Fastener Corrosion in Pressure-Treated Lumber: Measured Rates, Prices, and What to Buy
Fastener Corrosion in Pressure-Treated Lumber: Measured Rates, Prices, and What to Buy - A step into the workshop.
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Pry a board off an aging deck sometime and look at what's left of the nails. If it went up after 2004 with whatever galvanized hardware was cheapest, odds are decent you'll find shanks necked down like chewed pencils and heads that pop off at a tap from the cat's paw. The wood's usually fine. The metal lost. That's backwards from how most people think about outdoor durability, and it isn't an accident. The lumber changed twenty years ago, the fasteners mostly didn't, and marketing filled the gap with salt-spray hours that don't mean much inside wet wood. Here's what the measured numbers say, and what a hundred of each fastener actually costs.
The 2004 Switcheroo
For thirty years, pressure-treated meant chromated copper arsenate. CCA left U.S. residential use around the end of 2003, and I won't pretend I miss arsenic in playscape lumber. But its replacements, alkaline copper quaternary (ACQ) and copper azole, traded one problem for another. Both lean hard on copper as the biocide, and copper is murder on steel. The USDA Forest Products Laboratory's review of the published data (report FPL-GTR-220) puts ACQ and copper azole at 2 to 19 times more corrosive than CCA, depending on the metal and the environment. The American Galvanizers Association pegs both at more than double CCA, which is exactly why the industry's coating recommendation jumped from G90 to the thicker G185.
The mechanism tells you when you're safe. Corrosion in treated wood is wet chemistry, driven by cupric ions from the preservative; the chromates in old CCA actually inhibited it. And it needs water. FPL's data shows corrosion effectively stops below roughly 15 to 20 percent wood moisture content. Hardware in wood that stays dry can coast for decades. A shaded deck that never dries out is a different planet.
The Numbers Vendors Won't Print
FPL drove fasteners into treated Southern pine and held it a year at 27 °C and 100 percent humidity, soaked to about 24.6 percent moisture. Read off their chart, so call these approximate: hot-dip galvanized corroded around 33 μm a year in ACQ and 29 in copper azole, against about 16 in old CCA and 4 to 5 in untreated pine. Double the old rate. Roughly seven times bare pine.
Two wrinkles. First, in FPL's test the zinc on galvanized nails corroded faster than bare steel in every treatment; the zinc still shields the steel under it, but it's being spent on a clock. A different study, Kear 2009, found the opposite ordering, and FPL says the ranking is test-method-dependent, so I'll hand you both and pick neither. Second, do the arithmetic on the coating. G185 means 1.85 ounces of zinc per square foot, about 40 μm per side, per the American Galvanizers Association. Divide 40 by 33 and worst-case zinc life in saturated ACQ is on the order of one to three years. In dry service, decades. That spread is the honest answer, and it's why FPL warns, citing Jin and Preston, that accelerated hot-and-humid lab tests correlate poorly with fasteners actually living in treated wood. Remember that next time a box brags about ASTM B117 salt-spray hours.
Your Zip Code Does Half the Work
FPL also bolted a deck-moisture model onto the corrosion model and computed first-year corrosion at the worst spot on a galvanized fastener in a worst-case shaded ACQ deck, city by city. Phoenix: 5 μm. Chicago: 11. Miami and Minneapolis: 12. Baltimore: 16. Atlanta: 17. New Orleans: 20. Seattle: 26. Hilo, Hawaii: 45. Same fastener, same lumber, a nine-fold spread. Rain distribution beats rain totals, too: Miami and New Orleans both catch 2.12 meters a year, but New Orleans spreads it across the calendar and corrodes 67 percent harder. Central Texas isn't on that table, but I do know our boards cup and check by the time football season starts, opening little gutters that hold water right at the fastener.
The structural consequence is worse than the rates sound. At FPL's measured wet-service rates for hot-dip galvanized, 10 μm a year in CCA versus 60 in ACQ, an 8d nail's connection capacity collapses toward zero in roughly 25 years at the ACQ rate, while the same nail at the CCA rate keeps over half its strength for decades. Same nail. Different chemistry. Different deck.
Oak and Cedar Fight Dirty
Tannin-rich species are a second front. FPL measured steel corrosion in water extracts, and white oak at pH 4.3 with 1,020 mg/L of tannin chewed steel at 279 μm a year. Extract tests exaggerate absolute rates, so use them comparatively, but the surprise buried in the data is that at a given pH, more tannin actually inhibits corrosion. Acidity does the damage, and the literature FPL cites calls wood below pH 4.0 potentially dangerous to metals; western redcedar runs 2.9 to 4.7. So cedar corrodes fasteners because it's acidic and stains because it's tannic. Iron plus tannin plus water makes iron tannate, the blue-black compound that was literally ink for centuries, and per FPL's finishing bulletin it shows up "the first morning after rain or dew." Even a hammer strike or a swipe of steel wool leaves enough iron to bloom.
