Stress Crack in Meg lid

Underwaterbear · 27th November 2005, 21:48

WOW! Have I got this straight:

The MEG head you guys have got cracks in is Al ?

And this ally head is studded with Stainless Steel fittings ?

And you dive this thing in the salty ocean ?

And how often does the maintenance manual say you should pull the stainless fittings to clean up the corrosion from the threads of the fittings ?

As anyone who has ever worked with a yacht with an aluminium mast will be able to tell you - any SS fittings should come off every 12 months for inspection and cleaning and rejointing with Duralac or some similar anti corrosive paste. The fittings will grow a white powdery oxide and they will eventually break whatever they are screwed into...especially if the section is not massive.

How thick is that bulkhead into which the fittings are screwed on the meg? If it's around 2-3 mm 18mths is probably about right. If the manual does not specify periodic removal of SS fititings for inspection for dissimilar metals corrosion I would be asking why not.

Good luck.

AB

***diverklondike*** · 27th November 2005, 23:37

Has anyone brought this to Leon's attention??

craigc · 29th November 2005, 05:12

My head is plastic not Al. I'm away from home for the next few days so can't check the serial number but from memory it was seventy-something.

Craig

mattmexico · 30th November 2005, 00:43

Quote: (Originally Posted by abowie)

Let me get this right....the photo you posted at the start of this thread shows a crack in an ALUMINIUM head, NOT a delrin one?

Yo there

Most afirmative, its ali, but can't be verified ... its in the mail ...

Matt

RonMicjan · 2nd December 2005, 00:47

Hey Matt, thought I was going to do the swap when I came down january, guess this leaves me more time to dive. The threads on the handset and hud "through head" fittings are 1/4 NPT with teflon tape isolating the SS fittings from the aluminum head, so dissimilar metal corrosion is not an issue. Leon is very up on that stuff and uses an insulator for any bimetal connections. Unlike my VR3 which uses SS screws into an aluminum housing, someday my computer will just be a small pile of white powder, sigh...when will they learn

nice avatar matt, it really looks like you, must have been right during the hurricane eh?

***jradomski*** · 2nd December 2005, 03:31

Quote: (Originally Posted by RonMicjan)

Hey Matt, thought I was going to do the swap when I came down january, guess this leaves me more time to dive. The threads on the handset and hud "through head" fittings are 1/4 NPT with teflon tape isolating the SS fittings from the aluminum head, so dissimilar metal corrosion is not an issue. Leon is very up on that stuff and uses an insulator for any bimetal connections. Unlike my VR3 which uses SS screws into an aluminum housing, someday my computer will just be a small pile of white powder, sigh...when will they learn

nice avatar matt, it really looks like you, must have been right during the hurricane eh?

Ron using "teflon tape" is NOT a way to islolate 2 types of metals.. The teflon tape is mearly a lubricant and will fill in gaps to avoid spiral bypass.. The way npt type fittings seals is theat you are forcing a taperd efitting until the threads tightly seal with each other..
No matter how you put it you will have continuity between the two metals if the fitting is sealing properly..

If you want to limit dissimiliar metals (not eliminate) you have to go to a straight fitting (which by nature is loose) and coat it with a lubricant (most of it will stay in place), the sealing is done by an oring surface...

Lastly the other option is to use a sacrificial surface like a helicoil, but the you would have to use a straight thread with an oring..

Here is an explanation on how seals are made

http://www.colder.com/Downloads/NPT.pdf

Genesis · 2nd December 2005, 03:42

The other option is a bushing made of a non-conductive material - assuming whatever pressure ratings are necessary can be acheived.

Dissimilar metals in contact, especially in sea water, is not a good thing - there's no real solid way around the problem; galvanic action is inevitable. This is especially true when one of the metals is aluminum, which is HIGHLY anodic to virtually everything else. In fact, aluminum is nearly as anodic as zinc! (Typically -0.76 in ev, .vs. -0.98 for zinc) 304 passivated stainless is -0.05, 316 passivated is 0.00

It doesn't get much worse in terms of a combination than Aluminum and SS....

***jradomski*** · 2nd December 2005, 04:13

Quote: (Originally Posted by Genesis)

The other option is a bushing made of a non-conductive material - assuming whatever pressure ratings are necessary can be acheived.

Dissimilar metals in contact, especially in sea water, is not a good thing - there's no real solid way around the problem; galvanic action is inevitable. This is especially true when one of the metals is aluminum, which is HIGHLY anodic to virtually everything else. In fact, aluminum is nearly as anodic as zinc! (Typically -0.76 in ev, .vs. -0.98 for zinc) 304 passivated stainless is -0.05, 316 passivated is 0.00

It doesn't get much worse in terms of a combination than Aluminum and SS....

