Surface Treatment Comparison: Zinc Plating vs HDG vs Zinc Flake

Zinc electroplating

ISO 4042 addresses electroplated coating systems for fasteners, including zinc and zinc alloys. A finish specification should identify the coating system, any conversion layer, sealer and lubricant, rather than only a colour.

The standard includes requirements and recommendations to reduce hydrogen-embrittlement risk. For high strength fasteners, agree the process controls and applicable verification with the supplier before choosing an electroplated finish.

Hot-dip galvanizing

ISO 10684 covers spun hot-dip galvanized coatings on coarse-threaded steel fasteners from M8 to M64, up to class 10.9 for bolts, screws and studs and class 12 for nuts. Its scope should not be extended automatically to class 12.9 bolts.

Specify the mating thread arrangement and check fit after coating. The coating and nut thread allowances belong in the assembly specification, not just in a note describing the desired appearance.

Zinc flake coatings

ISO 10683 covers non-electrolytically applied zinc flake systems. These systems are used on high strength fasteners to address internal hydrogen-embrittlement concerns associated with coating processes. Preparation and application still need controlled procedures.

Confirm the complete coating system, including topcoat and lubricant. A trade name alone does not define corrosion performance, friction behaviour or suitability for the intended joint.

Compare quotations on the same basis

Give each supplier the same service environment, corrosion test and acceptance criteria, thread-fit requirements, friction requirements and quantity. Ask for the process specification and relevant test results. Compare the quoted coating system and inspection scope, not colour or price alone.