Hot-dip galvanized round bars are produced by a continuous sequence of surface preparation, molten zinc immersion and post-treatment, and each stage controls a different part of the final coating quality. The process is applied to carbon and low-alloy steel bars in the 5 mm to 200 mm diameter range, using equipment that can handle straight lengths rather than coiled strip. Understanding the sequence makes it easier to specify the right coating and to judge the inspection results that come with each delivery.
Step 1: Substrate Selection and Surface Pretreatment
The substrate is chosen for strength rather than for coating behaviour. Carbon steel grades such as Q235, Q345 and 45# are the usual choices, and low-alloy grades such as 20Cr are used where hardenability is needed. Bars are normally supplied in diameters from 5 mm to 200 mm and cut to the ordered length before galvanizing.
Degreasing: alkaline solution, typically based on sodium hydroxide, or an organic solvent removes oil and grease that would otherwise stop the zinc from wetting the steel.
Pickling: hydrochloric or sulfuric acid removes mill scale and rust so that a clean, active steel surface is exposed.
Rinsing: residual acid must be rinsed away before fluxing to avoid drag-out into the next bath.
Step 2: The Core Hot-Dip Galvanizing Stage
After pickling the bars are immersed in a flux bath, usually a mixture of zinc chloride and ammonium chloride. The flux forms a zinc chloride salt film that prevents re-oxidation during transfer and helps the molten zinc to wet the steel. The bar then enters the zinc pot.
Zinc bath: the molten zinc is held at 430 to 460 degrees C, with zinc purity of at least 99.5 percent.
Immersion: bars are dipped vertically and the dwell time is adjusted to the diameter so that the coating builds evenly around the full circumference.
Air knife: as the bar leaves the pot, high-pressure air wipes off surplus zinc and sets the coating mass.
Cooling: natural air cooling or water quenching solidifies the coating and develops the zinc-iron alloy layer beneath the pure zinc layer.
Step 3: Post-Treatment and Quality Inspection
Passivation: immersion in a chromate or chromium-free solution forms a conversion film that slows white rust during storage.
Oiling: a sprayed anti-rust oil protects the bar in humid storage and during shipment.
Coating thickness: checked with a magnetic thickness gauge at several points along the bar.
Appearance: visual inspection removes bars with zinc nodules, missed plating or surface cracks.
Adhesion: a bend test or hammer test confirms that the coating stays bonded to the steel.
Critical Control Points
| Parameter | Control value | Effect if out of range |
|---|---|---|
| Pickling acid | Hydrochloric acid 15-20 percent at 50-80 degrees C | Over-pickling attacks the substrate; under-pickling leaves scale |
| Zinc purity | 99.5 percent minimum | Impurities raise coating brittleness |
| Iron in the zinc bath | Below 0.03 percent | Higher iron makes the alloy layer thick and brittle |
| Zinc bath temperature | 430-460 degrees C | Too cold gives poor wetting; too hot accelerates iron pickup |
| Cooling rate | Circulating water for large diameters | Fast uneven cooling risks stress cracking |
Common Defects and Corrective Actions
| Defect | Corrective action |
|---|---|
| Uneven zinc layer | Adjust the bar lifting speed and recalibrate the air knife |
| Missed plating | Strengthen pickling and renew the flux bath |
| Excessive coating thickness | Shorten immersion time and raise the zinc bath temperature slightly |
Frequently Asked Questions
Q: What diameter range can be hot-dip galvanized as round bar?
Bars from about 5 mm to 200 mm in diameter are handled on a routine basis. Small diameters need shorter immersion times and gentler handling, while large diameters need controlled cooling to avoid internal stresses.
Q: Why is the bar dipped vertically rather than horizontally?
Vertical dipping lets the molten zinc drain evenly along the full length and prevents the zinc from pooling on one side. It also makes the air-knife wipe more uniform around the circumference.
Q: What coating mass is normal for a galvanized round bar?
Coating mass is agreed with the customer and depends on the exposure class. Heavier coatings are specified for outdoor and coastal service, while lighter coatings are used where the bar will be painted or used indoors.
Q: How is coating adhesion tested in production?
The two routine checks are the bend test, in which the bar is bent and the coating is examined for flaking, and the hammer test, in which the coating is struck and inspected for lifting. Both are acceptance tests, not destructive material tests.
Q: Can galvanized round bars be welded after galvanizing?
Yes, but the zinc coating must be removed locally at the weld zone and the area re-protected afterwards. Welding through zinc produces fume and porosity, so local stripping and good extraction are standard practice.
Q: What causes a brittle or flaking coating?
A brittle coating usually points to excessive iron in the zinc bath, an over-thick alloy layer from too long an immersion, or over-pickling of the substrate. Monitoring iron content below 0.03 percent and controlling immersion time prevents most cases.