Hot dip galvanizing specifications
General hot-dip galvanizing (HDG) is a metallurgical process in which steel or cast iron parts are immersed in a bath of molten zinc at an approximate temperature of 450 °C / 842 °F to form a coating of zinc-iron alloys and pure zinc, providing cathodic and barrier protection against atmospheric corrosion. The result is a metallurgical bond between the zinc and the steel substrate, with a coating thickness typically ranging between 45 µm / 1.77 mils and 120 µm / 4.72 mils, controlled by the withdrawal speed, bath composition, and immersion time.
Process Parameters
Section titled “Process Parameters”| Operating Parameter | Typical Value | Unit |
|---|---|---|
| Molten zinc bath temperature | 445 – 455 °C / 833 – 851 °F | – |
| Immersion time | 2 – 10 min | – |
| Bath composition | Zn 98.5–99.5 %, Pb <1 %, Al 0.005–0.02 % | % by weight |
| Resulting coating thickness | 45 – 120 µm / 1.77 – 4.72 mils | – |
| Withdrawal speed | 0.5 – 1.5 m/s / 1.6 – 4.9 ft/s | – |
| Steel preheating temperature | 60 – 150 °C / 140 – 302 °F | – |
The intermetallic reaction generates gamma (Fe₃Zn₁₀) alloy layers with 75–78 % Zn, delta (FeZn₁₀) with 88–93 % Zn, and zeta (FeZn₁₃) with 94–95 % Zn, followed by an outer pure zinc layer (eta) that solidifies upon cooling. The addition of aluminum suppresses the brittleness of the zeta layer and improves surface brightness.
Applicable Materials
Section titled “Applicable Materials”The process is suitable for most carbon steels, low-alloy steels, and cast irons. The reactivity with molten zinc varies according to the chemical composition, particularly the silicon (Si) and phosphorus (P) contents, which influence the final thickness and microstructure.
- Structural carbon steels: ASTM A36, A572, A283, general construction steels (Si ≤0.03 % or 0.15–0.25 % for predictable thicknesses).
- Pipe steels: ASTM A53 Gr. A/B, API 5L, EN 10255; common galvanized dimensions match Schedule 40 and 80 series (diameters from 10.3 mm / 0.405 in to 610 mm / 24 in).
- Low-alloy steels: with controlled contents of Si+P ≤0.04 % to avoid excessively thick alloys (Sandelin effect).
- Gray and ductile cast iron: ASTM A48, A536; require prior shot blasting treatment to remove surface graphite and ensure adhesion.
- Forged and hot-rolled parts: rolling scale must be removed by acid pickling and mechanical cleaning.
Advantages and Limitations
Section titled “Advantages and Limitations”| Advantages | Limitations |
|---|---|
| Active cathodic protection even at edges and scratches (zinc corrodes preferentially). | Continuous service temperature limitation to 200 °C / 392 °F; above this, layer detachment occurs due to accelerated diffusion. |
| Long service life: 20–50+ years depending on the environment, with no intermediate maintenance. | Non-uniform coating thickness on complex geometries; risk of buildup on threads and small holes. |
| Integral coating (internal and external on hollow profiles) prevents under-film corrosion. | Possible hydrogen embrittlement in high-strength steels (tensile strength >1100 MPa / 160 ksi) if pickling is not controlled. |
| Economical process for large series and moderately sized parts (kettles 7–20 m / 23–66 ft in length). | Thermal distortion in thin-walled parts or asymmetric sections due to stresses generated during cooling. |
| Excellent adhesion (metallurgical bond, not just mechanical) and abrasion resistance. | Not applicable on site; requires transport to a galvanizing plant. Maximum part size is limited by bath dimensions. |
| Exposed zinc layer forms a zinc carbonate patina, self-regenerating and environmentally stable. | Initial aesthetic appearance (spangle) may not be acceptable without subsequent chromating or painting treatment. |
Selection Guide
Section titled “Selection Guide”The international standard ISO 1461 (equivalent to BS EN ISO 1461 and ASTM A123/A153) specifies minimum coating thicknesses based on the substrate thickness. To select the appropriate protection system, the following should be considered:
- Exposure environment: rural areas (corrosivity C1-C2 according to ISO 9223) require a minimum thickness of 45–55 µm / 1.77–2.17 mils; industrial or marine environments (C3-C5) require 70–85 µm / 2.76–3.35 mils or more, and are often supplemented with paint (duplex system).
- Substrate type: steels with design thickness ≤1.5 mm / 0.06 in only need 45 µm / 1.77 mils, while parts over 6 mm / 0.24 in require at least 85 µm / 3.35 mils.
- Expected service life: for durations exceeding 30 years in aggressive conditions, a minimum thickness of 85 µm / 3.35 mils or a duplex system (HDG + paint) is recommended.
