B411 Shaft Pitting

[Ralph Johnstone]

When you cold work austenitic stainless steels, especially the 316 and 304 varieties, you also get martensitic transformations that are magnetic

No, I'm afraid you're now confusing the hardness of cold worked austenite with that of martensite which again, can only be formed by quenching the austenite from an elevated temperature. Think high temperature if you want to change the lattice structure.

This shaft can actually cold work during use.

If the shaft is being plastically deformed (cold worked) during use then it's really getting the bejabbers beat out of it.

These alloys cold work easily, in fact its one of the difficulties in machining them.

Machinists avoid cold working austenite by keeping their tools sharp.

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

I think you're getting way too much of your information from the internet. Go back to your textbooks, the ones with real authors, and you'll get a more accurate picture of whats what.

 

[dLj]

No, I'm afraid you're now confusing the hardness of cold worked austenite with that of martensite which again, can only be formed by quenching the austenite from an elevated temperature. Think high temperature if you want to change the lattice structure.



If the shaft is being plastically deformed (cold worked) during use then it's really getting the bejabbers beat out of it.



Machinists avoid cold working austenite by keeping their tools sharp.



I think you're getting way too much of your information from the internet. Go back to your textbooks, the ones with real authors, and you'll get a more accurate picture of whats what.

See post #27 later.

dj

 

[Ralph Johnstone]

On that note, I think we'll have to agree to disagree.

 

[JamesG161]

Ooooh, not so good. Insufficient nickel

There 3 non-rare Earth magnetic metals.
1) Iron = Fe
2) Cobalt = Co
3) Nickel = Ni

I would suggest a stronger Magnet to test the shaft, to confirm.

So what I see, Shaft is OK.

I did not SEE the full shaft. The area is the pictures is not badly corroded.
Jim...

PS: An expert in corrosion carried in his pocket, a Buffalo Nickel, to check Stainless Steel in the field. Quick check method.
PSS: Stress CRACK Corrosion is not characterized by pitting. It would take very high temperatures in Salt water to see them.
The Cutlass bearing can be a point of stress, if the shaft is not alligned properly.

 

[dlochner]

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.

@RitSim A couple of questions. Am I correct in assuming the Cutless bearing is in a skeg and not in a strut? Do you know if the boat was initially built with a standard stuffing box? Or was the dripless seal added by a prior owner?

If the dripless seal was added by a PO, then this is the culprit. A standard old fashioned stuffing box functions by allowing small amounts of water to pass through the shaft log. This small amount of water is sufficient to keep the water oxygenated and prevents crevice corrosion. An unvented dripless seal will not allow water to enter and leave or pass through in sufficient quantity, thus the water becomes oxygen depleted. The solution is to install a standard stuffing box or a vented drip less seal like the ones from PYI.

The vent in a vented seal will allow water to enter and leave the shaft log as the stern of the boat squats and rises while motoring or sailing, there is a place for the water to go, namely up the vent hose. Without a vent, the water in the shaft log just stays there, there is no place for it to go as the stern squats. Because there is no or very little water exchange the water in the tube becomes oxygen deprived.

The amount of water surrounding the shaft is very low. A Cutless bearing only has a few narrow grooves to allow water passage and enough play to allow a thin film of water around the shaft to lubricate the turning shaft. It would not take long to for the shaft to consume the limited oxygen in that area.

Shafts with struts allow greater water flow around the shaft and since the Cutless bearing is open at both ends water can flow through with little restriction. The strut also allows a larger volume of water to flow around the shaft in the shaft log. With a vented dripless seal a great deal of water will flow through the shaft log when in reverse, so much water that it often pushes water up the vent and out spraying what every is beneath the vent.

The preventive fix might be to change the seal to a vented seal or a standard stuffing box. And while everything is apart, a new shaft and Cutless bearing.

 

[Ralph Johnstone]

There 3 non-rare Earth magnetic metals.
1) Iron = Fe
2) Cobalt = Co
3) Nickel = Ni

Good point (iron should be body centred cubic) in that if you were ever to get into a really hi % of nickel, you may then become magnetic even if everything were still autenitic and face centred cubic ?

I would suggest a stronger Magnet to test the shaft, to confirm.

How do you see a stronger magnet identifying either excess nickel content or ferrite as the cause of magnetic attraction in the original shaft ? Just a stronger attraction regardless of what causes it. Or are you going to one up me yet ?

 

[dLj]

So I was perusing the literature on the magnetic properties of 304 and 316 due to cold working.
Here you go:

And for those interested in reading more:

FAQ 3: Magnetic Effects of Stainless Steels

Enjoy!

dj

p.s. Oh, and just for your edification @Ralph Johnstone - the text book that I would have had in the back of my mind talking about the room temperature martensitic transformation would have been "Handbook of Stainless Steels" by Peckner and Bernstein pages 4-30 through 4-31 if you'd care to read up on the metallurgy of this transformation.

