Piping Fitting Leak / Corrosion issue...

[quizzical1]

Hi All,

This is piping in a hot water recirc loop. Any ideas on what would be causing the leaking and also the buildup? Been installed for 2 years.

 

[EdStainless]

The problem is that mixing caused issues with the brass, and then once the corrosion is going the weak 303 and cast SS materials join in.
This is a common issue that people have, trust me we see this with some frequency.

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P.E. Metallurgy, consulting work welcomed

 

[quizzical1]

Thanks ES,

Dumb question - will passivation or EP the SS help retard the condition?

 

[EdStainless]

Nope.
The corrosion potential between the SS and Cu alloys is large enough that it will drive the process.
As long as the water is electrically conductive you have formed a battery and the anode (the part that dissolves) is the Cu alloys.
Hot water systems can be difficult.
I have seen corrosion is all SS systems (not the tubes but 303 or cast parts) and pure Cu can suffer very high erosion rates.
When I have seen all SS it is usually all 316L with no cast parts in it.

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P.E. Metallurgy, consulting work welcomed

 

[quizzical1]

Thank you Ed

 

[quizzical1]

Last dumb question:

If we switch the 303 SS Body to brass / bronze there are internal components that are 303SS and a 17-4PH SS retaining ring holding the parts in.
We manufacture other brass valves using that retaining ring and a 302SS spring and never see any corrosion with water at any temperature.
Could we still use SS the internals?

 

[EdStainless]

If the total amount (wetted surface area) of the more noble metal (SS in this case) is small enough then you should not see any serious issues. There still could be some localized galvanic corrosion at contact points, but they would not leak and should function for a long time.
I would back off from using 303. I know it machines well, but its corrosion resistance is trash. There are other options.
I hope that your 17-4PH is aged at 1050F or 1100F and not some lower temperature.
And I am a little surprised that you have 'never' seen stress corrosion cracking failures in the 302 springs. But if there are other metals nearby that are less corrosion resistant they could be providing galvanic protection to the 303 and 302 parts.
There are many cases where equipment is build with mixed metallurgy, and the tricks are
1. making sure that the differences are not too great
2. respecting the area ratios
3. knowing which parts are likely to corrode and making sure that they are the thickest ones

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P.E. Metallurgy, consulting work welcomed

 

[quizzical1]

Thanks for your help, Ed!

What other options would you recommend to 303SS?
The retaining ring is actually 15-7 MO with a RH950 temper. I know that NACE MR0103 requires it be tempered back from 43 HRC to 30-32 HRC hardness when used in oilfield applications to prevent SSC. Would that benefit here also?
As for the 302SS springs - we've never had any failures nor actively looked for SCC so I can't say for sure...

 

[TugboatEng]

The 302 spring may be protected by the anodic effect of all of that 303 stainless around it. Upgrading to a more resistant stainless such as 304 may create conditions that do support cracking in the spring.


Edit: double check voice to text before posting.

 

[EdStainless]

If the ring and spring are working stick with them as is, but at least you know to keep an eye on them now.
Yes, if you ever saw issues with the ring going to 1050F ( at bit softer) would be a big help.
There aren't a lot of good spring options. As long as you are in Cu alloy bodies I would expect the galvanic difference to protect the 302 spring.
If you wanted to look at changes to the 303, there are a couple that I can suggest.
There are some 304 grades with enhanced machining that have reasonable corrosion resistance, the Outokumpu Prodec comes to mind as one. The other option is to look at lean duplex stainless steels (2101, 2202, and so on). Even though they are a lot stronger the dual phase structure actually makes them machine very well, for a stainless.

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P.E. Metallurgy, consulting work welcomed

 

[quizzical1]

Thank you!

I really appreciate you all taking the time to help.