Stainless Steel Bolt Grades: 18-8 vs 316 vs 410 Explained
You're specifying fasteners for a dock rebuild, a coastal deck, or marine-adjacent metal framing and the hardware store shelf says "stainless" on three different boxes with three different prices. All three look the same. None of them are interchangeable.
Getting the grade wrong in a saltwater environment doesn't mean surface rust — it means pitting corrosion, red staining, and structural failure within two to five years. This guide gives you the alloy breakdown, tensile data, and application rules for every common stainless fastener grade.
What Makes Stainless Steel "Stainless"
Plain carbon steel rusts because iron reacts with oxygen and water to form iron oxide (rust). Stainless steel resists this by forming a thin, self-repairing chromium oxide passive layer on the surface. You need at least 10.5% chromium content for that layer to form reliably — that's the minimum for any stainless designation.
Beyond that baseline, different alloy additions change corrosion resistance, strength, weldability, and cost. The three grades that appear on fastener shelves — 304, 316, and 410 — use different formulas for different jobs.
Type 304 (18-8): The General-Purpose Grade
Type 304 is the most widely produced stainless steel in the world — roughly 50% of all stainless output. The "18-8" shorthand refers to its chromium-nickel composition: approximately 18% Cr and 8% Ni. It is austenitic, meaning it is non-magnetic in the annealed condition and cannot be hardened by heat treatment.
In fastener form, 304 has a minimum tensile strength of about 70,000–85,000 psi (depending on wire diameter and cold-working). That's lower than Grade 5 carbon steel (120,000 psi) and well below Grade 8 (150,000 psi). Use 304 stainless where corrosion resistance, not load, is the design driver.
Where 304 fails: chloride environments. Chloride ions — present in ocean air, pool water, road de-icing salt, and some chemical cleaning products — penetrate the passive oxide layer and cause pitting corrosion. You'll see it as small dark pits surrounded by rust staining. Once pitting starts, it accelerates.
Type 316: The Marine Grade
Type 316 starts with the same 18-8 austenitic base as 304, then adds 2–3% molybdenum (Mo). Molybdenum is the critical element: it stabilizes the passive film against chloride attack, dramatically raising the pitting resistance equivalence number (PREN). That single addition makes 316 the standard for marine, coastal, and chemical processing applications.
Tensile strength for 316 is comparable to 304 — 70,000–85,000 psi in standard fastener form. You don't get extra strength from the molybdenum addition; you get corrosion performance.
316 vs 316L
316L is a low-carbon variant (max 0.03% C vs 0.08% for standard 316) designed to prevent carbide precipitation during welding. For bolts, screws, and most structural fasteners, the distinction is irrelevant — the lower carbon content does not meaningfully affect strength or corrosion resistance in the fastener context. Don't pay a premium for 316L if you're not welding.
316 and Pressure-Treated Lumber
Modern pressure-treated lumber uses copper-based preservatives — ACQ (Alkaline Copper Quaternary), CA (Copper Azole), and MCA (Micronized Copper Azole). These preservative systems are significantly more corrosive to zinc-plated fasteners and even hot-dipped galvanized (HDG) hardware than the old CCA-treated lumber. For wet or coastal PT applications, 316 stainless is the safest specification. In dry inland climates, HDG fasteners meeting ASTM A153 remain code-compliant for most residential deck work.
Type 410: Strength Over Corrosion
Type 410 is martensitic stainless — an entirely different metallurgical family. Unlike austenitic 304 and 316, martensitic grades can be hardened by heat treatment (quenching and tempering), reaching tensile strengths up to 185,000 psi. They are also magnetic.
The trade-off is corrosion resistance. Type 410 contains only 11.5–13.5% Cr and no nickel. That's barely above the 10.5% minimum for the stainless designation, and it performs accordingly — roughly one-third the pitting resistance of 304.
You'll find 410 stainless in two common fastener applications:
- Wood screws and deck screws: The hardness from heat treatment allows the screw tip to self-drill or cut threads into wood and composite decking. Type 304 is too soft for this in most production processes.
- Self-tapping screws for sheet metal: Same logic — 410 can be formed with a cutting flute or Tek point that 304 cannot hold reliably.
