Galvanizing in H2S environment

[Scipio]

I've got one of those 'depends on who you ask' type questions. There's been a few instances where I've installed equipment in facilities processing sour natural gas, anywhere from 50 ppm up to 30% H2S, and pressures upwards of 2200 psig. Some equipment (like fin-fan coolers) and a lot of structural steel use galvanizing to protect steel from external corrosion, other components are painted. I've been advised by a couple people not to use galvanizing, as in the event of an H2S release on site, H2S will react with the zinc in galvanizing and cause it to blacken and eventually flake off, however doesn't seem to be a problem with something like an inorganic zinc-rich epoxy coating. At the time I was pretty new to H2S environments and didn't question it, now I'm looking back and wondering how much of that advice was based on fact.

Question becomes, all other things being equal, is galvanizing really more vulnerable to H2S attack than zinc based epoxies? Any thoughts?

 

[TVP]

ZnS readily forms in the presence of H₂S. I have no direct experience, but I would assume that the zinc coating (hot-dip, electrogalv, galvanneal, etc.) would be attacked quicker than the zinc-filled epoxy coating.