1794ACN15 Flex I/O Adapters Hanging

Hi,
We have a 15 year old installation with 6 lines of 7 flex I/O adapters hanging off dual redundant PLC 5's. We've never had any major issues apart from the odd card failure until now. We're now starting to get various items of plant faulting because the PLC code is showing an output firing but it's not reaching the field item via flex I/O. A reset of the supply on the adapter for the particular I/O module cures the issue, there's no need to cycle power on the particular card. All the BNC connections look to be sound. I don't think it's PLC related because we have 6 identical lines and a couple of them are showing issues. As with any intermittent fault it's a process of elimination. We've cleaned all the card pins and bases as we though it may be an issue with H2S attack on the connections (sewerage plant). We've only just discovered that an adapter reset does the trick so were considering a change out of all 7 adapters next.
Anyone experienced similar?
Thanks
Jim

Morning Jim,

We have one major installation with 1794ACN15 adapters, and the main issues we get are with the BNC connectors, usually find that once we have an issue, we then have a blitz to check and remake/replace the connectors. But then we could have a vibration problem, rather than corrosion.

H2S could be a problem, especially on waste water plants of that vintage.
What is the ventilation like where the racks are mounted?
Are some more enclosed than others, hence getting more exposure to corrosion?
I have seen other PLCs attacked by corrosion in WWTP and not always obvious on the electronics. If you have one or two that are susceptible, change the adapter but also check the ventilation...

Thanks John,
Yes might be an idea to have a good look at he BNC's rather then just check they're tight. All that adapters are in similar control panels with reasonable ventilation/cooling. We've had to change a large number of small relays in the past due to H2S attack just hoping that flex I/O isn't going the same way.

Jim