Cruisers Forum
 


Join CruisersForum Today

Reply
  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 20-06-2019, 05:34   #1
Registered User

Join Date: Mar 2007
Boat: Privilege 482
Posts: 389
When does stainless steel surface corrosion/pitting become a structural issue?

So we recently dropped the rudder to replace a lower bearing. Noticed some areas of corrosion and pitting on the stainless steel (316?) rudderstock. This area is just above where the stock exits the rudder body and is in the area where the bearing sits (and slightly above it). The rudderstock is solid and 39mm in diameter. Some of the small pitting holes allow me to insert a small pin about 2-3mm.

One suggested remediation was to clean and prep the holes and then fill with a metal epoxy to prevent the continuation of the corrosion. This is fine in theory, but my concern is when does the pitting become severe enough to impinge on the structural capacity of the ruder shaft?

Where's a metallurgical engineer when you need one?

Cheers- Matt
Attached Thumbnails
Click image for larger version

Name:	20190620_120550.jpg
Views:	146
Size:	421.9 KB
ID:	194310  
__________________

__________________
Boats, kids, and all that jazz.
teneicm is offline   Reply With Quote
Old 20-06-2019, 05:56   #2
Marine Service Provider
 
boatpoker's Avatar

Join Date: Nov 2008
Location: Toronto, Ontario or Bahamas
Boat: Benford 38 Fantail Cruiser
Posts: 3,750
Re: When does stainless steel surface corrosion/pitting become a structural issue?

That shaft is done !
Here is a shaft x-ray of a single pinhead size pit. It shows the pit is many hundreds of time bigger sub surface.

The second photo shows what the inside of your shaft will look like when it breaks.

(Not a metallurgist but a Certified Marine Corrosion Analyst)
Attached Images
  
__________________

__________________
That hysterical laughter you hear as you sail a way in your "new" boat ..... is the seller.
boatpoker is offline   Reply With Quote
Old 20-06-2019, 06:45   #3
Certifiable Refitter/Senior Wannbe
 
Wotname's Avatar

Cruisers Forum Supporter

Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: C.L.O.D.
Posts: 9,162
Re: When does stainless steel surface corrosion/pitting become a structural issue?

Quote:
Originally Posted by boatpoker View Post
That shaft is done !
Here is a shaft x-ray of a single pinhead size pit. It shows the pit is many hundreds of time bigger sub surface.

The second photo shows what the inside of your shaft will look like when it breaks.

(Not a metallurgist but a Certified Marine Corrosion Analyst)
THIS!

FWIW, that isn't surface corrosion, it is crevice corrosion - an entirely different animal! What you are looking at is "the tip of the iceberg".
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
Wotname is offline   Reply With Quote
Old 20-06-2019, 10:36   #4
Registered User

Join Date: Apr 2007
Location: Australia
Boat: Island Packet 40
Posts: 2,614
Images: 7
Re: When does stainless steel surface corrosion/pitting become a structural issue?

It largely comes down to how the shaft is being loaded in use. If it's a spade rudder where the shaft is being subjected to bending stresses failure is more probable than if it supported at both bottom and top and consequently the loading is only shear and torsional.

If it was carbon steel you could just grind out the pits and weld and reshape the shaft but welding on stainless is a chancy business.
RaymondR is offline   Reply With Quote
Old 20-06-2019, 11:59   #5
Registered User
 
Wind River's Avatar

Join Date: Oct 2012
Location: San Diego
Boat: Hudson Force 50 Center Cockpit
Posts: 313
Re: When does stainless steel surface corrosion/pitting become a structural issue?

Quote:
Originally Posted by boatpoker View Post
That shaft is done !
Here is a shaft x-ray of a single pinhead size pit. It shows the pit is many hundreds of time bigger sub surface.

The second photo shows what the inside of your shaft will look like when it breaks.

(Not a metallurgist but a Certified Marine Corrosion Analyst)
I suspect mine would look the same as this x-ray picture or worse. There was a small pinhole in the end of the rudder stock. I started picking at it with a scribe until it opened up and I could stick a small pocket screwdriver 1/2" into the solid stock. That was enough for me, I didn't need the x-ray but it would be interesting to see just how far it goes.

