valid (for example, ASTM A167 was withdrawn in 2014). A good guide to specifying stainless steel is the SSINA Designer Handbook: Specifications for Stainless Steel, a pdf copy of which can be downloaded from the SSINA website.

Using stainless steel on a project is not straight forward, and involves careful design considerations to establish the best design configuration and SAE Type and finish to specify; calling for site-cleaning and passivating of installations may be a prudent preemptive consideration.

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 The image heading this blog is from the website of Penn Stainless Products, Inc..