[TheForge] Stainless contamination

[James Binnion]

It is the presence of chromium oxide that is formed on the surface of  
stainless steel that makes it "stainless"  Other alloying components  
will help in very crrosive environments but the most important one is  
the chromium.  If you weld, grind wire brush or polish with an  
abrasive you end up disturbing the chromium oxide layer and you then  
need to passivate it to restore the "stainless" qualities. This is  
typically done with a mineral acid (nitric acid is typical) that  
dissolves the iron from the surface of the stainless and leaves  
behind a chromium oxide or chromium  surface that rapidly oxidizes  
and makes the surface "stainless"  again. If your bar top was wire  
brushed or polished and then plastic sealed without passivating then  
you will see the rust on the surface as the iron is oxidized. There  
are citric acid based passivation solutions out there like CitriSurf  
that are a lot safer to use than the nitric acid based ones. If you  
weld, grind, wire brush etc  most stainless alloys you need to  
passivate it before exposing it to corrosive environments like the  
outdoors.


James Binnion
jbin at well.com