Teneicm - As Boatpokers picture shows, the crevice corrosion works its way on the inside. The outermost "shell" looks unaffected at the break point while the inside has turned to dust.

Replace the stock or at the very least have it x-rayed. If it is indeed just surface corrosion (and I doubt that) have it passivated before putting it back into service.
__________________
Follow my refit on Facebook at S/V Wind River.
Wind River is offline   Reply With Quote
Old 20-06-2019, 18:20   #6
Registered User

Join Date: Mar 2007
Boat: Privilege 482
Posts: 389
Re: When does stainless steel surface corrosion/pitting become a structural issue?

Yeah, it's the not knowing just how bad it is that's the issue. If we were just coastal cruising I might chance it but as we're gearing up to go across the Indian Ocean next year, I don't think it's worth the risk. Anyone have a good link to instructions on how best to pull apart the rudder to get at the shaft? I'd prefer not to destroy the whole thing and have to rebuild it from the ground up......
__________________
Boats, kids, and all that jazz.
teneicm is offline   Reply With Quote
Old 20-06-2019, 18:53   #7
Registered User

Join Date: Apr 2007
Location: Australia
Boat: Island Packet 40
Posts: 2,614
Images: 7
Re: When does stainless steel surface corrosion/pitting become a structural issue?

My understanding of crevice corrosion is that it occurs when water is retained in a crack or other flaw where capillary action holds water until the oxygen in it is depleted at which time the steel is unable to form the non corrosive metallic oxide layers which protect the underlying steel.

Since the pits tend to be linear and parallel with the longitudinal axis of the rod it appears most improbable that there is any crack in the rod. Fatigue cracks tend to the circumferential rather than axial.

In the circumstances it appears more probable that the pits are caused by electrolysis in which case they will tend to a half spherical shape which is not a highly significant producer of stress risers as are notch bottomed cracks.

You could try taking a die grinder and grinding a shallow groove down along the pits and use dye check crack detector to see whether any cracks have propagated down into the parent metal.

If it is electrolysis then replacing the rudder without finding the underlying cause will just repeat the problem.
RaymondR is offline   Reply With Quote
Old 20-06-2019, 22:38   #8
Marine Service Provider
 
boatpoker's Avatar

Join Date: Nov 2008
Location: Toronto, Ontario or Bahamas
Boat: Benford 38 Fantail Cruiser
Posts: 3,750
Re: When does stainless steel surface corrosion/pitting become a structural issue?

Quote:
Originally Posted by RaymondR View Post
My understanding of crevice corrosion is that it occurs when water is retained in a crack or other flaw where capillary action holds water until the oxygen in it is depleted at which time the steel is unable to form the non corrosive metallic oxide layers which protect the underlying steel.

Since the pits tend to be linear and parallel with the longitudinal axis of the rod it appears most improbable that there is any crack in the rod. Fatigue cracks tend to the circumferential rather than axial.

In the circumstances it appears more probable that the pits are caused by electrolysis in which case they will tend to a half spherical shape which is not a highly significant producer of stress risers as are notch bottomed cracks.

You could try taking a die grinder and grinding a shallow groove down along the pits and use dye check crack detector to see whether any cracks have propagated down into the parent metal.

If it is electrolysis then replacing the rudder without finding the underlying cause will just repeat the problem.
I have no idea what someone means when they use the word "electrolysis". That term does not apply to corrosion. Corrosion involving electricity in marine metals is limited to only two types, galvanic or stray current. Neither are indicated by the OP's photo. This is clearly crevice corrosion.
__________________
That hysterical laughter you hear as you sail a way in your "new" boat ..... is the seller.
boatpoker is offline   Reply With Quote
Old 21-06-2019, 01:24   #9
Registered User

Join Date: Apr 2007
Location: Australia
Boat: Island Packet 40
Posts: 2,614
Images: 7
Re: When does stainless steel surface corrosion/pitting become a structural issue?

