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I've read that it is bad to use treated wood for a trailer bunk for a pontoon boat because the chemicals would react with the metal and cause problems.

If I am going to put plastic slides on the bunk, would it be ok to use treated wood then, since the metal would not actually be touching the wood?

  • I think this might boil down to a question for Chemistry SE, since the issue now will be (I think) any reaction between the chemical in the treated wood and the chemical makeup of the plastic. – Don Branson Jun 14 '18 at 21:34
  • What metal? Aluminum if fairly non-reactive. I have never had the wood outlast the metal. – paparazzo Jun 14 '18 at 23:29
  • Just a fun fact that I learnt at the weekend that you treat boats differently for salt and freshwater to preserve them, worth bearing in mind. – Aravona Jun 18 '18 at 12:24
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The problem is that the chemicals in the treated wood are corrosive to metal, especially when wet. Beyond, the boat, it might not be so good for your trailer either.

The two most popular chemicals for wood pressure treatment are alkaline copper quaternary (ACQ) and copper azole (CA), which are both active corrosion materials.

Only two corrosion protection systems are recommended for use with these pressure treatment chemicals: hot-dip galvanized steel and stainless steel.

In Contact with Treated Wood

Not only do ACQ and CA lack corrosion-inhibiting chromium and arsenic, they have much higher concentrations of copper than CCA — not a bad thing to ward off most fungal growth, but just awful if you happen to be a less noble metal than copper. Also, some ACQ and CA formulas contain ammonia; those are even more corrosive.

...

Copper actually isn’t corrosive in a dry, inert environment, but what deck is never wet? And when water contacts ACQ- or CA-treated wood, it lifts copper ions from the preservative. As the copper-tainted water seeps onto metal hardware, a galvanic reaction causes the hardware to corrode.

Fighting Fastener Corrosion

I can't find any evidence either for or against whether it reacts with plastic.

Your other (more expensive) option is to use redwood instead, that is also rot resistant and lacks the corrosive chemicals.

  • That's good information. The parts on the trailer that will be touching the wood are galvanized, so hopefully I'll be ok there. – Homer Jun 20 '18 at 19:33
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I believe your question begins with a false premise, that using treated wood has some advantage and one possible negative on a boat trailer.

It is true that that wood treatments will interact badly with some metals.

But also consider, there is not much wood on a boat trailer. Any untreated wood on your boat trailer is going to have several years of useful life. There are also rubber parts with a similar life span, not to mention oxidization to the metal.

Your trailer is going to need maintenance. Replacing untreated wood as it ages, is probably one of the least expensive and time consuming maintenance tasks, you will have. Adding treated wood, is going to increase risk to more expensive and difficult to address issues.

Also consider, how much of the time that wood will actually be exposed to the weather, for the longest times of the worst weather, there is going to be a boat over the top protecting the wood and the trailer.

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