new stainless rudder gudgeon / metal ??

[Northeaster]

I have a question of mixing metals. I am aware of the problem of galvanic corrosion, when mixing some metals. So, when having a new rudder gudgeon made, I asked for 316 stainless, as the rudder post/pintle is stainless. However, and a retired machinist, who has 20 years in various boat building / propeller type work, advised me that it is not a good idea to use the same type of metal, in applications where they come in contact - such as teh pintle / gudgeon. He suggested putting a bronze bearing inside the tube part of the gudgeon, so the stainless rudder contacts the bronze, instead of stainless.

[capngregg]

I'm not a metallurgist but dissimilar metals that are mated together are the ones that suffer the most from galvanic corrosion which is aggravated by the introduction of a reactive substance such as saltwater.

Stainless on stainless isn't bad. Stainless on Bronze is not too bad either. Real dissimilar metal corrosion happens when you put aluminum with steel and so on. The molecular composition of the metals (remember that proton-neutron, positive-negative thing from chemistry class) and their "potential difference" in electrical terms is what causes galvanic corrosion with regard to dissimilar metals.

That being said, bronze bushings with stainless pintles would probably wear very nicely. (good match)

[camaraderie]

Ditto that...stainless to stainless is the way to go. What reason did he give you for suggesting otherwise? Look at the pintle/gudgeon sets sold commercially...not done his way.

[Faster]

For low temp/low friction assemblies like the gudgeon/pintles on a rudder, SS on SS should be just fine. Stainless is particularly bad for galling if forced together at high friction, and while that may be where your machinist was coming from, it shouldn't be the case here.

A quieter installation would be nylon or similar bushing in the SS gudgeon, but that is more custom, and would actually wear a little faster. Also some nylons/other plastics, when submerged, absorb water and expand, creating tight sleeves and extra friction.

Titanium is always nice... light weight, strong, corrosion resistant (grinny thingy here)

[Northeaster]

I think that he felt that stainless on stainless had more of a chance to sieze together, over time, in salt water. I think his experience and judgement was probably based on fittings that are solidly bolted together (ie nuts/bolts, etc). He said, that in his experience, which does not involve sailboats, that was kind of a well known rule -- who knows!!

Since my brother called tonight, and has already fabricated the new piece, out of stainless (as he had not yet read my latest email requesting a bronze bushing) - it looks like stainless it is!!!

[sailingdog]

Stainless on stainless is definitely the way to go...but it would be best if they're both the same grade of stainless. 316 is better than 304, which is more prone to chloride stress cracking in a marine environment.

Titanium is a much better metal, but a bitch to work with and much more expensive...

The idea of using a nylon, delrin, or other plastic bushing is nice, but generally not necessary.

[eherlihy]

Interesting problem... I've been (slowly) reading Nigel Calder's book (as some of you know) and he has an excellent chapter on Corrosion, Bonding and Lightning protection (good lord, did I really write that? - I gotta get a life! )

The issue is that Stainless needs to oxidize in order to resist corrosion. "If stainless steel is used in an area where stagnant water can collect, sooner or later it's passivity will break down, and it will become active (normally in isolated pinholes and crevices) and it will begin to 'feed' on itself, just as the copper in a brass screw feeds on the zinc." [emphasis is Calder's] I would suspect that the microscopic nooks and crannies created by creating a pintle/grudgeon assembly from flat stock and rods could create such an environment. Bronze, depending upon the alloy, would actually be less succeptable to corrosion in this environment based on the table on P199 of the book.

Were this my boat, I would try to tie both the rudder grudgeon and pintle assembly to a zinc.