My application is a salt water environment with near stagnant seawater. The application requires inconel 625 studs. Temperature and duration of the environment is unknown.

Currently the designed washer is SS 316L with the hex jam nut undetermined. According to my knowledge and the galvanic chart, the voltage range of the two is below

Inconel 625: +.10 to -.04
300 Series Stainless Steels: -.00 to -.15

Worst case, the difference is .25. I've been told that not going above .15 is generally the rule of thumb.

First question is does anyone have an idea of the amount of time that the washer and possibly the nut if the nut is SS 316L would corrode?

Second question is if my only options for the nut are either inconel or SS, which one would you use?

Third question is how does anyone feel about the use of a monel washer and/or nut?

Also note that the preferred answer (my knowledge/opinion) is to have the washer design changed to inconel and use inconel nuts.

Thanks for the help.

If it were me, I would use no 316. For any parts.
625 and K-500 would both be suitable and they are very similar in galvanic potential.

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Plymouth Tube

316/316L is not suitable for long time seawater service.

625 is great, could be a little more than needed.

The 6% molybdenum stainless steels, such as AL-6XN (UNS N08367) have given good long-term seawater service and are less expensive than is 625 (UNS N06625) They are available from distributor stocks in sheet, plate, bar, pipe and pipe fittings. Threaded fasteners are commonly made to order. Welding is usually done with 625 bare wire (N06625) or 112 covered electrodes (W86112). The best weld filler choice is, in my opinion, Inco-Weld 686CPT (N06686). That would be for 625, AL-6XN or, for that matter, Hastelloy C-276 base metal.

The limiting factor will be crevice corrosion where there is metal-to-metal contact.

I would be leary of mixing either 625 or AL-6XN with any copper alloy. The copper alloy may corrode preferentially. Monel, regardless, is not suited for applications near land where the seawater is contaminated with sewage. That is, contaminated with sulphur compounts.

Hmmmmm.....

I always thought it was the ammonia compounds in sewage tainted water that were the evil doers for copper alloys - not that sulphur compounds are ever forgiving to CU.

rmw

Ammonia may crack the Cu alloy but sulfer from sewage corrodes them. Copper alloy heat exchanger tubing not so good in rivers for this reason. The Susquehanna comes to mind.