K-Monal Shaft material pitting

[GasProcess]

There are two pumps for lifting sea water for cooling purpose. ( One running + one standby ). Recently we have replaced the pump shaft for for Sea Water service from SS to K-Monal. The new shaft indicated signs of light pitting when pump was kept idle for about 8 months in the water. Is this ok or some thing wrong with shaft material.

The original Shaft material for the same pump was some grade of SS which is as good as new even after many years of service under same conditions.

Thanks

 

[rolledalloy]

It is my understanding that in stagnant or slow moving seawater the Monel K-500 is prone to fouling/pitting. Flowing seawater is less severe. Also the Monel type materials are more sensitive to sulfides in seawater than are stainless steels. What type of stainless was used previously? Do you believe it was a 300 series, duplex, 6-Moly.

Special Metals has a website and the data sheet for Monel K-500 discusses stagnant seawater.

http://www.specialmetals.com.

 

[unclesyd]

I used K-Monel on boats that were not tied to the dock a good percentage of the time. The problem was the same as you have and as stated in the previous post. The pitting on boat shafts was especially pronounced in the packing and cutlass bearing areas. For boats that were inactive I have used Aquamet 22 (22-13-5) with excellent results. On boats that are on the go Aquamet 17 (17/4) was the choice.


My pump story:
We used 6 pumps for River Water, quite brackish, that originally had 410/420 shafts. Some years ago we changed to 17/4, actually Aquamet 17, as all pumps were in regular use. As time progressed the amount of river water usage decline and we were using the intermittently and as result had a few problems with 17/4. The pumps are down to three and the shafts have been changed to Aquamet 22 (22-13-5). At my last check all were doing quite well in intermettant service.

The only vertical pumps I've seen using K-Monel were two we inherited from a sister company.

 

[EdStainless]

K500 is also going to be sensitive to the galvanic envronment. What alloy are the impellers and bowls?
Where are the pits relitve to attachments?
Is the shafting annealed and aged, or cold finihsed and direct aged?

My experience with 22 has been poor, but then I have been working in reducing environments, not open seawater.

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[GasProcess]

EdStainless

The impeller and bowl are Aluminum Bronze ( 76 : 11) and age hardened by hot working. The pitting is all over the portion of shaft which was submerged under water

Also could you please clarify about 22 & 17/4

Our earlier shft appears to be 300 sereis but I am not sure. IS there any way it can be told by just looking or scratching a bit

 

[rorschach]

aquamet 22 is 22% chrome, 13% nickel, and 5% Manganese. you can get the information here:

http://www.aquamet.com/aquamet22/

17/4 is 17% Chrome, 4% Nickel. it is a age hardening stainless steel. you can read about Aquamet 17 here:

http://www.aquamet.com/aquamet17/