rudder shaft corrosion

Troy Scott · Post by **Troy Scott** » Dec 15th, '09, 00:13

Folks,

I've been searching the archives for anything about corrosion of the stainless steel rudder shaft where it exits the top of the rudder. I haven't found the answers I need.

I have noticed some pitting on my rudder shaft just above where it exits the rudder. Other than that, the rudder system seems to be in good condition. I have removed the rudder stuffing box and I see no corrosion there. I believe the pitting is limited to just an area about .75" long, extending upward from the fiberglass. The pitting doesn't seem deep. I doubt the integrity of the rudder shaft is in question at this time. I doubt the shaft is damaged inside the rudder. My concern is to learn what caused this, and how to prevent any continued development of this corrosion. I know about crevice corrosion. I don't know for sure, but I suspect this is something a little different. I wonder if there is something else happening here. Should there be a zinc on this shaft? Could copper bottom paint have exacerbated this situation? Should I just blast it clean and later let the epoxy barrier coat cover the stainless steel in that area? Obviously just leaving it bare is not an option, as it would soon be encrusted with marine life.

Any information, comments and/or suggestions will be welcome.

Oswego John · Post by **Oswego John** » Dec 15th, '09, 02:02

Troy Scott writes:

Should there be a zinc on this shaft? Could copper bottom paint have exacerbated this situation? Should I just blast it clean and later let the epoxy barrier coat cover the stainless steel in that area? Obviously just leaving it bare is not an option, as it would soon be encrusted with marine life.

A zinc would be fine. I just don't know where you would fit it on the upper post.

Apparently, copper bottom type paint hasn't exacerbated the lower part of the rudder post, even with the mixture of SS and bronze at the gudgeon and pintle area.

If you don't think that the corrosion is structurally damaging, maybe the easiest, quickest solution is to clean the affected area well with an air powered needle scaler. Then fill the craters with a tig welder using a filler that is compatible with SS #304. Then lap the welded area with succeedingly finer degrees of emory cloth until it shines.

There are mixed feelings about covering SS with anything that prevents its exposure to the oxygen in air.

Good luck,

That should say emery cloth.

O J

tmsc · Post by **tmsc** » Dec 15th, '09, 13:20

Troy,

Is the rudder post tied into the bonding system? If it is, it should be protected by the prop shaft zinc.

Steve Laume · Post by **Steve Laume** » Dec 15th, '09, 17:26

I have never liked the perry nut zinc arrangement on Raven. If there was room for a shaft zinc that would be better. I have thought about mounting a zinc somewhere on the hull or rudder.

If you were to through bolt a pair of zincs to the rudder does anyone think a heavy wire taking several wraps around the rudder shaft and then being bolted to the zincs would be effective.

You could even embed the wire to reduce drag on the way to the zincs. The wrapped wire could be epoxied in place too. If you ground down the top edge of the rudder it would make a very clean installation with zincs that would stay in place and last much longer than that silly perry nut.

If solid copper wire was used would that create any problems?

There has got to be a better zinc arrangement. If you do protect the rudder shaft is that well connected to the rest of the boat? I am trying to remember that part, Steve.

Jim Davis · Post by **Jim Davis** » Dec 15th, '09, 18:00

To properly connect a hull zinc that would protect the shaft and prop you should use a shaft brush. http://www.boatelectric.com/shftbrBE.htm
I would also tie the zinc to the engine ground. For a rudder shaft you could also use a shaft brush or perhaps a length of stranded copper wire hose clamped to the rudder shaft. If wire is used it probably should be a frequent inspection point.

One alternative to the shaft brush on the drive shaft would be a jumper over the coupling and tieing to the engine ground. This may not be a real good idea as you would be relying on the oil coated gears in the transmission for electrical circuit to the zinc(s).

John Stone · Post by **John Stone** » Dec 17th, '09, 17:50

Troy,
My rudder has been down for two months and I have inspected it closely. I have the same pitting in the same place. I researched this thoroughly and though it could be galvanic corrosion I believe what you are seeing is "immersion pitting corrosion."