- Subsequent deformability: Fe-Zn alloy layers are brittle; if bending or forming is required after galvanizing, steels with low silicon content (Si ≤0.03 %) should be evaluated to obtain thinner and more ductile layers (≤70 µm / 2.76 mils of alloy).
Parameter Tables by Material
Section titled “Parameter Tables by Material”Thickness requirements according to ISO 1461 for steel and cast iron
| Substrate thickness (mm) | Material | Minimum coating thickness (µm / mils) | Minimum zinc mass (g/m² / oz/ft²) | Typical corrosion rate in rural environment (µm/year / mils/year) |
|---|---|---|---|---|
| >6 | Structural steel | 85 / 3.35 | 610 / 2.00 | 1–2 / 0.04–0.08 |
| >3 to ≤6 | Structural steel | 70 / 2.76 | 505 / 1.66 | 1–2 / 0.04–0.08 |
| >1.5 to ≤3 | Structural steel | 55 / 2.17 | 395 / 1.29 | 1–2 / 0.04–0.08 |
| ≤1.5 | Structural steel | 45 / 1.77 | 325 / 1.07 | 1–2 / 0.04–0.08 |
| ≥6 | Gray or ductile cast iron | 70 / 2.76 | 505 / 1.66 | 2–3 / 0.08–0.12 |
| <6 | Gray or ductile cast iron | 60 / 2.36 | 430 / 1.41 | 2–3 / 0.08–0.12 |
| Any thickness | Centrifuged parts | 45 / 1.77 | 325 / 1.07 | 1–2 / 0.04–0.08 |
Galvanized steel pipe according to ASTM A53 (Schedule 40) – application examples
| Nominal size (in) | Outside diameter (mm / in) | Nominal inside diameter (mm / in) | Nominal linear weight (kg/m / lb/ft) | Typical coating (µm / mils) |
|---|---|---|---|---|
| ½ | 21.3 / 0.840 | 15.8 / 0.62 | 1.26 / 0.85 | 45–55 / 1.77–2.17 |
| 1 | 33.4 / 1.315 | 26.6 / 1.05 | 2.50 / 1.68 | 45–55 / 1.77–2.17 |
| 2 | 60.3 / 2.375 | 52.5 / 2.07 | 5.43 / 3.65 | 55–70 / 2.17–2.76 |
| 4 | 114.3 / 4.500 | 102.3 / 4.03 | 16.06 / 10.79 | 70–85 / 2.76–3.35 |
| 6 | 168.3 / 6.625 | 154.1 / 6.07 | 28.23 / 18.97 | 85–100 / 3.35–3.94 |
| 8 | 219.1 / 8.625 | 202.7 / 7.98 | 42.49 / 28.55 | 85–100 / 3.35–3.94 |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the temperature of the zinc bath in hot-dip galvanizing?
Section titled “What is the temperature of the zinc bath in hot-dip galvanizing?”The molten zinc bath is maintained at approximately 450 °C / 842 °F during the hot-dip galvanizing process, which ensures a proper metallurgical reaction between the zinc and the steel.
What minimum coating thickness is required for structural steel over 6 mm according to ISO 1461?
Section titled “What minimum coating thickness is required for structural steel over 6 mm according to ISO 1461?”For steel with a thickness greater than 6 mm, the minimum galvanized coating thickness is 85 µm / 3.35 mils, ensuring effective cathodic protection against corrosion.
What is the expected service life of a hot-dip galvanized part in a rural environment?
Section titled “What is the expected service life of a hot-dip galvanized part in a rural environment?”In rural environments with low pollution, the galvanized coating can protect the steel for more than 50 years, because the corrosion rate of zinc is typically only 1–2 µm per year (0.04–0.08 mils/year).
Up to what temperature can galvanized steel be exposed without coating degradation?
Section titled “Up to what temperature can galvanized steel be exposed without coating degradation?”The maximum recommended temperature for continuous service of hot-dip galvanized steel is 200 °C / 392 °F; above this temperature, the zinc layer may detach due to differences in iron and zinc diffusion.
How much zinc is consumed per square meter in a typical 85 µm coating?
Section titled “How much zinc is consumed per square meter in a typical 85 µm coating?”A zinc coating of 85 µm thickness is equivalent to approximately 610 g/m² (2.00 oz/ft²) of zinc, which provides a physical barrier and cathodic protection to the steel substrate.
What is the typical corrosion rate of zinc in an industrial environment compared to a rural one?
Section titled “What is the typical corrosion rate of zinc in an industrial environment compared to a rural one?”In an industrial environment, the corrosion rate of zinc can increase to 4–8 µm per year (0.16–0.31 mils/year), compared to 1–2 µm per year in a rural environment, which reduces the service life of the coating to approximately 15–25 years for the same 85 µm thickness.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/ansi-steel-pipes-d_305.html
- efunda.com: https://www.efunda.com/processes/plastic_molding/molding_dip.cfm
- manufacturingguide.com: https://www.manufacturingguide.com/en/node/6071