I have a client who wants to use blackened stainless steel lawn headers to separate different landscape items; for example decomposed granite from washed gravel or grass. I am concerned about the longevity of the “blackened” finish. Does anyone have any experience on the expected life of the finish in an exterior application? This project is in Northern CA.
It’s been quite a few years, so this information may not be current, and it was an interior application for blackened steel. But, all those caveats aside, the consensus among the blackened steel experts I contacted was “don’t do it”. The color can’t be guaranteed to be consistent or to last. They suggested coating the steel instead.
I’d also be concerned about the chemicals used for the blackening leaching into the soil - no idea if they’d be harmful or not, but would definitely want to know before proceeding.
A number of companies produce colored stainless steel sheet for exterior applications. Two I’ve specified products from in the past are Rigidized and Rimex. More could be found here: http://www.4specs.com/s/05/05-7010.html
I don’t know how their finishes would hold up for a lawn header application though. I’m assuming surface abrasion from the decomposed granite or gravel would be an issue.
Could the designers be thinking of Cor-10 steel?
We recently had a very interesting presentation in our office from someone at Rimex, a company that produces a wide range of stainless steel surface treatments. I suggest you contact that company.
For once I may have some useful information on a topic. Last October I had a similar request on a project. My research come up with the following:
There are basically three ways to blacken stainless steel. One involves caustic solutions such as sodium hydroxide and nitrates in a high temperature bath (~300 deg. F) which convert the surface to magnetite. This process is similar to “bluing” steel as with gun barrels.
A second is similar but the conversion method is performed at a lower temperature (220-245 deg. F) which is below the boiling point of the solution and gives off less toxic fumes.
There is a third process which is applied at room temperature. It is not an oxide conversion but is the deposition of a copper selenium-compound. Copper selenide is itself toxic https://www.guidechem.com/msds/20405-64-5.html and must be handled carefully.
Further, and as might be expected, the more environmentally friendly the process is the less durable the finish.
“For once I may have some useful information on a topic”…I disagree James, I typically read your posts each time, perhaps it is because you were a friend of the late Ralph Liebing (both a mentor and an adversary). Keep posting.
Jerome, I hope you mean that I do occasionally have useful information to share. 
Yes, Ralph was an amazing gentleman and it is to my great advantage and pleasure to have known him.
Agreed, Ralph was a good friend and it was my privilege to have also called him friend and mentor. (and, James, you DO have useful information to share - occasionally - grin)