Corrosion in cooling water tubes

[prc]

Corrosion is noticed in cooling water tube of a transformer cooler.It is a shell-tube heat exchanger with cooling water inside the tube.Tube is 90:10 cupro-nickel with water velocity of 0.45 metre per second.River water(not polluted) is used . Tube dia 16/14mm. After nearly 2-3 years of service localised corrosion finally result in pin hole formation in the tube.Some say it is crevice corrosion.Can any one throw some more light?What is the reason for this? How to prevent it?

 

[rob768]

Hard to say without photo's. Are there deposits under which crevice-like corrosion could have initiated? Is there turbulence, causing local exceeding of allowable velocity?

 

[EdStainless]

What material is the rest of the HX (the tubesheet and head)?
How near the end of the tubes are the pits?
What do the tubes look like? Are there any surface deposites/residues?
The tubes should be a nice uniform dark color.

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[prc]

The deposits/residues are not uniform.It is greenish,bluish or black and not everywhere.Tube sheet is naval brass.Pitting is somewhere at middle of tubes,more predominant on lower half of the circular heat exchanger(horizontally mounted )No turbulance. Deposits started at crevice like corrosion points.

 

[EdStainless]

Green and Blue are not good colors on Cu alloys. You have active corrosion. It could be realted to ammonia or sulfur compounds in the watter, but most likely microbiological.

You need to extract a tube for evaluation by an expert.

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Rust never sleeps
Neither should your protection

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[prc]

Thank you Edstainless. I have taken out tube.What all tests to be done to find the presence of Ammonia or sulphur compounds or microbiological?Is low velocity create this type of corrosion ?

 

[EdStainless]

Where are you? You need a good local lab to do some work for you.
Low flow in Cu alloys normaly only is a problem is there is fouling. High flow minimizes the fouling, but can lead to other problems.

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Rust never sleeps
Neither should your protection

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[prc]

I am in India. I checked the material of the tube at a lab and found the same meet with spec of Cu90 Ni10.Water analysis shows no abnormality.My understanding of corrosion is also as explained by EdStainless.There is no fouling in tube. But the tube supplier says low velocity can cause crevice corrosion.Is there any sense in it.How to proceed further?

 

[EdStainless]

Did you look at the microstructure? What about other element? I can't remember, but there are some secondary phases that can form in CuNi and result in lowered corrosion resistance.

Something sounds wrong with your situation. Keep looking.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

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