Full Comparison Table
| Property | 304 (18-8) | 316 | 410 |
|---|---|---|---|
| Chromium % | 18–20 | 16–18 | 11.5–13.5 |
| Nickel % | 8–10.5 | 10–14 | — |
| Molybdenum % | — | 2–3 | — |
| Tensile strength | 70–85 ksi | 70–85 ksi | 80–185 ksi |
| Heat hardenable | No | No | Yes |
| Magnetic | Slightly (worked) | Slightly (worked) | Yes |
| Chloride resistance | Moderate | High | Low |
| ISO designation | A2 | A4 | — |
| Relative cost | $ | $$–$$$ | $ |
| Typical use | General hardware | Marine/coastal | Wood screws |
How to Read Stainless Fastener Markings
Stainless bolts are rarely stamped with a grade the way SAE carbon steel bolts are (no radial lines = Grade 2, 3 lines = Grade 5, etc.). Instead, look for:
- "18-8" or "304" on the packaging or stamped on larger bolt heads — confirms austenitic general-purpose grade
- "316" or "A4" — confirms molybdenum-bearing marine grade (ISO A4 = Type 316)
- "A2" — ISO designation for Type 304; common on metric stainless fasteners from European manufacturers
- Property class suffix on metric: "A4-70" means Type 316, 700 MPa (~101,000 psi) minimum tensile; "A4-80" means 800 MPa (~116,000 psi)
Unmarked stainless screws — especially wood screws and sheet metal screws — are almost always 410. Hex head bolts marketed as "marine grade" should be verified as 316 by the supplier, not assumed.
Application Decision Tree
└─ Within 2 miles of salt water → 316 (required)
└─ Pool water, chemical exposure → 316 (required)
└─ PT lumber, coastal region → 316 (safest)
└─ PT lumber, inland, dry climate → HDG or 304
└─ General outdoor, no chlorides → 304 (fine)
└─ Wood screws into structural lumber → 410 (typical)
└─ Self-tapping into metal sheet → 410 (hardened tip)
Price Reality Check
A box of 100 × 3-inch #10 deck screws in 316 stainless runs roughly $25–$40, versus $10–$15 for comparable 304 or coated carbon steel. On a 500 sq ft deck requiring 500–700 screws, upgrading from 304 to 316 adds $50–$100 to the fastener budget. Compare that to the cost of a deck refinish or structural repair in five years. It's not close.
For structural bolts — 1/2-inch HDG carriage bolts for a ledger connection — the jump from HDG to 316 stainless is larger: roughly 3× the unit cost. In that application, confirm with your AHJ whether 316 is required or whether HDG per ASTM A153 is acceptable for the exposure class.
Identify Your Fastener Now
Not sure what grade your existing stainless fastener is? Use our free identifier tool — enter the head type, drive, and measurements to get the full designation.
Open Identifier →Recommended Tool
316 Stainless Deck Screws — Marine Grade
For coastal decks, docks, or any PT lumber in salt-air environments, start with verified 316 stainless fasteners. These are the grade to specify when the box has to say more than just "stainless."
Shop 316 Stainless Screws on Amazon →Frequently Asked Questions
- What is the difference between 18-8 and 316 stainless steel bolts?
- 18-8 stainless (Type 304) contains 18% chromium and 8% nickel. It resists corrosion well in most indoor and mild outdoor environments. 316 stainless adds 2–3% molybdenum to the 18-8 formula, which dramatically improves resistance to chloride pitting — the corrosion mechanism in saltwater, pool water, and chemical environments. Use 316 within two miles of saltwater, in contact with pressure-treated lumber in coastal areas, or anywhere chlorides are present.
- Is 410 stainless stronger than 304 or 316?
- 410 stainless (martensitic) can be heat-treated to higher hardness than 304 or 316, giving it tensile strength up to 185,000 psi in hardened condition vs. 70,000–85,000 psi for annealed 304. However, 410 has far less corrosion resistance — roughly one-third of 304's resistance — because it contains no nickel and only 11.5–13.5% chromium. It is used for wood screws and self-tapping screws where hardness matters more than corrosion resistance.
- Can I use 304 stainless for deck screws near the ocean?
- No — not within about two miles of the ocean. Saltwater spray contains chloride ions that attack the passive oxide layer on 304 stainless, causing pitting corrosion and eventual red staining or fastener failure. Use 316 stainless for any coastal deck project. Building codes specifically require 316 or hot-dipped galvanized hardware within defined distances of salt water.
- What do A2 and A4 mean on stainless fasteners?
- A2 and A4 are ISO/European designations printed on metric stainless fasteners. A2 corresponds to Type 304 (18-8). A4 corresponds to Type 316 (the molybdenum-bearing marine grade). A metric bolt stamped "A4-70" means 316 stainless, property class 70 (minimum 700 MPa tensile strength, approximately 101,000 psi).
- What stainless grade is required for pressure-treated lumber?
- Current pressure-treated lumber uses copper-based preservatives (ACQ, CA, MCA) that are highly corrosive to zinc-plated hardware in wet conditions. For interior or dry climates, hot-dipped galvanized (HDG) meeting ASTM A153 is typically code-compliant. For coastal or wet environments, 316 stainless is the safest choice. Check your local building code and manufacturer specifications.