Dear freinds for repairing a highl

[mimi1017]

Dear freinds
for repairing a highly corroded impeller made from duplex stainless steel which is working in a sea water pump i suggest to use IN625 or Hastalloy C276 overlay for rebulding and restoring blades. The impeller blades has crevice corrossion and lost thickness during many years of working. I am not sure about the overlaying procedure and corrosion resistance of overlay over duplex SS.any comments about this or alternative solutions. We already considered using ceramic paste repairing.

 

[weldstan]

Duplex SS has been weld overlayed with both materials as stated. Whether you should do this or remake the impellers from a more corrosion resistant alloy is a question only you can make.

 

[EdStainless]

I would so with a temporary repair and order new impeller from C276.
Welding over an alloy that is having corrosion issues if often a short term fix.
Sooner or later corrosion will undercut the overlay.

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P.E. Metallurgy, Plymouth Tube

 

[mimi1017]

@ weldstan thanks. is there any consideration about weld overlay? some general guidlines used for duplex SS welding also should be used here?

@edstainless the pump is an old version. it works for years and now the owner does not like to disassemble the whole shaft and impeller. i think using IN625 or C276 it can work for another 10 years. they have already repaired with ceramic paste and it works for another 4 years. however i think welding is much better than polymer coating. the only concern is about welding procedure and HAZ corossion in welding overlay.

 

[weldstan]

If you weld overlay won't you need to machine afterwards? After machining or grinding, will there be exposed duplex because of too thin an overlay? If so reconsider the ceramic application, at least it lasted 4 years.

 

[cap1a79]

At first, are you sure that the type of corrosion for the impeller is crevice? Crevice corrosion happens in stagnant zone at the gas between two components. I believe the applicable damage mechanism will be erosion or cavitation. Anyway...refer to the below..

Referring to clause 6.12.2.3 of API 610, it is as below. (Note that DSS is ferrous material.)

"6.12.2.3 Ferrous pressure boundary and impeller castings shall not be repaired by welding, peening, plugging, burning in or impregnating, except as allowed by the following.
a) Weldable grades of steel castings may be repaired by welding in accordance with 6.12.3. Weld repairs shall
be inspected according to the same quality standard used to inspect the casting."

Unless you are ready to comply with 6.12.3 which requires full documents of welding even for the partial repair, you had better forget the option of weld but consider ceramic coating and replace it periodically as weldstan recommended. By the way, generally, Ni-al bronze (ex., C95800) has much higher corrosion resistance in sea water and much preferred while super DSS (Not DSS) is used only up to 30 C in sea water service based on some major oil & gas standards.





Lee SiHyoung,

 

[mimi1017]

@cap1a79 the pump is vertical one. the api 610 is about centrifugal pumps. I am not sure if it can be applied to this standard. However as you and weldstan mentioned it is important the duplex SS does not exposed after welding overlay. We have all range of inspection available. Moreover we try to do welding using pulsed cold TIG welding to lower HAZ . However i will consider ceramic coating. I just looked at welding procedure of duplex ss the interpass tempreture and heat input have some limitations. We will consider it.

 

[cap1a79]

If the cause of the damage is due to erosion(Not crevice), have you considered potable HVOF gun to form tungsten carbide or chromium carbide coating on DSS?

Lee SiHyoung,

 

[mimi1017]

Hi i think it is cavitation. excuse me for misleading. about HVOF most of the time i doubt about proper bonding to the tips of blades. I think it is not a good surface and substrate for proper bonding of HVOF. you can see the corossion and erosion on the surface in this picture. the blades are big and tips are destroyed .which needs also buildup/

 

[cap1a79]

1) Cavitation - If the cause of damage mechanism is cavitation, the most powerful prevention method is to control NPSH. Material selection to prevent cavitation cannot be a significant solution although it can mitigate a little bit. (Refer to API 571 clause 4.2.15.)

2) Erosion - If the cause of damage mechanism is erosion, you need to check velocity guideline & the presence of solid particle. (Refer to API 571 clause 4.2.15 as well.)

3) Sea water corrosion - Generally, in sea water service, the use of DSS is challengeable nowadays. Refer to API 938-C clause 5.2 & 5.3. For new pump, I recommend Ni-Al bronze (C95800) usually.

4) As the basic material design is quite challengeable now, you may need to upgrade the material to replace the DSS if you want permanent solution.


