B411 Shaft Pitting

[RitSim]

I am changing my Volvo molded rubber shaft seal. After removing the shaft coupling at the transmission, I pulled some shaft out of the shaft log and saw pitting of the shaft where the shaft is in the cutlass bearing. The pitting is not deep. I am guessing that this is O2 deprived corrosion. The boat is in brackish water April to Nov. and usually gets used 1/week. Does the group have any thoughts on preventing this? Cause?

One Pic is before 400 grit polish.

 

[sail sfbay]

Likely O2 deprived corrosion when the oxide surface film breaks down, then the exposed metal loses its non-corrosive properties. The exposed metal will break down, forming a small hole or pit. This pitting tends to develop in areas where water flow is restricted. Do you have a sacrificial shaft anode? Causes of shaft corrosion and mitigation measures are discussed in this article under "Corrosion".

Troubleshooter: Shaft Failures

Some failures on cruising boats occur gradually, providing warning signs to those who pay attention. Gradually increasing vacuum pressure in the fuel filter

www.passagemaker.com

 

[dLj]

How do you put your zincs on this shaft? I would look carefully at how that is being done.

You say the pits are not deep. Are you able to reface this shaft so that it is completely smooth again? I would want to remove all pitting as the pits will facilitate further corrosion. If you can't do this, then you need to make a decision as to how you sail and the risk you wish to run of this shaft degrading further and potentially breaking. The surface pits provide fatigue initiation sites that could lead to fracture of the shaft.

If I couldn't resurface this shaft there are 2 decision points I'm have to make:

1) Coastal cruising with the ability to keep an eye on this shaft at least once a year, then I may not worry too much about it. I'd inspect each year and keep an eye on the progression.

2) Taking longer trips, especially if I was going off-shore, I'd replace the shaft.

In all cases, review of how the zincs are being used is advised.

dj

 

[sail sfbay]

Agree with dLj's recommendation no. 2 to replace it. That is what I did on a power boat I purchased that had two 2 inch diameter shafts and is the general consensus in the thread below.

Pitted Prop Shaft Repair ? - Cruisers & Sailing Forums

Yes, I know I should just replace it. I'm trying to save some money where it's possible, my financial outlook isn't good now but in a couple of years it will be better, I'm just tired of waiting to go

www.cruisersforum.com

 

[Ralph Johnstone]

I pulled some shaft out of the shaft log and saw pitting of the shaft where the shaft is in the cutlass bearing.

It's still showing corrosion forward of where the cutlass bearing was sitting and the corrosion aft of the cutlass is worse.

Not too much question here but it looks like the shaft is just a poor grade of stainless

OR

as others have said, where do the zincs go ? ? ? ? If no zincs, then the prop would REALLY be in bad shape.

A little more info would go a long way towards identifying and curing the problem:

- are you the first owner since new (1999) ?

- what have previous haul outs looked like re the shaft ?

- what is the condition of the shaft inside the boat ?

- do you have zincs and where are they mounted ?

- what is the condition of the prop and what material is it ?

 

[dlochner]

It certainly looks like crevice corrosion. We know the cause, deoxygenated water and the metals in the shaft start corroding each other. The question is why is there inadequate water flow through the cutless bearing? Usually these bearings have grooves in them to allow water flow. Also the shaft is surprising clean in the corroded area.

I'm not familiar with Volvo shaft seals, are they vented? or do you have to burp them when launched? When the boat is launched a air bubble can form in the seal which prevents water from flowing in and out. The PYI shaft seals all have vent to make certain water can flow through the seal.

On most of the Beneteau that I have seen the prop shaft exits through a skeg with the prop quite close to the skeg. The anode would be located else where. This corrosion would not be prevented by an anode. The anode is there to protect the bronze bits under water. I also believe Beneteau uses SS through hull fittings and not bronze, but I could be incorrect on this.

 

[RitSim]

To answer the questions posed- The zinc is mounted aft the propeller. I acquired this boat three years ago and since the seal was leaking I wanted to change it out. Still going to do that but I believe the real cause was a nut on the aft port motor mount lowered itself about 1/2" and pulled the shaft and created the seal leak.

I did take the shaft completely out. After cleaning the pits are only under the cutlass bearing. They are not deep maybe <.005". The "corrosion aft of the bearing mentioned above was just marine scale with no pitting under it.

The Volvo seal has a flush that comes from a thru hull with natural circulation . I did notice last year that even when I removed the seal to readjust it almost no water came back thru the shaft log - will look into that also.

Have inquired about the cost of a new shaft also.

