Corrosion Protection Process Comparison for Power Engineering Fasteners

Why Anti-Corrosion Process Selection Matters

Power engineering fasteners endure prolonged outdoor exposure. Coastal salt spray, industrial acid rain, high temperature, and high humidity each accelerate metal corrosion. A wrong anti-corrosion choice can cause bolts to develop red rust at half their design life, and high-strength bolts may face hydrogen-induced delayed fracture.

By the end you will know how to match anti-corrosion processes to environmental corrosivity categories and bolt strength grades, and be able to select the right process for your power engineering project.

Selecting an anti-corrosion process is not about picking the most expensive or thickest coating, it is about matching three core variables: environmental corrosivity category, bolt strength grade, and post-coating fit accuracy requirements.

ISO 9223 Corrosivity Category Quick Reference

Category	Environment Description	Typical Scenario	Carbon Steel Annual Corrosion Loss	Suitable Anti-Corrosion
C1	Very Low	Dry indoor	≤1.3 μm	black oxide/Phosphating sufficient
C2	Low	Rural areas	1.3–25 μm	zinc plated
C3	Medium	Urban industrial areas	25–50 μm	hot-dip galvanized ≥45μm
C4	High	Coastal inland transition	50–80 μm	Hot-dip galvanizing ≥85μm or Dacromet
C5	Very High	Coastal high salt spray	80–200 μm	Dacromet or Sherardizing
CX	Extreme	Offshore platforms	200–700 μm	Dacromet + topcoat or stainless steel

Hot-dip galvanizing coating is sufficient for C3 and below. For C4 and above, hot-dip galvanizing begins to struggle. For C5 and above, if anti-corrosion compatibility is not considered and hot-dip galvanizing is used directly, extensive red rust within 5 years is a high probability event.

In-Depth Comparison of Three Processes

Hot-Dip Galvanizing (HDG)

Principle: The workpiece is immersed in molten zinc at 450°C. Zinc and iron undergo a metallurgical reaction, forming a composite coating of multiple zinc-iron alloy layers plus a pure zinc layer. The coating is metallurgically bonded to the substrate.

Advantages: Most mature process, lowest cost, thick coating (45–85μm), covers all surfaces including internal holes and threads.

Critical Shortcoming: Bolts of grade 10.9 and above have a risk of hydrogen embrittlement. Hydrogen atoms penetrate the steel substrate during the pickling process, accumulating in high-stress areas and leading to delayed fracture. ISO 898-1:2013 §9.7 explicitly states that bolts with HRC≥32 are not recommended for electroplating. For hot-dip galvanizing, pickling time must be strictly controlled and dehydrogenation is mandatory (200°C × 4h or more). Many small manufacturers do not implement this properly.

Dacromet

Principle: The workpiece is dip-coated with an inorganic coating containing zinc and aluminum flakes, then baked at 300°C to cure, forming a zinc-aluminum composite coating. The entire process involves no pickling, completely eliminating hydrogen embrittlement.

Advantages: Thin coating (5–15μm) does not affect thread fit, no hydrogen embrittlement, salt spray test can reach 1,000h, suitable for high-strength bolts.

Disadvantages: Poor wear resistance (coating hardness ~HV200, lower than hot-dip galvanizing’s ~HV300), cost is higher than hot-dip galvanizing, poor coating conductivity (unsuitable for grounding connectors).

Sherardizing

Principle: The workpiece is mixed and sealed with zinc powder, heated to 380–450°C. Zinc atoms thermally diffuse into the steel surface, forming a zinc-iron alloy layer. It is a diffusion layer, not a coating.

Advantages: Uniform coating (no accumulation on threads), no hydrogen embrittlement, salt spray can reach 1,200h, high surface hardness (~HV400), highest thread fit accuracy.

Disadvantages: Long production cycle (3–6 hours per batch), high cost for large workpieces, efficiency decreases for sizes above M20.

Parameter Matrix

Comparison Dimension	Hot-Dip Galvanizing (HDG)	Dacromet	Sherardizing
Anti-Corrosion Category	C3–C4	C4–C5	C4–C5
Coating Thickness	45–85 μm	5–15 μm	15–75 μm
Salt Spray Test (NSS)	500–800h	480–1,000h	600–1,200h
Hydrogen Embrittlement Risk	Yes (Grade 10.9+)	No	Low (Near None)
Thread Fit	Requires tolerance allowance	No impact	No impact (Most Precise)
Surface Hardness	~HV300	~HV200	~HV400
Coating Conductivity	Good	Poor	Good
Relative Cost	1×	1.5–2×	1.2–1.8×
Applicable Specifications	Various	Various	Optimal for M20 and below
Reference Standard	ISO 1461 §4 / ASTM A123	ISO 10683 / GB/T 5267.2	EN 13811 / ASTM A1059

Selection Decision Framework

It is not about “which process is best,” but “which one matches your operating conditions.”

If your condition is…	Recommended Process	Reason
C3–C4 Inland + Grade 8.8 Bolts	Hot-Dip Galvanizing	Best cost, 20–30 year lifespan inland
C3–C4 Inland + Grade 10.9 Bolts	Dacromet or Hot-Dip Galvanizing (with dehydrogenation proof)	Prioritize hydrogen embrittlement avoidance
C4–C5 Coastal + Any Grade	Dacromet or Sherardizing	Hot-dip galvanizing in C5 may show red rust within 5–8 years
C5 Offshore Wind + Grade 10.9	Dacromet + epoxy topcoat	Dual protection
M20 and below + High Thread Fit	Sherardizing	Most uniform coating, highest precision
Budget Sensitive + C3 or below	Hot-Dip Galvanizing	Meets requirements at lowest cost

Next Steps

To finalize your anti-corrosion process selection, prepare the following:

• [ ] Determine the ISO 9223 corrosivity category for your project site (C1–CX).
• [ ] Identify the bolt strength grade (e.g., 8.8, 10.9) and any hydrogen embrittlement constraints.
• [ ] Specify thread fit requirements and whether post-coating tolerance adjustments are acceptable.
• [ ] Estimate the total fastener quantity and budget to compare initial cost vs. lifecycle cost.

For further assistance, contact our engineering team or explore our fastener products.