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Lag Bolts Rust: Does Stainless Outlast Galvanized?

Ask any contractor who’s been called back to a sagging pergola and you’ll hear the same punch-line: “The lag bolts looked fine—until they snapped in half.” The culprit is rarely the wood; it’s rust that hollowed out the fastener from the inside. Choosing between stainless and galvanized lag bolts is therefore not a cosmetic decision but a design-life calculation. So how do the two finishes really compare in years, not marketing claims?
Hot-dip galvanized lag bolts carry a zinc armor roughly 2.5 mils (0.0025 in.) thick, fused to the steel at 450 °C. The American Galvanizers Association predicts maintenance in a suburban climate at 60-plus years, and even in salty tropical air the coating is expected to protect for 50 years before 5 percent red rust appears. That longevity assumes the zinc is intact; where the coating is scratched, the sacrificial layer continues to shield the exposed steel, though at a gradually declining rate. In short, galvanized wins on cost per corrosion-free year: a ½ × 6-inch bolt costs about one-third of its stainless twin.
Stainless steel lag bolts—most commonly alloy 304—rely on 18 percent chromium to form an invisible, self-healing oxide film. Laboratory salt-fog tests rate 304 stainless at 1,000 hours before any visible corrosion, and field studies on coastal boardwalks show negligible section loss after 25 years. Upgrade to 316 stainless and the molybdenum boost pushes the envelope even in marine splash zones. The catch is price: a single 316 lag bolt can cost five times the galvanized equivalent, yet for many homeowners the “install once, forget forever” promise outweighs the upfront pain.

Where the choice tilts is environment and load. Galvanized coatings lose thickness at about one micron per year in moderate climates, so a 65-micron coat eventually thins to the point where threads begin to seize. Stainless, by contrast, loses essentially zero thickness; instead, it can suffer crevice corrosion if starved of oxygen—think lag bolts buried in wet PT lumber wrapped by a joist hanger. In that scenario a thick galvanized coat may actually outperform a stainless bolt trapped without air.
Mechanics also matter. Galvanized threads are slightly oversized by the zinc layer; installers must chase nuts or re-tap if precise torque is required. Stainless threads are dimensionally clean but prone to galling under high RPM, so hand-driving with a dab of anti-seize is recommended. Mixing metals—galvanized bracket with stainless bolt—invites galvanic corrosion; isolate with nylon washers or choose matching alloys.
Bottom line? For decks in Kansas or pergolas in Phoenix, hot-dip galvanized lag bolts deliver 50–80 years of service at little cost. If your project drips with salt spray or you simply refuse to revisit the job, 304 (or 316) stainless is the true lifetime option. In dollars per rust-free year, galvanized wins on spreadsheets; stainless wins on peace of mind.