Surface Anti-Corrosion Technology
Sherardizing: Long-Life Zinc Diffusion Protection for Steel Components
Sherardizing, also called powder zinc diffusion coating, is a solid-state thermal diffusion process. Zinc atoms penetrate the steel surface at elevated temperature and react with the iron substrate to form a metallurgically bonded iron-zinc alloy layer dominated by the intermetallic phases FeZn7 and FeZn13. The result is not a film sitting on top of the steel: it is a graded alloy system that becomes part of the component itself.
How the Process Works
At elevated temperature, zinc atoms migrate under Fick's diffusion law. With the aid of an activator, they enter through the micro-defects and pores of the steel surface and move inward, building a dense and uniform layer.
Zinc reacts with the iron substrate to form stable intermetallic compounds, while the zinc powder and activator sustain a continuous supply of active zinc atoms into the base material.
The finished system comprises a mixed layer of pure zinc and intermetallics, a zinc-iron alloy layer and a substrate diffusion zone. The layers are tightly interlocked, so corrosion protection and bonding strength are achieved at the same time.
What Buyers Gain Beyond Corrosion Resistance
Compared with electro-galvanizing and hot-dip galvanizing, the diffusion-formed alloy layer changes the economics of protection over the full service life of the asset.
The zinc-iron alloy layer bonds to the substrate with 300–350 MPa structural stability and 600–700 MPa adhesion, rated adhesion grade 1, so the coating does not peel or flake.
Coating thickness depends only on temperature and holding time, not on part geometry, so threads, inner walls, blind holes and grooves receive the same layer without losing fit.
No acid pickling and no electrolytic deposition step, so no hydrogen is introduced. High-strength fasteners can be coated without embrittlement cracking risk.
The thermal diffusion process forms an alloy layer without weakening the substrate. Structural strength is maintained, and fatigue strength under alternating loads is improved.
At HV 200–400, the layer withstands abrasion and vibration far better than hot-dip zinc at HV 71–265, extending service life of moving and loaded parts.
The closed solid-state process produces no harmful discharge, uses zinc powder with high utilisation and consumes less energy than conventional coating lines.
Measured Performance Against Conventional Zinc Coatings
| Property | Electro-galvanizing | Hot-dip galvanizing | Powder sherardizing |
|---|---|---|---|
| Coating thickness | 5–30 μm | 5–100 μm | 20–120 μm |
| Zinc layer hardness | HV 75–85 | HV 80–90 | HV 200–400 |
| Bond type | Mechanical | Metallurgical | Metallurgical |
| Adhesion grade | Grade 4 | Grade 3 | Grade 1 |
| Heat resistance | General | Good | Very good |
| Hydrogen embrittlement | Very strong | Relatively strong | None |
| Service life at 20 μm | 3–5 years | 5–20 years | 15–25 years |
Independent neutral salt spray testing to GB/T 10123, carried out by a quality supervision station in Tianjin: sherardized samples showed no rust after 1,080 hours, while hot-dip galvanized samples rusted at 96 hours. Nano-composite sherardizing raises inner-layer hardness to HV 404–587 when higher wear and high-temperature oxidation resistance are required.
Where Sherardized Steel Is Specified
The technology is chosen wherever maintenance access is expensive and corrosion failure is not an option.
Compliance With International and Chinese Standards
- ISO 14713-3:2017 — zinc coatings, guidance for sherardizing
- ISO 17668:2016 / EN ISO 17668:2016 — zinc diffusion coatings on iron
- EN 13811:2003 — zinc diffusion coatings on iron products
- BS 4921 — sherardizing of iron and steel
- ASTM A1059/A1059M — zinc alloy thermal diffusion coatings
- ASTM B695 / MIL-C-81562 — mechanically deposited zinc coatings
- JB/T 5067-1999 — powder sherardizing of iron and steel parts
- JB/T 8418-2008 — powder metallizing
- GB/T 35505-2017 — quality control of sherardizing for steel structures
- TB/T 3274-2011 — railway concrete beam fittings, multi-alloy co-diffusion
- SY/T 7353-2016 — thermal diffusion zinc protection for metal components
- GB/T 6461-2002 — rating of specimens after corrosion testing
Batch-wise coating thickness reports, salt spray records and hardness verification can be provided with delivery, and material certificates are traceable to heat number.
Frequently Asked Questions
At an equal 20 micron thickness, service-life evaluation gives electro-galvanizing 3–5 years, hot-dip galvanizing 5–20 years and powder sherardizing 15–25 years. In the salt spray test cited above, sherardized samples reached 1,080 hours without red rust.
Yes. Thickness is controlled by temperature and holding time and is independent of part shape and position. Precision vacuum sherardizing keeps dimensional and form tolerance intact, so no re-tapping is needed after treatment.
Yes. The process introduces no hydrogen, so hydrogen embrittlement sensitivity is rated none, compared with very strong for electro-galvanizing and relatively strong for hot-dip galvanizing.
Yes. ISO 14708-3:2017 describes this duplex system approach, where a sherardized item receives an additional coating layer for heavy-duty service. Sherardized steel also provides a strong base that extends the durability of the paint applied over it.
Part drawings or photos, base material grade, dimensions and weight, required coating thickness, applicable standard, quantity and destination port. A technical proposal is normally returned within two working days.
Send Your Parts for a Sherardizing Assessment
Share drawings, base material and target design life. MESCO will recommend a coating thickness and standard for your service environment, provide samples for evaluation, and quote per your project terms.