Overview
ISO 17668:2016 - Zinc diffusion coatings on ferrous products - Sherardizing - Specification defines minimum requirements for zinc diffusion (sherardized) coatings applied to ferrous articles. The standard covers the sherardizing process outcome - zinc‑iron inter‑metallic layers - and specifies minimum coating thicknesses for six coating classes intended to provide protection against corrosion and wear. It does not set requirements for base‑metal surface finish or for post‑treatments/organic over‑coatings (duplex systems), although informative annexes provide guidance on post‑treatments and corrosion performance.
Key topics and technical requirements
- Coating scope: Specifies minimum thickness requirements for six classes of sherardized zinc diffusion layers (zinc‑iron alloy coatings) on ferrous products.
- Process parameters (informative): Sherardizing is typically performed in closed, slowly rotating or fixed containers at about 300 °C to 500 °C, normally below zinc’s melting point (419 °C). Resulting coating thicknesses of around 10 µm to 75 µm (and higher if required) are achievable.
- Surface condition: The standard does not mandate base‑material surface finish or roughness; the base material should, however, be clean prior to sherardizing.
- Inspection and sampling: Acceptance inspection is carried out at the sherardizer’s works before delivery and includes appearance assessment and thickness testing. A control sample is taken per inspection lot; sample sizes vary by batch (e.g., batch sizes 4–500 → sample 3; above 10 000 → sample 20).
- Thickness measurement methods: Routine methods include the magnetic method (ISO 2178) and electro‑magnetic method (ISO 2808 / ISO 3882); the gravimetric method (ISO 1460) may be used to determine coating mass and calculate thickness.
- Acceptance criteria and appearance: Coating appearance is typically grey (matt or lustrous) with characteristic roughness; superficial scratches and white zinc corrosion products during storage are not cause for rejection. Orange‑brown discoloration at delivery is not permitted unless agreed.
Practical applications and users
Who uses ISO 17668:2016:
- Sherardizers and coating service providers for process control and contractual specifications.
- OEMs and purchasers specifying durable corrosion protection on ferrous components (gears, housings, fittings).
- Quality engineers and inspection personnel for acceptance testing and sampling plans.
- Designers and coating specifiers using sherardized layers as primer/base coats in duplex systems.
Practical benefits:
- Provides clear, industry‑recognized thickness classes for procurement and quality assurance.
- Recommends appropriate measurement methods for routine plant control and acceptance testing.
- Helps align supplier and purchaser expectations for corrosion/wear performance.
Related standards
- ISO 2178 - Measurement of coating thickness (magnetic method)
- ISO 1460 - Gravimetric determination of coating mass (hot‑dip context)
- ISO 2808 / ISO 3882 - Paints/film thickness and electro‑magnetic measurement methods
- ISO 14713 (informative) - Guidance on corrosion protection of steel by zinc coatings
Keywords: ISO 17668:2016, sherardizing, zinc diffusion coatings, zinc‑iron inter‑metallic layers, coating thickness, corrosion protection, ferrous products, magnetic method, gravimetric method, acceptance inspection.
