CS corrosion of a form I have never seen.

[StoneCold]

We recently opened the tube side of a carbon steel heat exchanger. Cooling water runs through the tubes and hot R22 refrigerant is condenced on the shell. The tubes are copper but the shell, tubesheet and bonnets are carbon steel. There was corrosion on the tube sheet and on the piping coming into the exchanger. The corrosion was mostly black nodules sticking into the pipe and a black brittle layer about 1/16 of an inch thick on the tube sheet. The crazy part is that when you brake the black layer off the metal underneath is "white". It looks like it was sand blasted. The "white" metal underneath is pretty porous, like cast iron. Once the black layer is removed the surface will rust over in a few minutes. There is no suflur smell. Could it be anerobic bacteria? The ph of the water runs around 9. Our filtering system on the cooling water catches quite a bit of bacterial material. Last sample of the cooling water showed bacterial levels at 100,000 CFU/ml and sulfer reducing bacteria were less than 100 CFU/ml.
Why would the metal underneath be perfectly clean?

Thanks
StoneCold

 

[EdStainless]

The metal is clean because it is a site with active corrosion. As the metal is removed you get the 'clean' look.


= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[unclesyd]

You have what I call the bugs of rust. Your problem probably starts with an iron bacteria attack
on the steel or it can then progress into what used to be called a differential aeration cell depending on your water treatment. Take a piece of PH paper and pop one of the tubercles off and quickly apply the strip, you should see an acid reaction.

There are several other possibilities though I suspect that this is what you are seeing. With the current available water treatments, this problem is hard to keep under control if you have any volume of water.

I would get with your water chemical supplier and make him aware of the conditions you found as the next step is the bugs start building condominiums.

 

[StoneCold]

Thanks EdStainless and UncleSyd.
I contacted my Nalco guy and we are going to make some changes. We are adding a second biocide for the bugs.
We are going to double our current Phoshponate feed until it is used up and then switch to a Phosphosuccinic Oligimer.
A third step I am adding is corrosion coupons to the system. We run blind right now.

Unclesyd I already closed up the exchanger so I was not able to try the Ph paper test but we are opening another one later this week and I will try it on that one. If it is a bug issue I think they are past the condo stage and on to "estate lots".

Thanks Guys. I knew it had to be bad when the metal underneath was so shiny, I have just never seen that.

StoneCold.