Mating mild steel and stainless rudder post flanges

I have a 316L upper rudder post and need to mate it to a mild steel rudder. Is it best to have two different flanges above and below with isolation between them (that would worry me - plastic sleeves and rudder forces?!), or simply weld a stainless flange on top of the mild steel rudder and have the same mating flange materials directly bolted to each other?

 

Galvanic potential between the two metals can put the carbon steel at significant risk for aggressive corrosion.

At my power plant we face this all of the time

We use non-conductive gaskets, non-conductive shims under washers, and non-conductive sleeves on the fasteners. The goal is absolutely no metal to metal contact. We do a check after assembly the ensure very high electrical resistance between the two metals.

Note that good sheet gasket material (synthetic fibers with buna/nitrile/epdm/etc binder) can withstand huge amounts of compression, is a good electrical insulator and is very stable. I would use either 1/16" or 1/32" thick material. This stuff would be much better than plastic shim material. If you do not have access to pre-made sleeves, you can even use this stuff to wrap around the fasteners before sliding them into the slightly over-sized holes that you will need to use. Just make sure the material is butted tight along the lengthwise slit and is long enough. Of course for the sleeves, 2 liter drink bottle or milk jug plastic would also work ok since the sleeve is not heavily loaded like the flat surfaces.

 

I would make it all stainless or if that is not possible consider ss/fiberglass/epoxy hybrid.

 

Ray I'd build the rudder from steel and the rudder shaft and upper coupling from SS in this case. Potential Corrosion rates are firstly ratios of areas and secondly altered by cathodic protection.
providing everything in contact with the water is painted properly with epoxy and has an anode kept up it will make very little difference.

Be aware that 316L is weaker than higher carbon 316 in shear and tensile and has no better corrosion resistance. If you do use 'L' then make sure you use the correct figures for your shaft design for the allowable stress.


[Added]
For good measure use a compound that excludes water from the mating surfaces of the coupling and the bolt holes. Something like 3M 5200 is good but waterproof grease is ok too.

 

A little stainless 101:

316 and 316L are better for corrosion resistance than 304 / 304L. The 316s were supposed to be good enough for salt water exposure, and initially they we pretty good. However, modern foundries have gotten more precise in their additives, and todays batches tend to have less moly than older 316s. The moly is an expensive additive, but it happens to be a key ingredient that helps avoid pitting in salt water applications.

So to save a few bucks, modern foundries are turning out the "salt water proof" stainless that is not reliable for a prime original intended application. Higher moly alloys are available, but typically at much extra $$$$$.

As far as the low carbon versions, these are typically more expensive and generally considered better (especially when being welded).

Finish (cold worked, hot worked, annealed, etc) is what makes a big difference for strength. All four (304, 304L, 316L, 316L) are generally soft with a yield of 30 ksi (yield is more important in most applications, it is where things start to bend) unless "strain hardened". These steels strain harden quite a bit and yield of up to 85 ksi or so is available.

I personally do not like the strain hardened versions much as they tend to gall very badly and they go from bent to broke pretty quick.

 

Actually the low carbon 316 L is around 17% weaker (yield) than 316. ( 316L has a yield strength of 25ksi compared with 316 at 30 ksi).

If the design is for 316 never substitute 316L and always check that fabricators don't use 316L as a substitute believing it is actually a better material.

I've seen a number of specifications where the shafts were labelled 316/316L and they are quite different. You'll even find stainless steel suppliers putting 316/316L on a single specification sheet with the values for 316 only.

 

MJ is correct for 316 /316L plate made to ASTM A240 and most pipe (A312 for example). These ratings are pretty close to what you see for the softest, annealled, versions if multiple forms are available.

AK Steel however, advertises their plate with a 42 ksi yield (probably more than a little cold work).

I found a couple of vendors selling "dual certified" 316/316L bar at 60 ksi yield (probably "cold rolled").

Again, the yield is highly dependent on the finish / form (cold worked, hot worked, annealed).

If you are calculating stresses, you really do need to know what spec the material was made to. For some applications we even require Certified Material Test Reports (CMTRs). Usually when we get the CMTRs, it shows the material exceeds the minimum by a good margin.