Crevice Corrosion - Is My Rudder Toast ?

[delmarrey]

Originally Posted by afmstm

If you have stray current in the marina, the worst thing you can do is bond all the immersed metal bits. What happens is the stray current flows into one of the metal bits, through the bonding and exits another metal bit. The metal bit where the current flows out will be eaten by electrolysis. See here for a good, intro article: Grounding.
You should consider de-bonding any metal through hulls that do not need to be electrically connected to anything else. Since your rudder post was not bonded, that implies stray current corrosion was not the culprit. Could it have been bonded in the past?

Stainless steel is unfortunately self-corroding in anaerobic sea water. You should have a dedicated zinc for the rudder, either on the shaft, or failing that in a nearby zinc mounted in the hull and connected to the shaft by a tinned braid or shaft brush. This zinc should not be connected to anything else so you reduce the chances of stray current corrosion....

Regarding crevice corrosion. Your photo showing the band of pitting is pretty scary. I would do more than take a file to it - take a dremel tool with a pointed grinder, and just like a dentist going after a cavity, dig into some of the holes. You have to determine how deep they go. Just like a dentist, your goal is to get to the bottom of the cavity and clean it out. It's OK to remove material -after all, if it's pitted it's not adding much strength, and also, it's unlikely you are removing a large percentage using a dremel tool. I have done this on chainplates and found some holes penetrate several mm.

If the band of pitting does indeed extend deep into the metal, you can calculate how much strength you have lost. Basically, you need to measure the depth of the pitting, and use that to calculate the % loss in cross sectional area. Strength varies more or less with area (strictly speaking, it's also related to the distance from the center of rotation, but just ignore that for now).

Getting a welder to look at it seems like a good idea, although of course welded metal is often more corrosion prone.

The concern I would have on my own rudder shaft is that crevice corrosion has eaten away the metal inside the rudder where you can't see it....

Good luck....

I would agree with this 100%! But welding I would not.
1) It'll burn the fiber glass.
2) It'll actually create a weak area in the tube, unless it were heat treated afterwords, which you can't do being attached to the rudder FG.

If it were solid, it would weight a ton (not quite)

Since it's submerged I wonder if the water has gone down into the rudder itself.

[Eleven]

Don't weld. The weld is never quite the same material as the component, no matter how well they are matched. Usually the weld will migrate to the parent material. The Only way to slow it down is ZIncs, (which give up their material to anything lower in the electrolytic food chain).
My rudder fabrication was seriously damaged because the zincs weren't there on purchase, and I didn't know any better. I lost one rudder in f6. Cats are lucky that it often takes more than one thing to go wrong.

[Cheechako]

Yeah.... I dont think I would weld it either. You need to see how deep it is. If not too deep maybe epoxy coat it after getting rid of all the pourous material. Is this a spade rudder or supported rudder?
I have seen a prop shaft that completely corroded away to nothing in less than a year in a hot marina. Was part of a marine corrosion seminar I took years ago.
Bonding is something that seems to be a black art and has as many opinions pro and con as anchors! However, if the current scheme is allowing that much corrosion, I would try bonding.... why not!

[Matt Johnson]

The corrosion was from a different marina then where I'm at (at least two years ago). I was thinking that I'm going to grind it down this weekend and see how far I need to dig before I hit good metal. If it's not much, I'll polish it out and apply a zinc to the shaft.

[delmarrey]

I am really surprised that the boat was built this way w/o a zinc already setup.

"If it were mine", I'd attach a zinc on the transom below the water line with silicon bronze thru bolts into the inside and then attach a wire inside to the bolts and then perminetly to the rudder shaft allowing enough for swing and motion so it wouldn't brake. Much like you see on powerboats.

Bonding with the other metals throws off the system. In order for bonding to work properly the metals all have to be similar in size and type. Or you would need an expensive cathodic monitoring system.

[barnakiel]

From my limited experience with SS, I would neither weld not epoxy it. If it is rod then provided the damage is superficial only, I would grind, polish, passivise and leave bare.

If it is a tube (I doubt it) then I would replace the shaft/rudder and build a new one with full/rod shaft in SS, bronze(?) or perhaps monel. Naming these metals since I know propeller shafts can be utilised.

Welding can weaken the thing, covering in epoxy may lead to oxygen starvation. IMHO.

b.