Quote:
Originally Posted by boatpoker View Post
I have no idea what someone means when they use the word "electrolysis". That term does not apply to corrosion. Corrosion involving electricity in marine metals is limited to only two types, galvanic or stray current. Neither are indicated by the OP's photo. This is clearly crevice corrosion.
Electrolysis is caused by the flow of stray currents the EMF source is not necessarily generated by the metals involved. Galvanism is the generation of an EMF by dissimilar metals in an electrolyte. It could be either but it is pitting which is more likely related to electric current flow rather than oxygen deprivation.

Crevice corrosion generally occurs in a place where there is limited refreshment of the fluid such as cracks. Circular pits generally do not provide the confining condition, be mindful that they start as shallow depressions.
RaymondR is offline   Reply With Quote
Old 21-06-2019, 03:36   #10
Marine Service Provider
 
boatpoker's Avatar

Join Date: Nov 2008
Location: Toronto, Ontario or Bahamas
Boat: Benford 38 Fantail Cruiser
Posts: 3,750
Re: When does stainless steel surface corrosion/pitting become a structural issue?

Quote:
Originally Posted by RaymondR View Post
Electrolysis is caused by the flow of stray currents the EMF source is not necessarily generated by the metals involved. Galvanism is the generation of an EMF by dissimilar metals in an electrolyte. It could be either but it is pitting which is more likely related to electric current flow rather than oxygen deprivation. its jsut a catchy word that has been improperly glammed onto just like the ladies in the hair removal business who are also not trained in corrosion.

Crevice corrosion generally occurs in a place where there is limited refreshment of the fluid such as cracks. Circular pits generally do not provide the confining condition, be mindful that they start as shallow depressions.
Absolutely not one single Certified Marine Corrosion Analyst will use the word "electrolysis" which describes the decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an induced current being passed through an electrolyte. . You need to get your information from more reliable sources.

https://www.nace.org/resources/gener...anic-corrosion

file:///C:/Users/Wallace/Documents/SURVEY%20NOTES/MOISTURE%20CONTENT/WEB%20PAGE%202/electrolysis.htm

People educated in these matters will not use that word. As I said. if it involves electricity in a boat you are strictly limited to galvanic or stray current corrosion. When I wrote my certification exam I was told use of that word was an automatic failure My instructors were both consultants to the USCG on corrosion.

I also get a lot of free instruction in these matters from my buddy, dock neigbour and consultant to the US Navy on their electric warships, he retired year as President of IEEE although he still consults with the New Panama Canal on their electrical systems.
__________________
That hysterical laughter you hear as you sail a way in your "new" boat ..... is the seller.
boatpoker is offline   Reply With Quote
Old 21-06-2019, 14:26   #11
Registered User

Join Date: Sep 2007
Location: Queensland, Australia
Boat: None at present--between vessels. Ex Piver Loadstar 12.5 metres
Posts: 930
Re: When does stainless steel surface corrosion/pitting become a structural issue?

If you can see it, it is an issue. Replace it.
Mike Banks is offline   Reply With Quote
Old 21-06-2019, 15:09   #12
Registered User

Join Date: Feb 2012
Location: Moorea Island, French Polynesia
Boat: Cal-40
Posts: 163
Re: When does stainless steel surface corrosion/pitting become a structural issue?

Well considering how important that damn thing is and that it will only fail when itís used and the fact that itís only used in moments when you need it...... I think I would change or repair that.
Siberian Sea is offline   Reply With Quote
Old 21-06-2019, 15:33   #13
Registered User

Join Date: Apr 2007
Location: Australia
Boat: Island Packet 40
Posts: 2,614
Images: 7
Re: When does stainless steel surface corrosion/pitting become a structural issue?

Replacing an entire rudder is not a trivial repair. It involves the machining of a new shaft, the welding on of a plate or tangs, the forming of the shape from a filler material, the glassing over of the filler material, sanding and painting. It's a major and potentially costly repair and in this case may not be necessary.

The solution is to find the cause of the pitting and then act accordingly. If the pitting is the extension of cracks in the stainless steel shaft then the rudder will need to be replaced.

