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Opinions on a large CuNi pipe containing sewater

Opinions on a large CuNi pipe containing sewater

Opinions on a large CuNi pipe containing sewater

(OP)
Hi everyone smile

On a ship, I have a large CuNi 90/10 pipe that contains seawater. The Seawater has little to no circulation. I know that we need to circulate oxygen-rich water for a prolonged time to establish a sufficient oxide layer on the pipe. From the yard during the new build, this was not performed properly.

Questions:
1. What is your opinion on the inner surface of the pipe. Do we have an inner surface that "looks" ok?
2. Or do we have a problem with excessive corrosion? Have not taken UT measurements of the wall thickness. I think it is difficult to analyze based on the pictures. The surface feels mostly smooth.
When I rub my finger on the surface it turns brown/ Black.
3. The last picture shows an area between 7 to 9 o clock that looks dark grey/brown. Is this the surface we want to have?
3. What is the ideal surface characteristics/colure of a CuNi pipe that runs seawater and has a well-established protective oxide layer?
4. I have taken a water sample since some of the water had dark particles. The result from the water sample showed 58% corrosion product of copper. The pipe was not properly flushed after commissioning. So maybe some of these particles from the water sample are corroded products from welding/grinding during installation. ?

I have attached some photos.

For info, the pipe has been drained for seawater about 2 weeks ago. I think this is the reason for the greenish stripe at the bottom of the pipe where some water hasn't been drained.

Other relevant info:
Age pipe: 6 years
Medium: seawater
Material: CuNi 90/10 with min. 1.5% Fe
Thickness: 4.5mm
Weld: Seamless full penetration.
Diameter: DN300

Thank you



RE: Opinions on a large CuNi pipe containing sewater

Cu-Ni does not rely on a protective oxide layers like stainless steel do. Stagnation should not cause issues provided sulfides are low.

RE: Opinions on a large CuNi pipe containing sewater

In general brown is good and black is bad, but that can be very relative.
And sulfides are the kiss of death.
If polluted water was run through it when it was new, then there is likely some damage.
I have heard of people chemical cleaning and re-passivation (you are forming mixed copper oxides/hydroxides not a real oxide layer).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Opinions on a large CuNi pipe containing sewater

(OP)
Thank you for reply.

I have measured the Sulfide levels and they are not present,

Have you any ideas regaring the chemicals used to clean the pipe and accelerate the re-passivisation?

If anyone else has any opinions on the photos, please comment smile

RE: Opinions on a large CuNi pipe containing sewater

Has there been any loss of pipe wall thickness ? .... If not, you are home free !!

In my opinion, this would be fairly easy and inexpensive to determine in your case ....

Is there any location in the system where there is a higher liquid velocity than other parts ?

90/10 CuNi is very corrosion resistant to seawater but it is relatively soft and has a very, very low allowable Code stress level

MJCronin
Sr. Process Engineer

RE: Opinions on a large CuNi pipe containing sewater

Quote (EdStainless)

(you are forming mixed copper oxides/hydroxides not a real oxide layer)

Correct; the deep green product is malachite, one form of copper oxide/hydroxide. Another form is azurite, a turquoise blue compound that is also not adherent to the metal. In brass HX tubes these colours offer visual clues about the temperature at which corrosion happened.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

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