[delmarrey]

Originally Posted by barnakiel

From my limited experience with SS, I would neither weld not epoxy it. If it is rod then provided the damage is superficial only, I would grind, polish, passivise and leave bare.

If it is a tube (I doubt it) then I would replace the shaft/rudder and build a new one with full/rod shaft in SS, bronze(?) or perhaps monel. Naming these metals since I know propeller shafts can be utilised.

Welding can weaken the thing, covering in epoxy may lead to oxygen starvation. IMHO.

b.

FYI 2-7/8" solid round SS is 22.09 # per ft.

[barnakiel]

Originally Posted by delmarrey

FYI 2-7/8" solid round SS is 22.09 # per ft.

Price no issue with things like rudder stock, keel bolts, chainplates, thru-hulls, etc. (?)

Sniffing around scrap / boat/yards may result in finding a top quality prop shaft that can easily be converted into something else.

At least this is my attitude to the bare basics.

b.

[delmarrey]

Originally Posted by barnakiel

Price no issue with things like rudder stock, keel bolts, chainplates, thru-hulls, etc. (?)

Sniffing around scrap / boat/yards may result in finding a top quality prop shaft that can easily be converted into something else.

At least this is my attitude to the bare basics.

b.

Sorry! That's not price, that's weight per foot, in pounds or about 30 kg per meter.

[barnakiel]

Originally Posted by delmarrey

Sorry! That's not price, that's weight per foot, in pounds or about 30 kg per meter.

Options?

b.

[svholdfast]

FunJohnson, I had to dig my Metal Corrosion in Boats book out after looking at your photos. It appears you have pitting corrosion of stainless steel the grain structure is under attack and has been wakened unfortunate there is no easy fix the stock should be replaced.
sorry,
Hold Fast

I would recommend Aquamet 22.

[delmarrey]

Originally Posted by barnakiel

Options?

b.

SS schedule 80 pipe is used quite often for rudders. I did do a solid 4" Titainium rudder shaft once for a racer. Quite expensive!

Mine is a 3-1/2" SS pipe now 30 years old. I had it out a few years ago and did a Penetrant Testing and found nothing. Not bad after years of racing.

A solid shaft is a bit over kill in weight for sailboats. Power boat rudders take a lot more stress.


.

[barnakiel]

Originally Posted by delmarrey

SS schedule 80 pipe (...)

OK, I take it schedule 80 stands for some sort of super-super-super thick wall, extruded, not welded then?

I grew up where rudder posts were bronze, then SS, always rod, never pipe. Someone told me monel is OK too, but that's an opinion.

In fact pipe will have some benefits - like being able to use oversize diameters and big bearings thus limiting loads.

b.

[delmarrey]

Originally Posted by barnakiel

OK, I take it schedule 80 stands for some sort of super-super-super thick wall, extruded, not welded then?

I grew up where rudder posts were bronze, then SS, always rod, never pipe. Someone told me monel is OK too, but that's an opinion.

In fact pipe will have some benefits - like being able to use oversize diameters and big bearings thus limiting loads.

b.

Pipe comes in sizes with different wall thicknesses. The pipe size is based on the inside diameter of standard (schedule 40) pipe. The common thick wall pipe is schd 80, so it's EZ to get and works great if in SS for marine rudders. Schd 120 is even better but hard to get as well as expensive.

Race boats do like to keep their weight down. I've even built Titanium anchors for some racers just so they can qualify. A bit useless though.

BTW Monel® would be good but it's spendy too.

One more note: Mechanical tubing is different then pipe. It's measured on the outside with the wall thickness as secondary and can be either welded seam or full extrusion. It's also fairly round unlike pipe which is a bit egg shaped, especially in the lighter schedules.

And rudder pipes are usually filled with a filler of some sort. to keep the water out.
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[barnakiel]

Originally Posted by delmarrey

Pipe comes in sizes with different wall thicknesses. (...).

THX for explaining the matters. I will use a thickwalled pipe then next time our windvane shaft breaks (oh yes, it will).

I remember Najad started using aluminum for stock some time ago, I have also seen carbon rudder stock somewhere on the web.

Reading your post I thought if Titanium would do(?) Maybe a titanium pipe , thick-wall (if such a thing exists) would be both light and strong?

b.