Zinc Plated or Dacromet Screws? Choosing a Coating That Protects High-Strength Screws

Surface treatment protects fasteners from corrosion, which would otherwise reduce the thread cross-section and clamping force.

Selection should consider the operating environment, corrosion requirements and strength grade. For high-strength screws and self-drilling screws, which have higher hardness, the effect of the coating process on hydrogen embrittlement risk must also be taken into account.

1. Alkaline and Acid Processes for Zinc Plated Screws

Zinc plated screws offer low cost and a uniform finish, making them the most widely used fastener products. Depending on the bath chemistry, zinc electroplating is divided into acid zinc and alkaline zinc.

Alkaline zinc plating is now widely used across the fastener industry. When high-strength screws of class 10.9 and above are zinc plated, alkaline zinc is recommended, followed by hydrogen relief baking in accordance with ISO 4042.

2. Dacromet Screws for High-Strength Fasteners

Dacromet is a zinc-aluminum flake coating applied by dip-spin coating and then cured in an oven. Because the process involves no acid pickling or electrolysis, Dacromet screws carry a very low risk of hydrogen embrittlement and generally offer better corrosion resistance than zinc plated screws. They are widely used for high-strength screws and automotive parts. Geomet and similar zinc flake coatings belong to the same category.

3. Hot-Dip Galvanized Screws and Phosphate Coating

Hot-dip galvanized screws are produced by immersing fasteners in molten zinc to form a thick coating, making them suitable for outdoor steel structures and infrastructure. Because the coating is thick, thread clearance must be considered when pairing with nuts. Phosphate coating forms a phosphate layer finished with oil, providing basic rust protection and lubrication for indoor use or as a base for painting.

4. Choosing a Coating for Self-Drilling Screws

Self-drilling screws are case hardened and have high surface hardness. Common coatings include zinc plating and zinc flake. For outdoor applications such as roofing and cladding, coatings with higher corrosion resistance are recommended. If zinc plating is used, hydrogen relief baking must be properly carried out.

5. Verifying Corrosion Resistance

The corrosion resistance of different coatings is generally compared by salt spray testing under standards such as ISO 9227, measured by the hours before white rust and red rust appear. When comparing options, make sure the same test standard and conditions are used, and consider the actual operating environment and expected service life.

With over 40 years in the fastener industry, PATTA applies consistent quality standards from requirement review, inspection and testing to mass production and global delivery. If you are evaluating zinc plated screws, Dacromet screws or other coating options, our team is happy to recommend solutions suited to your application.

👉 Contact us: https://www.patta.com/contact/

6. FAQ

Q1: What is the difference between zinc plated screws and Dacromet screws?
A: Zinc plated screws go through acid pickling and electrolysis, which generate hydrogen. Dacromet screws are coated by dipping and curing without electrolysis, so the risk of hydrogen embrittlement is very low and corrosion resistance is generally better.

Q2: Should high-strength screws use alkaline or acid zinc plating?
A: Alkaline zinc is recommended. It provides a more uniform coating across the thread and allows hydrogen to escape more easily during baking, helping reduce the risk of hydrogen embrittlement.

Q3: Where are hot-dip galvanized screws used?
A: Hot-dip galvanized screws have a thick, weather-resistant coating suited to outdoor steel structures and infrastructure. Thread clearance must be considered when pairing them with nuts.