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Old 02-10-2016, 08:22 AM   #41
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Repairing pitted prop and rudder shafts is a process that's not uncommon for larger shafts, it's called cladding. An article in this month's Professional BoatBuilder magazine on the subject of shaft alignment includes a sidebar on cladding. http://highseasyachtservice.com/wp-c...-highlight.pdf. I have no connection with the featured shop, other than as a journalist. It's cost effective for more larger, specialized and more expensive shafts.

This corrosion has all the earmarks of crevice corrosion, which occurs when stainless steel alloys are starved of oxygen, which frequently occurs under packing material and inside shaft logs. Less corrosion resistant stainless steel alloys such as 304 and even 316 are more prone to this, while proprietary shaft alloys such as Aqualoy 22 and equivalents are much more corrosion resistant.

You've now mated a highly corrosion resistant, and very noble on the galvanic scale, alloy, Inconel, which is great stuff by the way, and ideal for wet exhaust systems, to a less noble shaft which is probably 300 series stainless steel. The two are separated by roughly 100-170 mV in the galvanic series, which isn't a huge difference (that's good) and they use similar parent metals, nickel, chrome and iron, which is also good. The farther apart metals and alloys are on the galvanic series, the less compatible they are. The Inconel is located in the most corrosion prone area, in the shaft log, which again is also good. The less noble stainless will, however, be sacrificial to the Inconel, which will make it more important than ever to ensure you keep these rudders galvanically protected with zinc or aluminum anodes, either attached to the rudders directly or via a bonding system, which is also connected to sacrificial anodes. I've never seen a stainless-alloy shaft cladded with Inconel, however, in this welder's world and line of work it maybe common. In marine cladding the goal is to choose a weld material that is as close to the shaft alloy make up as possible.

As an aside, any stainless steel alloy that is welded must be of the low carbon variety, denoted by an L suffix, such as 304L or 316L. Welding stainless steel alloys that are not low carbon results in a phenomenon known as carbide precipitation or weld migration, which in turn produces a corrosion prone perimeter around a weld, this can often be seen as a "halo" of rust around a weld. I've seen many stainless steel tank failures that were the result of carbide precipitation.

I'm very reluctant to use graphite packing in any seawater application. Graphite is the most noble "metal" on the galvanic series, above even platinum, titanium and Inconel, which means when it is in contact with these or virtually any other metal, and immersed in an electrolyte (seawater in this case) the other metal will be anodic or sacrificial, and it will corrode. Conventional packing, particularly for rudder shafts which move little and never get hot, is more than adequate.
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Old 02-10-2016, 10:22 AM   #42
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Peripheral to the repair of Angus's shaft, Northern Spy has been directly involved with spec details and maintenance protocols for the USN. Now he is directly involved with specs and maintenance for some of the biggest pump shafts in the world. I will not steal his thunder on this one but marine and industrial shaft repairs are truly in his ballpark.

A PM or two with him may prove worthwhile on this subject. Regarding your sidebar, proper temperature control and bead thickness of the laid on cladding material is important and has a detailed spec of its own dependent upon shaft diameter, MOC, IG welding machine and depth of penetration.
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Old 02-11-2016, 06:49 AM   #43
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Sunchaser, thanks, I look forward to learning from Spy, as I said in my post, I'm sure [he and] the shop had positive experiences with the Incaloy weld material approach, it's just not one I've encountered in the yachting world. I've done work with the navy and CG and have learned they do many things differently for a variety of reasons.

Re. the cladding process, I have no doubt there are a slew of details and requirements that must be followed. I didn't have room to detail it all in the sidebar, however, that shop had to go through rigorous analysis and testing, and demonstrate their proficiency to become certified to carry out repairs on ABS inspected vessels, which they do routinely.
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Old 02-11-2016, 12:43 PM   #44
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Thank you both for the additional info. The shop that performed this work says Inconel 625 has become their go-to alloy for these kinds of repairs, including for rudder shafts in salt and fresh water. Hopefully, they were not blowing smoke my way.

I use two large anodes per rudder and will bond the steering housings, which will include the shafts, to the bonding system. When I bought the boat, one of the rudder housings was not bonded and there was significant corrosion of the SS bolts holding the shaft log in place.

The main reason I want to use graphite packing on the rudder shafts is that I never want to see another leak there. It's a big job to get to the stuffing boxes. I see many references on line to using Inconel cladding or thermal spray on shafts in marine environments and graphite packing with SS shafts. I'm far from an expert, but would think that Inconel 625's very close proximity to both SS and graphite on the galvanic scale (Inconel 625 is even closer to graphite than SS) would minimize the likelihood of significant corrosion.
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Old 02-11-2016, 02:00 PM   #45
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I kind of thought about that too. I agree there may be a margin. But rudder stock loads are dependent upon the type, size, and arrangement of the rudder and (number of) bearing. It will have inherently different mechanical loads than a prop or pump shaft.

