The chemistry that eats your deck: treated lumber vs. the wrong fasteners.

Your frame is a battery waiting to happen

For decades, pressure-treated lumber used chromated copper arsenate — CCA — and the fastener rules were simpler. CCA was phased out for residential decking in 2004, replaced by copper-based treatments: alkaline copper quaternary (ACQ), copper azole (CA), and micronized copper formulations you'll see labeled MCA or similar. They protect wood well. They are also, by design, rich in copper ions in a wet, salty, oxygenated environment — which is exactly what a deck frame is, year-round.

Put ordinary steel in contact with that wood and you get galvanic corrosion: the copper chemistry pulls electrons from the less noble metal, and the fastener corrodes from the inside out. The zinc coating on a standard electroplated deck screw buys you a few seasons. The black oxide on a bulk-bin structural screw buys you less. What you see from above is a board that mysteriously cups or pops; what you see from below is a rust bloom around a screw head that has lost half its shank.

This isn't a cosmetic problem. A joist hanger, a post base, or a ledger bolt that corrodes through is a connection failure — the same category of failure as undersized footings (see TB-102) or a ledger done wrong (see TB-101). The wood looks fine. The metal doesn't.

What "rated for treated lumber" actually means

Code and manufacturer testing converge on a short list of acceptable metals for contact with modern preservatives. Hot-dip galvanized steel with a heavy zinc coating — the G185 spec you'll see on engineered connectors — is the baseline for hangers, post bases, and structural screws in a treated frame. Stainless steel (typically 304 for most deck environments; 316 where salt or persistent wetting is expected) is the premium option: slower, more predictable, and the right call at the ledger and anywhere failure would be catastrophic. Polymer-coated or ceramic-coated structural screws from manufacturers who publish third-party testing for ACQ/CA contact are acceptable when the coating is intact and the fastener is driven correctly — which is why we don't mix brands casually on primary connections.

What doesn't belong: electroplated "deck screws" on joist hangers, drywall screws anywhere, powder-coated hardware not listed for exterior treated contact, and aluminum flashing in direct contact with wet treated lumber (aluminum and copper are a particularly unhappy pair — galvanic corrosion accelerates in both directions).

Why brand matters less than the stamp on the box

Two screws can look identical and carry opposite instructions. One says "for use with pressure-treated lumber" and lists the coating or galvanizing class; one says "interior/exterior" and quietly omits treated compatibility. Hangers are worse: a generic stamped hanger and a tested connector from Simpson Strong-Tie or USP share a silhouette but not a load rating or a corrosion warranty in treated wood.

We treat fasteners like lumber grades — specified on the drawing, purchased to that spec, and installed per the manufacturer's fastener schedule (hangers want nails of a particular diameter and length; post bases want bolts torqued to a pattern; structural screws want pilot holes and depth). Substituting "something galvanized from the truck" is how you get a deck that passes inspection on framing day and fails on chemistry three years later.

Composite decking doesn't get you off the hook

It's a common assumption: composite surface, so corrosion doesn't matter. But the frame under every composite deck is still pressure-treated pine or Douglas fir. The hidden fasteners clip into that frame. The posts, beams, and joists are treated. The chemistry doesn't care what you walk on. In fact, composite's tighter surface and the moisture that still reaches the frame through gaps and end cuts can keep the treated zone wet longer than an open wood deck — same rules, same hardware.