First of all, how does wood become "pressure treated"? Pressure treatment is a process that forces chemical preservatives into the wood. First, the wood is placed inside a closed cylinder, then the cylinder is placed under a vacuum to draw water and air from the wood cells and then pressure is used to force the preservatives into the wood. Some of the types of waterborne preservatives include, but are not limited to: Chromated Copper Arsenate (CCA), Alkaline Copper Quat (ACQ), Micronized Copper Quat (MCQ), and Copper Azole (CA).
While the chemicals in pressure treated wood are very good at inhibiting rot and insects, they also increase the corrosion/oxidation of metals used in construction. The copper in the treatment reacts with the aluminum and steel in a galvanic reaction that corrodes the metals.
Chromated Copper Arsenate (CCA) treated wood. CCA-treated wood has been in use since the 1930s with it being voluntarily phased out of residential wood applications in 2003/2004 due to health concerns when it came in contact with people. The decades of experience with fasteners installed into CCA-treated wood that show fasteners have a long service life. An added benefit of CCA is that it contains chromium, which is a corrosion inhibitor. Even though this treatment does not degrade fasteners as quickly as other treatments, the use of galvanized or stainless steel fasteners is still recommended.
Alkaline Copper Quat (ACQ) treated wood. ACQ has three times the amount of copper than CCA-treated wood. A study done by Simpson Strong-Tie found that ACQ is twice as corrosive to metals as CCA, and some other studies have found it to be three times as corrosive. As a result, the use of aluminum building components in contact with ACQ-treated wood is not recommended. When aluminum is used for flashings or door and window frames, a physical barrier or spacer material should be added to prevent direct contact of alkaline copper treated wood and aluminum products. Fasteners should be either Stainless steel (Type 304 and 316) or meet ASTM-A153 (for hot-dip fastener products) and ASTM- A 653 (Coating Designation G-185 for hot-dip connector and sheet products).
Micronized Copper Quat (MCQ) treated wood. According to Great Southern Wood Preserving, Inc., MCQ treated products may be used in direct contact with aluminum building products when limited to code-compliant construction applications that provide proper water drainage and do not allow the wood to be exposed to standing water or water immersion. Additionally, galvanized protected connectors coated at a G90 level or its equivalent (0.90 oz./ft²) and other hardware and fasteners which meet building codes for exterior use can be used in direct contact with wood treated with MCQ preservatives.
Copper Azole (CA) treated lumber. According to ProWood, spacer materials or other physical barriers should be used to prevent direct contact between CA-treated wood and aluminum products. When using CA-treated wood in close proximity to aluminum products, such as aluminum siding, flashing, furniture, and door and window frames, a 1/4" minimum space must be allowed for between the wood and the aluminum products. Polyethylene or nylon spacers can be used to maintain the 1/4" spacing. Fasteners should be either Stainless steel (Type 304 and 316) or meet ASTM-A153 (for hot-dip fastener products) and ASTM- A 653 (Coating Designation G-185 for hot-dip connector and sheet products).
The point to remember when dealing with pressure treated wood is fasteners and other metal building elements used in construction need to be of the proper material. Additionally, keeping the wood dry goes a long way to inhibit metal corrosion. Keeping these subsurface materials dry relies on a great designer, experienced contractor, and knowledgeable Owner's representative. But this discussion is for another time.