Extending the life of exhaust components

[mikep996]

When I picked up my repaired exhaust collector the other day, the owner of the shop explained that he had added an attachment lug so the collector could be connected to the boat's bonding system. There are no such lugs on the original collectors and they do not contact any metal portion of the engine or other components. He also said that when he fabricates such systems, he adds a fitting to allow a zinc to be installed in the collector. He says that will dramatically increase the life of the metal exhaust components. Sounds sensible to me so I will connect both my collectors to the bonding system.

Although I currently plan to change to fiberglass collectors in the next year or two, he made a pretty good case for cupro-nickel collectors - his preferred metal for such systems. The cost for a pair of C-N collectors for the 53 MY is half the price of a pair made of FG. He says the cupro-nickel is much more corrosion resistant to salt water than is stainless steel and that a conservative service life would be at least 10 years range.

I'm no expert on any of this but I found it interesting and worth passing on - especially the part about bonding and Zinc-ing the collectors.

[Dick]

I got about two years out of the "patch" to my monel collector. So, that's consistent with what you've been told. Incidently, before re-welding, the shop sent a sample of the material in for testing. It was 68.9% NI and 27.9% CU. It tested as monel 400 or better. So, what your shop is really telling you is to stick with the monel. It surprises me that fabrication costs for monel are cheaper than glass. That's contrary to the estimates I rec'd.

Interesting info on zincs and bonding. Although, based upon Genesis' prev response, I'm still not sure I understand the bonding rqmt as the monel collector is completely isolated from everything. It's attached only to hoses. But, I got the same advice from my shop!

Dick

[Genesis]

.. and still corroded!

Without a connection, bonding is neither necessary nor will it do anything.

[mikep996]

Well, I decided to call him back a little while ago for some clarification. His view, and advice, is that since the metal collectors are partially filled with salt water all the time - and stagnant salt water most of the time (which is worse) they are susceptable to electrolysis involving the rest of the boat and, for that matter, the entire marina. He said it has nothing to do with galvanic corrosion because, as you pointed out, it is not conntected to anything metal. It is stray current corrosion that will be reduced by bonding/zinc-ing. If there is no stray current issues on the boat/in the marina, then there is no benefit to bonding/zincing.

I decided to consult the Pascoe site and he seems to support that view. His article on corrosion,

www.yachtsurvey.com/corrosion.htm

includes this statement: "The general rule is that anytime a piece of metal plumbing or hardware is isolated in a system, as with a sea strainer that is joined by two hoses is electrically isolated, needs to be wired into the system. This can be done by daisy chaining items together, but it's a good idea not to include too many items in a chain. Obviously, at any point where a connection is broken, all those items upstream will be unprotected."

So I guess I'll string 2 feet of #8 wire from each collector to the bonding system and be done with it. As near as I can tell, it'll either help, or it won't!

[Genesis]

To have a stray current problem you must have current flowing through the metal involved. Not possible here, unless something is radically wrong.

Ditto on galvanism, except there the mechanism is dissimilar metals.

Now - if you have welded some metal with the WRONG FOCKING ROD, then you just created a galvanic cell! NOW you've got a problem.

Don't do that!

Oh, by the way, if you DO do that then you need to make sure that zinc is immersed the ENTIRE time, and I'd put a zinc in it - I most certainly don't want Monel or SS connected to the bonding system without GOOD protection, because its more noble than a lot of other metals - which means that the collectors could conceivably cause OTHER underwater metals to corrode on the boat!

[saltshaker36]

When I repowered, I installed custom stainless surge tubes. The tubes have bonding tabs and a fitting for a zinc pencil. This is the way Rybovich makes all their metal exhaust components. I was on a 61' Buddy Davis and the surge tubes were set up the same. Same thing on a Jim Smith and a Whiticar.

I agree that this seems unnecessary unless these components are always below the water line. This is not the case on my boat and I don't think it was on all the other boats either. There must be a reason why it seems to be a common practice.

BTW my monel collectors from my 8V71's appeared to be original (27yrs old) and in good shape when I pulled them for the repower.

Jack Sardina

[Genesis]

.. collectors were neither bonded nor did they have a zinc fitting.

They lasted nearly 20 years.

The water injection bells finally were the failure point. After 20 years of being cycled from 1000F exhaust gas to under 150F gas+water where the interface is at the shower head, I don't think that's a bad service record at all.

[mikep996]

Earlier this morning, I measured the resistance and the voltage present between the exhaust collector on the other engine and ground. The resistance was 3.4 Ohms. I checked bonded but not metal-to-metal connected components (Seacocks) and the resistance to ground was .4 Ohms. Voltage reading from collector to ground was 1.5Mv. Voltage reading from seacock to ground was 0.

I don't know enough electrical theory to know if these resistance/volt readings are significant in this application, but they indicate that there is some electrical difference between the un-bonded collector and other metal components. Whether it is enough to cause any kind of problem, I have no clue.

[Genesis]

.. the resistance is, and if there's an offset on the engine from ground (shouldn't be, but you don't know without checking)

What did you use as your ground reference, etc?

[mikep996]

My ground references were the engine block and the bonding bolt on the engine mount frame. They showed no difference between them. Similar to what you and others have noted , this was an original collector from 1980 so at 25 years, it didn't do badly at all regardless of (non) grounding, (non) bonding, galvanism, electrolysis, crevice corrosion, or any of the other salt water demons. I'd be happy for the next set to get 25 years.

Maybe I'll just leave it the heck alone!

And speaking of the next set, I like the idea of Fiberglass but I have to admit I like the appearance of that cupro-nickel. It looks very boat-enginey if that makes the slightest bit of sense.