Dacromet-Coated Socket Head Cap Screws

2026-11-03

1. Introduction

Socket head cap screws,standardized in ISO 4762 / DIN 912 — are the bolts of choice wherever high clamping loads, compact heads, and precise torque control come together. Because they are typically manufactured in property classes 8.8, 10.9 and especially 12.9, these bolts operate at a high percentage of their yield strength, and that same strength makes them exceptionally vulnerable to hydrogen embrittlement, stress corrosion cracking, and rust initiated at the head fillet and hex socket walls.

Dacromet coating — the non-electrolytic zinc-aluminum flake system defined by ISO 10683 and ASTM F1136 — addresses precisely these weaknesses. In an 8–12 μm thin film it delivers 720 to 1,000 hours of salt spray resistance to first red rust (ASTM B117 NSS), creates no hydrogen embrittlement risk during processing, and preserves the precise hex socket geometry and 6g thread tolerance that socket head cap screws depend on.

2. Bolt Geometry and Design Features

A socket head cap screw is a full-strength bolt with a cylindrical head and an internal hexagon socket drive:

Cylindrical headwith a knurled circumferential surface; head diameter dk ≈ 1.5 × d — roughly 30–40% smaller than a hexagon head bolt of the same size, allowing compact, close-pitched bolt patterns;
Head-to-shank fillet, the fatigue-critical transition zone where stress concentration is highest;
Full thread or partial thread(smooth shank) styles; partial-thread screws place the shear plane in the unthreaded shank for higher shear capacity;
Point variants: flat (chamfered) point standard; dog pointand cone point for location and locking functions.

Compared with hex bolts, SHCS generate higher preload for the same torque thanks to the efficient socket drive, and they are installed with hex keys, torque wrenches, or powered hex drivers — making them standard bolts in machine tools, injection molds, hydraulic manifolds, jigs and fixtures, and robotics.

3. Corrosion Performance Comparison

Coating on class 8.8–12.9 SHCS	Thickness	Salt spray to red rust (ASTM B117)	Hydrogen embrittlement risk	Notes
Black oxide (blackened), oiled	< 1 μm	< 24 h	None	Cosmetic only; oil washes off
Zinc phosphate + oil	3–5 μm	≈ 48–72 h	None	Common "black" finish, limited life
Electro-galvanized Zn12, Cr(VI)-free	12 μm	≈ 192 h	High (12.9 not recommended)	Socket and threads thin-plated
Zinc-nickel alloy electro-plate	8–12 μm	≈ 480–720 h	Moderate	Higher cost; needs post-bake
Dacromet DAC 320 (Class 2)	8 μm	≈ 720 h	Virtually none	Silver gray
Dacromet DAC 500 / black	10–12 μm	≈ 1,000 h	Virtually none	Black finish matches blackened series

6. Standards and Typical Product Range

Key standards: ISO 4762 / DIN 912 (socket head cap screws), ISO 898-1 (mechanical properties), ISO 10683:2018 (non-electrolytic zinc flake coatings), ASTM F1136, ASTM B117, ISO 4042 (electroplating and HE guidance), DIN 267-28 and GB/T 70.1 / GB/T 5267.3.

Parameter	Common Range
Diameter	M3 – M42 (No.4 – 1-1/2")
Length	6 – 300 mm, full thread or partial thread
Property class	8.8 / 10.9 / 12.9
Head style	Standard (ISO 4762), low head (DIN 6912), knurled head
Point	Flat / dog point / cone point
Coating	DAC 320 / DAC 400 / DAC 500; silver gray or black; Cr(VI)-free
Thread tolerance	6g, threads coated without re-tapping

7. Applications and Selection Guidance

1.Match coating to strength: for class 10.9 and 12.9 bolts, insist on non-electrolytic zinc flake (Dacromet / Geomet) to eliminate hydrogen embrittlement;

2.Replace black oxide intelligently: specify black Dacromet when customers want the blackened look with real rust protection;

3.Verify coating classby NSS hours (DAC 320 ≈ 720 h; DAC 500 ≈ 1,000 h) and request the coating manufacturer's test report;

4.Confirm Cr(VI)-free passivation(trivalent chromium) for RoHS / ELV compliance;

5.Check friction data: obtain the nut factor and coefficient-of-friction report for torque-controlled assembly;

6.Complete the system: use coated nuts, washers, and tapped inserts of compatible potential to avoid mixed-potential fastener sets.

8. Conclusion

Dacromet-coated socket head cap screws unite the mechanical virtues of the strongest general-purpose bolts — compact cylindrical heads, efficient hex socket drive, high preload capability — with a coating system engineered for exactly their weak points: no hydrogen embrittlement on 12.9 steel, coverage into the socket and crevice sites, self-healing passivation against pitting and red rust, and friction stability for precise assembly. Specified correctly, they deliver 720–1,000 salt spray hours in an 8–12 μm film, and bring the dark, matte appearance of blackened bolts into genuinely corrosion-proof service.

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