If there is adequate strength with the remaining diameter, one could consider skim cutting and using a Belzona (1111 super metal is common) build up instead of weld filler.
Belzona is likely the best epoxy for a shaft repair. Why not get a copper or nickel metal or even use stone dust like powdered clay , portland cement, limestone and use another epoxy?

I did a minor fill job on one of my shafts using powdered limestone and Loctite epoxy.
Can't say much about the repair. It does not leak. I also switched to the GFO packing. Mine was also crevice corrosion but only on prop shafts, neither rudder had that problem.

This video appears to be a worthwhile cheaper repair and also no issue with dissimilar metals.
This example video shows a really heavily undercut shaft, likely does not need to be that deep. Do you think the Belzona 1111 adds back any missing shaft strength from the heavy undercut?
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Old 02-11-2016, 02:27 PM   #46
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Belzona is likely the best epoxy for a shaft repair. Why not get a copper or nickel metal or even use stone dust like powdered clay , portland cement, limestone and use another epoxy?

I did a minor fill job on one of my shafts using powdered limestone and Loctite epoxy.
Can't say much about the repair. It does not leak. I also switched to the GFO packing. Mine was also crevice corrosion but only on prop shafts, neither rudder had that problem.

This video appears to be a worthwhile cheaper repair and also no issue with dissimilar metals.
This example video shows a really heavily undercut shaft, likely does not need to be that deep. Do you think the Belzona 1111 adds back any missing shaft strength from the heavy undercut?
Around 100 to 116 seconds on that video it was easily discernible at the then slow rpm that the shaft being worked on was not straight. Sure hope that out of straight shaft was used for demo purposes only... and it was not yours!
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Old 02-11-2016, 03:58 PM   #47
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Around 100 to 116 seconds on that video it was easily discernible at the then slow rpm that the shaft being worked on was not straight. Sure hope that out of straight shaft was used for demo purposes only... and it was not yours!
Not mine, just a video showing how this can be done I think. It does look to have run out.

When I repaired my shaft, it looked similar to those rudder shafts. Basically I cleaned up the shaft best i could to bright metal, and filled in the pits with the stone dust and epoxy. Then just hand smoothed with a file and sand paper.
Prior to this, people had used flax packing, and the shafts had some wear and leaked all the time. So at a haul out I decided to improve the situation. Now it does not leak. When I got the boat, the amount of water coming in was spraying quite a bit, only the bilge pumps kept it floating. Now that was back in 1998, so it has been a lot of years gone by.
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Old 02-11-2016, 04:24 PM   #48
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I remember some more now. The rough shafts, I would put in flax packing, then the packing quickly wore due to the roughness and was always leaking,
So my epoxying smoothed the metal surface and I switched to GFO cause it was softer, less wear on the epoxy and I figured would work better,and it did, One reason i used stone dust, I wanted a surface that would maintain its hardness when rubbed by the packing, And also I was concerned about mixing metals and galvanic currents. But likely that would not have been a problem.

Using GFO packing, maybe even softer sealers like polyurethanes would fill in the shaft crevices and work just as well. Who knows. I probably did some light filing of the surface, but you know GFO packing as long as the surface is concentric enough, it wont care if the surface is contoured a little. Your not building up a shaft surface to bear the weight and forces of a ball bearing. All you want is it to seal and not leak.

Here is another Belzona video. Belzona is tough enough to have a bearing pressed onto a shaft.
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Old 02-12-2016, 06:36 AM   #49
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When shafts leak chronically at stuffing boxes the first step that users often take is to reach for an exotic packing. In most cases the leak isn't the result of the packing, it's an issue with the shaft's surface, and in this case that's exactly what happened, the shaft was pitted, making it impossible for the shaft to seal properly with the packing. Now that it's smooth, good old-fashioned waxed flax will work fine. If you want, you can go with Teflon, however, graphite is both unnecessary and potentially harmful. Properly installed, greased (I prefer Lubrimatic marine wheel bearing grease) rudder stuffing lasts for years and hundreds if not thousands of hours, again because there's so little movement and not heat generation.

I agree, the Inconel and shaft alloy aren't that far apart galvanically-speaking, so probably not a big issue, still I'd make certain under any circumstances anodes were in good shape and the shaft and stuffing box are bonded with good, clean, tight connections.