New question - why are the pits only under the cutlass bearing - it seems that there must be something in the rubber that is promoting the pitting corrosion. Maybe sulfur? Googled that and indeed elemental sulfur (from rubber curing) does promote pitting corrosion in 316 stainless steel and nitronic 60.

 

[Ralph Johnstone]

Googled that and indeed elemental sulfur (from rubber curing) does promote pitting corrosion in 316 stainless steel and nitronic 60.

Remember, that statement did come from the "net". Elemental sulphur is a new one on me. Did you notice the author ? I would first talk to several marine suppliers (if possible) of shafts to get a better idea of what's the best.

I'd still have to guess from this distance that the corrosion is the product of a poorer grade of SS. With the amount of work you're investing in this project, I think it would be well worth it to invest in a new top quality shaft regardless of the price (and it won't be cheap).

 

[dlochner]

Remember, that statement did come from the "net". Elemental sulphur is a new one on me. Did you notice the author ? I would first talk to several marine suppliers (if possible) of shafts to get a better idea of what's the best.

I'd still have to guess from this distance that the corrosion is the product of a poorer grade of SS. With the amount of work you're investing in this project, I think it would be well worth it to invest in a new top quality shaft regardless of the price (and it won't be cheap).

There are some places to cut corners and stretch a useful life and other places, not so much. I'd put the pitted prop shaft in the latter category.

Pull the shaft, replace the cultless bearing and rest easy. A few years back I replaced a shaft and bought from DeepBlueYachtSupply.com The price was about the best I could find for the shaft and the bearing. Service and shipping was good. They also have a handy tool to remove the flange. Haven't tried yet, but having deal with stubborn flanges, this looks to be a investment of $39.

Boat Propellers, Prop Shaft Seals, Shafts, Zincs, and Marine Hardware

Lowest Prices, Largest inventory of boat props, inboard prop shaft seals, zinc anodes, inboard shafts, cutlass bearings, and marine hardware.

www.deepblueyachtsupply.com

 

[RitSim]

Researching a new shaft now. I suggested the Sulfur angle because it is obvious the corrosion is just under the cutlass bearing. The rest of the shaft is free of pits and is subject to the same water passing the cutlass bearing. The shaft is magnetic.

 

[dlochner]

Researching a new shaft now. I suggested the Sulfur angle because it is obvious the corrosion is just under the cutlass bearing. The rest of the shaft is free of pits and is subject to the same water passing the cutlass bearing. The shaft is magnetic.

The Johnson Duramax bearings use nitrile rubber. This does note have sulfur in it. I took a quick look at the research on sulfur and SS and it seems the big issue is sulfur compounds in the gas and oil business. Crude oil and natural gas often contain sulfur.

The most likely cause is deoxygenated water that is trapped in the Cutlass bearing. A while back a corrosion specialist I talked to told me one of his jobs in the marine industry was turning prop shafts on boats that were laid up. The concern was crevice corrosion as a result of the stagnant water in the shaft log.

I happened to come across this article, it may be helpful when you change the cutlass bearing.

Replacing A Cutlass Bearing (Sleeve Bearing) - Marine How To

The Cutlass Bearing Replacing a cutlass bearing is not a tough project but does require some thought. On some boats the bearing is inside the dead wood making replacement more of a task than when it's mounted in a strut. These

marinehowto.com

 

[Ralph Johnstone]

The shaft is magnetic.

Ooooh, not so good. Insufficient nickel, which most likely means insufficient everything else as well (except iron).

the big issue is sulfur compounds in the gas and oil business

Been there and seen that. Compounds and not elemental sulphur. Not going to ionize and get to work when in the elemental state.

Researching a new shaft now.

Regardless of what the final price comes out to be, I have never looked back and regretted paying a fortune for a job. However, I have often looked back and regretted cutting corners on a job.

As @dlochner said:

There are some places to cut corners and stretch a useful life and other places, not so much.

 

[dLj]

Ooooh, not so good. Insufficient nickel, which most likely means insufficient everything else as well (except iron).

Yeah, this is a common misconception. The austenitic stainless steels are only nonmagnetic when fully annealed. It would be expected for this shaft to have a certain amount of cold working that would alter that state. I've seen these alloys look just like steel as far as magnetic properties... And they are perfectly well made... this is not a test for this alloy that has any bearing on it's performance. If one were to be concerned that it's the wrong alloy, or the alloy was not made correctly (an incredibly rare occurrence) then you'd need to use an alloy identification method, like a hand held XRF gun or other method...