I agree the bushing is the best solution if there is room..

mverick · 2nd December 2005, 04:23

Quote: (Originally Posted by jradomski)

I agree the bushing is the best solution if there is room..

Or, if the fitting is Stainless. Then make the head Stainless...

I know the Cannister is AL. But it is isolated with O rings.

RonMicjan · 2nd December 2005, 04:51

well joe, i have to disagree with you, the t tape does just fine keeping the parts apart, try it yourself and see. you dont like soldering molex pins either, but i, with my own eyes, see that it works. I know its a sealer on taper thread bud, dont try to teach your grandpa how to suck eggs.

dont think this means we wont be able to take long warm showers to the wee hours of the morning though...

27th November 2005, 21:48	#31 (permalink)
Underwaterbear A Prismer in Megland Current Rebreather/s: Prism Topaz Other Rebreather/s: Join Date: May 2005 Location: Seattle, WA Posts: 185	Re: Stress Crack in Meg lid WOW! Have I got this straight: The MEG head you guys have got cracks in is Al ? And this ally head is studded with Stainless Steel fittings ? And you dive this thing in the salty ocean ? And how often does the maintenance manual say you should pull the stainless fittings to clean up the corrosion from the threads of the fittings ? As anyone who has ever worked with a yacht with an aluminium mast will be able to tell you - any SS fittings should come off every 12 months for inspection and cleaning and rejointing with Duralac or some similar anti corrosive paste. The fittings will grow a white powdery oxide and they will eventually break whatever they are screwed into...especially if the section is not massive. How thick is that bulkhead into which the fittings are screwed on the meg? If it's around 2-3 mm 18mths is probably about right. If the manual does not specify periodic removal of SS fititings for inspection for dissimilar metals corrosion I would be asking why not. Good luck. AB
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27th November 2005, 23:37	#32 (permalink)
*diverklondike* Moderator Current Rebreather/s: Inspiration Classic Other CCR Other Rebreather/s: Inspiration Classic Other CCR Join Date: Oct 2005 Location: Delaware, USA Posts: 1,556	Re: Stress Crack in Meg lid Has anyone brought this to Leon's attention??
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29th November 2005, 05:12	#33 (permalink)
craigc New Member Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Nov 2005 Posts: 2	Re: Stress Crack in Meg lid My head is plastic not Al. I'm away from home for the next few days so can't check the serial number but from memory it was seventy-something. Craig
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30th November 2005, 00:43	#34 (permalink)
mattmexico Rebreather World Writer Current Rebreather/s: Megalodon Classic Kiss Optima RB80 / Clone Other Rebreather/s: Inspiration Classic Evolution Prism Topaz Sport Kiss RB80 / Clone Join Date: Aug 2005 Location: Mexico, Q.Roo, Playa del Carmen Posts: 760	Re: Stress Crack in Meg lid Quote: (Originally Posted by abowie) Let me get this right....the photo you posted at the start of this thread shows a crack in an ALUMINIUM head, NOT a delrin one? Yo there Most afirmative, its ali, but can't be verified ... its in the mail ... Matt __________________ N 20' 37.617' W 87' 04.693' www.andreaswmatthes.com
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2nd December 2005, 00:47	#35 (permalink)
RonMicjan Cap Ron scourge of the NW Current Rebreather/s: Megalodon Dolphin Home Build Other Rebreather/s: Sport Kiss Classic Kiss Dolphin Home Build Join Date: Feb 2005 Location: Cape Disapointment, The Graveyard of the Pacific Posts: 1,082	Re: Stress Crack in Meg lid Hey Matt, thought I was going to do the swap when I came down january, guess this leaves me more time to dive. The threads on the handset and hud "through head" fittings are 1/4 NPT with teflon tape isolating the SS fittings from the aluminum head, so dissimilar metal corrosion is not an issue. Leon is very up on that stuff and uses an insulator for any bimetal connections. Unlike my VR3 which uses SS screws into an aluminum housing, someday my computer will just be a small pile of white powder, sigh...when will they learn nice avatar matt, it really looks like you, must have been right during the hurricane eh? __________________ Ron "Life is pain princess, anyone who tells you different, is selling something", The Dread Pirate Wesley. www.tmishop.com Diving bits etc.
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2nd December 2005, 03:31	#36 (permalink)
