Rudder Stock Crevice Corrosion

[Cruisin Cat]

Just dropped the rudders on my 3 yo cat and found severe corrosion of the stocks inside the lower submerged bearings....photo below.

Boatyard says it probably due to the quality of the stainless steel used but manufacturer says it's due to stray electric currents while in marinas.

Anyone had experience with this sort of corrosion?

[hellosailor]

I'm no expert but that looks more like galvanic than crevice corrosion. Could be the rudder stocks were made of pot metal, but if it is a galvanic issue, you should be able to test for stray currents to confirm/refute that.

When I say pot metal, there was an incident maybe 25 years ago where some flatware turned up in the US, highly radioactive. Apparently the source for the stainless included a cesium supply from a hospital medical device, conveniently thrown in the recycling bin with the rest of the "metal boxes". So these days? With the price of stainless?

Beneteau may be the sane ones, using newfangled and untrusted [sic] carbon fiber instead of stainless, to eliminate those questions.

Zanshin had a rudder stock fail and IIRC spent some time looking into this. He might have some idea of what bit you.

[fstbttms]

Originally Posted by Cruisin Cat

Just dropped the rudders on my 3 yo cat and found severe corrosion of the stocks inside the lower submerged bearings....photo below.

Boatyard says it probably due to the quality of the stainless steel used but manufacturer says it's due to stray electric currents while in marinas.

Anyone had experience with this sort of corrosion?

Of course the manufacturer will pass the blame along. But here's the thing- crevice corrosion and electrolytic corrosion are not related. The manufacturer is either ignorant or blowing smoke up your transom. And my bet is it's the latter.

[skipmac]

At first glance the photo looks like crevice corrosion if that is the part of the rudder post that was up inside the boat. A better or different grade of stainless might be less susceptible to crevice corrosion but not immune, it's just the nature of the metal. Put stainless in an oxygen deprived environment and it will lose the protective oxide layer and corrode.

[model 10]

Looks like a quality of material problem. Like 304 stainless instead of 316, but in 3 years? Wow! It looks like 20 year old stuff. It's pretty hard to bond a rudder stock to ground so perhaps both sides are right.

[Stumble]

Cruisin,

You really need to have an expert in corrosion, or metallurgy look at that. I see both types of corrosion regularly, but can't really tell which is more likely. A lot depends on exacally where it is relative to the boat, and exacally what happened to it. One of the major problems with stainless underwater is that if the stock breaks the surface of the water the stainless can actually become galvanically active versus itself.

[bglad]

I vote crevice corrosion caused by poor choice of construction material. Most examples of galvanic corrosion I see affect the whole part not just the section contained in a tight space where the water turns anoxic. I have seen the same thing on propeller shafts used by a US production builder who opted to install Aquamet 17 versus Aquamet 22. The shafts looked good except where you couldn't see them...

[Cruisin Cat]

Thanks everyone for your comments on this. The boatyard agrees the cause is most likely poor quality stainless used by the manufacturer. They're replacing the stocks with good quality stainless (can't remember the designation of the ss just now). I'll know in a couple of years if this fixes the problem

[Andrew Troup]

You say the corrosion took place "inside the lower submerged bearings"

If the surface of these 'bearings' contacting the shafts is non-conductive (eg non-metallic nor containing significant amounts of, say, a carbon compound), and given the strictly localised nature of that corrosion to that contact zone, there's virtually zero chance that the manufacturer is correct to blame stray currents, and I'd say it puts a definite questionmark against their expertise and/or integrity.

[Cruisin Cat]

Just found the info on the stainless that the boatyard is using to make my new rudder stocks...they said it's 2205.

[Cruisin Cat]

Originally Posted by Andrew Troup

You say the corrosion took place "inside the lower submerged bearings"

If the surface of these 'bearings' contacting the shafts is non-conductive (eg non-metallic nor containing significant amounts of, say, a carbon compound), and given the strictly localised nature of that corrosion to that contact zone, there's virtually zero chance that the manufacturer is correct to blame stray currents, and I'd say it puts a definite questionmark against their expertise and/or integrity.

Yes the corrosion is inside the lower stock bearing/sleeve which is made of non conducting material. Certainly points to a material quality problem.

[Andrew Troup]

Originally Posted by Cruisin Cat

Just found the info on the stainless that the boatyard is using to make my new rudder stocks...they said it's 2205.

That's good.

Are they replacing the bearing inners as well?

You wouldn't want to get contamination corrosion from the residues: at the very least they would want to be very thoroughly cleaned and scraped.

[Nicholson58]

Originally Posted by skipmac

At first glance the photo looks like crevice corrosion if that is the part of the rudder post that was up inside the boat. A better or different grade of stainless might be less susceptible to crevice corrosion but not immune, it's just the nature of the metal. Put stainless in an oxygen deprived environment and it will lose the protective oxide layer and corrode.

SPOT ON.

Oxygen depleted zone. Same thing happens in the stuffing box of the prop shaft. Here is mine after 28 years. I had it built up with plasma sprayed SS weld & then turned it to size in the shop. Oxygen depletion also causes the attack on your chain plates just below deck level. I replaced those with Titanium.

better grade of SS will help.

[Cruisin Cat]

Originally Posted by Andrew Troup

That's good.

Are they replacing the bearing inners as well?

You wouldn't want to get contamination corrosion from the residues: at the very least they would want to be very thoroughly cleaned and scraped.

Good point about replacing/cleaning the bearings to stop contamination from the residue of the old ss, I hadn't thought about that.
The boatyard said that apart from using the 2205 ss, they will "upgrade the size slightly". I guess that means a larger diameter stock and hence new or bored out bearings...but I will check.

[Weyalan]

Originally Posted by Cruisin Cat

Just found the info on the stainless that the boatyard is using to make my new rudder stocks...they said it's 2205.

This is a great choice. The cost of these sort of items (rudder stocks, shafts, etc) is mostly in the machining / labor, so specifying a top of the range duplex rather than 316 is, IMHO, well worth the few extral dollars in material costs.