needle valve corrosion resistance

[litwin]

Hello!
Some needle valve manufacturers use 316 steel as the body material and 630 or 440C steel as the stem.
As a mechanic, I understand their choice (the manufacturer provides a difference in hardness between the body and the stem). However, what about corrosion resistance? I think steel 630 and 440C are worse than steel 316 in terms of corrosion resistance. Or is this a problem for the stem is not important?

 

[EdStainless]

By nature needle valves often see very high flow velocity and cavitation in the throat, so having higher strength stem materials is prudent. If corrosion resistance is a big issue then the 316 body is not good enough either and and upgrade of all parts would be in order.
Contrary to popular belief 316 does not have that great of corrosion resistance.

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P.E. Metallurgy

 

[moltenmetal]

Depends of course on what sort of corrosion caused by what agent. But absolutely, if the stem were to gall in the seat because the two materials were selected improperly, the valve's corrosion resistance would be a moot point as the thing wouldn't work long enough to find out whether or not it corroded.

There are other strategies used to handle the tribology in these valves- non rotating stem designs for instance, or soft seats or stem tips etc.. There are a host of different designs used, each with its own unique advantages and disadvantages. Some of them are specifically used because they make it easier to make all wetted parts the valves out of corrosion resistant materials.