Hydrogen Sulfide - nailed that in one. It is evil evil, and will dissolve traces on circuit boards very rapidly, even in low atmospheric concentrations.
For mitigating this (and I've had to in the past), there are a couple solutions, but they all involve some common steps.
1) Seal ALL conduits entering the cabinet. Completely.
2) Seal ALL mounting holes through the cabinet.
Now for specifics - Fresh Air
1) Close off any external vents on the cabinet, except for one smallish exhaust. Run a duct outside the building, with a blower, and positively pressurize the cabinet so there is always fresh outside air exiting through the exhaust vent.
Another option -
2) after completely sealing the cabinet, add an isolated barrier heat exchanger, either air to air or water to air, something like from
http://www.norenproducts.com/
Yet another - AC Unit
3) Add a closed cycle AC unit to the cabinet.
Of the three options, #1 is the least expensive, and I've found to be the absolute best. Heat exchangers are the next best. I hate cabinet A/C units. #2 and #3 will only shift the problem, and delay it for a few years, as the copper in the heat exchanger and the A/C unit will also slowly disintegrate, and require replacement.