17-4 ph or F6NM stem in a 316 valve body

[Johnwoo1991]

Is it wrong to use a 17-4 PH (or r any other martensitic stainless steel) stem in a valve made entirely of AISI 316? Perhaps for reasons of corrosion resistance and galvanic corrosion? Leaving aside the fact that 17-4 PH is much superior in terms of mechanical resistance.

 

[TugboatEng]

Pumps are closer to my wheelhouse so I'll use that term but valves are similar.

17-4 PH has significantly higher corrosion resistance than other martensitic grades. It's commonly used as a shaft material in otherwise 316ss trimmed pumps that I work with.

In saltwater environments the 17-4 shaft holds up well in bronze bodies. We don't use 316 stainless pumps for salt water service.

I often see 400 series stainless steel valve stems in iron/steel valves severely pitted after salt spray exposure.

 

[Johnwoo1991]

Pumps are closer to my wheelhouse so I'll use that term but valves are similar.

17-4 PH has significantly higher corrosion resistance than other martensitic grades. It's commonly used as a shaft material in otherwise 316ss trimmed pumps that I work with.

In saltwater environments the 17-4 shaft holds up well in bronze bodies. We don't use 316 stainless pumps for salt water service.

I often see 400 series stainless steel valve stems in iron/steel valves severely pitted after salt spray exposure.

Thanks, do you think that using a SS400 stem in a 316 valve body could lead to galvanic corrosion, moreover?

 

[TugboatEng]

Is the fluid conductive? If so, yes there will be a galvanic corrosion issue.

You need to look at the ratios of surface area exposed. A little bit of 316 and a lot of 400 series is ok. A little 400 and a lot of 316 will result in rapid loss of the 400 series material if the fluid is conductive.

In a valve, if dissimilar metals are used, the stem is one item that needs to be more noble.

Graphite packing may also cause corrosion issues. Look for non-graphite type packings if your fluid is conductive.

 

[Danlap]

In a NACE environment, it is not recommended to use 17-4PH. This argument of course open for multiple interpretation or debate since NACE doesn't forbid specific material and only regulates maximum hardness of 22 HRC.
Have experience broken 17-4PH several times on gate valve subject for high load (relatively high torque to close usually equipped with hydraulic actuator) in addition to sulfide stress cracking and also hydrogen embrittlement.

this is the one that 'connecting' spindle with the wedge.

for smaller hand operated gate valve and ball valves, it is still quite common to use 17-4PH.
for particular case as picture, we've replaced with Nitronic 50.

it is really depending on what the application is

Regards,
D