AISI 316 stainless steel belongs to the iron alloys classification, while Monel K-500 belongs to the nickel alloys.

In a galvanic couple, the metal higher in the series (or the smaller) represents the anode, and will corrode preferentially in the environment. The metal with the lower number will act as an anode and will corrode preferentially.

SS 316L is in number 53 in the table

MONEL 400 is in number 63 in the table

In principle 316L will be more active or anodic than MONEL K-500

luis

What is the electrolyte between them? If this is dry then there can not be corrosion. If the 316 sensor passes through a K-500 (is it aged hardened?) fitting and that is getting splashed frequently with something other than just clean water then you may have an issue.
If it is an environment where you might develop crevice corrosion in 316 against 316, then the couple will accelerate the corrosion.

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P.E. Metallurgy, consulting work welcomed

It is dry inside a thermowell, so no chance of galvanic corrosion. However, metal dusting, etc. are quite common external corrosion of thermowells.

DHURJATI SEN

Where is this thermowell installed? If there is no electrolyte between them no galvanic corrosion will occur as EdStainless and Dhurjati Sen both said, by the other side if for instance from K-500 side if we are in the presence of a flue gas, we should think in another type of corrosion other than galvanic.

luis

Quote (Dhurjati Sen)

However, metal dusting, etc. are quite common external corrosion of thermowells.

I’m not overly familiar with K500 but given it’s mechanical properties and ASME IID listing I don’t see how it could be used in a system where metal dusting would be of any concern.

Huub
- You never get what you expect, you only get what you inspect.