Lee SiHyoung,

 

If it comes to overlay with Inconel/Hastelloy (really the same family with different trade names) then Hastelloy C-22 filler is the preferred option. Exposed DSS adjacent to more noble weld deposits are unavoidable, giving galvanic impetus to corrosion process(es).

I recommend you have a root cause failure analysis done before deciding on a fix. I notice ripples at upper left in the picture, and none of the frosted surface characteristic of cavitation erosion (but have the surfaces already been ground?)

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

 

[EdStainless]

Cap, I also like NAB in seawater service, but if this failure is from erosion or cavitation then NAB would have a much shorter life since it is a much softer material.

Mimi,
You do need a real failure analysis done.
If it is cavitation then there are likely some operation issues that must be addressed.
If it is erosion then changing alloys will not help, Your 2205 (I am guessing that it the case version) is one of the most erosion resistant alloys out there.
If this is erosion/corrosion then you an alloy with higher corrosion resistance and higher strength. Something like 625 comes to mind. Though the best bet might be another stainless (no need for duplex) unit with weld overlay of a Co based alloy.

Whatever temp repair is done they need to start working on the new impeller. They have no choice about this.

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P.E. Metallurgy, Plymouth Tube

 

[weldstan]

Has a definite look of erosion corrosion. Whereas you stated that the pump has given many years of service, have operating parameters changed over time, like a few years prior to the initial ceramic applied repair?

 

[cap1a79]

Dear Ed,
Thanks for the comment. Erosion issue for NAB (C95800) can be covered by operating guideline as below 22.9 m/s and cavitation can be controlled by correct NPSH for NAB.Please note that I mentioned NAB(C95800) as starting consideration for sea water corrosion, rather than comparing it with DSS under single erosion or cavitaion aspect.)

1. Cavitation issue for NAB (C95800) - Control NPSH

2. Erosion issue for NAB - follow up velocity guideline keeping no solid particles. The maximum velocity is o.k. up to 22.9 m/s based on the below reference. Also, the use of NAB(C95800) for pump is a standard for one of the major oil & producing companies as demonstrated by operation practice.

https://www.copper.org/applications/marine/seawater/seawater_corrosion.html

Lee SiHyoung,

 

[mimi1017]

Dear friends
I look at the other data. it seems the failure mechanism is erosion corrosion caused by sand and glass type particles available in sea water in this region. by combining this with highly agitated water in the surface of blades you can see many scratches in the root of blades. the black areas in photo are remaining ceramic polymer after these years after repair. so i think NAB is not the case.
@welstan i think there is no change in operation. the same case happens for some other pumps in this area like this type of erosion corrosion.
After this my concern about using Hastalloy C276 for overlay and buildup the impeller blades is corrosion resistance of weld and HAZ. Galvanic corrosion of HAZ and duplex SS versus the Nickel based overlay alloy. furthermore the base material is not exactly 2205. it is older version from EBARA pump japanese company. SCS 11 which has lower Mo and N compare to the 2205.
i am thinking about doing some corossion test for dissimilar weld of this repair. maybe Galvanic corrosion test. what do you think?
@ ironic metallurgist about C22 we don't have it now. we can use hastalloy C276. I think this is better In625. because IN625 has high Fe in dilution by base metal and make the corrosion inferior.

 

[mimi1017]

@Cap1a79 Beside erosion corrosion the cavitation also happens in edges of blades.

 

[cap1a79]

As you have mentioned that sand and glass are present with some scratches and cavitation, you may start with reviewing modification of process condition at first (ex., sand removal facility & velocity checkup, adjusting NPSH) before material selection, unless you can afford to change the materials for all the upstream & downstream which are exposed to same condition.

Lee SiHyoung,

 

Solid particles like sand will cause abrasive wear.
Erosion corrosion is strictly a fluid problem, although there could be more than one thing going on here.

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

 

[mimi1017]

About this erosion corossion and cavitation in impeller what is the best material for rebuilding the impeller? In625 , Hastalloy C22 , C276
i notice becase the base metal has PREN<40 it would be a galvanic corossion cell when using the Ni base layers over duplex stainless steel. the base metal is old version even PREN is lower than 2205. so my concern is how to evaluate the weld and HAZ for galvanic and erosion ?

 

[weldstan]

If this is primarily an abrasive problem and it well may be and you are unable to remove the sand/abrasives, hard surfacing is your most effective repair remedy. Unfortunately refiguring the impellers may prove difficult. Again your previous ceramic repair and maybe changing ceramic used might be your best repair scenario.