Corrosion Engineering Questions

[Larry Christianson]

Greetings All,
I live in the Great Lakes region and own a small marine service company. I
do mostly electrical service on small to midsize pleasure craft almost all
consisting of either wood or glass. Because I also like to feed the kids
(and myself) I will occasionally take on other installation jobs when asked.
I have a customer with a steel hull schooner who wants me to hook up a fuel
oil heater in his main cabin. I have already purchased intake and exhaust
flues made of stainless steel thinking this would be the best choice to
prevent corrosion. This combination would be no problem with a glass or
wooden boat but has me thinking further about corrosion between dissimilar
metals and high moisture and eventually salt water and air. Can anyone lend
me some guidance in this little problem of mine? If I can't use S.S. than
what are my other options (aluminum or galvanized maybe)? If I caulk with
5200 would this insulate the two metals enough to prevent corrosive action?
Thank You,
Larry Christianson
Modern Marine Service

[David Wilkinson]

You could try chapter XI in Skene's Elements of Yacht Design on
"Galvanic Series of metals". This lists the common metals in order from
Electro-Negative to Electro-Positive. He says you will only have
problems if two metals are too far apart in the list. You should be OK
because Mild Steel and Stainless steel are quite close together and he
says "You can use aluminium, mild steel and stainless steel judiciously
together". If you are still worried then aluminium is next to mild steel
in the list and could be the best choice.

In article <CvA%2.565$Uz6....@news.megsinet.net>, Larry Christianson
<lar...@megsinet.net> writes

--
David Wilkinson

[Mark Hiscock]

> oil heater in his main cabin. I have already purchased intake and exhaust
> flues made of stainless steel thinking this would be the best choice to
> prevent corrosion. This combination would be no problem with a glass or
> wooden boat but has me thinking further about corrosion between dissimilar
> metals and high moisture and eventually salt water and air. Can anyone
lend
> me some guidance in this little problem of mine? If I can't use S.S. than
> what are my other options (aluminum or galvanized maybe)? If I caulk with
> 5200 would this insulate the two metals enough to prevent corrosive
action?

I wouldn't think that stainless or mild steel would be that far apart on the
Galvanic scale. It's darned unhealthy (for the boat) to mix aluminum and
steel of any kind. It's been a few years since I've even thought about this
sort of problem, but you should be able to find a galvanic scale on the net
somewhere. Basicly, find the two metals you want to use on the listing, and
if they're next to each other (or in the same line), then the corrosion
damage due to the different metals will be minimal/non-existant. If they're
several steps away... well, avoid it like the plague.

As a side note, the price of stainless is so low nowadays that you could
build a buffer plate of some sort using stainless steel bolts and plating
arrangement with minimal cost increase compared to mild steel. If you're
really worried about it, you could look into getting a 'bi-metallic' plate
for the actual connection. Basicly, it's two dissimmilar metals fused in
such a way that the galvonic process is suppressed between them. This sort
of treatment is used on ships and such with steel hulls and aluminum
superstructures.

Good luck.

Mark Hiscock
Naval Architect
TIME Marine, Inc.
Houston, TX
http://www.flash.net/~txintl