 

[dLj]

PS: An expert in corrosion carried in his pocket, a Buffalo Nickel, to check Stainless Steel in the field. Quick check method.

@JamesG161 I've been spinning this one around in my head and for the life of me I can't figure out what the Buffalo Nickel would have to do with checking Stainless Steel in the field.

The Buffalo Nickel was produced between 1913 and 1938 and was made of 75% copper and 25% nickel. This is also known as alloy UNS C71300. While I can't afford a Buffalo Nickel, I could afford a small piece of this alloy if I knew it would be beneficial.

Can you elaborate on how your corrosion expert used this Buffalo Nickel? I don't see how it could be related... but, I'm always interested in learning new tricks!

dj

 

[JamesG161]

The Buffalo Nickel was produced between 1913 and 1938 and was made of 75% copper and 25% nickel.

It has just enough Nickel content to slide off an Iron pipe, in the field, slowly. Thus noticeable drag.

When then testing, using the coin on a 316 SS pipe, no drag and coin falls like on a plastic pipe.

At the time, as young engineer, I had forgotten Nickel had magnetic properties, and was duly impressed.
Jim...

PS: If you have a Buffalo Nickel, use a strong magnetic on it, you will see it move slightly.

 

[JamesG161]

How do you see a stronger magnet identifying either excess nickel content or ferrite as the cause of magnetic attraction in the original shaft ? Just a stronger attraction regardless of what causes it. Or are you going to one up me yet ?

Ok you asked and I have a few types of these Rare Earth Magnets.
You know now we use magnets to put stuff on refrigerators , like notes etc?
Try these magnets to hang note, as an April fools joke...
https://www.amazon.com/DIYMAG-Magne...ncoding=UTF8&refRID=82VKHZRKMEM55N306H7A&th=1

I tried to hang a bag on our boat's cocktail table, in the down position [magnets on both sides], but the slick surface and sea state, cause the bag to slide down slowly.
Jim..

 

[JamesG161]

Have inquired about the cost of a new shaft also.

To avoid any type of Stress Corrosion Cracking [SCC] or general over all corrosion buy your new shaft with this Metal type..
https://www.aquamet.com/aquamet-17
Jim...

PS: It takes a very high stress to have SCC happen on a 316 SS shaft.

 

[dLj]

It has just enough Nickel content to slide off an Iron pipe, in the field, slowly. Thus noticeable drag.

When then testing, using the coin on a 316 SS pipe, no drag and coin falls like on a plastic pipe.

At the time, as young engineer, I had forgotten Nickel had magnetic properties, and was duly impressed.
Jim...

PS: If you have a Buffalo Nickel, use a strong magnetic on it, you will see it move slightly.

Hahaha - that's pretty cool! I'll have to pick up a small piece of that alloy and play with it. Although I see no real advantage over a known magnet, it still would be really fun to do! Maybe I'll pick up a cheap Buffalo Nickel at a coin show...

dj

 

[dLj]

To avoid any type of Stress Corrosion Cracking [SCC] or general over all corrosion buy your new shaft with this Metal type..
https://www.aquamet.com/aquamet-17
Jim...

PS: It takes a very high stress to have SCC happen on a 316 SS shaft.

Not necessarily. This mechanism is an insidious one. You do need stress, that's the S part of SCC. But you also have the second C, corrosion part of the term. When you get corrosion attacking the grain boundaries, under conditions of stress, then the corrosion actually supplies a portion of the stress. Corrosion product occupies greater space then the metal. You have a small crack with corrosion product at the tip, that corrosion product pushes the grain boundary outwards causing the crack to open slightly more, that exposes new metal that corrodes and you have a self propagating system. Because the new exposed metal is at the base of a crack, covered in corrosion product, there is limited oxygen available to form the protective oxide and you are off to the races. In layman's terms, you essentially unzipper the material along the grain boundaries.

All the above said however, I completely agree with you regarding this specific shaft and SCC is not going to be the concern.

Just a FYI - the aquamet-17 you mention is essentially 17-4 PH stainless steel. The mechanical properties listed on their web site indicate they are using an H1150 heat treat condition for this alloy - that is an absolutely fantastic alloy and heat treat combination and indeed, you will never have any corrosion problems with that alloy in that heat treated condition. I have specific literature showing this alloy in that heat treated condition showed absolutely no signs of corrosion after something like 6 and 1/2 years of marine environmental exposure. It is a rated material for submerged marine application. Something 316 is not rated for, even though we all use it that way...

dj

 

[JamesG161]

I did not want to get into the Stress on his shaft versus SCC.
____
I still suggest that @RitSim observed "pitting" is from inadequate Zinc protection.

