Galvanic corrosion question

[2boatsinMI]

Was talking to a friend and he mentioned that he had replaced the transom in his tinny. As a long time lurker on this forum I asked him what kind of wood he used. To my horror he said pressure treated. I promptly advised him of the corrosion danger and his question was how long before damage was severe. I am hoping one of the aluminum experts will chime in.

 

[minuteman62-64]

Seems like on the timing there'd be a number of variables. How wet was the PT when installed? Was PT sealed/coated? Is aluminum transom skin painted where in contact with PT?

I've never used it in my boat, but, everything I've read says don't. So, if it were mine I'd remove it at first opportunity.

 

[sphelps]

I've seen some of the new pt 2x s' eat through reg galvanized nails in a year ...

 

[jigngrub]

Not galvanic corrosion when PT wood is used on aluminum, it's dissimilar metal corrosion... when the copper in the PT wood reacts with the aluminum.

In your friends case there could be a double jeopardy of corrosion, both dissimilar metal and galvanic corrosion if water is trapped between the inner transom skin and the wood. Galvanic corrosion occurs when water is trapped and the oxygen is depleted in it, the water then turns acidic and that's when you get the galvanic corrosion. Since your friend has the potential for twice the corrosion, I'd put the time line at twice as fast if there isn't some sort of barrier between the PT wood and the aluminum and the PT wood has been allowed to saturate with water.

 

[2boatsinMI]

Thanks guys, I'll relay the info and hopefully he'll replace it with the sealed exterior plywood.

 

[jigngrub]

If he'll paint the inside transom skin with a self etching primer before installing the new wood he won't have to worry about the galvanic corrosion that will happen with any wood.

 

[Ned L]

...."Not galvanic corrosion when PT wood is used on aluminum, it's dissimilar metal corrosion... when the copper in the PT wood reacts with the aluminum."...
......... I believe what you are talking about are really one and the same thing. Galvanic corrosion takes place when two metals that are differing on the galvanic scale are put in an electrolyte (water, especially salt water being much more so).
If you put dissimilar metals together in a completely dry environment nothing will happen.
The first question I would ask here is,..'is the boat in fresh or salt water?' Fresh water, less of a problem, salt water more of a problem. Aluminum and copper ( in the PT wood) are far apart on the galvanic scale. Put them together in an electrolyte and the less noble metal (aluminum) will corrode. This is galvanic corrosion.

 

[DavidMoore]

What Ned said.

 

[jigngrub]

If you put dissimilar metals together in a completely dry environment nothing will happen.
.

Try that some time with copper and aluminum and get back with us on how well it worked for you.

 

[Ned L]

Try that some time with copper and aluminum and get back with us on how well it worked for you.

Hmmm,.... Should I say that nothing in OUR lifetime will happen?
What am I missing here? Remove the electrolyte and you won't have galvanic corrosion.
Toss a piece of aluminum flashing on top of a piece of copper flashing in your dresser drawer and see how fast the aluminum corrodes.
Zinc is even "less noble" than aluminum. Almost every penny in all of our pockets are copper on a zinc core, no big corrosion issue there.

Edit:
Better example- This is why you don't need 'zincs' or anodes on "trailer boats". No electrolyte no galvanic corrosion.

Galvanic corrosion is a specific type of corrosion consisting of dissimilar metals in an electrolyte. Another specific type of corrosion is "stray current corrosion", which results from improper wiring on either a boat or (typically) a pier at a marina. 'Zincs' offer no protection for stray current corrosion. And then there is just plain "oxidation" which is corrosion that simply takes place in the presence of O2.