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Feedwater heater tube-to-tubesheet weld corrosion

Feedwater heater tube-to-tubesheet weld corrosion

(OP)
Hello everybody,

maybe one of you can lend me your thoughts on a reoccurring damage in one of our feedwater heaters.
We found a significant amount of heavily corroded tube-to tubesheet welds (see picture).
The affected welds were found on the inlet to the tubes as well as on the outlet.
No damage was found in the tubes them-self or the tubesheet, so not the typical pattern of erosion-corrosion where you would expect the worst corrosion shortly behind the welds in the tubes.
We also performed DPI and radiography on the welds to rule out manufacturing defects or cracks, found nothing.
Process conditions are the following:
Tubeside: fully deionised Boiler feedwater, conductivity before cation filter = 6 microS/cm, conductivity after = 0.26 microS/cm => pH~9.3, , ammonia added for pH control, O2 around 5 ppb, but could also have been higher in times as there were problems with the deaerator (O2 content is not regularely monitored), no oxygen scavenger added
Velocity in tubes 1.5-1.6 m/S, temperature 170-230oC, pressure 170barg
Metallurgy: Tubes P235GH (mild carbon steel), tubesheet 15NiCuMoNb (high strength carbon steel), welds are TIG welded with C-0.5%Mo filler and PWHT at 580oC (all confirmed by PMI checks)

With the process conditions I don't think typically FAC would be a problem here.
The only thing I can think of is galvanic corrosion, where the weld is somehow anodic to the tubesheet/tubes.
What are your thoughts? Ever seen anything like this in this service before?

Thank you for your help!

RE: Feedwater heater tube-to-tubesheet weld corrosion

Is the attack of the welds, or is it the portion of tube that was heated in the weld process?
It looks to be an issue with the composition/structure where the weld and tube meet each other.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Feedwater heater tube-to-tubesheet weld corrosion

(OP)
Ed,

from the various stages of damage on different tubeholes I'd say its more like an attack on the weld metal, not only on the HAZ. In one location (sorry no photo) the weld was corroded through and leaked but no damage to the tube material itself.

Daniel Breyer
Inspection Engineer

http://lnkd.in/dFcQA-w

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