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Views: 0 Author: Site Editor Publish Time: 2026-08-26 Origin: Site
A carbon steel bolt (grade 4.8–12.9) coated with a zinc layer that corrodes sacrificially to protect the steel underneath.
Type | Coating thickness | Typical use | Cost level |
|---|---|---|---|
Electro-galvanized (zinc plated) | 5–12 µm | Indoor, light outdoor, furniture, appliances | Low |
Hot-dip galvanized (HDG) | 45–85 µm | Outdoor structures, transmission towers, marine-adjacent | Medium |
Mechanical galvanized | 10–40 µm | Springs, hardened parts | Medium |
Key point: the zinc coating provides the corrosion protection; the carbon steel core provides the strength.
An alloy steel containing chromium (≥10.5%) that forms a self-healing passive oxide layer. The corrosion resistance is in the material itself, not a coating — so scratches and cuts do not create rust paths.
Grade | Common designation | Tensile strength | Typical use |
|---|---|---|---|
304 (A2) | A2-70 / A2-80 | ~700 MPa (A2-70) | General outdoor, food, mild chemical |
316 (A4) | A4-70 / A4-80 | ~700 MPa (A4-70) | Marine, coastal, chloride exposure |
Factor | Electro-galvanized (zinc) | Hot-dip galvanized (HDG) | Stainless 304 (A2) | Stainless 316 (A4) |
|---|---|---|---|---|
Corrosion resistance | Good (indoor), limited outdoor | Very good (thick coating) | Excellent | Excellent (best for chloride/salt) |
Strength available | 4.8–12.9 (up to 1,200 MPa) | 8.8–10.9 typical | A2-70 ≈ 700 MPa | A4-70 ≈ 700 MPa |
Scratch/abrasion behavior | Zinc layer can be damaged | Coating tough, can chip at edges | Self-healing passive layer | Self-healing passive layer |
Marine / coastal use | ❌ Short lifespan | Good with thick coating, but coating damage needs repair | Good, but 304 can pit in aggressive salt | ✅ Best choice |
Chemical exposure | ❌ | Good | ✅ Best choice | |
Cost (relative) | 1× | 1.3–1.8× | 3–4× | 4–6× |
Reusability | Limited | Limited | High | High |
Galvanic compatibility | With carbon steel | With carbon steel | ⚠️ Can corrode faster when paired with carbon steel in wet conditions | ⚠️ Same caution |
Mistake 1: Using stainless bolts when strength is the requirement. A 304 A2-70 bolt is only about as strong as a grade 5.8 carbon steel bolt. If your joint needs 8.8 or 10.9 strength, standard stainless will fail — you need high-strength carbon steel (galvanized) or a high-strength stainless like A4-80, duplex, or precipitation-hardened grades.
Mistake 2: Using zinc-plated bolts outdoors and expecting years of service. Electro-galvanized coating (5–12 µm) is designed for indoor environments. Outdoors, especially with rain or humidity, it typically rusts within 1–3 years. For outdoor use, specify hot-dip galvanized (45–85 µm) instead.
Mistake 3: Mixing stainless bolts with carbon steel components in wet environments. Galvanic corrosion accelerates when dissimilar metals contact in an electrolyte (water). If you must mix, isolate with insulating washers or use compatible materials throughout the joint.
Mistake 4: Confusing "stainless" with "rust-proof." Stainless can still pit or stain — 304 in aggressive coastal salt environments is a classic failure. That is what 316 (with molybdenum) is for.
Your situation | Recommended choice |
|---|---|
Indoor machinery, furniture, appliances | Electro-galvanized 8.8 |
Outdoor structure, moderate climate | Hot-dip galvanized 8.8/10.9 |
Coastal, marine, chemical plant | Stainless 316 (A4-70/A4-80) |
Food processing | Stainless 304/316 |
High-strength requirement (dynamic load) | Galvanized 10.9 (or higher-strength stainless) |
Budget-critical, low corrosion risk | Electro-galvanized 4.8/8.8 |
Long service life, minimal maintenance | Stainless 316 or HDG |
Hex bolts (full thread): DIN 933 / ISO 4017 / GB/T 5783
Hex bolts (partial thread): DIN 931 / ISO 4014 / GB/T 5782
Property classes: ISO 898-1 (carbon steel 4.8–12.9), ISO 3506 (stainless A2/A4)
Coating: ISO 4042 (electroplated), ISO 10684 (hot-dip galvanized fasteners)
Specifying the standard plus the finish in your inquiry eliminates most supply confusion — e.g., "DIN 933, M12×40, 8.8, hot-dip galvanized per ISO 10684."
Ningbo Weifeng Fastener Co., Ltd. supplies both galvanized (electro, hot-dip, Dacromet) and stainless steel (304/316) bolts in grades 4.8–12.9, to DIN, ISO and GB standards. As a fastener manufacturer serving global buyers since 2003, we can recommend the right material and finish for your environment — browse our hex bolt range or send your inquiry with the application details, and we will confirm the correct specification and samples.
Is stainless steel stronger than galvanized steel bolts? Not necessarily. Standard stainless (A2-70) is about equivalent to carbon steel grade 5.8. Galvanized high-strength bolts (8.8/10.9) are significantly stronger. Strength and corrosion resistance are separate properties — choose each for what you need.
How long do galvanized bolts last outdoors? Electro-galvanized: 1–3 years in humid outdoor conditions. Hot-dip galvanized: typically 20–50+ years in rural/suburban atmospheres, less in aggressive coastal or industrial environments.
Does stainless steel rust? Under most conditions, no — the chromium oxide layer is self-healing. But 304 can pit in aggressive chloride (salt) environments; 316 contains molybdenum and resists chlorides far better.
Can I use galvanized bolts in a marine environment? Only hot-dip galvanized with a thick coating, and expect coating degradation over time. For long-term marine service, stainless 316 is the standard recommendation.
Why do stainless bolts cost so much more? Alloying (chromium, nickel, molybdenum) and processing cost more than zinc coating on carbon steel. The premium buys corrosion resistance with no maintenance — often cheaper over the product lifecycle.
Do you offer both galvanized and stainless bolts? Yes. We manufacture hex bolts and other fasteners in carbon steel (4.8–12.9) with electro-galvanized, hot-dip galvanized, black oxide or Dacromet finishes, and in stainless steel 304/316. Send us your application and we will recommend the most cost-effective option.
Sources: ISO 898-1 (carbon steel property classes), ISO 3506 (stainless steel fastener mechanical properties), ISO 4042 (electroplated coatings), ISO 10684 (hot-dip galvanized fasteners), ASTM A153/A153M. Application guidance based on common engineering practice for fastener corrosion protection.