FPL's coating hierarchy for these woods: shiny electroplated zinc is the thinnest, skip it; mechanically galvanized zinc contains iron and will likely stain anyway; hot-dip lasts longest but the zinc globs clog screw recesses. Their conclusion, and mine, is that stainless is the best choice, particularly for screws. The Western Red Cedar Lumber Association agrees: 304 for general work, 316 for salt air, and never electroplated or copper, which reacts with cedar's oils and streaks. If a stain beats you anyway, saturated oxalic acid at 5 percent or stronger pulls it, but the fix is temporary if any iron remains, and it can leave oak pink if it sits too long.
The Bill, Per Hundred
These are verified July 2026 prices from online specialty retailers; big-box shelves vary by channel and quantity, so check yours before you budget.
| Fastener | Where and how sold | ≈ $/100 |
|---|---|---|
| Hot-dip galvanized ring-shank nails (Maze, ASTM A153) | Pro-Tech Products, $66.95 per 12 lb | $5–6 |
| Polymer-coated deck screws (Grip-Rite PrimeGuard Plus, 3½") | Riley Building Supplies, $41.99 per 264-count box | ~$16 |
| 316 stainless deck screws, #10 × 3" | The Fastener Depot, $0.81 each | $81 |
| 305 stainless deck screws, #10 × 3" | The Fastener Depot, $0.87 each | $87 |
(The nail math: Do it Best lists 8d ring-shanks at 555 per 5-pound box, about 111 nails a pound.)
Two surprises in there. At The Fastener Depot, 316, the grade carrying 2 to 3 percent molybdenum that 304 lacks, is cheaper than 305 at the same size. The "316 always costs more" rule dies at the SKU level, even though Eagle Claw's own comparison shows about a 36 percent 316 premium at its prices. Shop the vendor, not the metallurgy. And the real cliff isn't between stainless grades; it's coated versus stainless at roughly five to one. BuildingAdvisor's broader rule of thumb puts stainless at three to eight times the cost of triple-zinc fasteners.
Now put service life against price. Stainless: in Baker's 14-year embedded-fastener study, corrosion on the 304- and 316-type nails was smaller than the measurement uncertainty. That result comes from CCA-era wood, and there's no 14-year ACQ dataset in FPL's compilation, so stretching it to modern lumber is inference, though Simpson's and retailers' testing back 304 and 316 as compatible with ACQ. Coated: the only years figure on offer is Grip-Rite's 10-year warranty, not lifetime, plus an ICC-ES report showing accelerated-test equivalence to hot-dip. Galvanized in wet ACQ: 40 μm of zinc against a measured 33 a year. You can do that math in your head.
What Goes in My Cart
The code floor, per PermitDeck's 2026 summary of IRC R317.3: anything touching treated wood must be hot-dip galvanized to ASTM A153, stainless 304 or 316, silicon bronze, or copper, with G185 minimum on connectors. PermitDeck is blunt about the alternative: standard G90 hangers "will corrode through in 3 to 7 years in ACQ." Polymer-coated screws are legal only through an ICC-ES evaluation report; Grip-Rite's is ESR-2914, earned under AC257's paired 1,440-hour accelerated tests against an ASTM A153 Class D hot-dip benchmark, per Simpson Strong-Tie's engineering blog. And per Simpson's own FAQ, never mix stainless and galvanized in one connection.
One shelf note. Much of what box stores sell in 2026 is micronized copper azole, and YellaWood says its corrosion behavior is similar to CCA and untreated wood. That's the vendor's claim, on vendor-cited testing. The micronized product FPL actually measured was the quat version, a different formulation, and it still ate galvanized steel about four times faster than untreated pine. I don't bet a deck on marketing copy.
So, verdicts. A deck you want to outlive the mortgage, anywhere the wood stays wet, gets stainless, and 316 if you can smell salt from the porch. Eighty-one dollars a hundred stings until you price pulling and re-fastening a spongy deck. A fence in Phoenix, a covered porch, anything that genuinely stays dry, runs coated screws with an ICC-ES report or double-dipped ring-shanks with a clear conscience, because dry wood barely corrodes anything. That's not hedging. That's the right fastener for the job, and for once a government lab already ran the numbers so you don't have to trust the side of the box.