Why Zinc Coating Uniformity Determines Galvanized Sheet Performance
A galvanized sheet is only as good as the evenness of its zinc layer. Where the coating runs thick, the panel may crack or flake during bending; where it runs thin, the same panel rusts early. Uniformity therefore governs both the forming behaviour and the service life of hot-dip and electrogalvanized products, and continuous lines monitor it closely against the coating-mass tolerances defined in ASTM A653/A653M and the general requirements of ASTM A924/A924M.
Uneven deposition is almost never caused by one variable. It is the combined result of substrate surface condition, galvanizing process parameters, bath or electrolyte chemistry and the mechanical state of the line equipment. Working through these four areas in order is the fastest route to a stable coating.
Pre-treatment: residual oil, oxide scale and unsuitable roughness block or distort zinc nucleation.
Process parameters: bath temperature, immersion time, strip entry geometry and air knife settings decide how much zinc remains on the strip.
Bath and electrolyte chemistry: iron, lead, cadmium and heavy-metal impurities change zinc fluidity and deposition rate.
Equipment condition: worn rollers, clogged nozzles and oxidised conductor rolls create local thin or thick bands.
Pre-Treatment: Degreasing, Pickling and Substrate Roughness
Degreasing is the first place where a non-uniform coating is created. Rolling oil and rust-preventive oil left on the strip physically separate zinc from steel, producing bare spots or an abnormally thin coating. A combined alkaline spray and immersion sequence removes them reliably, followed by acid pickling to strip the oxide scale and a neutralisation rinse.
| Stage | Typical parameter | Consequence if out of range |
|---|---|---|
| Alkaline degreasing | 2-5 % alkaline solution, 60-80 °C, 3-5 min | Residual oil causes bare spots; over-long treatment etches the surface |
| Acid pickling | Hydrochloric acid 10-20 %, 20-40 °C, 5-10 min | Under-pickling leaves scale; over-pickling pits the steel and traps zinc |
| Neutralisation and rinse | Weak alkaline rinse to neutral pH | Residual acid causes flash rust and patchy adhesion |
| Substrate roughness | Ra 0.8-3.2 µm for hot-dip substrates | Too rough concentrates zinc in valleys; too smooth weakens mechanical keying |
The pickling window is narrow for a reason. Under-pickling leaves a residual scale film that prevents zinc from bonding in patches, while over-pickling removes so much substrate that pits form; those pits then act as sinks and fill with extra zinc, so the coating ends up thick where it should be thin. Roughness must be set during rolling. If the rolled surface falls outside the recommended window, light grinding or a change of roll surface finish is the correct correction, not a change in bath chemistry.
Hot-Dip Control: Bath Temperature, Immersion Time and Air Knife Settings
In hot-dip galvanizing the amount of zinc that stays on the strip is decided in two moments: the seconds spent inside the zinc bath, and the fraction of a second in which the air knife wipes the strip as it leaves. Both must be tightly controlled.
| Parameter | Recommended setting | Failure mode when it drifts |
|---|---|---|
| Zinc bath temperature | 445-455 °C, fluctuation within ±5 °C | Above 470 °C zinc drains away by gravity; below 430 °C it spreads poorly |
| Immersion time, thin strip | 10-15 s for thickness ≤ 2 mm | Too short gives incomplete coverage; too long builds a brittle alloy layer |
| Immersion time, heavy strip | 15-25 s for thickness 2-6 mm | Uneven zinc-iron growth and a rough surface |
| Strip entry angle | 85-90°, close to vertical | Angled entry makes one area contact the bath first and deposit unevenly |
| Strip entry speed | 1-2 m/min, held stable | Fast entry does not allow the bath to spread; slow entry over-immerses areas |
| Air knife pressure | 0.2-0.3 MPa for 50-80 g/m²; 0.1-0.2 MPa for 100-150 g/m² | Too low leaves excess zinc; too high cuts through the coating on thin strip |
| Air knife distance and angle | 100-150 mm, perpendicular to the strip within ±2° | Unequal distance between the two sides gives side-to-side thickness differences |
| Nozzle gap condition | Gap error ≤ 0.1 mm, cleaned on schedule | Zinc dross clogging creates strong and weak air zones and streaky coating |
The air knife is the single most powerful lever on coating uniformity. A nozzle set slightly closer on one side, tilted by more than two degrees, or partly blocked by dross will write a permanent thickness pattern onto the coil that no downstream process can remove. The gap should be checked with a feeler gauge and the slot cleaned on a fixed routine, not only when coating-mass readings begin to drift.
Electrogalvanizing: Current Density and Electrolyte Balance
Electrolytic lines deposit zinc ion by ion, so the current distribution across the strip becomes the coating distribution. Local current density that is too high causes rapid zinc accumulation; a density that is too low leaves the coating thin.
Current density: 10-30 A/dm², adjusted to the electrolyte system in use.
Electrode symmetry: equal distance from both electrodes to the strip, tolerance ≤ 5 mm.
Edge control: auxiliary anodes or shielding plates at the strip edges, where current naturally concentrates.
Zinc ion concentration: 30-60 g/L; too low starves deposition, too high promotes zinc dust that roughens the surface.
pH: 3-5 for acidic electrolytes, 8-10 for alkaline systems; large swings cause nodular growth.
Temperature: 20-50 °C; overheating destabilises the electrolyte and flattens deposition uniformity.
Bath Chemistry, Filtration and Equipment Maintenance
Impurities quietly destroy uniformity. Iron above roughly 0.05 % forms zinc-iron dross that adheres to the strip and raises bumps; lead above about 0.03 % and cadmium above about 0.01 % reduce bath fluidity so the zinc no longer spreads evenly. Dross must be skimmed on schedule and bath chemistry checked continuously. In electrolytic lines, heavy-metal impurities such as iron, copper and nickel should stay below 0.1 g/L, because they deposit preferentially and crowd out the zinc; filtration to 5 µm or electrolytic purification keeps them in check.
Mechanical condition matters just as much. Worn or misaligned sink and stabiliser rolls make the strip wobble or touch the roll, thinning the coating along the contact line, so rolls should be polished periodically and levelled to within 0.5 mm/m. In electrogalvanizing, conductor rolls carrying an oxide film or a loose contact interrupt current locally, so their surfaces must be cleaned and the contact pressure kept even.
Frequently Asked Questions
Q: What is the most influential single parameter for zinc layer uniformity?
On hot-dip lines it is air knife control; pressure, distance, angle and nozzle gap condition together decide the final coating profile.
Q: Which zinc bath temperature should be maintained?
Keep the bath between 445 °C and 455 °C with fluctuation held within ±5 °C. Higher temperatures thin the coating by drainage and lower ones stop it spreading.
Q: Can a substrate that is too rough be corrected on the galvanizing line?
No. Roughness is set during rolling, so it must be corrected in the mill or by light grinding before galvanizing.
Q: What impurity limits apply to a hot-dip zinc bath?
Iron should stay at or below 0.05 %, lead at or below 0.03 % and cadmium at or below 0.01 %.
Q: Why does coating build up at the edges of electrolytically galvanized strip?
Current concentrates at the edges. Auxiliary anodes or shielding plates spread the current and restore an even deposit.
Q: How often should air knife nozzles be cleaned?
Clean them on a fixed routine rather than on demand, and re-check the gap whenever the error approaches 0.1 mm.