Hmmm I did some quick research. From the attached article, it sounds like the NEW treated wood is way worse than the OLD treated wood on aluminum.
After reading the full article...I'd be pulling all of the treated wood ( old or new) out and replacing it, too. regards, Rich
https://joneakes.com/jons-fixit-database/2049-OVERVIEW-Living-with-Pressure-Treated-Wood
There are two important things to know about the new preservatives like ACQ. The new formulations contain much more copper than CCA does, which make them about twice as corrosive to many metals. That means we need to think twice about the fasteners we use as we enter the domain of building material incompatibility. Actually by 2008 we have evolved from ACQ-C to ACQ-D which is more fastener friendly, but still requires a careful choice of brackets, nails and screws.
Untreated nails are totally unacceptable while hot dipped galvanized nails and brackets are now considered a "minimum" protection for the metal; before it was just "recommended". Be careful, not all fasteners and brackets have the same thickness of zinc in the galvanization process and hence not the same degree of protection against corrosion. Prior to the introduction of high copper concentrate PTW quality galvanization was considered to be G60, or 0.60 oz of zinc/square foot of metal. How about the special green "deck" screws? If they are labeled "ACQ OK" they are in fact equivalent to double hot dipped galvanized fasteners, but not up to the SS 304 standards. Now, Simpson Strong-Tie makes their "standard" material G90 -- 50% thicker zinc and they actually recommend for such projects as outdoor decks to use their ZMAX brand which is twice the protection at G185.
Actually for severe moisture areas, or salt water areas or just for those who want to play safe, the fasteners and brackets should be stainless steel (grade 304 or better). In fact in any area where the wood is constantly wet, like on the coast, you should be using SS fasteners and SS brackets in all outdoor structures. How do you know if a screw or bracket is Stainless Steel 304 or better if it is not identified as such? Put a magnet to it. If the magnet sticks it may technically be in the category of Stainless Steel but it has an iron content, and it can rust. If the magnet will not stick, it is SS 304 or better.
Another common technique has recently been developed of putting a rubber type of barrier membrane between the high copper concentrate PTW and the galvanized steel brackets. This prevents the direct wood steel contact on the bracket surface where water can easily sit. Several different "membranes" are showing up in the stores to accomplish this, such as Simpson's T-PTBARRIER. This is a good half way measure short of using expensive and hard to locate Stainless Steel brackets and supports. Always use galvanized fasteners with galvanized brackets, and stainless steel fasteners with stainless steel brackets -- don't mix metals!
ALUMINUM
One other note, the copper in ACQ treated lumber will react very strongly with any aluminum, corroding aluminum fasteners and aluminum brackets very quickly. Even Aluminum siding will blister if mounted in contact with ACQ strapping or used as trim on an ACQ deck. Always separate the aluminum from the treated wood. Some people use rubber membranes as above, but that leaves fastener problems. Aluminum siding is best installed with aluminum nails, but aluminum nails cannot be driven into ACQ treated wood. The work around is to attach 1x3 untreated strapping to the treated wood with galvanized nails and then the aluminum siding to the untreated strapping with short aluminum nails. The strapping is weather protected and should not rot.
Just in case you were hoping that the ACZA or other treatments with ammonia carriers may get us out of these problems, be forewarned that they have proven to be even more corrosive to fasteners! To date only the borax treatments are less corrosive, but they leach out and leave the wood unprotected. Even here the original CA-A has evolved removing the Borax in exchange for more copper giving what is called Copper Azole CA-B; less leaching, more fastener corrosion. Things are indeed constantly changing seeking better performance with adjusted trade-offs. It just makes it a bit difficult trying to understand what it is that we are buying at any given moment.
There is of course the possibility of working with a wood that is harder than cedar and even more resistant to rot, a good contender for the job if we can get a good supply : Larch.
The second issue with high copper concentrate PTW like ACQ and CA-B is that the copper can cause problems for micro-organisms in fresh water lakes. Many ecologists are opposed to the use of ACQ for docks or boardwalks in cottage country and I think they are right. So what do you use? The old CCA treated wood would be a good alternative, if you can find it. Otherwise you may be forced to use cedar, which isn't as strong and won't last terribly long in water. Or you will have to go with metal, plastic or concrete.
As for working with the lumber, we should continue to follow the rules for any wood with any chemicals on it, even painted cedar: wear gloves when working, wear a dust mask if you are breathing dust, and don't burn it -- take it to the landfill instead.
The whole reason for using pressure treated wood is to extend the life span of wood structures, and save a lot of trees while we are at it. I am sure that the chemicals the industry offers us will continue to evolve as we find safer and safer alternatives that still get the job done while reducing the material incompatibility.