If it is the pitting is caused by electrolytic or galvanic corrosion (no matter what one wishes to call it, so lets just say it's the flow of an electrical current) the very small amount of metal which has been removed contributes an insignificant reduction in the strength of the shaft.

The other factor to be considered is could the pits act as stress risers which might lead to the initiation of a crack if left un-repaired?

The usual remedy for avoidance of stress risers it to avoid "notches". In the case of threads the thread roots are rounded and with hatches in a boat radiused corners and in the case of your shaft rounding the bottoms of the pits with a die grinder and maybe filling the pits with an epoxy metal filler. Do not weld them as in stainless this is just as likely to create cracks.

The dye check process is a two part product used to identify cracks in metal objects. It comes in a couple of spray cans. The first part is sprayed on and capillary draws it into and retains it in the crack. Then the excess is wiped off. The part is then sprayed with the second part, which is usually white in colour. The white component draws out the first part from the crack and changes colour, usually to red, identifying the path of the crack. It's fairly cheap and easy to do.
RaymondR is offline   Reply With Quote
Old 21-06-2019, 15:35   #14
Registered User
 
Reefmagnet's Avatar

Join Date: May 2008
Location: puɐןsuǝǝnb 'ʎɐʞɔɐɯ
Boat: Nantucket Island 33
Posts: 3,344
Re: When does stainless steel surface corrosion/pitting become a structural issue?

Quote:
Originally Posted by boatpoker View Post
Absolutely not one single Certified Marine Corrosion Analyst will use the word "electrolysis" which describes the decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an induced current being passed through an electrolyte. . You need to get your information from more reliable sources.

https://www.nace.org/resources/gener...anic-corrosion

file:///C:/Users/Wallace/Documents/SURVEY%20NOTES/MOISTURE%20CONTENT/WEB%20PAGE%202/electrolysis.htm

People educated in these matters will not use that word. As I said. if it involves electricity in a boat you are strictly limited to galvanic or stray current corrosion. When I wrote my certification exam I was told use of that word was an automatic failure My instructors were both consultants to the USCG on corrosion.

I also get a lot of free instruction in these matters from my buddy, dock neigbour and consultant to the US Navy on their electric warships, he retired year as President of IEEE although he still consults with the New Panama Canal on their electrical systems.

That's interesting.



The common description of electrolysis goes along the lines that it is "a process that converts electrical energy into chemical energy"


For example...

Wikipedia:
Quote:
In chemistry and manufacturing, electrolysis is a technique that uses a direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential.

I suspect that the different reference to electrolytic corrosion in this situation is simply used to avoid confusion with galvanic corrosion.



Anyhoo, that is a very interesting picture of severe corrosion. I did a double take when I saw it because a few years ago I rebuilt my rudder and noticed some minor crevice corrosion around the shaft just at the point where it entered the body of the rudder. In my case I ground it out, threw on some weld and dressed it with the grinder back to shape. I had to think for a moment and wonder if I didn't just paper mache over a bigger problem!
Reefmagnet is offline   Reply With Quote
Old 21-06-2019, 16:22   #15
Registered User
 
Moonflow's Avatar

Join Date: Sep 2018
Location: Sydney, Australia
Boat: Boden Elizabeth
Posts: 3
Re: When does stainless steel surface corrosion/pitting become a structural issue?

Check out Hilma Sailing experience
https://www.facebook.com/29411976439...6730207466182/
__________________

Moonflow is offline   Reply With Quote
Reply

Tags
corrosion, stainless steel, steel

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Repair of surface pitting on saidrive sowwaninii Propellers & Drive Systems 7 06-01-2019 07:34
Steel hull pitting Mollymawk Construction, Maintenance & Refit 17 04-12-2017 07:26
Preventing Stainless Steel from Pitting bcboomer Construction, Maintenance & Refit 30 12-05-2015 10:57
Does Stainless Steel Finish Matter For Corrosion Protection Or Anything Else ? rebel heart Construction, Maintenance & Refit 24 18-02-2014 09:03
Surface Pitting on Clevis Pins svtatoosh Deck hardware: Rigging, Sails & Hoisting 4 08-12-2011 07:28



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 09:34.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2019, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.