How the Coating Forms
Hot-dip galvanizing is a metallurgical reaction process rather than a coating operation. Cleaned and fluxed steel is immersed in molten zinc, conventionally held at about 445 to 465 °C, and zinc diffuses into the steel surface while iron diffuses outward. The result is a layered coating: an inner layer of zinc-iron intermetallic phases alongside the steel, and an outer layer of nearly pure zinc that solidifies as the part is withdrawn.
Two dynamic balances govern the process. The first is thermal, as the cold workpiece absorbs heat from the bath and its surface temperature rises. The second is the zinc-iron exchange, which becomes significant once the surface passes roughly 200 °C and continues until the part cools below that temperature after withdrawal. Thickness is fixed at the moment the reaction effectively stops on cooling, which is why immersion time, withdrawal speed and cooling practice all influence the result.
Factors That Drive Coating Thickness
| Factor | Effect on coating growth | Notes for ordering |
|---|---|---|
| Steel thickness and heat content | Thicker steel stays hot longer and grows a thicker coating | Drives the thickness class used in the requirement table |
| Silicon content of the steel | Contents around 0.03% to 0.12% greatly accelerate growth | Reactive heats produce thick matte grey coatings that may need bath adjustment |
| Phosphorus content of the steel | Above about 0.03% accelerates growth further | Residual levels should be declared on the mill certificate |
| Surface condition | Rough or heavily blasted surfaces take up more zinc | Abrasive blast grade influences the coating mass achieved |
| Bath temperature and immersion time | Higher temperature and longer immersion increase growth up to a limit | Process parameters are adjusted rather than pushed beyond the ductility limit |
| Withdrawal and cooling | Slow drainage leaves heavier and uneven deposits | Drainage design of the part matters as much as the process |
Requirement Tables by Steel Thickness Group
GB/T 13912 and ISO 1461 use the same structure: the requirement depends on the thickness of the steel being galvanized, and compliance is judged on a mean value together with a minimum local value. Coating mass is quoted alongside thickness because the gravimetric test measures mass, and the conversion of one micrometre of zinc to about 7.14 g/m² links the two.
| Steel thickness group | Minimum local thickness | Minimum mean thickness | Mean coating mass |
|---|---|---|---|
| Below 1.5 mm | 35 µm | 45 µm | 320 g/m² |
| 1.5 mm up to 3 mm | 45 µm | 55 µm | 395 g/m² |
| 3 mm up to 6 mm | 55 µm | 70 µm | 505 g/m² |
| 6 mm and above | 70 µm | 85 µm | 610 g/m² |
The bath itself is subject to a purity requirement in the standard, and the zinc used must meet the specified minimum content. Additions are controlled to keep the coating ductile, because alloying elements that accelerate growth also embrittle the layer.
Why More Thickness Is Not Always Better
Coating ductility falls as the alloy layers thicken. A coating that is far above the requirement can crack and flake when the part is bent, dropped or cold formed after galvanizing.
Thick coatings on threaded fasteners change the thread fit, so fastener coatings are specified separately with allowance for the coating thickness.
Very thick coatings are more likely to be uneven, with heavy runs and nodules that interfere with assembly and with coating thickness measurement.
Where the environment demands more protection than the standard table gives, the answer is usually a duplex system with a paint layer on top of the galvanizing, or a change of coating family, rather than an uncontrolled increase in zinc thickness.
Inspection, Repair and Acceptance
Coating mass is determined gravimetrically to ISO 1460 or GB/T 1839, which is the reference method when a dispute arises. Stripping is done on a defined area with a defined solution and the result is converted to thickness.
Coating thickness on site is measured by the magnetic method of ISO 2178, with a calibrated gauge and a defined number of readings per reference area, reporting the mean and every individual value.
Adhesion is verified by knife or bend testing, and appearance is recorded, remembering that a matte grey finish is expected on reactive steel and is not a defect in itself.
Damaged areas from welding, cutting, handling or site work are cleaned and coated with a zinc-rich repair product applied to the practice described in ASTM A780, with the repair area recorded on the inspection sheet.
Production is controlled through the process parameters recorded during dipping, the steel chemistry declared on the incoming certificate, and the coating test results for the lot. Documentation normally includes an EN 10204 3.1 certificate covering the steel and a coating report giving the test method and results.
Frequently Asked Questions
Q: What determines the required coating thickness?
A: The thickness of the steel being galvanized. Steel above 6 mm requires a minimum mean of 85 µm, while steel below 1.5 mm requires 45 µm, because thinner steel holds less heat and grows a thinner coating.
Q: Why does galvanized steel sometimes come out dull grey instead of bright?
A: A matte grey finish indicates a thick alloy layer, usually caused by reactive silicon or phosphorus in the steel. It shows the coating grew faster than normal and is not by itself a defect.
Q: Can the coating be too thick?
A: Yes. Above roughly 150 to 200 µm the coating becomes brittle and can crack or flake during handling and forming, so the requirement table values should be met without excessive overshoot.
Q: How is the coating checked without cutting a part?
A: By magnetic thickness gauge on the surface to ISO 2178, or by stripping a small defined area and weighing to determine coating mass to ISO 1460 or GB/T 1839 when a definitive result is needed.
Q: Does a heavier coating give a proportionally longer life?
A: Service life tracks coating mass while the coating remains intact, so a heavier coating lasts longer in the same atmosphere, provided it stays adherent and ductile and the cut edges and damaged areas are properly repaired.
