PLC board corrosion

Where I work, we had a problem with corrosion on the Siemens Microbox PLC's. Siemens sent one of their engineers in from Germany, he went around doing air samples trying to figure out what was going on. Air samples showed no problem. We also had the same problem with hard drives, not sure why the problem was with MicroBox PLC's and Hard drives, other electronics didn't seem to have the problem.

What we ended up doing is spraying the circuit boards with a coating we got from the local electronics supplier, IIRC they called it conformal coating. I don't know if it was a certain flux or solder, but some boards had this problem, others didn't. McMaster Carr used to sell an insulating varnish for electric motor coils, I would think it would work good too. Coating the boards changed the life of the boards from months to years.

Good Luck

RogerN

Something corrosive is outgasing and being trapped in your tightly closed box. When PVC breaks down it can outgas HCl. Some silicones outgas acetic acid. Other plastics can outgas formic acid or formaldehyde.

Tom Gardner fired this volley in news:EI- dnW19uaKzXp3MnZ2dnUVZ snipped-for-privacy@giganews.com:

Tom, I don't know what the insulation system is on those cables. More likely than letting in (excessive) moisture, the cable is gassing off HCl as PVC in it decomposes. It takes only trace amounts of HCl and moisture in the air - especially trapped - to cause serious bare-metal corrosion.

Also, if there's any H2S in the atmosphere from the gas furnaces, that can contribute a lot to corrosion, assuming any of that air can get into the enclosure. You'll notice the edges of mirrors turning black if H2S is the culprit, and the corrosion of tin/lead or RoHS lead-free solders due to H2S will be almost jet black, and 'velvety' looking.

LLoyd

Thanks Roger, I'll take your advise. The flux thing is the only thing that makes sense.

So, maybe being so sealed up can cause problems. I don't have any other electronics having problems for many years and this one is only one year old and the only one in a sealed box.

No problems with the mirrors in the bathrooms. The only air exchange in the box is through the Greenfield. The only other electronics that has failed has been the PS on a Chinese DRO on a BP. I looked at the boards with a loupe and didn't see ANY issues.

Many years ago I worked on flight hardware for NASA. I recommend using

============================ Hey Tom,

We had a multi-lane bi-directional stepper switch ( 18P25T ) in a salt plant, that would corrode to un-usability every six months, and the switch and our labour to replace it was considerable. It sat in a closed but not sealed cabinet, and there was no direct heat in the machine room, so the temperature varied quite a bit by season. I put a Plexiglas window in the swing lid of 12" X 12" X9" deep gasketed cabinet and a 60 watt "Rough Service" light bulb with a simple light dimmer to make the bulb just glow at about 25 watts. Took twice as long to do this the one time, including installing, moving, re-wiring, and caulking the wire-cable so the box was essentially air-tight. In the 9 more years that I was still in the trade, the switch stepped without failure, and used the same light bulb.

Worked for me.

Brian Lawson

Ive had pretty good luck with Krylon transparent spray for distributors or similar. Its a very thin spray "paint"

Some guys swear by this. Ive not used it..but have had pretty good reports on it.

it has the benifit of killing existing corrosion on circuit boards

"conformal" coatings are good..but make future service problematic. They basically varnish the boards and glue everything together.

If there is something ON the boards that is creating a problem..you have just sealed it under the coating..and it will spread under it.

Gunner

Gunner

The methodology of the left has always been:

1. Lie
2. Repeat the lie as many times as possible
3. Have as many people repeat the lie as often as possible
4. Eventually, the uninformed believe the lie
5. The lie will then be made into some form oflaw
6. Then everyone must conform to the lie

Can the Everclear still go in the punch?

How about a 25w to begin with? How do you judge how much is enough, temp?

How big is the case? Ambient temperature of the location? 15 to 30 degrees warmer is usually enough.

Hey Tom,

A 25 watt bulb at 120 volts will have the life span of a 25 watt bulb, while a bulb of any higher wattage dialed down will have a significantly longer life span.

I'm only guessing at what the actual wattage was it was operating at, but just above a glow.

Brian Lawson

I get it!