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Reaction between black oxide finished s.s. screws, and Cor-ten (weathering steel) sheeting

Hi,

I'm an architect. We are designing a building using 18-ga. corrugated Corten siding (may not be "Corten" -- perhaps another weathering steel very similar). Typically when you see Corten used in architecture, the fasteners are hidden. In our case that is not affordable... we want to use a very simple attachment and system. So we're proposing to screw the siding directly to aluminum subgirts, with a separating silicone tape between.

The fasteners are to be high-strength stainless steel with a bonded washer. The PROBLEM is that those stainless steel hex heads are going to stand out very strongly against the dark, rusted Corten wall. So we're looking for cheap, exterior-grade ways to finish those stainless steel heads. So far what we've found is a black oxide finish. It looks very good, and is sufficiently durable. We're going to install it in a mock-up to test, but what I am curious to know (and having trouble learning) is whether, given knowledge of the chemistry, or personal experience, of black oxide, stainless steel, and Cor-ten or weathering steel, you would expect there to be a reaction between the black oxide and the steel.

If you think it *is* a problem, then please let me know if there are alternate finishes to darken/blacken the stainless steel that you would recommend for this application! I looked at snap-on caps but they're too finicky for this 6-story wall. We're concerned that paint will be chipped off when the screws are driven -- but perhaps it's still a viable option.

Thanks for your time -- any advice would be *much* appreciated. This is to be in New York City.

George Showman
architect - Brooklyn, New York, USA
March 25, 2010