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?
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.
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.
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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.
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.
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.
I think some plastics have some bad chemicals just like treated wood. Untreated wood in a water environment will rot, warp and eventually break. No doubt treated wood exposed to any metal will have some unexpected (or expected) effects. In either case, it may be the water you are in that contributes to the problem. I've seen the problems of salt water on boats, trailers, lawn furniture (which is not normally submersed in water) and even aluminum window frames. Some of those items have never been submersed, but rather just close to salt water. Maintenance, inspection and repair is probably the solution. Just how often and in what manner is the question - and solution. I've had treated bunks for 14 years that gave their usefulness but never harmed the boat hull. But, very few times was the boat in salt water, and when it was it was washed down from bow to stern and the engine flushed immediately. So far, so good.
