Delta-Tone Coated Class 10.9 Bolts | 500–1000 h+ Salt Spray · 300°C · No Hydrogen Embrittlement
When you need class 10.9 strength together with salt-spray life, the answer is not stainless but Delta-Tone, a zinc-flake coating. The process is non-electrolytic, so no hydrogen is introduced and no baking step is needed. Film thickness is only 4–10 µm, which lets the parts mate on the standard thread — unlike hot-dip galvanising, where the nut has to be opened up by 0.2–0.4 mm. Neutral salt spray reaches 500–1000 h+ and temperature capability about 300°C. Our class 10.9 hex bolts, socket screws and studs can all ship Delta-Tone coated; for demanding export markets we default-recommend Delta-Tone on anything at class 8.8 and above.
Properties & Specifications
| Property Class | 10.9 (quenched and tempered 35CrMo); also applied to 8.8 and 12.9 |
| Tensile Strength | 1000 MPa nominal · 940 MPa minimum yield |
| Hardness | 32 – 39 HRC |
| Coating | Delta-Tone / zinc-flake (Geomet-type) non-electrolytic |
| Coating Thickness | 4 – 10 µm |
| Neutral Salt Spray | 500 – 1000 h+ to red rust |
| Service Temperature | ≈ 300°C |
| Hydrogen Embrittlement | None — no electrolytic step, so no hydrogen bake required |
| Thread Fit | Mates on the standard thread; no nut oversizing needed |
| Relative Cost | Medium–high (vs electro-plated zinc: low) |
Material Advantages
No hydrogen uptake: pickling-induced delayed fracture is the biggest plating risk on 10.9 / 12.9 parts, and a non-electrolytic coating avoids it by construction
Thin film (4–10 µm): no thread allowance on the bolt and no oversize nut, so nothing fails to assemble on the shop floor
About 300°C service capability, far above the 80°C limit of electro-plated zinc — usable on exhaust and engine-perimeter hardware
500–1000 h+ neutral salt spray gives long-life protection outdoors and under de-icing salt, at a lower cost than 316 stainless
Stable friction when bolts and nuts are coated in the same batch, so torque scatter stays under control
Heavy-metal-free process, cleaner than hot-dip galvanising and easier to pass export compliance audits
Applications
Automotive chassis — suspension brackets, subframes and brake brackets in de-icing-salt service
Outdoor high-strength joints — solar mounts, towers, guard rails and lighting columns
Work attachments on construction and farm machinery
Anchor bolts and pipe-rack supports at coastal plants
Engine-perimeter and exhaust hardware (about 300°C capability)
The default corrosion route we recommend on class 8.8 and above for demanding Western markets
Grade Selection
| Class / Grade | Service temperature | Notes |
|---|---|---|
| Electro-Zinc 蓝白锌 | ≤ 80°C | About 48–72 h salt spray, low cost, medium hydrogen risk (bake mandatory at 10.9+) |
| Yellow Zinc / Zn-Ni 彩锌 | ≤ 80°C | 96–240 h salt spray for automotive parts and general outdoor use, low-to-medium cost |
| HDG 热浸锌 | ≤ 250°C（短期更高） | 1000 h+ salt spray but 50–100 µm thick — nuts need 0.2–0.4 mm oversize; low hydrogen risk |
| Delta-Tone 达克罗 | ≤ 300°C | 500–1000 h+ salt spray, 4–10 µm film, no hydrogen embrittlement, standard thread fit — the default for high-strength parts |
| Black Oxide 发黑 | 随基材 Substrate only | Rust prevention only, no salt-spray claim; unsuitable for corrosive service; the 12.9 default |
Frequently Asked Questions
Two reasons. First, hydrogen: pickling and plating drive hydrogen into the steel and without baking this is a delayed-fracture risk at 10.9 and above, whereas Delta-Tone has no electrolytic step and introduces none. Second, thread fit: hot-dip galvanising needs nuts opened up by 0.2–0.4 mm, while a 4–10 µm Delta-Tone film mates on the standard thread. That is why we default-recommend Delta-Tone from class 8.8 up for demanding Western markets.
Decide on corrosion life and how the parts assemble. Hot-dip gives 1000 h+ salt spray and about 250°C, suits steelwork, guard rails and transmission towers, but its 50–100 µm coating needs thread allowance or re-tapping and oversize nuts. Delta-Tone gives 500–1000 h+ and about 300°C with a thin film, so it suits high-strength bolts and equipment plants that need standard-thread mating.
Because 316 cannot reach class 10.9. A4-80 stays at the 800 MPa level, while a Delta-Tone coated 10.9 bolt delivers 1000 MPa tensile with 500–1000 h+ salt spray at a lower cost than 316. This is stated explicitly in the selection guide.
Name the standard and the hours, for example 'Delta-Tone, 720 h to red rust per ISO 9227 neutral salt spray'. Salt-spray life varies with film thickness and process (500–1000 h+ is the usual industry range), and 'finish: zinc plated' is called out in our own article as the vaguest spec in an enquiry — under that same name, salt-spray life spans 48 to 1000+ hours and price differs three-fold.
Get Grade Selection & Quotation
Submit your duty parameters (temperature, pressure, media, speed) — engineers reply with grade selection and quotation within 24 hours.