Been there and seen that. Compounds and not elemental sulphur. Not going to ionize and get to work when in the elemental state.

I completely agree. The concern with sulfur is in the oil and gas industries where there are a lot of sulfur based compounds that will attack these alloys. @dlochner is on the right path, this is likely due to a breakdown of the passive oxide layer causing pitting corrosion.

dj

 

[Ralph Johnstone]

Yeah, this is a common misconception. The austenitic stainless steels are only nonmagnetic when fully annealed. It would be expected for this shaft to have a certain amount of cold working that would alter that state.

It's been a few years, but I don't seem to particularly remember SS austenite transforming to ferrite while being cold worked. In fact, highly unlikely you'll get any crystal lattice transformation without the application of elevated temperatures.

 

[JamesG161]

If you ever rotate your shaft, there is little chance of Stress Crack Corrosion. Sulphur or not.

Pitting should be only caused by insufficient Zinc protection.

You need an additional Shaft Zinc too!

Although you do not show a full shaft picture, my old shaft from the Previous Owner Pitted too.
Why,?
No Zinc on the shaft!

They had a new boat move into their marina. Zapped their Zincs.

So what I see, Shaft is OK.
Jim

 

[Ralph Johnstone]

So what I see, Shaft is OK.

I get the impression from @RitSim that he did have a zinc attached on the aft side of the prop:

To answer the questions posed- The zinc is mounted aft the propeller.

I'm still not willing to give the shaft a clean bill of health. Too many items indicating poor SS quality, particularly the pitting under the cutlass bearing. If that situation were to occur again after the old shaft were re-installed, the pitting each year would soon chew up the new bearing. By the time you've removed the coupling from the shaft and extracted it, there's really not that much more expense (relatively speaking) to install a new shaft. If the shaft does have quality problems, he'll pay the same again down the road.

 

[dLj]

It's been a few years, but I don't seem to particularly remember SS austenite transforming to ferrite while being cold worked. In fact, highly unlikely you'll get any crystal lattice transformation without the application of elevated temperatures.

When you cold work austenitic stainless steels, especially the 316 and 304 varieties, you also get martensitic transformations that are magnetic. With these alloys it takes very little cold work to get that transformation. So you don't need ferrite. As you point out, that will not likely happen unless you get elevated temperatures - for example when welding.

This shaft can actually cold work during use. These alloys cold work easily, in fact its one of the difficulties in machining them.

The internet is filled with misinformation on this - just a FYI....

I was once evaluating a piece manufactured by an Italian company and a bunch of us were sitting around when the part arrived and we were trying to guess why the part had not performed as desired and one of the group said let's check it with a magnet. It was supposed to be - hmmm don't recall now, either 304 or 316 - and we all took turns putting a magnet on the part, compared the magnetism against a known piece of steel and we all proclaimed it could not be stainless steel as it had the same attraction to the magnet as the steel. We then went through all the full analysis: microstructural, chemical, complete root cause, and indeed, it was exactly the alloy that it was supposed to be and the only thing that came out was it had been cold worked. There were no material anomalies what-so-ever. The poor performance was positively identified as a specific misuse in the machine during use.

Using a magnet on any piece of stainless steel that has been in use, standing rigging, shafts or whatever, especially if if undergoes loading in use, has the potential, and likelihood, of altering the magnetic response to a magnet. And that's assuming that the part was originally installed with no cold work. You actually have to be very careful in manufacturing to not instill any cold work in these alloys, in fact, if that's a design requirement, you usually need to do an anneal step as one of the final steps in manufacturing.

fwiw

dj

 

[dlochner]

Using a magnet on any piece of stainless steel that has been in use, standing rigging, shafts or whatever, especially if if undergoes loading in use, has the potential, and likelihood, of altering the magnetic response to a magnet.

I wish the guys at the scrap metal yard understood this, I would have received a better scrap price for all the SS rigging and bits and pieces I sell them.

 

[dLj]

I wish the guys at the scrap metal yard understood this, I would have received a better scrap price for all the SS rigging and bits and pieces I sell them.

Go to a scrap yard that has a hand held XRF and have them test it in front of you. And, not to be overly cynical, I'd guess they do understand this and are simply using the current internet bullsh!t to buy low and sell high...

dj

 

[dlochner]

Go to a scrap yard that has a hand held XRF and have them test it in front of you. And, not to be overly cynical, I'd guess they do understand this and are simply using the current internet bullsh!t to buy low and sell high...

dj

Actually I think they can barely read. Nice guys tho'. For the volume of metal I recycle it would only matter to a few dollars, not worth the trouble.