Pencil Zink

[Harrier]

I had a duh moment   I replaced the pencil zink on my heat exchanger yesterday and woke up this morning thinking to myself "you idiot". 
When replacing, I thought to myself it would be a good idea to coat the threads in lanacote to both help seal the joint and make later removal go smoothly.  I'm sure most of you already see the issue.  Zincs need electrical contact to function properly.  Its possible, and even likely, that there is still good contact between the threads, but I'd prefer not to risk it. 
The whole issue got me to thinking about how I could envision many good intentioned individuals doing something similar or even worse such as wrapping the threads in teflon tape.  These innocent mistakes can be costly, so I figured I'd take a warning shot across your bow and remind you to not insulate your pencil zink from the component it is protecting!

[wes]

Interesting, especially since as a brand new 27 owner (that is to say I'm brand new; the 1988 boat is not) I have no idea where this "pencil zinc" is located. One more thing to worry about! More clues, please....

Wes

[Harrier]

I have the Westerbeke 20B.  The heat exchanger used on this engine is made from several different materials so it is an electro-chemical worst case scenario.  To protect the heat exchanger there is a small zinc attached to the end of a brass plug that gets threaded into the jacket of the heat exchanger.  You can get copies of the engine manuals from Westerbeke that explain the replacement.  http://www.westerbeke.com/Products/ProductOperatorManual.aspx?ManualType=1&ID=3.
Not sure what type of engine you have but this method of protection is not uncommon.

[Allure2sail]

Pencil zinc removal heads up....
One piece of advice that I received at a diesel seminar was to always use two wrenches when removing pencil zincs. One wrench on the boss sticking out of the heat exchanger (usually a hex or at least two flats on it) and the other wrench on the pencil zinc it self. Work the two wrenches so the stress is not put on where the boss is brazed to the the body of the exchanger. The instructor said that the threads on the zinc bind up and you will break off the boss instead of unscrewing the zinc. That is an expensive item to replace and sometimes the threaded boss cannot be repaired. I always put some kind of penetrating oil a few days before I attempt it (I like PDQ) to help unscrew it without breaking.
Bruce
Allure

[NateD]

I don't know much about zincs on boats, and I know nothing about Westerbeke motors, but how about using a dielectric grease? It will maintain electrical conductivity, not sure how it will hold up to salt water/heat.

[skip1930]

Electrical Conductivity.
Comprehensible Corrosion: Keeping Your Electrons From Jumping Ship.

Chapter 31 in Dave Gerr's book 'The Nature of Boats'.

" And not all zinc is equal. "Use military grade zinc Mil Spec, Mil-A-18001 J [or higher letter].
Buy only zinc intended for marine corrosion protection. Many common zinc alloys are useless.

There is considerable confusion about the proper name for galvanic corrosion - the only proper term. Frequently it's called electrolysis, or electrolytic corrosion. Both are misnomers. Electrolysis is the chemical or chemical breakdown of the electrolyte - the fluid medium that transfers ions between metals. Obviously, in our case, electrolysis would be breakdown of sea water - not of much concern. Electrolytic corrosion is the corrosion produced by externally generated electric currents. It's also known as stray current corrosion. Stray current corrosion can be extremely serious. Check it with an meg-ohms meter. "

skip. ctrl key down, tap + to enlarge print.

" Making Your Zincs Work.
It takes some smarts to make zincs work.

1~You must install zincs to protect your boat's metal fittings or hull.

2~The zincs must be in tight, clean electric clean contact with the metal components they're protecting. [If they're not electrically contacted to the bonding system or metal hull, they're useless.]

3~The surface of the zincs must be exposed to water. You can't paint a zinc anode, ever! You want it exposed, and you want it to corrode. [They're not called "sacrificial Zincs" for nothing]

If your boat's zincs aren't wearing away they're either way, way too large [unlikely indeed], they're not in proper contact with the metals they're protecting, or they're painted over. In any of these cases the boat's zincs would be useless.

Properly sized, zincs should last about one year, at which time they'll be, oh, nearly half gone. You have to remember to check your zincs whenever possible to see that they're firmly attached and corroding properly. You also have to remember to install new zincs at the beginning of every season. Forget, and you'll likely end up replacing your prop or shaft instead-not sound economy. "

[Harrier]

I dont have Dave Gerr's book, but I thought this site has a good practical explanation of the forms of corrosion and how they may pertain to yachting. 
http://coxengineering.sharepoint.com/Pages/Metallurgy.aspx


Curious about the "Many common zinc alloys are useless" comment?  As long as the material is less noble than what you are protecting, which in this case is brass, shouldn't it work just fine?  The zinc in a zinc alloy is still zinc, no? Perhaps the less percentage of zinc in the alloy, the shorter it lasts?