H2S and other compounds are rough on metal in the presence of oxygen! I work on disposal pumps and found that even the smallest opening can draw in oxygen. Most of the time you cannot see a leak of any sort, but once the pump is up and running it can start to draw in oxygen. Hey I also found that oilfield water chemistry places can hook you up with dissolved O2 test kits that read 0-100ppm of dissolved O2 for pretty cheap. The last I purchased was 50 tests for $20.