As you know SS requires a fresh supply of dissolved oxygen to the surface to provide protection. Under most conditions, all it requires is for water to move across the metal. But, in the stern-tube the water does not flow over that part of the rudder post because it is cut-off.

My primary source of information is Everett Collier's "The Boatowner's Guide to Corrosion," a very good book by the way. From Collier, "if we want to use stainless steel below the waterline, we must be sure that the surface of the metal is exposed to constantly flowing water . . ." He goes on to say "anywhere stagnate water accumulates--the protective coating will begin to break down. Over time, pinholes develop in the coating. These points of exposed metal begin to function as anodes to the cathodic surfaces around them. The result is pitting." He goes on to say this is very bad corrosion because it is concentrate in a small area and is often seen between shafts and bearing and in sterntubes.

SS is just not that good a metal for underwater use. It used to be that propeller shafts were made of bronze but they are almost all SS now. I suspect our rudder posts are also 304 but I am not certain of that. I think the best way to avoid it would be a bronze rudder post or at least a better grade of SS to include Aquamet 22--supposedly far superior to 316 though I am way outside my level of experience. Anyway, I am not prepared to spend that kind of money on the rudder post and the corrosion does not seem excessive to me. There is a lot of steel there. I'll let mine go and keep an eye on it.

Collier says there are three things we can do to reduce the likely hood of SS corrosion. 1) keep the water moving, 2) ensure good cathodic protection and 3), use high grade SS or some other more nobel material.

There are some other suggestions in the book about sandblasting and epoxy coating for SS in other applications. I think maybe sanding it smooth as you an and coating with bottom point is about as good as you can do. I am not sure of what else can be done. I can send you a picture the pitting I have if you are interested and want to compare it.

Good luck and let me know if you come up with another solution.

John

Troy Scott · Post by **Troy Scott** » Dec 17th, '09, 20:38

John,
the interesting thing is that the pitting is just above the fiberglass, but NOT in the space above that, in the stuffing box tube. It seems that there would be much better water flow over the space just above the fiberglass compared to higher, in the tube. This is why I was thinking that the pitting may have been associated with some metallic bottom paint. It would be easy for a well-meaning painter to coat that portion of the shaft. It would be nearly impossible to work the paint up into the stuffing box tube. I'm thinking that the most reasonable approach to arresting (or slowing) this pitting is to barrier coat that exposed area of the shaft before applying bottom paint, and to make absolutely certain to tie that metal to the other below-the-waterline metals. OR, I think I could probably install a zinc on the shaft above the rudder.

John Stone · Post by **John Stone** » Dec 18th, '09, 17:47

Troy,
I went out to the boat shed and checked my rudder. The pitted corrosion I have is 3-4 inches above the top of my rudder which puts it just inside the rudder tube assembly. Perhaps your rudder post corrosion is galvanic corrosion as you suggest.

Regards,
John

Evergreen · Post by **Evergreen** » Dec 19th, '09, 01:49

Three years ago we purchased a new rudder from Robinhood Marine for our then new to us CD36 - Evergreen.
At the end of each season we too have noticed some surface rust on the rudder shaft just above the fiberglass. I thought it was rather peculiar for a brand new rudder shaft. For lack of any other explanation, I assumed that the clean up after the fiberglass manufacturing process somehow compromised the integrity of the surface of the steel shaft. How we dealt with it: At the end of each season we have the marina polish and reseal the shaft. This has worked rather well (at least at relieving my anxiety) and so far we have no pitting developing.
Next summer we will be moving the boat to Robinhood where we will ask them what their opinion is on the matter.

Troy Scott · Post by **Troy Scott** » Dec 20th, '09, 12:30

Philip and Sharon,

Thank you very much for volunteering this information. I'm really interested in getting to the bottom of this.

Please elaborate. What exactly is involved in "we have the marina polish and reseal the shaft"?

It's pretty hard to get to anything but a small space just above the rudder, so it seems to me that any polishing would be pretty limited. Also, what is involved in resealing? Are they resealing where the shaft enters the rudder? or the stuffing box? or the surface of the stainless steel itself? What do they use to reseal the shaft?

One other thing: Why was a new rudder necessary?