Belzona is fine for making an otherwise pitted shaft smooth, however, it doesn't return lost strength like cladding does, and if the Belzona becomes dislodged it can be even worse, as it will admit water which it will trap, which will become stagnant, which will reinitiate the pitting process. It's an option, however, it's not a permanent fix like cladding.
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Old 02-12-2016, 06:53 AM   #50
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Thanks, Steve. I'll weigh this carefully. By "hundreds if not thousands of hours," I assume you mean the actual movement of the shafts? I had rejected Teflon because of its reputation for grooving shafts but maybe that's not a concern with the comparatively little movement for rudder shafts.
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Old 02-12-2016, 08:23 AM   #51
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See if yours is here,

https://www.google.com/search?q=bron...i7kmuaoMaQM%3A
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Old 02-12-2016, 12:09 PM   #52
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Thanks. Here are the rudders; tiller arms are above the shelves. Shafts are about 2 inch diameter.
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Old 02-12-2016, 12:21 PM   #53
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Are pitted rudder shafts something that also occurs on fresh water only boats?
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Old 02-12-2016, 12:50 PM   #54
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Are pitted rudder shafts something that also occurs on fresh water only boats?
Damn good question!

I know that (at least I'm pretty sure) galvanization/corrosion induced via water borne electric currents is diminished in fresh compared to salt water; due to increased electrolyte qualities of salt. However, I know nothing about pitting or crevice corrosion situations in fresh compared to salt waters???
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Old 02-13-2016, 09:47 AM   #55
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"Hundreds if not thousands of hours"...of running or underway time.

Teflon can work too well, and it can cause shaft grooving, however, and yes, that's mostly a non issue on rudders again because the movement is so small comparatively, there's no heat generation and so little friction.

Crevice corrosion can and does occur in both fresh and salt water. The chloride ion, i.e. salt exacerbates this process, however, I have encountered stainless steel crevice corrosion on many occasion in fresh water applications (including on the shaft of my electric tooth brush).

Remember, there are two primary types of boat-borne corrosion, galvanic, which involves connected dissimilar metals immersed in an electrolyte (water), which in turn generate current flow between them much like a battery - this is a slow corrosion process, months, years; and stray current corrosion, which is primarily the result of DC battery current leaking into water, bilge or sea. This is fast corrosion, think days.

Crevice corrosion occurs when stainless steel's oxide coating is compromised by a lack of exposure to oxygen, often in a crevice, under the fold of a T bolt stainless steel hose clamp for instance, after which it becomes active rather than passive. This creates pitting, and different parts of the pit generate voltage differences on the metal, between the bottom of the pit and nearby metal surface surrounding the pit for instance, and those differences sustain the corrosion process. While not the same as, this makes crevice corrosion more closely related to galvanic corrosion. It's slow, and occurs where stainless steel is exposed to stagnant water, under packing is one of the very best examples.

For more on stainless steel and corrosion mechanisms see this short column Stainless Steel Miracle Metal? | Steve D'Antonio Marine Consulting The opening photo is of a crevice corroded prop shaft.

If you want a longer article on the subject, one you can use of you are having difficulty falling asleep, see http://stevedmarineconsulting.com/wp...less-Small.pdf
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Old 02-14-2016, 06:41 PM   #56
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2 cents: I have had great luck passivating my stainless items before and after installation. CitriSurf is the product I use, I am sure there are others but this one is much safer than the nitric acid based ones and still does a great job.

A soak with this will put a new protective coating on your stainless where it was removed by machining and dissolve any iron/steel particles which may have found their way onto the surface during welding & grinding. Some claim that even using unplated steel tools can deposit iron on stainless and start pitting corrosion
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Old 02-15-2016, 07:31 AM   #57
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A new rudder with a bronze shaft might save repeat future hassles.
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Old 02-19-2016, 02:43 PM   #58
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Passivation is a worthwhile approach for many stainless steel corrosion issues. I'm not sure how much of a difference it would make with a submerged part that lives in the crevice corrosion zone, under packing and inside shaft logs. Polishing shafts, and any stainless steel hardware for that matter, to a mirror finish also lowers corrosion potential, as the smoother and harder surface offers fewer opportunities for pitting corrosion to take hold.

Bronze shafts are a thing of beauty, and immune to this type of corrosion. Since they are difficult to weld, however, it would be tough to attach a flange and mate them to these stainless steel rudder blades. Most bronze underwater hardware is cast, struts, shaft logs etc, and again it's very good and ideal for underwater applications.
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