*jradomski* Moderator Current Rebreather/s: Inspiration Classic Sport Kiss Optima rEvo Other CCR Home Build Other Rebreather/s: Inspiration Vision Evolution Megalodon Classic Kiss rEvo Other CCR Home Build Join Date: Mar 2005 Location: "Da" Bronx Posts: 2,885	Re: Stress Crack in Meg lid Quote: (Originally Posted by RonMicjan) Hey Matt, thought I was going to do the swap when I came down january, guess this leaves me more time to dive. The threads on the handset and hud "through head" fittings are 1/4 NPT with teflon tape isolating the SS fittings from the aluminum head, so dissimilar metal corrosion is not an issue. Leon is very up on that stuff and uses an insulator for any bimetal connections. Unlike my VR3 which uses SS screws into an aluminum housing, someday my computer will just be a small pile of white powder, sigh...when will they learn nice avatar matt, it really looks like you, must have been right during the hurricane eh? Ron using "teflon tape" is NOT a way to islolate 2 types of metals.. The teflon tape is mearly a lubricant and will fill in gaps to avoid spiral bypass.. The way npt type fittings seals is theat you are forcing a taperd efitting until the threads tightly seal with each other.. No matter how you put it you will have continuity between the two metals if the fitting is sealing properly.. If you want to limit dissimiliar metals (not eliminate) you have to go to a straight fitting (which by nature is loose) and coat it with a lubricant (most of it will stay in place), the sealing is done by an oring surface... Lastly the other option is to use a sacrificial surface like a helicoil, but the you would have to use a straight thread with an oring.. Here is an explanation on how seals are made http://www.colder.com/Downloads/NPT.pdf __________________ Joe Radomski CCR Trimix Instructor Trainer ANDI Instructor Trainer Director #10 All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated. Last edited by jradomski : 2nd December 2005 at 03:38.
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2nd December 2005, 03:42	#37 (permalink)
Genesis Bubbless Box of Death Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: Oct 2005 Location: Sunny Florida Posts: 1,394	Re: Stress Crack in Meg lid The other option is a bushing made of a non-conductive material - assuming whatever pressure ratings are necessary can be acheived. Dissimilar metals in contact, especially in sea water, is not a good thing - there's no real solid way around the problem; galvanic action is inevitable. This is especially true when one of the metals is aluminum, which is HIGHLY anodic to virtually everything else. In fact, aluminum is nearly as anodic as zinc! (Typically -0.76 in ev, .vs. -0.98 for zinc) 304 passivated stainless is -0.05, 316 passivated is 0.00 It doesn't get much worse in terms of a combination than Aluminum and SS....
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2nd December 2005, 04:13	#38 (permalink)
*jradomski* Moderator Current Rebreather/s: Inspiration Classic Sport Kiss Optima rEvo Other CCR Home Build Other Rebreather/s: Inspiration Vision Evolution Megalodon Classic Kiss rEvo Other CCR Home Build Join Date: Mar 2005 Location: "Da" Bronx Posts: 2,885	Re: Stress Crack in Meg lid Quote: (Originally Posted by Genesis) The other option is a bushing made of a non-conductive material - assuming whatever pressure ratings are necessary can be acheived. Dissimilar metals in contact, especially in sea water, is not a good thing - there's no real solid way around the problem; galvanic action is inevitable. This is especially true when one of the metals is aluminum, which is HIGHLY anodic to virtually everything else. In fact, aluminum is nearly as anodic as zinc! (Typically -0.76 in ev, .vs. -0.98 for zinc) 304 passivated stainless is -0.05, 316 passivated is 0.00 It doesn't get much worse in terms of a combination than Aluminum and SS.... I agree the bushing is the best solution if there is room.. __________________ Joe Radomski CCR Trimix Instructor Trainer ANDI Instructor Trainer Director #10 All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.
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2nd December 2005, 04:23	#39 (permalink)
mverick ScubaPimp Current Rebreather/s: Megalodon Other CCR Home Build Other Rebreather/s: Other CCR Home Build Join Date: Aug 2005 Posts: 510	Re: Stress Crack in Meg lid Quote: (Originally Posted by jradomski) I agree the bushing is the best solution if there is room.. Or, if the fitting is Stainless. Then make the head Stainless... I know the Cannister is AL. But it is isolated with O rings.
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2nd December 2005, 04:51	#40 (permalink)
RonMicjan Cap Ron scourge of the NW Current Rebreather/s: Megalodon Dolphin Home Build Other Rebreather/s: Sport Kiss Classic Kiss Dolphin Home Build Join Date: Feb 2005 Location: Cape Disapointment, The Graveyard of the Pacific Posts: 1,082	Re: Stress Crack in Meg lid well joe, i have to disagree with you, the t tape does just fine keeping the parts apart, try it yourself and see. you dont like soldering molex pins either, but i, with my own eyes, see that it works. I know its a sealer on taper thread bud, dont try to teach your grandpa how to suck eggs. dont think this means we wont be able to take long warm showers to the wee hours of the morning though... __________________ Ron "Life is pain princess, anyone who tells you different, is selling something", The Dread Pirate Wesley. www.tmishop.com Diving bits etc.
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