Bob Shulman · Post by **Bob Shulman** » Dec 20th, '09, 14:15

We are former Cape Dory owners (now have a Baba), but check out the CD board occasionally. On our Baba I was concerned about a lack of protection for the stainless rudder shaft. I cut a piece of copper foil (the same material used for SSB grounding), connected it with a hose clamp to the rudder shaft, and connected the other end inside the hull to a hull bonding zinc. The rudder shaft does not turn enough to damage the copper zinc; it just swings from side to side as the shaft rotates. It has been in place for two years with no problems.
Regards,
Bob S
Baba 35
"Timothy Lee"

Troy Scott · Post by **Troy Scott** » Dec 20th, '09, 18:01

Bob and ALL,

On my CD36, I have ample access to the SS shaft inside the boat. Just in case there has been a bonding issue in the past, I think I will use a SS hose clamp to attach a flexible copper braid to the SS rudder shaft above the rudder shaft stuffing box. I will connect that to the underwater bronze grounding plate through the bonding system. Thoughts? Also, I wonder what you guys think about my idea of smoothing and epoxy barrier coat sealing of the small area of the shaft that is pitting. The part of the shaft that is inside the rudder is already isolated from any appreciable oxygen, so I would only be extending the present situation upward by about an inch. I would like to think that this would be a good thing, and that it wouldn't just move the problem higher up the shaft......

Jim Davis · Post by **Jim Davis** » Dec 20th, '09, 19:15

I wouldn't tie it to the bronze grounding plate. If I were to tie it to anything it would be a hull zinc. If al the through hulls are bonded and connected to a zinc fine, if not all you are accomplishing is connecting SS to bronze, not a good idea from an electrolysis point of view.

John Stone · Post by **John Stone** » Dec 20th, '09, 19:39

Troy,
It makes sense to me to polish the pitted area and then try to protect it with a barrier coat. I would do some research to make sure you are polishing it properly and not further degrading the SS.

I am not so sure about tying the SS rudder post into the bonding system. I understand there are pros and cons to this. I do know that if bonded it needs to be tied into a sacrificial zinc as Jim Davis stated otherwise you can create the very electrical circuit you are trying to defeat. Right now, I would think, your rudder post is electrically isolated and that can be a good thing depending on your philosophy and the bonding system you are employing as well as the type of electronic equipment you are using. There are specific requirements for bonding and I expect many boats are not bonded properly to start with. Poorly done it can exacerbate stray current corrosion among other things. Again, I am no expert but whatever you choose to do you need to make sure you have no stray electrical currents and everything is bonded correctly otherwise you may make the problems worse. ABYC is a good starting point, and if you don't already have it, I would also check Nigel Calder's book "Boatowners Mechanical and Electrical Manual." Again, I would seek out some expert advice and weigh the pros and cons before you decide what to do.

John

Bob Shulman · Post by **Bob Shulman** » Dec 20th, '09, 20:12

Troy,
I think it's a good idea to protect the rudder shaft from galvanic corrosion by connecting it to the bonding system, but I don't think crevice corrosion will be prevented by bonding. We are in the process of replacing our chainplates due to crevice corrosion, even though the chainplates are bonded. I believe it's due to trapped moisture and a lack of oxygen where the chainplates pass through the deck. Rudders with stainless shafts would seem to be a place of concern for crevice corrosion, but short of pulling the rudder there is no way to inspect the entire shaft. Additionally, rudders on older boats may very likely have some intrusion of water, so the stainless grid inside the rudder might possibly be compromised. Since I have not heard about rudder failures on Babas I just sort of push it to the back of my mind. I certainly don't intend to pull the rudder and grind it open to check! Recently, I read an article by Beth Leonard about techniques for streering after a rudder failure; she noted a statistic about rudders failures that was pretty alarming, although I suspect many of these failures are more relevant to spade rudders. My understanding about crevice corrosion is that once it starts the stainless steel in the corroded area will have a greater tendency to continue corroding. Something about the individual components of the alloy reacting with each other. It may be galvanic corrosion at this point?
Regards,
Bob S
Baba 35
"Timothy Lee"

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