Even though he has a propellor Zinc, it is not enough for the shaft protection.

Zinc protects first, the closest direct contact metal. Thus, his propeller is well protected, but "links" directly.

He should add a shaft direct connected Zinc. This should stop future "pitting".

A very close up picture of that area would reveal SCC, if it is happening.
SCC, in his case, is not from Oxygen Starvation [He sails 1/week], but could only be from Stresses.

How can I say that?
The shinny area in not corroded.

Most I can offer with out more pictures.
Jim...

 

[Davidasailor26]

Most Beneteaus, and there are lots of them, don’t have shaft mounted zincs because of the limited space between the skeg and prop. They probably also use the same shaft material on most models. But we don’t hear about Beneteau shafts failing and props falling off every day. At what point is leaving well enough alone worth considering? If the corrosion is bad enough that it can’t be polished out without reducing shaft diameter then I’d replace. Otherwise I personally would probably just polish it out, make sure the water line into the shaft log is clear, and call it good.

 

[Ralph Johnstone]

The shaft is magnetic.

Time for one last look at this idea again which @RitSim first identified on his prop shaft and separate the wheat from the chaff. @dLj was good enough to supply a document from the Australian Stainless Steel Development Asociation (ASSDA) which seems a reputable association so I think it safe to go by their figures. After all, we're not getting paid for this and no children are going to die at the end of the day if we're wrong.

The one chart from the ASSDA which interested me was the following:

This indicates cold working does in fact cause austenite (non magnetic) to transform into ferrite (magnetic) as it is plastically deformed. This is indicated by the increase in permeability. Linear, funtionally related, who cares, it increases. But I'm from Missouri so I've got to see for myself.

Not having a lab to work in, it's down to the workshop and way back to basics.

I managed to find a 1/8" dia. SS stud which showed no signs of magnetic attraction as measured by my little retrieval magnet which has an amazing amount of pull for such a small magnet:

The next move was to start work hardening the 1/8" stud and keep an eye on it as I beat on it mercilessly with a ball peen hammer on the top of an anvil. Very little happened initially (when checked by the magnet) until it was really getting deformed:

At the above amount of plastic deformation, it was quite magnetic:

As shown above, I was then able to hang the relatively heavy magnetic device from the cold worked SS stud. The other end of the stud still showed no attraction to the magnet. I'm now satisfies that work hardening does in fact alter the austenite.

So what's the magnetic attraction (permeability) of SS items on a sailboat ? So off to the marina to check a few things. Randomly held out my little magnet to the pulpits of several sail boats as I passed and not a trace of attraction there.

Once on our boat, I checked every SS object I could find and found only two items which had some attraction: the 15 kg. SS claw anchor:

................... and the assorted forged SS fittings on the SS pushpit and pulpit.

Everything else on the boat: railings, hand holds, sheet SS covers, shroud wires, AND the propeller shaft showed no sign of magnetic attraction. Only the deep drawn SS sink showed minuscule attraction at the tightly bent corners, but none on the flat surfaces of the sink.

From this pleasant little passtime on a rainy, miserable afternoon, I think it safe to say that @RitSim 's prop shaft permeability is not due to previous work hardening but some other cause, which may include the fact that the shaft is a piece of CRAP. Any way you look at it, if it's got pitting under the cutlass bearing, I wouldn't risk doing the job all over again in the near future. High quality shafts don't pit from oxygen starvation.

 

[RitSim]

So its time to weigh in again.
1. I believe this corrosion is fairly new - certainly not 20 years in the making.
2. I looked back at my Fall notes and i had noted that the zinc, while still there, had degraded more than a previous year.
3. I am familiar with cold working and SS magnetic properties. My shaft is strongly magnetic indicating NOT 304 or 316. Could be 17-4 or a similar alloy which are strongly magnetic.
4. Could the very mild rotating contact between the shaft and the cutlass bearing be affecting the stainless steel anti corrosion layer? This with a weak zinc created the pits.

 

[JamesG161]

High quality shafts don't pit from oxygen starvation.

________

4. Could the very mild rotating contact between the shaft and the cutlass bearing be affecting the stainless steel anti corrosion layer?

No.

weak zinc created the pits.

Yes.
_____
If you have room for a shaft Zinc, put one on when you change your propellor Zinc.
Jim...

 

[JamesG161]

my little retrieval magnet which has an amazing amount of pull for such a small magnet:

Bet you a Buffalo Nickel that small magnet, is Rare Earth Magnet.

Neodymium magnet - Wikipedia

Jim..

 

[RitSim]

This has been a great discussion. Thanks to all